THE CHILDREN'S BOOK OF STARS THE CHILDREN'S BOOK OF STARS MITTON A. &C. BLACK [Illustration: THE MOON-CHILD MUST KEEP ON RUNNING ROUND HER. P. 11. ] THE CHILDREN'S BOOK OF STARS +-------------------------------------------------+ | BY THE SAME AUTHOR | | | | CHILDREN'S BOOK OF LONDON | | | | CONTAINING 12 FULL-PAGE ILLUSTRATIONS | | IN COLOUR BY JOHN WILLIAMSON | | PRICE =6s. = | | | | 'The stories are told in a way that is bound | | to rivet attention, and the historical sketches| | will leave a lasting impression on the minds | | of young readers which will be very useful | | when their studies in history become more | | advanced. '--_Scotsman. _ | | | | | | ANIMAL AUTOBIOGRAPHIES | | | | THE DOG | | | | WITH 12 FULL-PAGE ILLUSTRATIONS IN | | COLOUR BY J. WILLIAMSON | | | | PRICE =6s. = | | | | 'A true life history, written "out of the | | fulness of first-hand knowledge" by an author | | who is thoroughly acquainted with all the | | ways of "the friend of man. "'--_Glasgow | | Herald. _ | | | | 'The story is admirably told in clear and | | fascinating language. '--_Freeman's Journal. _ | | | | A. & C. BLACK. SOHO SQUARE. LONDON, W. | | | +-------------------------------------------------+ AGENTS AMERICA THE MACMILLAN COMPANY 64 & 66 FIFTH AVENUE, NEW YORK CANADA THE MACMILLAN COMPANY OF CANADA, LTD. 27 RICHMOND STREET WEST, TORONTO INDIA MACMILLAN & COMPANY, LTD. MACMILLAN BUILDING, BOMBAY 309 BOW BAZAAR STREET, CALCUTTA THE CHILDREN'S BOOK OF STARS BY G. E. MITTON AUTHOR OF 'THE CHILDREN'S BOOK OF LONDON, ' 'ANIMAL AUTOBIOGRAPHIES: THE DOG, ' ETC. LONDON ADAM AND CHARLES BLACK 1907 _Published September, 1907_ PREFACE It was the intention of the late Agnes Clerke to write the preface tothis 'Children's Book of Stars. ' Miss Clerke took a warm and sympatheticinterest in the authoress and her work, but her lamented death occurredbefore this kindly intention could be fulfilled. I cannot pretend to write adequately as her substitute, but I could notresist the appeal made to me by the author, in the name and for the sakeof her dear friend and mine, to write a few words of introduction. I am in no way responsible either for the plan or for any portion ofthis work, but I can commend it as a book, written in a simple andpleasant style, calculated to awaken the interest of intelligentchildren, and to enable parents otherwise ignorant or astronomy toanswer many of those puzzling questions which such children often put. DAVID GILL. AUTHOR'S NOTE This little work is the outcome of many suggestions on the part offriends who were anxious to teach their small children something of themarvels of the heavens, but found it exceedingly difficult to get holdof a book wherein the intense fascination of the subject was not lost inconventional phraseology--a book in which the stupendous facts werestated in language simple enough to be read aloud to a child withoutparaphrase. Whatever merit there may be in the present work is due entirely to myfriend Agnes Clerke, the well-known writer on astronomy; the faults areall my own. She gave me the impetus to begin by her warm encouragement, and she helped me to continue by hearing every chapter read as it waswritten, and by discussing its successor and making suggestions for it. Thus she heard the whole book in MS. A week after the last chapter hadbeen read to her I started on a journey lasting many months, and whileI was in the Far East the news reached me of her death, by which theworld is the poorer. For her sake, as he has stated, her friend SirDavid Gill, K. C. B. , kindly undertook to supply the missing preface. G. E. MITTON. CONTENTS CHAPTER I PAGE THE EARTH 1 CHAPTER II HANGING IN SPACE 13 CHAPTER III THE SHINING MOON 21 CHAPTER IV THE EARTH'S BROTHERS AND SISTER 32 CHAPTER V FOUR SMALL WORLDS 48 CHAPTER VI FOUR LARGE WORLDS 67 CHAPTER VII THE SUN 89 CHAPTER VIII SHINING VISITORS 103 CHAPTER IX SHOOTING STARS AND FIERY BALLS 120 CHAPTER X THE GLITTERING HEAVENS 135 CHAPTER XI THE CONSTELLATIONS 148 CHAPTER XII WHAT THE STARS ARE MADE OF 159 CHAPTER XIII RESTLESS STARS 170 CHAPTER XIV THE COLOURS OF THE STARS 176 CHAPTER XV TEMPORARY AND VARIABLE STARS 188 CHAPTER XVI STAR CLUSTERS AND NEBULĘ 197 ILLUSTRATIONS PRINTED IN COLOUR THE MOON-CHILD MUST KEEP ON RUNNING ROUND HER _Frontispiece_ FACING PAGE THE EARTH AND MOON HANGING IN SPACE 16 THE ENGLISH SUMMER AND WINTER 40 JUPITER AND ONE OF HIS MOONS 70 THE PLANET SATURN AND TWO OF HIS MOONS 78 FLAMES FROM THE SUN 100 THE COMET IN THE BAYEUX TAPESTRY 104 A STICK THRUST INTO THE WATER APPEARS CROOKED 114 CONSTELLATIONS NEAR THE POLE STAR 150 ORION AND HIS NEIGHBOURS 154 THE SPECTRUM OF THE SUN AND SIRIUS 168 ILLUSTRATIONS IN BLACK AND WHITE PAGE THE MOON _facing_ 24 AN ECLIPSE OF THE MOON 28 AN ECLIPSE OF THE SUN 29 THE MOON RAISING THE TIDES 30 COMPARATIVE SIZES OF THE PLANETS 35 DIFFERENT PHASES OF VENUS 51 ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY 55 MAP OF MARS _facing_ 56 ORBITS OF THE EARTH AND MARS 63 JUPITER AND HIS PRINCIPAL MOONS 72 SUN-SPOTS _facing_ 98 A GREAT COMET " 118 THE GREAT NEBULA IN ANDROMEDA " 202 THE CHILDREN'S BOOK OF STARS CHAPTER I THE EARTH It is a curious fact that when we are used to things, we often do notnotice them, and things which we do every day cease to attract ourattention. We find an instance of this in the curious change that comesover objects the further they are removed from us. They grow smaller andsmaller, so that at a distance a grown-up person looks no larger than adoll; and a short stick planted in the ground only a few feet awayappears as long as a much longer one at ten times the distance. Thisprocess is going on all round us every minute: houses, trees, buildings, animals, all seem larger or smaller in proportion to their distance fromus. Sometimes I have seen a row of raindrops hanging on a bar by thewindow. When the sun catches one of them, it shines so brilliantly thatit is as dazzling as a star; but my sense tells me it is a raindrop, andnot a star at all. It is only because it is so near it seems as brightand important as a mighty star very, very far away. We are so much accustomed to this fact that we get into a habit ofjudging the distance of things by their size. If we see two lightsshining on a dark night, and one is much larger than the other, we thinkthat the bright one must be nearer to us; yet it need not necessarily beso, for the two lights might possibly be at the same distance from us, and one be large and the other small. There is no way in which we cantell the truth by just looking at them. Now, if we go out on any finemoonlight night and look up at the sky, we shall see one object thereapparently much larger than any other, and that is the moon, so thequestion that occurs to us at once is, Is the moon really very muchlarger than any of the stars, or does it only seem so because it is verymuch nearer to us? As a matter of fact, the moon is one of the smallestobjects in view, only, as it is our nearest neighbour, it appears veryconspicuous. Having learned this, we shall probably look about to seewhat else there is to attract attention, and we may notice one starshining very brilliantly, almost like a little lamp, rather low down inthe sky, in that part of it where the sun has lately set. It is sobeautifully bright that it makes all the others look insignificant incomparison, yet it is not really large compared with the others, only, as it comes nearer to us than anything else in the sky except the moon, it looks larger than it has any right to do in comparison with theothers. After this we might jump to the conclusion that all the bright largestars are really small and near to us, and all the faintly shining oneslarge and far away. But that would not be true at all, for some brightones are very far away and some faint ones comparatively near, so thatall we can do is to learn about them from the people who have studiedthem and found out about them, and then we shall know of our ownknowledge which of them seem bright only because they are nearer thanthe others, and which are really very, very brilliant, and so stillshine brightly, though set in space at an almost infinite distance fromus. The sun, as we all know, appears to cross the sky every day; he gets upin the east and drops down in the west, and the moon does the same, only the moon is unlike the sun in this, that it changes its shapecontinually. We see a crescent moon growing every night larger andlarger, until it becomes full and fat and round, and then it growsthinner and thinner, until it dies away; and after a little while itbegins again, and goes through all the same changes once more. I willtell you why this is so further on, when we have a chapter all about themoon. If you watch the stars quietly for at least five minutes, you will seethat they too are moving steadily on in the same way as the sun andmoon. Watch one bright star coming out from behind a chimney-pot, andafter about five minutes you will see that it has changed its place. Yetthis is not true of all, for if we watch carefully we shall find thatsome, fairly high up in the sky, do not appear to move at all. The fewwhich are moving so slowly that they seem to us to stand still are at apart of the sky close to the Pole Star, so called because it is alwaysabove the North Pole of the earth. I will explain to you how to find itin the sky for yourselves later on, but now you can ask anyone to pointit out. Watch it. It appears to be fixed in one place, while the otherstars are swinging round it in circles. In fact, it is as if we on theearth were inside a great hollow globe or ball, which continually turnedround, with the Pole Star near the top of the globe; and you know thatif you put your finger on the spot at the top of a spinning globe orball, you can hold it there while all the rest of the ball runs round. Now, if you had to explain things to yourself, you would naturallythink: 'Here is the great solid earth standing still, and the sun andmoon go round it; the stars are all turning round it too, just as ifthey were fixed on to the inside of a hollow globe; we on the earth arein the middle looking up at them; and this great globe is slowlywheeling round us night by night. ' In the childhood of the world men believed that this was reallytrue--that the earth was the centre of the universe, that the sun andmoon and all the hosts of heaven were there solely to light and benefitus; but as the world grew wiser the wonders of creation were fathomedlittle by little. Some men devoted their whole lives to watching theheavens, and the real state of things was gradually revealed to them. The first great discovery was that of the daily movement of the earth, its rotation on its own axis, which makes it appear as if all theseshining things went round it. It is indeed a very difficult matter tojudge which of two objects is moving unless we can compare them bothwith something outside. You must have noticed this when you are sittingin a train at a station, and there is another train on the other side ofyours. For if one of the trains moves gently, either yours or the other, you cannot tell which one it is unless you look at the station platform;and if your position remains the same in regard to that, you know thatyour train is still standing, while the other one beside it has begun tomove. And I am quite sure that there is no one of us who has not, at onetime or another, stood on a bridge and watched the water running awayunderneath until we felt quite dizzy, and it seemed as if the water werestanding still and the bridge, with ourselves on it, was flying swiftlyaway backwards. It is only when we turn to the banks and find themstanding still, that we realize the bridge is not moving, and that it isthe running water that makes it seem to do so. These everyday instancesshow us how difficult it is to judge whether we are moving or an outsideobject unless we have something else to compare with it. And themarvellous truth is that, instead of the sun and moon and stars rollinground the earth, it is the earth that is spinning round day by day, while the sun and the stars are comparatively still; and, though themoon does move, yet when we see her get up in the east and go down inthe west that is due to our own movement and not to hers. The earth turns completely round once in a day and night. If you take anorange and stick a knitting-needle through it, and hold it so that theneedle is not quite straight up but a little slanting, and then twirl itround, you will get quite a good idea of the earth, though of coursethere is no great pole like a gigantic needle stuck through it, that isonly to make it easy for you to hold it by. In spinning the orange youare turning it as the earth turns day by day, or, as astronomers expressit, as it rotates on its axis. There is a story of a cruel Eastern King who told a prisoner that hemust die if he did not answer three questions correctly, and thequestions were very difficult; this is one of them: 'How long would it take a man to go round the earth if he never stoppedto eat or drink on the way?' And the prisoner answered promptly: 'If he rose with the sun and keptpace with it all day, and never stopped for a moment to eat or drink, hewould take just twenty-four hours, Your Royal Highness. ' For in thosedays it was supposed that the sun went round the earth. Everyone is so remarkably clever nowadays that I am sure there will besomeone clever enough to object that, if what I have said is true, therewould be a great draught, for the air would be rushing past us. But, asa matter of fact, the air goes with us too. If you are inside a railwaycarriage with the windows shut you do not feel the rush of air, becausethe air in the carriage travels with you; and it is the same thing onthe earth. The air which surrounds the earth clings to it and goes roundwith it, so there is no continuous breeze from this cause. But the spinning round on its own axis is not the earth's only movement, for all the time it is also moving on round the sun, and once in a wholeyear it completes its journey and comes back to the place from whence itstarted. Thus the turning round like a top or rotating on its axis makesthe day and night, and the going in a great ring or revolving round thesun makes the years. Our time is divided into other sections besides days and years. We have, for instance, weeks and months. The weeks have nothing to do with theearth's movements; they are only made by man to break up the months;but the months are really decided by something over which we have nocontrol. They are due to the moon, and, as I have said already, the moonmust have a chapter to herself, so we won't say any more about themonths here. If any friend of ours goes to India or New Zealand or America, we lookupon him as a great traveller; yet every baby who has lived one year onthe earth has travelled millions of miles without the slightest effort. Every day of our lives we are all flung through space without knowing itor thinking of it. It is as if we were all shut up in a comfortabletravelling car, and were provided with so many books and pictures andcompanions that we never cared to look out of the windows, so that hourby hour as we were carried along over miles of space we never gave thema thought. Even the most wonderful car ever made by man rumbles andcreaks and shakes, so that we cannot help knowing it is moving; but thisbeautiful travelling carriage of ours called the earth makes never acreak or groan as she spins in her age-long journey. It is alwaysastonishing to me that so few people care to look out of the window aswe fly along; most of them are far too much absorbed in their littlepetty daily concerns ever to lift their eyes from them. It is true thatsometimes the blinds are down, for the sky is thickly covered withclouds, and we cannot see anything even if we want to. It is true alsothat we cannot see much of the scenery in the daytime, for the sunshining on the air makes a veil of blue glory, which hides the stars;but on clear nights we can see on every side numbers of stars quite asinteresting and beautiful as any landscape; and yet millions of peoplenever look up, never give a thought to the wonderful scenery throughwhich their car is rushing. By reason of the onward rush of the earth in space we are carried over adistance of at least eighteen miles every second. Think of it: as wedraw a breath we are eighteen miles away in space from the point we wereat before, and this goes on unceasingly day and night. These astonishingfacts make us feel how small and feeble we are, but we can take comfortin the thought that though our bodies are insignificant, the brain ofman, which has discovered these startling facts, must in itself beregarded as one of the most marvellous of all the mysteries amid whichwe live. Well, we have arrived at some idea of our earth's position; we knowthat the earth is turning round day by day, and progressing round thesun year by year, and that all around lie the sentinel stars, scatteredon a background of infinite space. If you take an older boy or girl andlet him or her stand in the middle to represent the sun, then a smallerone would be the earth, and the smallest of all the moon; only in truthwe could never get anyone large enough to represent the sun fairly, forthe biggest giant that ever lived would be much too small in proportion. The one representing the sun must stand in the middle, and turn slowlyround and round. Then let the earth-child turn too, and all the time sheis spinning like a top she must be also hastening on in a big ring roundthe sun; but she must not go too fast, for the little moon-child mustkeep on running round her all the time. And the moon-child must keep herface turned always to the earth, so that the earth never sees her back. That is an odd thing, isn't it? We have never seen the other side of themoon, which goes round us, always presenting the same face to us. The earth is not the only world going round the sun; she has manybrothers and a sister; some are nearer to the sun than she is, and someare further away, but all circle round the great central light-giver inrings lying one outside the other. These worlds are called planets, andthe earth is one of them, and one of the smaller ones, too, nothing sogreat and important as we might have imagined. CHAPTER II HANGING IN SPACE If you are holding something in your hand and you let it go, whathappens? It falls to the ground, of course. Now, why should it do so?You will say: 'How could it do anything else?' But that is only becauseyou are hampered by custom. Try to shake yourself free, and think, Whyshould it go down instead of up or any other way? The first man who wasclever enough to find some sort of an answer to this question was thegreat philosopher Sir Isaac Newton, though he was not quite the first tobe puzzled by it. After years of study he discovered that every thingattracts every other thing in proportion to their masses (which is whatyou know as weight) and their distance from each other. In morescientific language, we should say every _body_ instead of every_thing_, for the word body does not only mean a living body, but everylump or mass of matter in the universe. The earth is a body in thissense, and so is the table or anything else you could name. Now as theearth is immeasurably heavier than anything that is on it, it pullseverything toward itself with such force that the little pulls of otherthings upon each other are not noticed. The earth draws us all towardit. It is holding us down to it every minute of the day. If we want tomove we have to exert another force in order to overcome this attractionof the earth, so we exert our own muscles and lift first one foot andthen the other away from the earth, and the effort we make in doing thistires us. All the while you are walking or running you are exercisingforce to lift your feet away from the ground. The pull of the earth iscalled gravitation. Just remember that, while we go on to something elsewhich is almost as astonishing. We know that nothing here on earth continues to move for ever;everything has to be kept going. Anything left to itself has a tendencyto stop. Why is this? This is because here in the world there issomething that fights against the moving thing and tries to stop it, whether it be sent along the ground or thrown up in the air. You knowwhat friction is, of course. If you rub your hands along any roughsubstance you will quickly feel it, but on a smooth substance you feelit less. That is why if you send a stone spinning along a carpet or arough road it stops comparatively soon, whereas if you use the sameamount of force and send it along a sheet of ice it goes on moving muchlonger. This kind of resistance, which we call friction, is one of thecauses which is at work to bring things to a standstill; and anothercause is the resistance of the air, which is friction in another form. It may be a perfectly still day, yet if you are bicycling you arebreaking through the air all the time, just as you would be throughwater in swimming, only the resistance of the air is less than that ofwater. As the friction or the resistance of the air, or both combined, gradually lessens the pace of the stone you sent off with such force, the gravitation of the earth begins to be felt. When the stone firststarted the force you gave to it was enough to overcome the gravitationforce, but as the stone moves more slowly the earth-pull asserts itself, and the stone drops down to the ground and lies still upon the surface. Now, if there were no friction, and therefore no resistance, there wouldbe no reason why anything once set moving should not go on moving forever. The force you give to any object you throw is enough to overcomegravitation; and it is only when the first force has been diminished byfriction that the earth asserts its authority and pulls the movingobject toward it. If it were possible to get outside the air and out ofreach of the pull of the earth, we might fling a ball off into space, and it would go on in a straight line until something pulled it toitself by the force of gravity. Gravitation affects everything connected with the earth; even our air isheld to the earth by gravitation. It grows thinner and thinner as we getfurther away from the earth. At the top of a high mountain the air is sothin that men have difficulty in breathing, and at a certain height theycould not breathe at all. As they cannot breathe in very fine air, it isimpossible for them to tell by personal experiment exactly where the airends; but they have tried to find out in other ways, and thoughdifferent men have come to different conclusions on the subject, it issafe to say that at about two hundred miles above the earth there isnothing that could be called air. Thus we can now picture our spinningearth clothed in a garment of air that clings closely about her, andgrows thinner and thinner until it melts away altogether, for there isno air in space. [Illustration: THE EARTH AND MOON HANGING IN SPACE] Now in the beginning God made the world, and set it off by a firstimpulse. We know nothing about the details, though further on you shallhear what is generally supposed to have taken place; we only know that, at some remote age, this world, probably very different from what it isnow, together with the other planets, was sent spinning off into spaceon its age-long journey. These planets were not sent off at random, butmust have had some particular connection with each other and with thesun, for they all belong to one system or family, and act and react oneach other. Now, if they had been at rest and not in movement, theywould have fallen right into the sun, drawn by the force of gravitation;then they would have been burned up, and there would have been an end ofthem. But the first force had imparted to them the impulse to go on in astraight line, so when the sun pulled the result was a movement betweenthe two: the planets did not continue to move in a straight line, neither did they fall on to the sun, but they went on a course betweenthe two--that is, a circle--for the sun never let them get right awayfrom him, but compelled them to move in circles round him. There is avery common instance of this kind of thing which we can see, or perhapsfeel, every day. If you try to sit still on a bicycle you tumble off, because the earth pulls you down to itself; but if, by using the forceof your own muscles, you give the bicycle a forward movement thisresists the earth-pull, and the result is the bicycle runs along theground. It does not get right away from the earth, not even two or threefeet above ground; it is held to the earth, but still it goes forwardand does not fall over, for the movement is made up of the earth-pull, which holds it to the ground, and the forward movement, which propels italong. Then again, as another instance, if you tie a ball to a stringand whirl it round you, so long as you keep on whirling it will not fallto the ground, but the moment you stop down it drops, for there isnothing to fight against the pull of gravitation. Thus we can picturethe earth and all the planets as if they were swinging round the sun, held by invisible strings. It is the combination of two forces thatkeeps them in their places--the first force and the sun's pull. It isvery wonderful to think of. Here we are swinging in space on a ball thatseems only large to us because we are so much smaller ourselves; thereis nothing above or below it but space, yet it travels on day by day andyear by year, held by invisible forces that the brain of man hasdiscovered and measured. Of course, every planet gives a pull at every other planet too, butthese pulls are so small compared with that of the sun that we need notat present notice them. Then we come to another point. We said thatevery body pulled every other body in proportion to their weights andtheir distance. Now, gravity acts much more strongly when things arenear together than when they are far away from each other; so that if asmaller body is near to another somewhat larger than itself, it ispulled by it much more strongly than by a very much larger one at aconsiderably greater distance. We have an instance of this in the caseof the earth and moon: as the earth responds to the pull of the sun, sothe moon responds to the pull of the earth. The moon is so comparativelynear to the earth that the earth-pull forces her to keep on going roundand round, instead of leaving her free to circle round the sun byherself; and yet if you think of it the moon does go round the sun too. Recall that game we had when the sun was in the middle, and the twosmaller girls, representing the earth and moon, went round it. Themoon-child turned round the earth-child, but all the while theearth-child was going round the sun, so that in a year's time the moonhad been all round the sun too, only not in a straight line. The moon issomething like a dog who keeps on dancing round and round you when yougo for a walk. He does go for the walk too, but he does much more thanthat in the same time. Thus we have further completed our idea of ourworld. We see it now hanging in space, with no visible support, held inits place by two mighty forces; spinning on year after year, attended byits satellite the moon, while we run, and walk, and cry, and laugh, andplay about on its surface--little atoms who, except for the brain thatGod has given them, would never even have known that they arecontinually moving on through endless space. CHAPTER III THE SHINING MOON 'Once upon a time, ' long, long ago, the earth was not a compact, round, hard body such as she is now, but much larger and softer, and as sherotated a fragment broke off from her; it did not go right away fromher, but still went on circling round with the motion it had inheritedfrom her. As the ages passed on both the earth and this fragment, whichhad been very hot, cooled down, and in cooling became smaller, so thatthe distance between them was greater than it had been before theyshrank. And there were other causes also that tended to thrust the twofurther from each other. Yet, compared with the other heavenly bodies, they are still near, and by looking up into the sky at night you cangenerally see this mighty fragment, which is a quarter the diameter ofthe earth--that is to say, a quarter the width of the earth measuredfrom side to side through the middle. It is--as, of course, you haveguessed--the moon. The moon is the nearest body to us in all space, andso vast is the distance that separates us from the stars that we speakas if she were not very far off, yet compared with the size of the earththe space lying between us and her is very great. If you went rightround the world at the thickest part--that is to say, in the region ofthe Equator--and when you arrived at your starting-point went off onceagain, and so on until you had been round ten times, you would only thenhave travelled about as far as from the earth to the moon! The earth is not the only planet which has a moon, or as it is called, asatellite, in attendance. Some of the larger planets have several, butthere is not one to compare with our moon. Which would you prefer if youhad the choice, three or four small moons, some of them not much largerthan a very big bright star, or an interesting large body like our ownmoon? I know which I should say. 'You say that the moon broke off from the earth, so perhaps there may besome people living on her, ' I hear someone exclaim. If there is one thing we have found out certainly about the moon, it isthat no life, as we know it, could exist there, for there is neither airnor water. Whether she ever had any air or water, and if so, why theydisappeared, are questions we cannot answer. We only know that now sheis a dead world. Bright and beautiful as she is, shedding on us a pale, pure light, in vivid contrast with the fiery yellow rays of the sun, yetshe is dead and lifeless and still. We can examine her surface with thetelescope, and see it all very plainly. Even with a large opera-glassthose markings which, to the naked eye, seem to be like a queerdistorted face are changed, and show up as the shadows of greatmountains. We can only see one side of the moon, because as I have said, she keeps always the same face turned to the earth; but as she swaysslightly in her orbit, we catch a glimpse of sometimes a little more onone side and sometimes a little more on the other, and so we can judgethat the unseen part is very much the same as that turned toward us. At first it is difficult to realize what it means to have no air. Besides supporting life in every breath that is drawn by livingcreatures, the air does numerous other kind offices for us--forinstance, it carries sound. Supposing the most terrific volcano explodedin an airless world, it could not be heard. The air serves as a screenby day to keep off the burning heat of the sun's rays, and as a blanketby night to keep in the heat and not let it escape too quickly. If therewere no air there could be no water, for all water would evaporate andvanish at once. Imagine the world deprived of air; then the sun's rayswould fall with such fierceness that even the strongest tropical sun weknow would be as nothing in comparison with it, and every green thingwould shrivel up and die; this scorching sun would shine out of a blacksky in which the stars would all be visible in the daytime, not hiddenby the soft blue veil of air, as they are now. At night the instant thesun disappeared below the horizon black darkness would set in, for ourlingering twilight is due to the reflection of the sun in the upperlayers of air, and a bitterness of deathly cold would fall upon theearth--cold fiercer than that of the Arctic regions--and everythingwould be frozen solid. It would need but a short time to reduce theearth to the condition of the moon, where there is nothing to shrivelup, nothing to freeze. Her surface is made up of barren, arid rocks, andher scenery consists of icy black shadows and scorching white plains. [Illustration: _Paris Observatory. _ THE MOON. ] The black shadows define the mountains, and tremendous mountains theyare. Most of them have craters. A crater is like a cup, and generallyhas a little peak in the middle of it. This is the summit of a volcano, and when the volcano has burst up and vomited out floods of lava anddébris, this has fallen down in a ring a little distance away from it, leaving a clear space next to the peak, so that, as the mountain ceasesvomiting and the lava cools down, the ring hardens and forms a circularridge. The craters on the moon are immense, not only in proportion toher size, but immense even according to our ideas on the earth. One ofthe largest craters in our own world is in Japan, and this measuresseven miles across, while in the moon craters of fifty, sixty, and evena hundred miles are by no means uncommon, though there are also hundredsand thousands of smaller ones. We can see the surface of the moon veryplainly with the magnificent telescopes that have now been made, andwith the best of these anything the size of a large town would beplainly visible. Needless to say, no town ever has been or ever will beseen upon the moon! All these mountains and craters show that at one time the moon must havebeen convulsed with terrific disturbances, far worse than anything thatwe have any knowledge of on our earth; but this must have been agesago, while the moon still probably had an atmosphere of its own. Now ithas long been quiet. Nothing changes there; even the forces that arealways at work on the earth--namely, damp and mould and water--alteringthe surface and breaking up the rocks, do not act there, where there isno moisture of any sort. So far as we can see, the purpose of the moonis to be the servant of the earth, to give her light by night and toraise the tides. Beautiful light it is, soft and mysterious--light thatchildren do not often have a chance of seeing, for they are generally inbed before the moon rises when she is at the full. We know that the moon has no heat of her own--she parted with all thatlong ago; she cannot give us glowing light from brilliant flames, as thesun does; she shines only by the reflection of the sun on her surface, and this is the reason why she appears to change her shape soconstantly. She does not really change; the whole round moon is alwaysthere, only part of it is in shadow. Sometimes you can see the dark partas well as the bright. When there is a crescent moon it looks as if itwere encircling the rest; some people call it, 'seeing the old moon inthe new moon's arms. ' I don't know if you would guess why it is we cansee the dark part then, or how it is lighted up. It is by reason of ourown shining, for we give light to the moon, as she does to us. The sun'srays strike on the earth, and are reflected on to the moon, so that themoon is lighted by earthshine as we are lighted by moonshine, and it isthese reflected earth-rays that light up the dark part of the moon andenable us to see it. What a journey these rays have had! They travelfrom the sun to the earth, and the earth to the moon, and then back tothe earth again! From the moon the earth must appear a much bigger andmore glorious spectacle than she does to us--four times wider across andprobably brighter--for the sun's light strikes often on our clouds, which shine more brilliantly than her surface. Once again we must use an illustration to explain the subject. Set alamp in the middle of a dark room, and let that be the sun, then take asmall ball to represent the earth and a smaller one for the moon. Placethe moon-ball between the lamp and the earth-ball. You will see that theside turned to the earth-ball is dark, but if you move the moon to oneside of the earth, then from the earth half of it appears light and halfdark; if you put it right away from the lamp, on the outer side of theearth, it is all gloriously lit up, unless it happens to be exactlybehind the earth, when the earth's shadow will darken it. This is thefull explanation of all the changes of the moon. [Illustration: AN ECLIPSE OF THE MOON. ] Does it ever fall within the earth's shadow? Yes, it does; for as itpasses round the earth it is not always at the same level, but sometimesa little higher and sometimes a little lower, and when it chances topass exactly behind it enters the shadow and disappears. That is what wecall an eclipse of the moon. It is nothing more than the earth's shadowthrown on to the moon, and as the shadow is round that is one of theproofs that the earth is round too. But there is another kind ofeclipse--the eclipse of the sun; and this is caused by the moon herself. For when she is nearest to the sun, at new moon--that is to say, whenher dark side is toward us, and she happens to get exactly between usand the sun--she shuts out the face of the sun from us; for though sheis tiny compared with him, she is so much nearer to us that she appearsalmost the same size, and can blot him right out. Thus the eclipses ofboth sun and moon are not difficult to understand: that of the moon canonly happen at full moon, when she is furthest from the sun, and it iscaused by the earth's shadow falling upon the moon; and that of the sunat new moon, when she is nearest to him, and it is caused by the solidbody of the moon coming between us and the sun. [Illustration: AN ECLIPSE OF THE SUN. ] Besides giving us light by night, the moon serves other importantpurposes, and the most important of all is the raising of the tides. Without the rising of the sea twice in every day and night our coastswould become foul and unwholesome, for all the dead fish and rottingstuff lying on the beach would poison the air. The sea tides scour ourcoasts day by day with never-ceasing energy, and they send a greatbreath of freshness up our large rivers to delight many people farinland. The moon does most of this work, though she is a little helpedby the sun. The reason of this is that the moon is so near to the earththat, though her pull is a comparatively small one, it is very stronglyfelt. She cannot displace the actual surface to any great extent, as itis so solid; but when it comes to the water she can and does displacethat, so that the water rises up in answer to her pull, and as the earthturns round the raised-up water lags behind, reaching backward towardthe moon, and is drawn up on the beach, and makes high tide. But it isstopped there, and meantime, by reason of the earth's movement, the moonis left far behind, and pulls the water to itself further on, when thefirst high tide relapses and falls down again. At length the moon getsround to quite the opposite side of the earth to that where she began, and there she makes a high tide too; but as she draws the water toherself she draws also the solid earth beneath the water to her in somedegree, and so pulls it away from the place where the first high tideoccurred, leaving the water there deeper than before, and so causing asecondary high tide. [Illustration: THE MOON RAISING THE TIDES. ] The sun has some influence on the tides too, and when moon and sun arein the same line, as at full and new moon, then the tides are highest, and are called spring tides; but when they pull in different directions, as when it is half-moon, then the tides are lowest and are called neaptides. CHAPTER IV THE EARTH'S BROTHERS AND SISTER The earth is not the only world that, poised in space, swings around thesun. It is one of a family called the Solar System, which means thesystem controlled and governed by the sun. When we look up at theglorious sky, star-studded night by night, it might seem to us that thestars move only by reason of the earth's rotation; but when men firstbegan to study the heavens attentively--and this is so long ago that therecord of it is not to be found--they noticed that, while every shiningobject in the sky was apparently moving round us, there were a few whichalso had another movement, a proper motion of their own, like the moon. These curious stars, which appeared to wander about among the otherstars, they called planets, or wanderers. And the reason, which waspresently discovered, of our being able to see these movements was thatthese planets are very much nearer to us than any of the real stars, and in fact form part of our own solar system, while the stars are atimmeasurable distances away. Of all the objects in the heavens theplanets are the most intensely interesting to us; for though removedfrom us by millions of miles, the far-reaching telescope brings some ofthem within such range that we can see their surfaces and discover theirmovements in a way quite impossible with the stars. And here, ifanywhere, might we expect to find traces of other living beings likeourselves; for, after all the earth is but a planet, not a very largenor a very small one, and in no very striking position compared with theother planets; and thus, arguing by what seems common-sense, we say, Ifthis one planet has living beings on its surface, may not the otherplanets prove to be homes for living beings also? Counting our ownearth, there are eight of these worlds in our solar system, and also anumber of tiny planets, called asteroids; these likewise go round thesun, but are very much smaller than any of the first eight, and stand ina class by themselves, so that when the planets are mentioned it isgenerally the eight large well-known planets which are referred to. If we go back for a moment to the illustration of the large lamprepresenting our sun, we shall now be able to fill in the picture withmuch more detail. The orbits of the planets, as their paths round thesun are called, lie like great circles one outside another at variousdistances, and do not touch or cut each other. Where do you suppose ourown place to be? Will it be the nearest to the sun or the furthest awayfrom him? As a matter of fact, it is neither, we come third in orderfrom the sun, for two smaller planets, one very small and the othernearly as large as the earth, circle round and round the sun in orbitslying inside ours. Now if we want to place objects around our lamp-sunwhich will represent these planets in size, and to put them in placescorresponding to their real positions, we should find no room largeenough to give us the space we ought to have. We must take the lamp outinto a great open field, where we shall not be limited by walls. Thenthe smallest planet, named Mercury, which lies nearest of all to thesun, would have to be represented by a pea comparatively close to thesun; Venus, the next, would be a greengage plum, and would be abouttwice as far away; then would come the earth, a slightly larger plum, about half as far again as Venus. After this there would be a lesserplanet, called Mars, like a marble. These are the first four, allcomparatively small; beyond them there is a vast gap, in which we findthe asteroids, and after this we come to four larger planets, mightyindeed as regards ourselves, for if our earth were a greengage plum, the first of these, Jupiter, would have to be the size of a football atleast, and the next, Saturn, a smaller football, while Uranus andNeptune, the two furthest out, would be about the size of the toyballoons children play with. The outermost one, Neptune, would be thirtytimes as far from the sun as we are. [Illustration: COMPARATIVE SIZES OF THE PLANETS. ] This is the solar system, and in it the only thing that shines by itsown light is the sun; all the rest, the planets and their moons, shineonly because the rays of light from the sun strike on their surfaces andare reflected off again. Our earth shines like that, and from the nearerplanets must appear as a brilliant star. The little solar system isseparated by distances beyond the realm of thought from the rest of theuniverse. Vast as are the intervals between ourselves and our planetaryneighbours, they are as nothing to the space that separates us from thenearest of the steady shining fixed stars. Why, removed as far from usas the stars, the sun himself would have sunk to a point of light; andas for the planets, the largest of them, Jupiter, could not possibly beseen. Thus, when we look at those stars across the great gulf of space, we know that though we see them they cannot see us, and that to them oursun must seem only a star; consequently we argue that perhaps thesestars themselves are suns with families of planets attached to them; andthough there are reasons for thinking that this is not the case withall, it may be with some. Now if, after learning this, we look again atthe sky, we do so with very different eyes, for we realize that some ofthese shining bodies are like ourselves in many things, and are shiningonly with a light borrowed from the sun, while others are mighty glowingsuns themselves, shining by their own light, some greater and brighter, some less than our sun. The next thing to do is to learn which are starsand which are planets. Of the planets you will soon learn to pick out one or two, and willrecognize them even if they do change their places--for instance, Venusis at times very conspicuous, shining as an evening star in the westsoon after the sun goes down, or us a morning star before he gets up, though you are not so likely to see her then; anyway, she is never foundvery far from the sun. Jupiter is the only other planet that compareswith her in brilliancy, and he shines most beautifully. He is, ofcourse, much further away from us than Venus, but so much larger that herivals her in brightness. Saturn can be quite easily seen as aconspicuous object, too, if you know where to look for him, and Mars issometimes very bright with a reddish glow. The others you would not beable to distinguish. It is to our earth's family of these eight large planets going steadilyround the same sun that we must give our attention first, before goingon to the distant stars. Many of the planets are accompanied bysatellites or moons, which circle round them. We may say that the sun isour parent--father, mother, what you will--and that the planets are thefamily of children, and that the moons are _their_ children. Our earth, you see, has only one child, but that a very fine one, of which she maywell be proud. When I say that the planets go round the sun in circles I am onlyspeaking generally; as a matter of fact, the orbits of the planets arenot perfect circles, though some are more circular than others. Insteadof this they are as a circle might look if it were pressed in from twosides, and this is called an ellipse. The path of our own earth roundthe sun is one of the most nearly circular of them all, and yet even inher orbit she is a good deal nearer to the sun at one time than another. Would you be surprised to hear that she is nearer in our winter andfurther away in our summer? Yet that is the case. And for the firstmoment it seems absurd; for what then makes the summer hotter than thewinter? That is due to an altogether different cause; it depends on theposition of the earth's axis. If that axis were quite straight up anddown in reference to the earth's path round the sun we should have equaldays and nights all the year round, but it is not; it leans over alittle, so that at one time the North Pole points towards the sun and atanother time away from it, while the South Pole is pointing first awayfrom it and then toward it in exactly the reverse way. When the NorthPole points to the sun we in the Northern Hemisphere have our summer. Tounderstand this you must look at the picture, which will make it muchclearer than any words of mine can do. The dark part is the night, andthe light part the day. When we are having summer any particular spot onthe Northern Hemisphere has quite a long way to travel in the light, andonly a very short bit in the dark, and the further north you go thelonger the day and shorter the night, until right up near the NorthPole, within the Arctic Circle, it is daylight all the time. You have, perhaps, heard of the 'midnight sun' that people go to see in the North, and what the expression means is that at what should be midnight thesun is still there. He seems just to circle round the horizon, neververy far above, but never dipping below it. When the sun is high overhead, his rays strike down with much more forcethan when he is low. It is, for instance, hotter at mid-day than in theevening. Now, when the North Pole is bowed toward the sun, the sunappears to us to be higher in the sky. In the British Isles he neverclimbs quite to the zenith, as we call the point straight above ourheads; he always keeps on the southern side of that, so that our shadowsare thrown northward at mid-day, but yet he gets nearer to it than hedoes in winter. Look at the picture of the earth as it is in winter. Then we have long nights and short days, and the sun never appears toclimb very high, because we are turned away from him. During the shortdays we do not receive a great deal of heat, and during the long nightthe heat we have received has time to evaporate to a great extent. Thesetwo reasons--the greater or less height of the sun in the sky and thelength of the days--are quite enough to account for the differencebetween our summer and winter. There is one rather interesting point toremember, and that is that in the Northern Hemisphere, whether it iswinter or summer, the sun is south at mid-day, so that you can alwaysfind the north then, for your shadow will point northwards. [Illustration: THE ENGLISH SUMMER (LEFT) AND WINTER (RIGHT). ] New Zealand and Australia and other countries placed in the SouthernHemisphere, as we are in the Northern, have their summer while we havewinter, and winter while we have summer, and their summer is warmer thanours, because it comes when the earth in its journey is three millionmiles nearer to the sun than in our summer. All this seems to refer to the earth alone, and this chapter should beabout the planets; but, after all, what applies to one planet applies toanother in some degree, and we can turn to the others with much moreinterest now to see if their axes are bowed toward the sun as ours is. It is believed that in the case of Mercury, in regard to its path roundthe sun, the axis is straight up and down; if it is the changes of theseasons must depend on the nearness of Mercury to the sun and nothingelse, and as he is a great deal nearer at one time than another, thismight make a very considerable difference. Some of the planets are likethe earth in regard to the position of their axes, but the two outermostones, Uranus and Neptune, are very peculiar, for one pole is turnedright toward the sun and the other right away from it, so that in onehemisphere there is continuous day all the summer, in the other there iscontinuous night, and then the process is reversed. But these littlepeculiarities we shall have to note more particularly in the account ofthe planets separately. There is a curious fact in regard to the distances of the planets fromthe sun. Each one, after the first, is, very roughly, about double thedistance from the sun of the one inside it. This holds good for all thefirst four, then there is a great gap where we might expect to findanother planet, after which follow the four large planets. Now, this gappuzzled astronomers greatly; for though there seemed to be no reason whythe planets should be at regular distances one outside the other, yetthere the fact was, and that the series should be broken by a missingplanet was annoying. So very careful search was made, and a thrill ofexcitement went all through the scientific world when it was known thata tiny planet had been discovered in the right place. But this was notthe end of it, for within a few years three or four more tiny planetswere observed not far from the first one, and, as years rolled on, oneafter another was discovered until now the number amounts to over sixhundred and others are perpetually being added to the list! Here was anew feature in the solar system, a band of tiny planets not one of whichwas to be compared in size with the least of those already known. Thelargest may be about as large as Europe, and others perhaps about thesize of Wales, while there may be many that have only a few square milesof surface altogether, and are too small for us to see. To account forthis strange discovery many theories were advanced. One was that there had been a planet--it might be about the size ofMars--which had burst up in a great explosion, and that these were thepieces--a very interesting and exciting idea, but one which proved to beimpossible. The explanation now generally accepted is a littlecomplicated, and to understand it we must go back for a bit. When we were talking of the earth and the moon we realized that oncelong ago the moon must have been a part of the earth, at a time when theearth was much larger and softer than she now is; to put it in thecorrect way, we should say when she was less dense. There is no need toexplain the word 'dense, ' for in its ordinary sense we use it every day, but in an astronomical sense it does not mean exactly the same thing. Everything is made up of minute particles or atoms, and when theseatoms are not very close together the body they compose is loose intexture, while if they are closer together the body is firmer. Forinstance, air is less dense than water, and water than earth, and earththan steel. You see at once by this that the more density a thing hasthe heavier it is; for as a body is attracted to another body by everyatom or particle in it, so if it has more particles it will be morestrongly attracted. Thus on the earth the denser things are reallyheavier. But 'weight' is only a word we use in connection with theearth; it means the earth's pulling power toward any particular thing atthe surface, and if we were right out in space away from the earth, thepulling power of the earth would be less, and so the weight would beless; and as it would be impossible always to state just how far away athing was from the earth, astronomers talk about density, which meansthe number of particles a body contains in proportion to other bodies. Thus the planet Jupiter is very much larger than the earth, but hisdensity is less. That does not mean to say that if Jupiter were in onescale and the earth in the other he would weigh less, because he is sovery much bigger he would outweigh the earth still; his total _mass_would be greater than that of the earth, but it means that a piece ofJupiter the same size as a piece of the earth would weigh less under thesame conditions. Now, before there were any planets at all or any sun, in the place ofour solar system was a vast gaseous cloud called a nebula, which slowlyrotated, and this rotation was the first impulse or force which God gaveit. It was not at all dense, and as it rotated a part broke off, andinheriting the first impulse, went on rotating too. The impulse wouldhave sent it off in a straight line, but the pull of gravity from thenebula held it in place, and it circled round; then the nebula, as itrotated, contracted a little, and occupied less space and grew denser, and presently a second piece was thrown off, to become in time anotherplanet. The same process was repeated with Saturn, and then with thehuge Jupiter. The nebula was always rotating and always contracting. Andas it behaved, so did the planets in their turn; they spun round andcooled and contracted, and the moons were flung off from them, just asthey--the planets--had been flung off from the parent nebula. Now, after the original nebula had parted with the mighty mass ofJupiter, it never again made an effort so great, and for a long timethe fragments that were detached were so small as hardly to be worthcalling planets; they were the asteroids, little lumps and fragmentsthat the nebula left behind. But as it still contracted in time therecame Mars; and having recovered a little, the nebula with more energygot rid of the earth, and next Venus, and lastly little Mercury, thesmallest of the eight planets. Then it contracted further, and perhapsyou can guess what the remainder of it is--the sun; and by spinning in aplastic state the sun, like the earth, has become a globe, round andcomparatively smooth; and its density is now too great to allow of itslosing any more fragments, so, as far as we can see, the solar system iscomplete. This theory of the origin of the planets is called the nebula theory. Wecannot prove it, but there are so many facts that can only be explainedby it, we have strong reason for believing that something of the kindmust have happened. When we come to speak of the starry heavens we shallsee that there are many masses of glowing gas which are nebulę of thesame sort, and which form an object-lesson in our own history. We have spoken rather lightly of the nebula rotating and throwing offplanets; but we must not think of all this as having happened in ashort time. It is almost as impossible for the human mind to conceivethe ages required for such slow changes as to grasp the great gulfs ofspace that separate us from the stars. We can only do it by comparison. You know what a second is, and how the seconds race past without ceasingday and night. It makes one giddy to picture the seconds there are in ayear; yet if each one of those seconds was a year in itself, what then?That seems a stupendous time, but it is nothing compared with the timeneeded to form a nebula into a planetary system. If we had five thousandof such years, with every second in them a year, we should then onlyhave counted one billion real years, and billions must have passed sincethe sun was a gaseous nebula filling the outermost bounds of oursystem! CHAPTER V FOUR SMALL WORLDS What must the sun appear to Mercury, who is so much nearer to him thanwe are? To understand that we should have to imagine our sun increasedto eight or nine times his apparent size, and pouring out far greaterheat and light than anything that we have here, even in the tropics. Itwas at first supposed that Mercury must have an extra thick covering ofclouds to protect him from this tremendous glare; but recentobservations tend to prove that, far from this, he is singularly freefrom cloud. As this is so, no life as we know it could possibly exist onMercury. His year--the time he takes to go round the sun and come back to thesame place again--is eighty-eight days, or about one-quarter of ours. Ashis orbit is much more like an ellipse than a circle, it follows that heis much nearer to the sun at one time than at another--in fact, when heis nearest, the size of the sun must seem three and a half timesgreater than when he is furthest away from it! Even at the best Mercuryis very difficult to observe, and what we can learn about him is notmuch; but, as we have heard, his axis is supposed to be upright. If sohis seasons cannot depend on the bend toward or away from the sun, butmust be influenced solely by the changes in his distance from the sun, which are much greater than in our own ease. There is some reason tobelieve, too, that Mercury's day and year are the same length. Thismeans that as the planet circles round the sun he turns once. If this isso the sun will shine on one half of the planet, producing anaccumulated heat terrific to think of; while the other side is plungedin blackness. The side which faces the sun must be heated to a pitchinconceivable to us during the nearer half of the orbit--a pitch atwhich every substance must be at boiling-point, and which no life as weknow it could possibly endure. Seen from our point of view, Mercury goesthrough all the phases of the moon, as he shines by the reflected lightof the sun; but this point we shall consider more particularly in regardto Venus, as Venus is nearer to us and easier to study. For a long timeastronomers had a fancy that there might be another planet even nearerto the sun than Mercury, perhaps hidden from us by the great glare ofthe sun. They even named this imaginary planet Vulcan, and some thoughtthey had seen it, but it is tolerably certain that Vulcan existed onlyin imagination. Mercury is the nearest planet to the sun, and also thesmallest, of course excepting the asteroids. It is about three thousandmiles in diameter, and as our moon is two thousand miles, it is not somuch bigger than that. So far as we are concerned, it is improbable weshall ever know very much more about this little planet. But next we come to Venus, our beautiful bright neighbour, whoapproaches nearer to us than any other heavenly body except the moon. Alas! when she is nearest, she like Mercury, turns her dark side towardus, coming in between us and the sun, so that we cannot observe her atall. Everyone must have noticed Venus, however carelessly they have looked atthe sky; but it is likely that far more people have seen her as anevening than a morning star, for most people are in bed when the sunrises, and it is only before sunrise or after sunset we can see Venuswell. She is at her best from our point of view when she seems to us tobe furthest from the sun, for then we can study her best, and at thesetimes she appears like a half or three-quarter moon, as we only see apart of the side from which the sunlight is reflected. She shines like alittle silver lamp, excelling every other planet, even Jupiter, thelargest of all. If we look at her even with the naked eye, we can seethat she is elongated or drawn out, but her brilliance prevents us fromseeing her shape exactly; to do this we must use a telescope. [Illustration: DIFFERENT PHASES OF VENUS. ] It is a curious fact that some planets shine much more brightly thanothers, without regard to their size--that is to say, the surface onwhich the sun's rays strike is of greater reflecting power in some thanin others. One of the brightest things in Nature that we can imagine isa bank of snow in sunlight; it is so dazzling that we have to look awayor wink hard at the sight; and the reflective power of the surface ofVenus is as dazzling as if she were made of snow. This is probablybecause the light strikes on the upper surface of the clouds whichsurround her. In great contrast to this is the surface of Mercury, whichreflects as dully as a mass of lead. Our own moon has not a highreflecting power, as will be easily understood if we imagine what theworld would be if condemned to perpetual moonlight only. It would, indeed, be a sad deprivation if the mournful cold light of the moon, welcome enough as a change from sunlight, were to take the place ofsunlight in the daytime. For a very long time astronomers could not discover what time Venus tookin rotating on her own axis--that is to say, what the length of her daywas. She is difficult to observe, and in order to find out the rotationit is necessary to note some fixed object on the surface which turnsround with the planet and comes back to the same place again, so thatthe time it takes in its journey can be measured. But the surface ofVenus is always changing, so that it is impossible to judge at allcertainly. Opinions differ greatly, some astronomers holding thatVenus's day is not much longer than an earthly day, while others believethat the planet's day is equal to her year, just as in the case ofMercury. Venus's year is 225 days, or about seven and a half of ourmonths, and if, indeed, her day and year are the same length, verypeculiar effects would follow. For instance, terrible heat would beabsorbed by the side of the planet facing the sun in the perpetualsummer; and the cold which would be felt in the dreary winter's nightwould far exceed our bitterest Arctic climate. We cannot but fancy thatany beings who might live on a planet of this kind must be differentaltogether from ourselves. Then, there is another point: even here onearth very strong winds are caused by the heating of the tropics; thehot air, being lighter than the cold air, rises, and the colder air fromthe poles rushes in to supply its place. This causes wind, but the windswhich would be raised on Venus by the rush of air from the icy side ofthe planet to the hot one would be tornadoes such as we could butfaintly dream of. It is, of course, useless to speculate when we know solittle, but in a subject so intensely interesting we cannot helpguessing a little. Venus is only slightly smaller than the earth, and her density is notvery unlike ours; therefore the pull of gravity must be pretty muchthere what it is here--that is to say, things will weigh at her surfaceabout the same as they do here. Her orbit is nearly a circle, so thather distance from the sun does not vary much, and the heat will not bemuch greater from this cause at one time of the year than another. As her orbit is tilted up a little she does not pass between us and thesun at each revolution, but occasionally she does so, and this passingis called a transit. Many important facts have been learned by watchingthese transits. Mercury also has transits across the sun, but as she isso much smaller than Venus they are not of such great importance. It wasby the close observation of Venus during her transits that the distancefrom the earth to the sun was first measured. Not until the year 2004will another transit of Venus occur. It is not difficult to imagine that the earth must appear a splendidspectacle from Venus, whence she is seen to great advantage. Whennearest to us she must see us like a little moon, with markings as thecontinents and seas rotate, and these will change as they are obscuredby the clouds rolling over them. At the North and South Poles will beglittering ice-caps, growing larger and smaller as they turn toward oraway from the sun. A brilliant spectacle! [Illustration: ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY. ] We might say with a sigh, 'If only we could see such a world!' Well, wecan see a world--not indeed, so large as Venus, yet a world that comesalmost as near to us as Venus does, and which, unlike her, is outside usin order from the sun, so that when it is nearest to us the fullsunlight is on it. This is Mars, our neighbour on the other side, and ofall the fascinating objects in the sky Mars is the most fascinating, forthere, if anywhere, should we be likely to discover beings likeourselves! Mars takes rather more than half an hour longer to rotate than we do, and as he is so much smaller than the earth, this means that he movesround more slowly. His axis is bent at nearly the same angle as ours is. Mars is much smaller than the earth, his diameter is about twice that ofthe moon, and his density is about three-quarters that of the earth, sothat altogether, with his smaller size and less density, anythingweighing a hundred pounds here would only weigh some forty pounds onMars; and if, by some miraculous agency, you were suddenly transportedthere, you would find yourself so light that you could jump enormousdistances with little effort, and skip and hop as if you were onsprings. [Illustration: _Memoirs of the British Astronomical Association. _ MAP OF MARS. ] Look at the map of Mars, in which the surface appears to be cut up intoland and water, continents and oceans. The men who first observed Marswith accuracy saw that some parts were of a reddish colour and othersgreenish, and arguing from our own world, they called the greenish partsseas and the reddish land. For a long while no one doubted that weactually looked on a world like our own, more especially as there wassupposed to be a covering of atmosphere. The so-called land and waterare much more cut up and mixed together than ours, it is true. Here andthere is a large sea, like that marked 'Mare Australe, ' but otherwisethe water and the land are strangely intermingled. The red colour of thepart they named land puzzled astronomers a good deal, for our land seenat the same distance would not appear so red, and they came at last tothe conclusion that vegetation on Mars must be red instead of green! Butafter a while another disturbing fact turned up to upset their theories, and that was that they saw canals, or what they called canals, on Mars. These were long, straight, dark markings, such as you see on the map. It is true that some people never saw these markings at all, anddisbelieved in their existence; but others saw them clearly, and watchedthem change--first go fainter and then darker again. And quite recentlya photograph has been obtained which shows them plainly, so they musthave an existence, and cannot be only in the eye of the observer, as themost sceptical people were wont to suggest. But further than this, oneastronomer announced that some of these lines appeared to be double, yetwhen he looked at them again they had grown single. It was like aconjuring trick. Great excitement was aroused by this, for if the canalswere altered so greatly it really did look as if there were intelligentbeings on Mars capable of working at them. In any case, if these arereally canals, to make them would be a stupendous feat, and if they areartificial--that is, made by beings and not natural--they show a veryhigh power of engineering. Imagine anyone on earth making a canal manymiles wide and two thousand miles long! It is inconceivable, but that isthe feat attributed to the Martians. The supposed doubling of thecanals, as I say, caused a great deal of talk, and very few people couldsee that they were double at all. Even now the fact is doubted, yetthere seems every reason to believe it is true. They do not all appearto be double, and those that do are always the same ones, while othersundoubtedly remain single all the time. But the canals do not exhaustthe wonders of Mars. At each pole there is an ice-cap resembling thosefound at our own poles, and this tells us pretty plainly something aboutthe climate of Mars, and that there is water there. This ice-cap melts when the pole which it surrounds is directed towardthe sun, and sometimes in a hot summer it dwindles down almost tonothing, in a way that the ice-caps at the poles of the earth never do. A curious appearance has been noticed when it is melting: a dark shadowseems to grow underneath the edge of it and extends gradually, and as itextends the canals near it appear much darker and clearer than they didbefore, and then the canals further south undergo the same change. Thislooks as if the melting of the snow filled up the canals with water, andwas a means of watering the planet by a system totally different fromanything we know here, where our poles are surrounded by oceans, and theice-caps do not in the least affect our water-supply. But, then, anotherstrange fact had to be taken into consideration. These straight linescalled canals ran out over the seas occasionally, and it was impossibleto believe that if they were canals they could do that. Other thingsbegan to be discussed, such as the fact that the green parts of Mars didnot always remain green. In what is the springtime of Mars they are so, but afterwards they become yellow, and still later in the season partsnear the pole turn brown. Thus the idea that the greenish parts are seashad to be quite given up, though it appeared so attractive. The idea nowgenerally believed is that the greenish parts are vegetation--trees andbushes and so on, and that the red parts are deserts of reddish sand, which require irrigation--that is to say, watering--before anything canbe grown on them. The apparent doubling of the canals may be due to thegreen vegetation springing up along the banks. This might form two broadlines, while the canal itself would not be seen, and when the vegetationdies down, we should see only the trench of the canal, which wouldpossibly appear faint and single. Therefore the arrangements on Marsappear to be a rich and a barren season on each hemisphere, the growthbeing caused by the melting of the polar ice-cap, which sends floodsdown even beyond the Equator. If we could imagine the same thing onearth we should have to think of pieces of land lying drear and dry anddead in winter between straight canal-like ditches of vast size. Alittle water might remain in these ditches possibly, but not enough towater the surrounding land. Then, as summer progressed, we should hear, 'The floods are coming, ' and each deep, huge canal would be filled upwith a tide of water, penetrating further and further. The water drawnup into the air would fall in dew or rain. Vegetation would spring up, especially near the canal banks, and instead of dreary wastes richgrowths would cover the land, gradually dying down again in the winter. So far Mars seems in some important respects very different from theearth. He is also less favourably placed than we are, for being so muchfurther from the sun, he receives very much less heat and light. Hisyears are 687 of our days, or one year and ten and a half months, andhis atmosphere is not so dense as ours. With this greater distance fromthe sun and less air we might suppose the temperature would be very coldindeed, and that the surface would be frost-bound, not only at thepoles, but far down towards the Equator. Instead of this being so, as wehave seen, the polar caps melt more than those on the earth. We canonly surmise there must be some compensation we do not know of thatsoftens down the rigour of the seasons, and makes them milder than weshould suppose possible. Of course, the one absorbing question is, Are there people on Mars? Tothis it is at present impossible to reply. We can only say the planetseems in every way fitted to support life, even if it is a littledifferent from our earth. It is most certainly a living world, not adead one like the moon, and as our knowledge increases we may some daybe able to answer the question which so thrills us. Our opportunities for the observation of Mars vary very greatly, for asthe earth's orbit lies inside that of Mars, we can best see him when weare between him and the sun. Of course, it must be remembered that theearth and the other planets are so infinitely small in regard to thespace between them that there is no possibility of any one of themgetting in such a position that it would throw a shadow on any other oreclipse it. The planets are like specks in space, and could notinterfere with one another in this way. When Mars, therefore, is in aline with us and the sun we can see him best, but some of these timesare better than others, for this reason--the earth's orbit is nearly acircle, and that of Mars more of an ellipse. [Illustration: ORBITS OF THE EARTH AND MARS. ] Look at the illustration and remember that Mars' year is not quite twoof ours--that is to say, every time we swing round our orbit we catchhim up in a different place, for he will have progressed less than halfhis orbit while we go right round ours. Sometimes when we overtake him he may be at that part which is furthestaway from us, or he may be at that part which is nearest to us, and ifhe is in the latter position we can see him best. Now at these, the mostfavourable times of all, he is still more than thirty-five millions ofmiles away--that is to say, one hundred and forty times as far as themoon, yet comparatively we can see him very well. He is coming nearerand nearer to us, and very soon will be nearer than he has been since1892, or fifteen years ago. Then many telescopes will be directed onhim, and much may be learned about him. For a long time it was supposed that Mars had no moons, and when DeanSwift wrote 'Gulliver's Travels' he wanted to make the Laputans dosomething very clever, so he described their discovery of two moonsattending Mars, and to make it quite absurd he said that when theyobserved these moons they found that one of them went round the planetin about ten hours. Now, as Mars takes more than twenty-four hours torotate, this was considered ridiculous, for no moon known then tookless time to go round its primary world than the primary world took toturn on its own axis. Our own moon, of course, takes thirty times aslong--that is a month contains thirty days. Then one hundred and fiftyyears later this jest of Dean Swift's came true, for two moons werereally discovered revolving round Mars, and one of them does actuallytake less time to complete its orbit than the planet does torotate--namely, a little more than seven hours! So the absurdity in'Gulliver's Travels' was a kind of prophecy! These two moons are very small, the outer one perhaps five or six milesin diameter, and the inner one about seven; therefore from Mars theouter one, Deimos, cannot look much more than a brilliant star, and theinner one would be but a fifth part the apparent width of our own moon. So Mars is not very well off, after all. Still, there is great variety, for it must be odd to see the same moon appearing three times in theday, showing all the different phases as it goes from new to full, eventhough it is small! Such wonderful discoveries have already been made that it is not toomuch to say that perhaps some day we may be able to establish some sortof communication with Mars, and if it be inhabited by any intelligentbeings, we may be able to signal to them; but it is almost impossiblethat any contrivance could bridge the gulf of airless space thatseparates us, and it is not likely that holiday trips to Mars will everbecome fashionable! CHAPTER VI FOUR LARGE WORLDS I have told you about the four lesser worlds of which our earth is one, and you know that beyond Mars, the last of them, there lies a vastspace, in which are found the asteroids, those strange small planetscircling near to each other, like a swarm of bees. After this therecomes Jupiter, who is so enormous, so superb in size compared with us, that he might well serve as the sun of a little system of his own. Youremember that we represented him by a football, while the earth was onlya greengage plum. But Jupiter himself is far less in comparison with thesun than we are in comparison with him. He differs from the planets wehave heard about up to the present in that he seems to glow with someheat that he does not receive from the sun. The illumination which makeshim appear as a star to us is, of course, merely reflected sunlight, andwhat we see is the external covering, his envelope of cloud. There is every reason to believe that the great bulk of Jupiter isstill at a high temperature. We know that in the depths of the earththere is still plenty of heat, which every now and then makes itspresence felt by bursting up through the vents we call volcanoes, theweak spots in the earth's crust; but our surface long ago cooled, forthe outside of any body gets cool before the inside, as you may havefound if ever you were trying to eat hot porridge, and circled round theedge of the plate with a spoon. A large body cools more slowly than asmall one, and it is possible that Jupiter, being so much larger than weare, has taken longer to cool. One reason we have for thinking this isthat he is so very light compared with his size--in other words, hisdensity is so small that it is not possible he could be made ofmaterials such as the earth is made of. As I said, when we study him through telescopes we see just theexterior, the outer envelope of cloud, and as we should expect, thischanges continually, and appears as a series of belts, owing to therotation of the planet. Jupiter's rotation is very rapid; though he isso much greater than the earth, he takes less than half the time theearth does to turn round--that is to say, only ten hours. His days andnights of five hours each seem short to us, accustomed to measurethings by our own estimates. But we must remember that everything isrelative; that is to say, there is really no such thing as fast or slow;it is all by comparison. A spider runs fast compared with a snail, buteither is terribly slow compared with an express train; and the speed ofan express train itself is nothing to the velocity of light. In the same way there is nothing absolutely great or small; it is all bycomparison. We say how marvellous it is that a little insect has all themechanism of life in its body when it is so tiny, but if we imagine thatinsect magnified by a powerful microscope until it appears quite large, the marvel ceases. Again, imagine a man walking on the surface of theearth as seen from a great distance through a telescope: he would seemless than an insect, and we might ask how could the mechanism of life becompressed into anything so small? Thus, when we say enormous or tiny wemust always remember we are only speaking by the measurements of our ownstandards. There is nothing very striking about Jupiter's orbit. He takes betweeneleven and twelve of our years to get round the sun, so you see, thoughhis day is shorter, his year is longer than ours. And this is not onlybecause his path is much larger, but because by the law of gravity themore distant a planet is from the sun the more slowly it travels, sothat while the earth speeds over eighteen miles Jupiter has only doneeight. Of course, we must be careful to remember the difference betweenrotation and revolution. Jupiter rotates much quicker than theearth--that is to say, he turns round more quickly--but he actually getsover the ground more slowly. The sun appears much smaller to him than itdoes to us, and he receives considerably less light and heat. There arevarious spots on his surface, and one remarkable feature is a dark mark, which is called the 'great red spot. ' If as we suppose what we see ofthe planet is merely the cloudy upper atmosphere, we should not expectto find anything permanent there, for the markings would change from dayto day, and this they do with this exception--that this spot, dark redin colour, has been seen for many years, turning as the planet turned. It was first noticed in 1878, and was supposed to be some great mountainor excrescence peeping up through the clouds. It grew stronger anddarker for several years, and then seemed to fade, and was not so easilyseen, and though still remaining it is now pale. But, most startlingto say, it has shifted its position a little--that is, it takes a fewseconds longer to get round the planet than it did at first. A fewseconds, you will say, but that is nothing! It does not seem much, butit shows how marvellously accurate astronomers are. Discoveries of vastimportance have been made from observing a few seconds' discrepancy inthe time the heavenly bodies take in their journeys, and the fact thatthis spot takes a little longer in its rotation than it did at firstshows that it cannot be attached to the body of the planet. It isimpossible for it to be the summit of a mountain or anything of thatsort. What can it be? No one has yet answered that question. [Illustration: JUPITER AND ONE OF HIS MOONS] When we get to the chapter on the sun, we shall find curiositiesrespecting the spots there as well. Jupiter has seven moons, and four of these are comparatively large. Theyhave the honour of having been the first heavenly bodies ever actuallydiscovered, for the six large planets nearest the sun have been known solong that there is no record of their first discovery, and of course ourown moon has always been known. Galileo, who invented the telescope, turned it on to the sky in 1610, when our King Charles I. Was on thethrone, and he saw these curious bodies which at first he could notbelieve to be moons. The four which he saw vary in size from twothousand one hundred miles in diameter to nearly three thousand sixhundred. You remember our own moon is two thousand miles across, so eventhe smallest is larger than she. They go round at about the same levelas the planet's Equator, and therefore they cross right in front of him, and go behind him once in every revolution. Since then the other threehave been discovered in the band of Jupiter's satellites--one a smallmoon closer to him than any of the first set, and two others furtherout. It was by observation of the first four, however, that veryinteresting results were obtained. Mathematicians calculated the timethat these satellites ought to disappear behind Jupiter and reappearagain, but they found that this did not happen exactly at the timepredicted; sometimes the moons disappeared sooner than they should havedone, and sometimes later. Then this was discovered to have somerelation to the distance of our earth from Jupiter. When he was at thefar side of his immense orbit he was much more distant from us than whenhe was on the nearer side--in fact, the difference may amount to morethan three hundred millions of miles. And it occurred to some clever manthat the irregularities in time we noticed in the eclipses of thesatellites corresponded with the distance of Jupiter from us. Thefurther he drew away from us, the later were the eclipses, and as hecame nearer they grew earlier. By a brilliant inspiration, this wasattributed to the time light took to travel from them to us, and thiswas the first time anyone had been able to measure the velocity or speedof light. For all practical purposes, on the earth's surface we holdlight to be instantaneous, and well we may, for light could travel morethan eight times round the world in one second. It makes one's brainreel to think of such a thing. Then think how far Jupiter must be awayfrom us at the furthest, when you hear that sometimes these eclipseswere delayed seventeen minutes--minutes, not seconds--because it tookthat time for light to cross the gulf to us! [Illustration: JUPITER AND HIS PRINCIPAL MOONS. ] Sound is very slow compared with light, and that is why, if you watch aman hammering at a distance, the stroke he gives the nail does notcoincide with the bang that reaches you, for light gets to youpractically at once, and the sound comes after it. No sound can travelwithout air, as we have heard, therefore no sound reaches us acrossspace. If the moon were to blow up into a million pieces we should seethe amazing spectacle, but should hear nothing of it. Light travelseverywhere throughout the universe, and by the use of this universalcarrier we have learnt all that we know about the stars and planets. When the time that light takes to travel had been ascertained by meansof Jupiter's satellites, a still more important problem could besolved--that was our own distance from the sun, which before had onlybeen known approximately, and this was calculated to be ninety-twomillions seven hundred thousand miles, though sometimes we are a littlenearer and sometimes a little further away. Jupiter is marvellous, but beyond him lies the most wonderful body inthe whole solar system. We have found curiosities on our way out: wehave studied the problem of the asteroids, of the little moon that goesround Mars in less time than Mars himself rotates; we have consideredthe 'great red spot' on Jupiter, which apparently moves independentlyof the planet; but nothing have we found as yet to compare with therings of Saturn. May you see this amazing sight through a telescope oneday! Look at the picture of this wonderful system, and think what it would belike if the earth were surrounded with similar rings! The first questionwhich occurs to all of us is what must the sky look like from Saturn?What must it be to look up overhead and see several great hoops orarches extending from one horizon to another, reflecting light indifferent degrees of intensity? It would be as if we saw several immenserainbows, far larger than any earthly rainbow, and of pure light, notsplit into colours, extending permanently across the sky, and now andthen broken by the black shadow of the planet itself as it came betweenthem and the sun. However, we must begin at the beginning, and find outabout Saturn himself before we puzzle ourselves over his rings. Saturnis not a very great deal less than Jupiter, though, so small are theother planets in comparison, that if Saturn and all the rest were rolledtogether, they would not make one mass so bulky as Jupiter! Saturn isso light--in other words, his density is so small--that he is actuallylighter than water. He is the lightest, in comparison with his size, ofany of the planets. Therefore he cannot be made largely of solid land, as our earth is, but must be to a great extent, composed of air andgaseous vapour, like his mighty neighbour. He approaches at times asnear to Jupiter as Jupiter does to us, and on these occasions he mustpresent a splendid spectacle to Jupiter. He takes no less thantwenty-nine and a half of our years to complete his stately march aroundthe sun, and his axis is a little more bent than ours; but, of course, at his great distance from the sun, this cannot have the same effect onthe seasons that it does with us. Saturn turns fast on his axis, but notso fast as Jupiter, and in turning his face, or what we call hissurface, presents much the same appearance to us that we might expect, for it changes very frequently and looks like cloud belts. The marvellous feature about Saturn is, of course, the rings. There arethree of these, lying one within the other, and separated by a fine linefrom each other. The middle one is much the broadest, probably about tenthousand miles in width, and the inner one, which is the darkest, wasnot discovered until some time after the others. As the planet swings inhis orbit the rings naturally appear very different to us at differenttimes. Sometimes we can only see them edgewise, and then even in thelargest telescope they are only like a streak of light, and this showsthat they cannot be more than fifty or sixty miles in thickness. The onewhich is nearest to Saturn's surface does not approach him within tenthousand miles. Saturn has no less than ten satellites, in addition tothe rings, so that his midnight sky must present a magnificentspectacle. The rings, which do not shine by their own light but byreflected sunlight, are solid enough to throw a shadow on the body ofthe planet, and themselves receive his shadow. Sometimes for daystogether a large part of Saturn must suffer eclipse beneath theencircling rings, but at other times, at night, when the rings are clearof the planet's body, so that the light is not cut off from them, theymust appear as radiant arches of glory spanning the sky. The subject of these rings is so complicated by the variety of theirchanges that it is difficult for us even to think about it. It is one ofthe most marvellous of all the features of our planetary system. Whatare these rings? what are they made of? It has been positively provedthat they cannot be made of continuous matter, either liquid or solid, for the force of gravity acting on them from the planet would tear themto pieces. What, then, can they be? It is now pretty generally believedthat they are composed of multitudes of tiny bodies, each separate, andcircling separately round the great planet, as the asteroids circleround the sun. As each one is detached from its neighbour and obeys itsown impulses, there is none of the strain and wrench there would be werethey all connected. According to the laws which govern planetary bodies, those which are nearest to the planet will travel more quickly thanthose which are further away. Of course, as we look at them from sogreat a distance, and as they are moving, they appear to us to becontinuous. It is conjectured that the comparative darkness of theinside ring is caused by the fact that there are fewer of the bodiesthere to reflect the sunlight. Then, in addition to the rings, enoughthemselves to distinguish him from all other planets, there are the tenmoons of richly-endowed Saturn to be considered. It is difficult togather much about these moons, on account of our great distance fromthem. The largest is probably twice the diameter of our own moon. One ofthem seems to be much brighter--that is to say, of higher reflectingpower--on one side than the other, and by distinguishing the sidesand watching carefully, astronomers have come to the conclusion that itpresents always the same face to Saturn in the same way as our own moondoes to us; in fact, there is reason to think that all the moons oflarge planets do this. [Illustration: THE PLANET SATURN WITH TWO OF HIS MOONS. ] All the moons lie outside the rings, and some at a very great distancefrom Saturn, so that they can only appear small as seen from him. Yet atthe worst they must be brighter than ordinary stars, and add greatly tothe variations in the sky scenery of this beautiful planet. Inconnection with Saturn's moons there is another of those astonishingfacts that are continually cropping up to remind us that, however muchwe know, there is such a vast deal of which we are still ignorant. Sofar in dealing with all the planets and moons in the solar system wehave made no remark on the way they rotate or revolve, because they allgo in the same direction, and that direction is calledcounter-clockwise, which means that if you stand facing a clock and turnyour hand slowly round the opposite direction to that in which the handsgo, you will be turning it in the same way that the earth rotates on itsaxis and revolves in its orbit. It is, perhaps, just as well to givehere a word of caution. Rotating of course means a planet's turning onits own axis, revolving means its course in its orbit round the sun. Mercury, Venus, Earth, Mars, Jupiter, and all their moons, as well asSaturn himself, rotate on their axes in this onedirection--counter-clockwise--and revolve in the same direction as theyrotate. Even the queer little moon of Mars, which runs round him quickerthan he rotates, obeys this same rule. Nine of Saturn's moons followthis example, but one independent little one, which has been namedPhoebe, and is far out from the planet, actually revolves in theopposite way. We cannot see how it rotates, but if, as we said just now, it turns the same face always to Saturn, then of course it rotates thewrong way too. A theory has been suggested to account for this curiousfact, but it could not be made intelligible to anyone who has notstudied rather high mathematics, so there we must just leave it, and putit in the cabinet of curiosities we have already collected on our wayout to Saturn. For ages past men have known and watched the planets lying within theorbit of Saturn, and they had made up their minds that this was thelimit of our system. But in 1781 a great astronomer named Herschel waswatching the heavens through a telescope when he noticed one strangeobject that he was certain was no star. The vast distance of the starsprevents their having any definite outline, or what is called a disc. The rays dart out from them in all directions and there is no 'edge' tothem, but in the case of the planets it is possible to see a disc with atelescope, and this object which attracted Herschel's attention hadcertainly a disc. He did not imagine he had discovered a new planet, because at that time the asteroids had not been found, and no onethought that there could be any more planets. Yet Herschel knew thatthis was not a star, so he called it a comet! He was actually the firstwho discovered it, for he knew it was not a fixed star, but it was afterhis announcement of this fact that some one else, observing itcarefully, found it to be a real planet with an orbit lying outside thatof Saturn, then the furthest boundary of the solar system. Herschelsuggested calling it Georgius Sidus, in honour of George III. , thenKing; but luckily this ponderous name was not adopted, and as the otherplanets had been called after the Olympian deities, and Uranus was thefather of Saturn, it was called Uranus. It was subsequently found thatthis new planet had already been observed by other astronomers andcatalogued as a star no less than seventeen times, but until Herschel'sclear sight had detected the difference between it and the fixed starsno one had paid any attention to it. Uranus is very far away from thesun, and can only sometimes be seen as a small star by people who knowexactly where to look for him. In fact, his distance from the sun isnineteen times that of the earth. Yet to show at all he must be of great size, and that size has actuallybeen found out by the most delicate experiments. If we go back to ourformer comparison, we shall remember that if the earth were like agreengage plum, then Uranus would be in comparison about the size of oneof those coloured balloons children play with; therefore he is muchlarger than the earth. In this far distant orbit the huge planet takes eighty-four of our yearsto complete one of his own. A man on the earth will have grown frombabyhood to boyhood, from boyhood to the prime of life, and lived longerthan most men, while Uranus has only once circled in his path. But in dealing with Uranus we come to another of those startlingproblems of which astronomy is full. So far we have dealt with planetswhich are more or less upright, which rotate with a rotation like thatof a top. Now take a top and lay it on one side on the table, with oneof its poles pointing toward the great lamp we used for the sun and theother pointing away. That is the way Uranus gets round his path, on hisside! He rotates the wrong way round compared with the planets we havealready spoken of, but he revolves the same way round the sun that allthe others do. It seems wonderful that even so much can be found outabout a body so far from us, but we know more: we have discovered thatUranus is made of lighter material than the earth; his density is less. How can that be known? Well, you remember every body attracts everyother body in proportion to the atoms it contains. If, therefore, therewere any bodies near to Uranus, it could be calculated by his influenceon them what was his own mass, which, as you remember, is the word weuse to express what would be weight were it at the earth's surface; andfar away as Uranus is, the bodies from which such calculations may bemade have been discovered, for he has no less than four satellites, ormoons. Considering now the peculiar position of the planet, we mightexpect to find these moons revolving in a very different way fromothers, and this is indeed the case. They turn round the planet atabout its Equator--that is to say, if you hold the top representingUranus as was suggested just now, these moons would go above and belowthe planet in passing round it. Only we must remember there is really nosuch thing as above and below absolutely. We who are on one side of theworld point up to the sky and down to the earth, while the people on theother side of the earth, say at New Zealand, also point up to the skyand down to the earth, but their pointings are directly the opposite ofours. So when we speak of moons going above and below that is onlybecause, for the moment, we are representing Uranus as a top we hold inour hands, and so we speak of above and below as they are to us. It was Herschel who discovered these satellites, as well as the planet, and for these great achievements he occupies one of the grandest placesin the rōle of names of which England is proud. But he did much morethan this: his improvements in the construction of telescopes, and hisdevotion to astronomy in many other ways, would have caused him to beremembered without anything else. Of Uranus's satellites one, the nearest, goes round in about two and ahalf days, and the one that is furthest away takes about thirteen and ahalf days, so both have a shorter period than our moon. The discovery of Uranus filled the whole civilized world with wonder. The astronomers who had seen him, but missed finding out that he was aplanet, must have felt bitterly mortified, and when he was discovered hewas observed with the utmost accuracy and care. The calculations made todetermine his path in the sky were the easier because he had been notedas a star in several catalogues previously, so that his position forsome time past was known. Everybody who worked at astronomy began toobserve him. From these facts mathematicians set to work, and, byabstruse calculations, worked out exactly the orbit in which he ought tomove; then his movements were again watched, and behold he followed thepath predicted for him; but there was a small difference here and there:he did not follow it exactly. Now, in the heavens there is a reason foreverything, though we may not always be clever enough to find it out, and it was easily guessed that it was not by accident that Uranus didnot precisely follow the path calculated for him. The planets all actand react on one another, as we know, according to their mass and theirdistance, and in the calculations the pull of Jupiter on Saturn and ofSaturn on Uranus were known and allowed for. But Uranus was pulled bysome unseen influence also. A young Englishman named Adams, by some abstruse and difficultmathematical work far beyond the power of ordinary brains, found out notonly the fact that there must be another planet nearly as large asUranus in an orbit outside his, but actually predicted where such aplanet might be seen if anyone would look for it. He gave his results toa professor of astronomy at Cambridge. Now, it seems an easy thing tosay to anyone, 'Look out for a planet in such and such a part of thesky, ' but in reality, when the telescope is turned to that part of thesky, stars are seen in such numbers that, without very carefulcomparison with a star chart, it is impossible to say which are fixedstars and which, if any, is an intruder. There happened to be no starchart of this kind for the particular part of the sky wanted, and thus along time elapsed and the planet was not identified. Meantime a youngFrenchman named Leverrier had also taken up the same investigation, and, without knowing anything of Adams' work, had come to the sameconclusion. He sent his results to the Berlin Observatory, where a starchart such as was wanted was actually just being made. By the use ofthis the Berlin astronomers at once identified this new member of oursystem, and announced to the astonished world that another large planet, making eight altogether, had been discovered. Then the Englishastronomers remembered that they too held in their hands the means formaking this wonderful discovery, but, by having allowed so much time toelapse, they had let the honour go to France. However, the names ofAdams and Leverrier will always be coupled together as the discoverersof the new planet, which was called Neptune. The marvel is that by purereasoning the mind of man could have achieved such results. If the observation of Uranus is difficult, how much more that ofNeptune, which is still further plunged in space! Yet by patience a fewfacts have been gleaned about him. He is not very different in size fromUranus. He also is of very slight density. His year includes one hundredand sixty-five of ours, so that since his discovery in 1846 he has onlyhad time to get round less than a third of his path. His axis is evenmore tilted over than that of Uranus, so that if we compare Uranus to atop held horizontally, Neptune will be like a top with one end pointingdownwards. He rotates in this extraordinary position, in the same manneras Uranus--namely, the other way over from all the other planets, but herevolves, as they all do, counter-clockwise. Seen from Neptune the sun can only appear about as large as Venusappears to us at her best, and the light and heat received are but onenine-hundreth part of what he sends us. Yet so brilliant is sunshinethat even then the light that falls on Neptune must be veryconsiderable, much more than that which we receive from Venus, for thesun itself glows, and from Venus the light is only reflected. The sun, small as it must appear, will shine with the radiance of a glowingelectric light. To get some idea of the brilliance of sunlight, sit neara screen of leaves on some sunny day when the sun is high overhead, andnote the intense radiance of even the tiny rays which shine through thesmall holes in the leaves. The scintillating light is more glorious thanany diamond, shooting out coloured rays in all directions. A small sunthe apparent size of Venus would, therefore, give enough light forpractical purposes to such a world as Neptune, even though to us a worldso illuminated would seem to be condemned to a perpetual twilight. CHAPTER VII THE SUN So far we have referred to the sun just so much as was necessary to showthe planets rotating round him, and to acknowledge him as the source ofall our light and heat; but we have not examined in detail thismarvellous furnace that nourishes all the life on our planet and burnson with undiminished splendour from year to year, without thought oreffort on our part. To sustain a fire on the earth much time and careand expense are necessary; fuel has to be constantly supplied, and menhave to stoke the fire to keep it burning. Considering that the sun isnot only vastly larger than all the fires on the earth put together, butalso than the earth itself, the question very naturally occurs to us, Who supplies the fuel, and who does the stoking on the sun? Before weanswer this we must try to get some idea of the size of this stupendousbody. It is not the least use attempting to understand it by plainfigures, for the figures would be too great to make any impression onus--they would be practically meaningless; we must turn to some othermethod. Suppose, for instance, that the sun were a hollow ball; then, ifthe earth were set at the centre, the moon could revolve round her atthe same distance she is now, and there would be as great a distancebetween the moon and the shell of the sun as there is between the moonand the earth. This gives us a little idea of the size of the sun. Again, if we go back to that solar system in which we represented theplanets by various objects from a pea to a football, and set a lamp inthe centre to do duty for the sun, what size do you suppose that lampwould have to be really to represent the sun in proportion to theplanets? Well, if our greengage plum which did duty for the earth wereabout three-quarters of an inch in diameter we should want a lamp with aflame as tall as the tallest man you know, and even then it would notgive a correct idea unless you imagined that man extending his armswidely, and you drew round him a circle and filled in all the circlewith flame! If this glorious flame burnt clear and fair and bright, radiating beams of light all around, the little greengage plum would nothave to be too near, or it would be shrivelled up as in the blast of afurnace. To place it at anything resembling the distance it is from thesun in reality you would have to walk away from the flaming light forabout three hundred steps, and set it down there; then, after havingdone all this, you would have some little idea of the relative sizes ofthe sun and the earth, and of the distance between them. Of course, all the other planets would have to be at correspondingdistances. On this same scale, Neptune, the furthest out, would be threemiles from our artificial sun! It seems preposterous to think that somespecks so small as to be quite invisible, specks that crawl about onthat plum, have dared to weigh and measure the gigantic sun; but yetthey have done it, and they have even decided what he is made of. Theresult of the experiments is that we know the sun to be a ball ofglowing gas at a temperature so high that nothing we have on earth couldeven compare with it. Of his radiating beams extending in all directionsfew indeed fall on our little plum, but those that do are the source ofall life, whether animal or vegetable. If the sun's rays were cut offfrom us, we should die at once. Even the coal we use to keep us warm isbut sun's heat stored up ages ago, when the luxuriant tropicalvegetation sprang up in the warmth and then fell down and was buried inthe earth. At night we are still enjoying the benefit of the sun'srays--that is, of those which are retained by our atmosphere; for ifnone remained even the very air itself would freeze, and by the nextmorning not one inhabitant would be left alive to tell the awful tale. Yet all this life and growth and heat we receive on the whole earth isbut one part in two thousand two hundred millions of parts that go outin all directions into space. It has been calculated that the heat whichfalls on to all the planets together cannot be more than one part in onehundred millions and the other millions of parts seem to us to be simplywasted. For untold ages the sun has been pouring out this prodigal profusion ofglory, and as we know that this cannot go on without some sort ofcompensation, we want to understand what keeps up the fires in the sun. It is true that the sun is so enormous that he might go on burning for avery long time without burning right away; but, then, even if he ishuge, his expenditure is also huge. If he had been made of solid coal hewould have been all used up in about six thousand years, burning at thepace he does. Now, we know that the ancient Egyptians kept careful noteof the heavenly bodies, and if the sun were really burning away he musthave been very much larger in their time; but we have no record of this;on the contrary, all records of the sun even to five thousand years agoshow that he was much the same as at present. It is evident that we mustsearch elsewhere for an explanation. It has been suggested that hisfurnace is supplied by the number of meteors that fall into him. Meteorsare small bodies of the same materials as the planets, and may belikened to the dust of the solar system. It is not difficult tocalculate the amount of matter he would require on this assumption tokeep him going, and the amount required is so great as to make itpractically impossible that this is the source of his supply. We haveseen that all matter influences all other matter, and the quantity ofmeteoric stuff that would be required to support the sun's expenditurewould be enough to have a serious effect on Mercury, an effect thatwould certainly have been noticed. There can, therefore, be no such massof matter near the sun, and though there is no doubt a certain number ofmeteors do fall into his furnaces day by day, it is not nearly enough toaccount for his continuous radiation. It seems after this as if nothingelse could be suggested; but yet an answer has been found, an answer sowonderful that it is more like a fairy tale than reality. To begin at the beginning, we must go back to the time when the sun wasonly a great gaseous nebula filling all the space included in the orbitof Neptune. This nebula was not in itself hot, but as it rotated itcontracted. Now, heat is really only a form of energy, and energy andheat can be interchanged easily. This is a very startling thing whenheard for the first time, but it is known as surely as we know anythingand has been proved again and again. When a savage wants to make a firehe turns a piece of hard wood very very quickly between hispalms--twiddles it, we should say expressively--into a hole in anotherpiece of wood, until a spark bursts out. What is the spark? It is theenergy of the savage's work turned to heat. When a horse strikes hisiron-shod hoofs hard on the pavement you see sparks fly; that is causedby the energy of the horse's leg. When you pump hard at your bicycle youfeel your pump getting quite hot, for part of the energy you are puttinginto your work is transformed into heat; and so on in numberlessinstances. No energetic action of any kind in this world takes placewithout some of the energy being turned into heat, though in manyinstances the amount is so small as to be unnoticeable. Nothing fallsto the ground without some heat being generated. Now, when this greatnebula first began its remarkable career, by the action of gravity allthe particles in it were drawn toward the centre; little by little theyfell in, and the nebula became smaller. We are not now concerned withthe origin of the planets--we leave that aside; we are onlycontemplating the part of the nebula which remained to become the sun. Now these particles being drawn inward each generated some heat, so asthe nebula contracted its temperature rose. Throughout the ages, overthe space of millions and millions of miles, it contracted and grewhotter. It still remained gaseous, but at last it got to an immensetemperature, and is the sun as we know it. What then keeps it shining?It is still contracting, but slowly, so slowly that it is quiteimperceptible to our finest instruments. It has been calculated that ifit contracts two hundred and fifty feet in diameter in a year, theenergy thus gained and turned into heat is quite sufficient to accountfor its whole yearly output. This is indeed marvellous. In comparisonwith the sun's size two hundred and fifty feet is nothing. It would takenine thousand years at this rate before any diminution could be noticedby our finest instruments! Here is a source of heat which can continuefor countless ages without exhaustion. Thus to all intents and purposeswe may say the sun's shining is inexhaustible. Yet we must follow outthe train of reasoning, and see what will happen in the end, in eras anderas of time, if nothing intervenes. Well, some gaseous bodies are farfiner and more tenuous than others, and when a gaseous body contracts itis all the time getting denser; as it grows denser and denser it at lastbecomes liquid, and then solid, and then it ceases to contract, as ofcourse the particles of a solid body cannot fall freely toward thecentre, as those of a gaseous body can. Our earth has long ago reachedthis stage. When solid the action ceases, and the heat is no more keptup by this source of energy, therefore the body begins to cool--surfacefirst, and lastly the interior; it cools more quickly the smaller it is. Our moon has parted with all her heat long ago, while the earth stillretains some internally. In the sun, therefore, we have an object-lessonof the stages through which all the planets must have passed. They haveall once been glowing hot, and some may be still hot even on thesurface, as we have seen there is reason to believe is the case withJupiter. By this marvellous arrangement for the continued heat of the sun we cansee that the warmth of our planets is assured for untold ages. There isno need to fear that the sun will wear out by burning. His brightnesswill continue for ages beyond the thoughts of man. Besides this, a few other things have been discovered about him. He is, of course, exceptionally difficult to observe; for though he is solarge, which should make it easy, he is so brilliant that anyoneregarding him through a telescope without the precaution of preparedglasses to keep off a great part of the light would be blinded at once. One most remarkable fact about the sun is that his surface is fleckedwith spots, which appear sometimes in greater numbers and sometimes inless, and the reason and shape of these spots have greatly exercisedmen's minds. Sometimes they are large enough to be seen without atelescope at all, merely by looking through a piece of smoked orcoloured glass, which cuts off the most overpowering rays. When they arevisible like this they are enormous, large enough to swallow many earthsin their depths. At other times they may be observed by the telescope, then they may be about five thousand miles across. Sometimes one spotcan be followed by an astronomer as it passes all across the sun, disappears at the edge, and after a lapse of time comes back again roundthe other edge. This first showed men that the sun, like all theplanets, rotated on his axis, and gave them the means of finding out howlong he took in doing so. But the spots showed a most surprising result, for they took slightly different times in making their journey round thesun, times which differed according to their position. For instance, aspot near the equator of the sun took twenty-five days to make thecircuit, while one higher up or lower down took twenty-six days, and onefurther out twenty-seven; so that if these spots are, as certainlybelieved, actually on the surface, the conclusion is that the sun doesnot rotate all in one piece, but that some parts go faster than others. No one can really explain how this could be, but it is certainly moreeasily understood in the case of a body of gas than of a solid body, when it would be simply impossible to conceive. The spots seem to keepprincipally a little north and a little south of the equator; there arevery few actually at it, and none found near the poles, but no reasonfor this distribution has been discovered. It has been noted that aboutevery eleven years the greatest number of spots appears, and thatthey become fewer again, mounting up in number to the next eleven years, and so on. All these curious facts show there is much yet to be solvedabout the sun. The spots were supposed for long to be eruptions burstingup above the surface, but now they are generally held to be deepdepressions like saucers, probably caused by violent tempests, and it isthought that the inrush of cooler matter from above makes them lookdarker than the other parts of the sun's surface. But when we use thewords 'cooler' and 'darker, ' we mean only by comparison, for in realitythe dark parts of the spots are brighter than electric light. [Illustration: _Royal Observatory, Greenwich. _ SUN-SPOTS. ] The fact that the spots are in reality depressions or holes is shown bytheir change of appearance as they pass over the face of the sun towardthe edge; for the change of shape is exactly that which would be causedby foreshortening. It sounds odd to say that the best time for observing the sun is duringa total eclipse, for then the sun's body is hidden by the moon. But yetto a certain extent this is true, and the reason is that the sun's ownbrilliance is our greatest hindrance in observing him, his rays are sodazzling that they light up our own atmosphere, which prevents us seeingthe edges. Now, during a total eclipse, when nearly all the rays arecut off, we can see marvellous things, which are invisible at othertimes. But total eclipses are few and far between, and so when one isapproaching astronomers make great preparations beforehand. A total eclipse is not visible from all parts of the world, but onlyfrom that small part on which the shadow of the moon falls, and as theearth travels, this shadow, which is really a round spot, passes along, making a dark band. In this band astronomers choose the bestobservatories, and there they take up their stations. The dark body ofthe moon first appears to cut a little piece out of the side of the sun, and as it sails on, gradually blotting out more and more, eagertelescopes follow it; at last it covers up the whole sun, and then amarvellous spectacle appears, for all round the edges of the black moonare seen glorious red streamers and arches and filaments of marvellousshapes, continually changing. These are thrown against a background ofpale green light that surrounds the black moon and the hidden sun. Inearly days astronomers thought these wonderful coloured streamersbelonged to the moon; but it was soon proved that they really are partof the sun, and are only invisible at ordinary times, because ouratmosphere is too bright to allow them to be seen. An instrument hasnow been invented to cut off most of the light of the sun, and when thisis attached to a telescope these prominences, as they are called, can beseen at any time, so that there is no need to wait for an eclipse. [Illustration: THE EARTH AS IT WOULD APPEAR IN COMPARISON WITH THEFLAMES SHOOTING OUT FROM THE SUN. ] What are these marvellous streamers and filaments? They are what theyseem, eruptions of fiery matter discharged from the ever-palpitating sunthousands of miles into surrounding space. They are for ever shootingout and bursting and falling back, fireworks on a scale too enormous forus to conceive. Some of these brilliant flames extend for three hundredthousand miles, so that in comparison with one of them the whole worldwould be but a tiny ball, and this is going on day and night withoutcessation. Look at the picture where the artist has made a little blackball to represent the earth as she would appear if she could be seen inthe midst of the flames shooting out from the sun. Do not make a mistakeand think the earth really could be in this position; she is only shownthere so that you may see how tiny she is in comparison with the sun. All the time you have lived and your father, and grandfather, and rightback to the beginnings of English history, and far, far further into thedim ages, this stupendous exhibition of energy and power has continued, and only of late years has anyone known anything about it; even now amere handful of people do know, and the rest, who are warmed and fed andkept alive by the gracious beams of this great revolving glowingfireball, never give it a thought. I said just now a pale green halo surrounded the sun, extending farbeyond the prominences; this is called the corona and can only be seenduring an eclipse. It surrounds the sun in a kind of shell, and there isreason to believe that it too is made of luminous stuff ejected by thesun in its burning fury. It is composed of large streamers or filaments, which seem to shoot out in all directions; generally these are not muchlarger than the apparent width of the sun, but sometimes they extendmuch further. The puzzle is, this corona cannot be an atmosphere in anyway resembling that of our earth; for the gravitational force of thesun, owing to its enormous size, is so great that it would make any suchatmosphere cling to it much more densely near to the surface, while itwould be thinner higher up, and the corona is not dense in any way, butthin and tenuous throughout. This makes it very difficult to explain; itis supposed that some kind of electrical force enters into the problem, but what it is exactly we are far from knowing yet. CHAPTER VIII SHINING VISITORS Our solar system is set by itself in the midst of a great space, and sofar as we have learnt about it in this book everything in it seemsorderly: the planets go round the sun and the satellites go round theplanets, in orbits more or less regular; there seems no place foranything else. But when we have considered the planets and thesatellites, we have not exhausted all the bodies which own allegiance tothe sun. There is another class, made up of strange and weird members, which flash in and out of the system, coming and going in all directionsand at all times--sometimes appearing without warning, sometimesreturning with a certain regularity, sometimes retiring to infinitedepths of space, where no human eye will ever see them more. Thesestrange visitors are called comets, and are of all shapes and sizes andnever twice alike. Even as we watch them they grow and change, and thendiminish in splendour. Some are so vast that men see them as flamingsigns in the sky, and regard them with awe and wonder; some cannot beseen at all without the help of the telescope. From the very earliestages those that were large enough to be seen without glasses have beenregarded with astonishment. Men used to think that they were signs fromheaven foretelling great events in the world. Timid people predictedthat the end of the world would come by collision with one of them. Others, again, fancifully likened them to fishes in that sea of space inwhich we swim--fishes gigantic and terrifying, endowed with sense andwill. It is perhaps unnecessary to say that comets are no more alive than isour own earth, and as for causing the end of the world by collision, there is every reason to believe the earth has been more than once rightthrough a comet's tail, and yet no one except scientific men evendiscovered it. These mysterious visitors from the outer regions of spacewere called comets from a Greek word signifying hair, for they oftenleave a long luminous trail behind, which resembles the filaments of awoman's hair. It is not often that one appears large and bright enoughto be seen by the naked eye, and when it does it is not likely to besoon forgotten. In the year 1910 such a comet is expected, a cometwhich at its former appearance compelled universal attention by itsbrilliancy and strangeness. At the time of the Norman Conquest ofEngland a comet believed to be the very same one was stretching itsglorious tail half across the sky, and the Normans seeing it, took it asa good omen, fancying that it foretold their success. The history of theNorman Conquest was worked in tapestry--that is to say, in what weshould call crewels on a strip of linen--and in this record the cometduly appears. Look at him in the picture as the Normans fancied him. Hehas a red head with blue flames starting from it, and several tails. Thelittle group of men on the left are pointing and chattering about him. We can judge what an impression this comet must have made to be recordedin such an important piece of work. [Illustration: THE COMET IN THE BAYEUX TAPESTRY. ] But we are getting on too fast. We have yet to learn how anyone can knowthat the comet which appeared at the time of the Norman Conquest is thesame as that which has come back again at different times, and aboveall, how anyone can tell that it will come again in the year 1910. Allthis involves a long story. Before the invention of telescopes of course only those comets could beseen which were of great size and fine appearance. In those days mendid not realize that our world was but one of a number and of no greatimportance except to ourselves, and they always took these blazingappearances in the heavens as a particular warning to the human race. But when astronomers, by the aid of the telescope, found that for onecomet seen by the eye there were hundreds which no mortal eye unaidedcould see, this idea seemed, to say the least of it, unlikely. Yet eventhen comets were looked upon as capricious visitors from outer space;odd creatures drawn into our system by the attraction of the sun, whodisappeared, never to return. It was Newton, the same genius whodisclosed to us the laws of gravity, who first declared that cometsmoved in orbits, only that these orbits were far more erratic than anyof those followed by the planets. So far we have supposed that the planets were all on what we should calla level--that is to say, we have regarded them as if they were floatingin a sea of water around the sun; but this is only approximatelycorrect, for the orbits of the planets are not all at one level. If youhad a number of slender hoops or rings to represent the planetaryorbits, you would have to tilt one a little this way and another alittle that way, only never so far but that a line through the centreof the hoop from one side to another could pass through the sun. The wayin which the planetary orbits are tilted is slight in comparison withthat of the orbits of comets, for these are at all sorts of angles--someturned almost sideways, and others slanting, and all of them areellipses long drawn out and much more irregular than the planetaryorbits; but erratic as they are, in every case a line drawn through thesun and extended both ways would touch each side of the orbits. A great astronomer called Halley, who was born in the time of theCommonwealth, was lucky enough to see a very brilliant comet, and thesight interested him so much that he made all the calculations necessaryto find out just in what direction it was travelling in the heavens. Hefound out that it followed an ellipse which brought it very near to thesun at one part of its journey, and carried it far beyond the orbit ofthe earth, right out to that of Neptune, at the other. Then he began tosearch the records for other comets which had been observed before histime. He found that two particularly bright ones had been carefullynoted--one about seventy-five years before that which he had seen, andthe other seventy-five years before that again. Both these comets hadbeen watched so scientifically that the paths in which they hadtravelled could be computed. A brilliant inspiration came to Halley. Hebelieved that instead of these three, his own and the other two, beingdifferent comets, they were the same one, which returned to the sunabout every seventy-five years. This could be proved, for if this ideawere correct, of course the comet would return again in anotherseventy-five years, unless something unforeseen occurred. But Halley wasin the prime of life: he could not hope to live to see his forecastverified. The only thing he could do was to note down exact particulars, by means of which others who lived after him might recognize his comet. And so when the time came for its return, though Halley was in hisgrave, numbers of astronomers were watching eagerly to see thefulfilment of his prediction. The comet did indeed appear, and sincethen it has been seen once again, and now we expect it to come back inthe year 1910, when you and I may see it for ourselves. When theidentity of the comet was fully established men began to search furtherback still, to compare the records of other previous brilliant comets, and found that this one had been noticed many times before, and once asI said, at the time of the Norman Conquest. Halley's comet is peculiarin many ways. For instance, it is unusual that so large and interestinga comet should return within a comparatively limited time. It is thesmaller comets, those that can only be seen telescopically, that usuallyrun in small orbits. The smallest orbits take about three and a halfyears to traverse, and some of the largest orbits known require a periodof one hundred and ten thousand years. Between these two limits liesevery possible variety of period. One comet, seen about the timeNapoleon was born, was calculated to take two thousand years to completeits journey, and another, a very brilliant one seen in 1882, mustjourney for eight hundred years before it again comes near to the sun. But we never know what might happen, for at any moment a comet which hastraversed a long solitary pathway in outer darkness may flash suddenlyinto our ken, and be for the first time noted and recorded, beforeflying off at an angle which must take it for ever further and furtherfrom the sun. Everything connected with comets is mysterious and most fascinating. From out of the icy regions of space a body appears; what it is we knownot, but it is seen at first as a hairy or softly-glowing star, and itwas thus that Herschel mistook Uranus for a comet when he firstdiscovered it. As it draws nearer the comet sends out some fan-likeprojections toward the sun, enclosing its nucleus in filmy wrappingslike a cocoon of light, and it travels faster and faster. From its headshoots out a tail--it may be more than one--growing in splendour andwidth, and always pointing away from the sun. So enormous are some ofthese tails that when the comet's head is close to the sun the tailextends far beyond the orbit of the earth. Faster still and faster fliesthe comet, for as we have seen it is a consequence of the law ofgravitation that the nearer planets are to the sun the faster they movein their orbits, and the same rule applies to comets too. As the cometdashes up to the sun his pace becomes something indescribable; it hasbeen reckoned for some comets at three hundred miles a second! Butbehold, as the head flies round the sun the tail is always projectedoutwards. The nucleus or head may be so near to the sun that the heat itreceives would be sufficient to reduce molten iron to vapour; but thisdoes not seem to affect it: only the tail expands. Sometimes it becomestwo or more tails, and as it sweeps round behind the head it has tocover a much greater space in the same time, and therefore it musttravel even faster than the head. The pace is such that no calculationscan account for it, if the tail is composed of matter in any sense as weknow it. Then when the sun is passed the comet sinks away again, and asit goes the tail dies down and finally disappears. The comet itselfdwindles to a hairy star once more and goes--whither? Into space soremote that we cannot even dream of it--far away into cold moreappalling than anything we could measure, the cold of absolute space. More and more slowly it travels, always away and away, until the sun, ashort time back a huge furnace covering all the sky, is now but a faintstar. Thus on its lonely journey unseen and unknown the comet goes. This comet which we have taken as an illustration is a typical one, butall are not the same. Some have no tails at all, and never develop any;some change utterly even as they are watched. The same comet is sodifferent at different times that the only possible way of identifyingit is by knowing its path, and even this is not a certain method, forsome comets appear to travel at intervals along the same path! Now we come to the question that must have been in the mind of everyonefrom the beginning of this chapter, What are comets? This question noone can answer definitely, for there are many things so puzzling aboutthese strange appearances that it is difficult even to suggest anexplanation. Yet a good deal is known. In the first place, we arecertain that comets have very little density--that is to say, they areindescribably thin, thinner than the thinnest kind of gas; and air, which we always think so thin, would be almost like a blanket comparedwith the material of comets. This we judge because they exercise no sortof influence on any of the planetary bodies they draw near to, whichthey certainly would do if they were made of any kind of solid matter. They come sometimes very close to some of the planets. A comet was sonear to Jupiter that it was actually in among his moons. The comet wasviolently agitated; he was pulled in fact right out of his old path, andhas been going on a new one ever since; but he did not exercise thesmallest effect on Jupiter, or even on the moons. And, as I said earlierin this chapter, we on the earth have been actually in the folds of acomet's tail. This astonishing fact happened in June, 1861. One eveningafter the sun had set a golden-yellow disc, surrounded with filmywrappings, appeared in the sky. The sun's light, diffused throughoutour atmosphere, had prevented its being seen sooner. This was apparentlythe comet's head. It is described as 'though a number of light, hazyclouds were floating around a miniature full moon. ' From this a cone oflight extended far up into the sky, and when the head disappeared belowthe horizon this tail was seen to reach to the zenith. But that was notall. Strange shafts of light seemed to hang right overhead, and couldonly be accounted for by supposing that they were caused by another tailhanging straight above us, so that we looked up at it foreshortened byperspective. The comet's head lay between the earth and the sun, and itstail, which extended over many millions of miles, stretched out behindin such a way that the earth must have gone right through it. The factthat the comet exercised no perceptible influence on the earth at all, and that there were not even any unaccountable magnetic storms ordisplays of electricity, may reassure us so that if ever we do againcome in contact with one of these extremely fine, thin bodies, we neednot be afraid. There is another way in which we can judge of the wonderful tenuity orthinness of comets--that is, that the smallest stars can be seenthrough their tails, even though those tails must be many thousands ofmiles in thickness. Now, if the tails were anything approaching thedensity of our own atmosphere, the stars when seen through them wouldappear to be moved out of their places. This sounds odd, and requires aword of explanation. The fact is that anything seen through anytransparent medium like water or air is what is called refracted--thatis to say, the rays coming from it look bent. Everyone is quite familiarwith this in everyday life, though perhaps they may not have noticed it. You cannot thrust a stick into the water without seeing that it lookscrooked. Air being less dense than water has not quite so strong arefracting power, but still it has some. We cannot prove it in just thesame way, because we are all inside the atmosphere ourselves, and thereis no possibility of thrusting a stick into it from the outside! Theonly way we know it is by looking at something which is 'outside'already, and we find plenty of objects in the sky. As a matter of fact, the stars are all a little pulled out of their places by being seenthrough the air, and though of course we do not notice this, astronomersknow it and have to make allowance for it. The effect is mostnoticeable in the case of the sun when he is going down, for theatmosphere bends his rays up, and though we see him a great glowing redball on the horizon, and watch him, as we think, drop gradually out ofsight, we are really looking at him for the last moment or two when hehas already gone, for the rays are bent up by the air and his imagelingers when the real sun has disappeared. [Illustration: A STICK THRUST INTO THE WATER APPEARS CROOKED. ] Therefore in looking through the luminous stuff that forms a comet'stail astronomers might well expect to see the stars displaced, but not asign of this appears. It is difficult to imagine, therefore, what thetail can be made of. The idea is that the sun exercises a sort ofrepulsive effect on certain elements found in the comet's head--that isto say, it pushes them away, and that as the head approaches the sun, these elements are driven out of it away from the sun in vapour. Thisaction may have something to do with electricity, which is yet littleunderstood; anyway, the effect is that, instead of attracting the mattertoward itself, in which case we should see the comet's tails stretchingtoward the sun, the sun drives it away! In the chapter on the sun we hadto imagine something of the same kind to account for the corona, and thecorona and the comet's tails may be really akin to each other, andcould perhaps be explained in the same way. Now we come to a strangerfact still. Some comets go right through the sun's corona, and yet donot seem to be influenced by it in the smallest degree. This may notseem very wonderful at first perhaps, but if you remember that a dashthrough anything so dense as our atmosphere, at a pace much less thanthat at which a comet goes, is enough to heat iron to a white heat, andthen make it fly off in vapour, we get a glimpse of the extreme finenessof the materials which make the corona. Here is Herschel's account of a comet that went very near the sun: 'The comet's distance from the sun's centre was about the 160th part ofour distance from it. All the heat we enjoy on this earth comes from thesun. Imagine the heat we should have to endure if the sun were toapproach us, or we the sun, to one 160th part of its present distance. It would not be merely as if 160 suns were shining on us all at once, but, 160 times 160, according to a rule which is well known to all whoare conversant with such matters. Now, that is 25, 600. Only imagine aglare 25, 600 times fiercer than that of the equatorial sunshine at noonday with the sun vertical. In such a heat there is no substance we knowof which would not run like water, boil, and be converted into smoke orvapour. No wonder the comet gave evidence of violent excitement, comingfrom the cold region outside the planetary system torpid and ice-bound. Already when arrived even in our temperate region it began to show signsof internal activity; the head had begun to develop, and the tail toelongate, till the comet was for a time lost sight of--not for daysafterwards was it seen; and its tail, whose direction was reversed, andwhich could not possibly be the same tail it had before, had alreadylengthened to an extent of about ninety millions of miles, so that itmust have been shot out with immense force in a direction away from thesun. ' We remember that comets have sometimes more than one tail, and a theoryhas been advanced to account for this too. It is supposed that perhapsdifferent elements are thrust away by the sun at different angles, andone tail may be due to one element and another to another. But if thecomet goes on tail-making to a large extent every time it returns to thesun, what happens eventually? Do the tails fall back again into the headwhen out of reach of the sun's action? Such an idea is inconceivable;but if not, then every time a comet approaches the sun he losessomething, and that something is made up of the elements which wereformerly in the head and have been violently ejected. If this be so wemay well expect to see comets which have returned many times to the sunwithout tails at all, for all the tail-making stuff that was in the headwill have been used up, and as this is exactly what we do see, thetheory is probably true. Where do the comets come from? That also is a very large question. Itused to be supposed they were merely wanderers in space who happened tohave been attracted by our sun and drawn into his system, but there arefacts which go very strongly against this, and astronomers now generallybelieve that comets really belong to the solar system, that their properorbits are ellipses, and that in the case of those which fly off at suchan angle that they can never return they must at some time have beenpulled out of their original orbit by the influence of one of theplanets. [Illustration: _Royal Observatory, Cape of Good Hope. _ A GREAT COMET. ] To get a good idea of a really fine comet, until we have the opportunityof seeing one for ourselves, we cannot do better than look at thispicture of a comet photographed in 1901 at the Cape of Good Hope. It isonly comparatively recently that photography has been applied to comets. When Halley's comet appeared last time such a thing was not thoughtof, but when he comes again numbers of cameras, fitted up with all thelatest scientific appliances, will be waiting to get good impressions ofhim. CHAPTER IX SHOOTING STARS AND FIERY BALLS All the substances which we are accustomed to see and handle in ourdaily lives belong to our world. There are vegetables which grow in theearth, minerals which are dug out of it, and elementary things, such asair and water, which have always made up a part of this planet since manknew it. These are obvious, but there are other things not quite soobvious which also help to form our world. Among these we may class allthe elements known to chemists, many of which have difficult names, suchas oxygen and hydrogen. These two are the elements which make up water, and oxygen is an important element in air, which has nitrogen in it too. There are numbers and numbers of other elements perfectly familiar tochemists, of which many people never even hear the names. We live in themidst of these things, and we take them for granted and pay littleattention to them; but when we begin to learn about other worlds we atonce want to know if these substances and elements which enter solargely into our daily lives are to be found elsewhere in the universeor are quite peculiar to our own world. This question might be answeredin several ways, but one of the most practical tests would be if wecould get hold of something which had not been always on the earth, buthad fallen upon it from space. Then, if this body were made up ofelements corresponding with those we find here, we might judge thatthese elements are very generally diffused throughout the bodies in thesolar system. It sounds in the highest degree improbable that anything should comehurling through the air and alight on our little planet, which we knowis a mere speck in a great ocean of space; but we must not forget thatthe power of gravity increases the chances greatly, for anything comingwithin a certain range of the earth, anything small enough, that is, andnot travelling at too great a pace, is bound to fall on to it. And, however improbable it seems, it is undoubtedly true that masses ofmatter do crash down upon the earth from time to time, and these arecalled meteorites. When we think of the great expanse of the oceans, ofthe ice round the poles, and of the desert wastes, we know that forevery one of such bodies seen to fall many more must have fallen unseenby any human being. Meteors large enough to reach the earth are not veryfrequent, which is perhaps as well, and as yet there is no record ofanyone's having been killed by them. Most of them consist of masses ofstone, and a few are of iron, while various substances resembling thosethat we know here have been found in them. Chemists in analyzing themhave also come across certain elements so far unknown upon earth, thoughof course there is no saying that these may not exist at depths to whichman has not penetrated. A really large meteor is a grand sight. If it is seen at night itappears as a red star, growing rapidly bigger and leaving a trail ofluminous vapour behind as it passes across the sky. In the daytime thisvapour looks like a cloud. As the meteor hurls itself along there may bea deep continuous roar, ending in one supreme explosion, or perhaps inseveral explosions, and finally the meteor may come to the earth in onemass, with a force so great that it buries itself some feet deep in thesoil, or it may burst into numbers of tiny fragments, which arescattered over a large area. When a meteor is found soon after its fallit is very hot, and all its surface has 'run, ' having been fused byheat. The heat is caused by the friction of our atmosphere. The meteorgets entangled in the atmosphere, and, being drawn by the attraction ofthe earth, dashes through it. Part of the energy of its motion is turnedto heat, which grows greater and greater as the denser air nearer to theearth is encountered; so that in time all the surface of the meteor runslike liquid, and this liquid, rising to a still higher temperature, isblown off in vapour, leaving a new surface exposed. The vapour makes thetrail of fire or cloud seen to follow the meteor. If the process went onfor long the meteor would be all dissipated in vapour, and in any caseit must reach the earth considerably reduced in size. Numbers and numbers of comparatively small ones disappear, and for everyone that manages to come to earth there must be hundreds seen only asshooting stars, which vanish and 'leave not a wrack behind. ' When ameteor is seen to fall it is traced, and, whenever possible, it is foundand placed in a museum. Men have sometimes come across large masses ofstone and iron with their surfaces fused with heat. These are in everyway like the recognized meteorites, except that no eye has noted theiradvent. As there can be no reasonable doubt that they are of the sameorigin as the others, they too are collected and placed in museums, andin any large museum you would be able to see both kinds--those whichhave been seen to come to earth and those which have been foundaccidentally. The meteors which appear very brilliant in their course across the skyare sometimes called fire-balls, which is only another name for the samething. Some of these are brighter than the full moon, so bright thatthey cause objects on earth to cast a shadow. In 1803 a fiery ball wasnoticed above a small town in Normandy; it burst and scattered stonesfar and wide, but luckily no one was hurt. The largest meteorites thathave been found on the earth are a ton or more in weight; others aremere stones; and others again just dust that floats about in theatmosphere before gently settling. Of course, meteors of this last kindcould not be seen to fall like the larger ones, yet they do fall in suchnumbers that calculations have been made showing that the earth mustcatch about a hundred millions of meteors daily, having altogether atotal weight of about a hundred tons. This sounds enormous, butcompared with the weight of the earth it is very small indeed. Now that we have arrived at the fact that strange bodies do comehurtling down upon us out of space, and that we can actually handle andexamine them, the next question is, Where do they come from? At one timeit was thought that they were fragments which had been flung off by theearth herself when she was subject to violent explosions, and that theyhad been thrown far enough to resist the impulse to drop down upon heragain, and had been circling round the sun ever since, until the earthcame in contact with them again and they had fallen back upon her. It isnot difficult to imagine a force which would be powerful enough toachieve the feat of speeding something off at such a velocity that itpassed beyond the earth's power to pull it back, but nothing that wehave on earth would be nearly strong enough to achieve such a feat. Imaginative writers have pictured a projectile hurled from a cannon'smouth with such tremendous force that it not only passed beyond therange of the earth's power to pull it back, but so that it fell withinthe influence of the moon and was precipitated on to her surface! Suchthings must remain achievements in imagination only; it is not possiblefor them to be carried out. Other ideas as to the origin of meteorswere that they had been expelled from the moon or from the sun. It wouldneed a much less force to send a projectile away from the moon than fromthe earth on account of its smaller size and less density, but thedistance from the earth to the moon is not very great, and anyprojectile hurled forth from the moon would cross it in a comparativelyshort time. Therefore if the meteorites come from the moon, the moonmust be expelling them still, and we might expect to see some evidenceof it; but we know that the moon is a dead world, so this explanation isnot possible. The sun, for its part, is torn by such giganticdisturbances that, notwithstanding its vast size, there is no doubtsufficient force there to send meteors even so far as the earth, but thechances of their encountering the earth would be small. Both thesetheories are now discarded. It is believed that the meteors are merelylesser fragments of the same kind of materials as the planets, circlingindependently round the sun; and a proof of this is that far moremeteorites fall on that part of the earth which is facing forward in itsjourney than on that behind, and this is what we should expect if themeteors were scattered independently through space and it was by reasonof our movements that we came in contact with them. There is no need toexplain this further. Everyone knows that in cycling or driving along aroad where there is a good deal of traffic both ways the people we meetare more in number than those who overtake us, and the same result wouldfollow with the meteors; that is to say, in travelling through spacewhere they were fairly evenly distributed we should meet more than weshould be overtaken by. You remember that it was suggested the sun's fuel might be obtained frommeteors, and this was proved to be not possible, even though there areno doubt unknown millions of these strange bodies circling throughoutthe solar system. There are so many names for these flashing bodies that we may get alittle confused: when they are seen in the sky they are meteors, orfire-balls; when they reach the earth they are called meteorites, andalso aerolites. Then there is another class of the same bodies calledshooting stars, and these are in reality only meteors on a smallerscale; but there ought to be no confusion in our thoughts, for all theseobjects are small bodies travelling round the sun, and caught by theearth's influence. When you watch the sky for some time on a clear night, you will seldomfail to see at least one star flash out suddenly in a path of thrillinglight and disappear, and you cannot be certain whether that star hadbeen shining in the sky a minute before, or if it had appeared suddenlyonly in order to go out. The last idea is right. We must get rid at onceof the notion that it would be possible for any fixed star to behave inthis manner. To begin with, the fixed stars are many of them actuallytravelling at a great velocity at present, yet so immeasurably distantare they that their movement makes no perceptible difference to us. Forone of them to appear to dash across the heavens as a meteor does wouldmean a velocity entirely unknown to us, even comparing it with the speedof light. No, these shooting stars are not stars at all, though theywere so named, long before the real motions of the fixed stars were evendimly guessed at. As we have seen, they belong to the same class asmeteors. I remember being told by a clergyman, years ago, that one night inNovember he had gone up to bed very late, and as he pulled up his blindto look at the sky, to his amazement he saw a perfect hail of shootingstars, some appearing every minute, and all darting in vivid trails oflight, longer or shorter, though all seemed to come from one point. Somarvellous was the sight that he dashed across the village street, unlocked the church door, and himself pulled the bell with all hismight. The people in that quiet country village had long been in bed, but they huddled on their clothes and ran out of their pretty thatchedcottages, thinking there must be a great fire, and when they saw thewonder in the sky they were amazed and cried out that the world must becoming to an end. The clergyman knew better than that, and was able toreassure them, and tell them he had only taken the most effectual meansof waking them so that they might not miss the display, for he was sureas long as they lived they would never see such another sight. A starshower of this kind is certainly well worth getting up to see, butthough uncommon it is not unique. There are many records of such showershaving occurred in times gone by, and when men put together and examinedthe records they found that the showers came at regular intervals. Forinstance, every year about the same time in November there is a starshower, not comparable, it is true, with the brilliant one the clergymansaw, but still noticeable, for more shooting stars are seen then than atother times, and once in every thirty-three years there is a speciallyfine one. It happened in fact to be one of these that the village peoplewere wakened up to see. Not all at once, but gradually, the mystery of these shower displays wassolved. It was realized that the meteors need not necessarily come fromone fixed place in the sky because they seemed to us to do so, for thatwas only an effect of perspective. If you were looking down a long, perfectly straight avenue of tree-trunks, the avenue would seem to closein, to get narrower and narrower at the far end until it became a point;but it would not really do so, for you would know that the trees at thefar end were just the same distance from each other as those betweenwhich you were standing. Now, two meteors starting from the samedirection at a distance from each other, and keeping parallel, wouldseem to us to start from a point and to open out wider and wider as theyapproached, but they would not really do so; it would only be, as in thecase of the avenue, an effect of perspective. If a great many meteorsdid the same thing, they would appear to us all to start from one point, whereas really they would be on parallel lines, only as they rushed tomeet us or we rushed to meet them this effect would be produced. Therefore the first discovery was that these meteors were thousands andthousands of little bodies travelling in lines parallel to each other, like a swarm of little planets. To judge that their path was not astraight line but a circle or ellipse was the next step, and this wasfound to be the case. From taking exact measurements of their paths inthe sky an astronomer computed they were really travelling round the sunin a lengthened orbit, an ellipse more like a comet's orbit than that ofa planet. But next came the puzzling question, Why did the earthapparently hit them every year to some extent, and once in thirty-threeyears seem to run right into the middle of them? This also was answered. One has only to imagine a swarm of such meteors at first hasteningbusily along their orbit, a great cluster all together, then, by thenear neighbourhood of some planet, or by some other disturbing causes, being drawn out, leaving stragglers lagging behind, until at last theremight be some all round the path, but only thinly scattered, while thebusy, important cluster that formed the nucleus was still much thickerthan any other part. Now, if the orbit that the meteors followed cut theorbit or path of the earth at one point, then every time the earth cameto what we may call the level crossing she must run into some of thestragglers, and if the chief part of the swarm took thirty-three yearsto get round, then once in about thirty-three years the earth muststrike right into it. This would account for the wonderful display. Solong drawn-out is the thickest part of the swarm that it takes a year topass the points at the level crossing. If the earth strikes it near thefront one year, she may come right round in time to strike into the rearpart of the swarm next year, so that we may get fine displays two yearsrunning about every thirty-three years. The last time we passed throughthe swarm was in 1899, and then the show was very disappointing. Here inEngland thick clouds prevented our seeing much, and there will not beanother chance for us to see it at its best until 1932. These November meteors are called Leonids, because they _seem_ to comefrom a group of stars named Leo, and though the most noticeable they arenot the only ones. A shower of the same kind occurs in August too, butthe August meteors, called Perseids, because they seem to come fromPerseus, revolve in an orbit which takes a hundred and forty-two yearsto traverse! So that only every one hundred and forty-second year couldwe hope to see a good display. When all these facts had been gatheredup, it seemed without doubt that certain groups of meteors travelled incompany along an elliptical orbit. But there remained still somethingmore--a bold and ingenious theory to be advanced. It was found that acomet, a small one, only to be seen with the telescope, revolved inexactly the same orbit as the November meteors, and another one, larger, in exactly the same orbit as the August ones; hence it could hardly bedoubted that comets and meteors had some connection with each other, though what that connection is exactly no one knows. Anyway, we can haveno shadow of doubt when we find the comet following a marked path, andthe meteors pursuing the same path in his wake, that the two have somemysterious affinity. There are other smaller showers besides these ofNovember and August, and a remarkable fact is known about one of them. This particular stream was found to be connected with a comet namedBiela's Comet, that had been many times observed, and which returnedabout every seven years to the sun. After it had been seen severaltimes, this astonishing comet split in two and appeared as two comets, both of which returned at the end of the next seven years. But on thenext two occasions when they were expected they never came at all, andthe third time there came instead a fine display of shooting stars, soit really seemed as if these meteors must be the fragments of the lostcomet. It is very curious and interesting to notice that in these star showersthere is no certain record of any large meteorite reaching the earth;they seem to be made up of such small bodies that they are alldissipated in vapour as they traverse our air. CHAPTER X THE GLITTERING HEAVENS On a clear moonless night the stars appear uncountable. You see themtwinkling through the leafless trees, and covering all the sky from thezenith, the highest point above your head, down to the horizon. It seemsas if someone had taken a gigantic pepper-pot and scattered them far andwide so that some had fallen in all directions. If you were asked tomake a guess as to how many you can see at one time, no doubt you wouldanswer 'Millions!' But you would be quite wrong, for the number of starsthat can be seen at once without a telescope does not exceed twothousand, and this, after the large figures we have been dealing with, appears a mere trifle. With a telescope, even of small power, many moreare revealed, and every increase in the size of the telescope shows morestill; so that it might be supposed the universe is indeed illimitable, and that we are only prevented from seeing beyond a certain point by ourlimited resources. But in reality we know that this cannot be so. Ifthe whole sky were one mass of stars, as it must be if the number ofthem were infinite, then, even though we could not distinguish theseparate items, we should see it bright with a pervading and diffusedlight. As this is not so, we judge that the universe is not unending, though, with all our inventions, we may never be able to probe to theend of it. We need not, indeed, cry for infinity, for the distances ofthe fixed stars from us are so immeasurable that to atoms like ourselvesthey may well seem unlimited. Our solar system is set by itself, like alittle island in space, and far, far away on all sides are other greatlight-giving suns resembling our own more or less, but dwindled to thesize of tiny stars, by reason of the great void of space lying betweenus and them. Our sun is, indeed, just a star, and by no means largecompared with the average of the stars either. But, then, he is our own;he is comparatively near to us, and so to us he appears magnificent andunique. Judging from the solar system, we might expect to find thatthese other great suns which we call stars have also planets circlinground them, looking to them for light and heat as we do to our sun. There is no reason to doubt that in some instances the conjecture isright, and that there may be other suns with attendant planets. It ishowever a great mistake to suppose that because our particular family inthe solar system is built on certain lines, all the other families mustbe made on the same pattern. Why, even in our own system we can see howvery much the planets differ from each other: there are no two the samesize; some have moons and some have not; Saturn's rings are quitepeculiar to himself, and Uranus and Neptune indulge in strange vagaries. So why should we expect other systems to be less varied? As science has advanced, the idea that these faraway suns must haveplanetary attendants as our sun has been discarded. The more we know themore is disclosed to us the infinite variety of the universe. Forinstance, so much accustomed are we to a yellow sun that we never thinkof the possibility of there being one of another colour. What would yousay then to a ruby sun, or a blue one; or to two suns of differentcolours, perhaps red and green, circling round each other; or to twosuch suns each going round a dark companion? For there are dark bodiesas well as shining bodies in the sky. These are some of the marvels ofthe starry sky, marvels quite as absorbing as anything we have found inthe solar system. It requires great care and patience and infinite labour before the verydelicate observations which alone can reveal to us anything of thenature of the fixed stars can be accomplished. It is only since theimprovement in large telescopes that this kind of work has becomepossible, and so it is but recently men have begun to study the starsintimately, and even now they are baffled by indescribable difficulties. One of these is our inability to tell the distance of a thing by merelylooking at it unless we also know its size. On earth we are used toseeing things appear smaller the further they are from us, and by longhabit can generally tell the real size; but when we turn to the stars, which appear so much alike, how are we to judge how far off they are?Two stars apparently the same size and close together in the sky mayreally be as far one from another as the earth is from the nearest; forif the further one were very much larger than the nearer, they wouldthen appear the same size. At first it was natural enough to suppose that the big bright stars ofwhat we call the first magnitude were the nearest to us, and the lessbright the next nearest, and so on down to the tiny ones, only revealedby the telescope, which would be the furthest away of all; but researchhas shown that this is not correct. Some of the brightest stars may becomparatively near, and some of the smallest may be near also. The sizeis no test of distance. So far as we have been able to discover, thestar which seems nearest _is_ a first magnitude one, but some of theothers which outshine it must be among the infinitely distant ones. Thuswe lie in the centre of a jewelled universe, and cannot tell even thesize of the jewels which cover its radiant robe. I say 'lie, ' but that is really not the correct word. So far as we havebeen able to find out, there is no such thing as absolute rest in theuniverse--in fact, it is impossible; for even supposing any body couldbe motionless at first, it would be drawn by the attraction of itsnearest neighbours in space, and gradually gain a greater and greatervelocity as it fell toward them. Even the stars we call 'fixed' are allhurrying along at a great pace, and though their distance prevents usfrom seeing any change in their positions, it can be measured bysuitable instruments. Our sun is no exception to this universal rule. Like all his compeers, he is hurrying busily along somewhere inobedience to some impulse of which we do not know the nature; and as hegoes he carries with him his whole cortčge of planets and theirsatellites, and even the comets. Yes, we are racing through space withanother motion, too, besides those of rotation and revolution, for ourearth keeps up with its master attractor, the sun. It is difficult, nodoubt, to follow this, but if you think for a moment you will rememberthat when you are in a railway-carriage everything in that carriage isreally travelling along with it, though it does not appear to move. Andthe whole solar system may be looked at as if it were one block inmovement. As in a carriage, the different bodies in it continue theirown movements all the time, while sharing in the common movement. Youcan get up and change your seat in the train, and when you sit downagain you have not only moved that little way of which you areconscious, but a great way of which you are not conscious unless youlook out of the window. Now in the case of the earth's own motion wefound it necessary to look for something which does not share in thatmotion for purposes of comparison, and we found that something in thesun, who shows us very clearly we are turning on our axis. But in thecase of the motion of the solar system the sun is moving himself, so wehave to look beyond him again and turn to the stars for confirmation. Then we find that the stars have motions of their own, so that it isvery difficult to judge by them at all. It is as if you were bicyclingswiftly towards a number of people all walking about in differentdirections on a wide lawn. They have their movements, but they all alsohave an apparent movement, really caused by you as you advance towardthem; and what astronomers had to do was to separate the true movementsof the stars from the false apparent movement made by the advance of thesun. This great problem was attacked and overcome, and it is now knownwith tolerable certainty that the sun is sweeping onward at a pace ofabout twelve miles a second toward a fixed point. It really matters verylittle to us where he is going, for the distances are so vast thathundreds of years must elapse before his movement makes the slightestdifference in regard to the stars. But there is one thing which we canjudge, and that is that though his course appears to be in a straightline, it is most probably only a part of a great curve so huge that thelittle bit we know seems straight. When we speak of the stars, we ought to keep quite clearly in our mindsthe fact that they lie at such an incredible distance from us that it isprobable we shall never learn a great deal about them. Why, men have noteven yet been able to communicate with the planet Mars, at its nearestonly some thirty-five million miles from us, and this is a mere nothingin measuring the space between us and the stars. To express thedistances of the stars in figures is really a waste of time, soastronomers have invented another way. You know that light can go roundthe world eight times in a second; that is a speed quite beyond ourcomprehension, but we just accept it. Then think what a distance itcould travel in an hour, in a day; and what about a year? The distancethat light can travel in a year is taken as a convenient measure byastronomers for sounding the depths of space. Measured in this way lighttakes four years and four months to reach us from the nearest star weknow of, and there are others so much more distant that hundreds--nay, thousands--of years would have to be used to convey it. Light which hasbeen travelling along with a velocity quite beyond thought, silently, unresting, from the time when the Britons lived and ran half naked onthis island of ours, has only reached us now, and there is no limit tothe time we may go back in our imaginings. We see the stars, not as theyare, but as they were. If some gigantic conflagration had happened ahundred years ago in one of them situated a hundred light-years awayfrom us, only now would that messenger, swifter than any messenger weknow, have brought the news of it to us. To put the matter in figures, we are sure that no star can lie nearer to us than twenty-five billionsof miles. A billion is a million millions, and is represented by afigure with twelve noughts behind it, so--1, 000, 000, 000, 000; andtwenty-five such billions is the least distance within which any starcan lie. How much farther away stars may be we know not, but it issomething to have found out even that. On the same scale as that we tookin our first example, we might express it thus: If the earth were agreengage plum at a distance of about three hundred of your steps fromthe sun, and Neptune were, on the same scale, about three miles away, the nearest fixed star could not be nearer than the distance measuredround the whole earth at the Equator! All this must provoke the question, How can anyone find out thesethings? Well, for a long time the problem of the distances of the starswas thought to be too difficult for anyone to attempt to solve it, butat last an ingenious method was devised, a method which shows once morethe triumph of man's mind over difficulties. In practice this method isextremely difficult to carry out, for it is complicated by so many otherthings which must be made allowance for; but in theory, roughlyexplained, it is not too hard for anyone to grasp. The way of it isthis: If you hold up your finger so as to cover exactly some object afew feet distant from you, and shut first one eye and then the other, you will find that the finger has apparently shifted very considerablyagainst the background. The finger has not really moved, but as seenfrom one eye or the other, it is thrown on a different part of thebackground, and so appears to jump; then if you draw two imaginarylines, one from each eye to the finger, and another between the twoeyes, you will have made a triangle. Now, all of you who have done alittle Euclid know that if you can ascertain the length of one side of atriangle, and the angles at each end of it, you can form the rest of thetriangle; that is to say, you can tell the length of the other twosides. In this instance the base line, as it is called--that is to saythe line lying between the two eyes--can easily be measured, and theangles at each end can be found by an instrument called a sextant, sothat by simple calculation anyone could find out what distance thefinger was from the eye. Now, some ingenious man decided to apply thismethod to the stars. He knew that it is only objects quite near to usthat will appear to shift with so small a base line as that between theeyes, and that the further away anything is the longer must the baseline be before it makes any difference. But this clever man thought thatif he could only get a base line long enough he could easily compute thedistance of the stars from the amount that they appeared to shiftagainst their background. He knew that the longest base line he couldget on earth would be about eight thousand miles, as that is thediameter of the earth from one side to the other; so he carefullyobserved a star from one end of this immense base line and then from theother, quite confident that this plan would answer. But what happened?After careful observations he discovered that no star moved at all withthis base line, and that it must be ever so much longer in order to makeany impression. Then indeed the case seemed hopeless, for here we aretied to the earth and we cannot get away into space. But the astronomerwas nothing daunted. He knew that in its journey round the sun the earthtravels in an orbit which measures about one hundred and eighty-fivemillions of miles across, so he resolved to take observations of thestars when the earth was at one side of this great circle, and again, six months later, when she had travelled to the other side. Then indeedhe would have a magnificent base line, one of one hundred andeighty-five millions of miles in length. What was the result? Even withthis mighty line the stars are found to be so distant that many do notmove at all, not even when measured with the finest instruments, andothers move, it may be, the breadth of a hair at a distance of severalfeet! But even this delicate measure, a hair's-breadth, tells its owntale; it lays down a limit of twenty-five billion miles within which nostar can lie! This system which I have explained to you is called finding the star'sparallax, and perhaps it is easier to understand when we put it theother way round and say that the hair's-breadth is what the whole orbitof the earth would appear to have shrunk to if it were seen from thedistance of these stars! Many, many stars have now been examined, and of them all our nearestneighbour seems to be a bright star seen in the Southern Hemisphere. Itis in the constellation or star group called Centaurus, and is thebrightest star in it. In order to designate the stars when it isnecessary to refer to them, astronomers have invented a system. To onlythe very brightest are proper names attached; others are noted accordingto the degree of their brightness, and called after the letters of theGreek alphabet: alpha, beta, gamma, delta, etc. Our own word 'alphabet'comes, you know, from the first two letters of this Greek series. Asthis particular star is the brightest in the constellation Centaurus, itis called Alpha Centauri; and if ever you travel into the SouthernHemisphere and see it, you may greet it as our nearest neighbour in thestarry universe, so far as we know at present. CHAPTER XI THE CONSTELLATIONS From the very earliest times men have watched the stars, felt theirmysterious influence, tried to discover what they were, and noted theirrising and setting. They classified them into groups, calledconstellations, and gave such groups the names of figures and animals, according to the positions of the stars composing them. Some of theseimaginary figures seem to us so wildly ridiculous that we cannotconceive how anyone could have gone so far out of their way as to inventthem. But they have been long sanctioned by custom, so now, though wefind it difficult to recognize in scattered groups of stars any likenessto a fish or a ram or a bear; we still call the constellations by theirold names for convenience in referring to them. Supposing the axis of the earth were quite upright, straight up and downin regard to the plane at which the earth goes round the sun, then weshould always see the same set of stars from the Northern and the sameset of stars from the Southern Hemispheres all the year round. But asthe axis is tilted slightly, we can, during our nights in the winter inthe Northern Hemisphere, see more of the sky to the south than we can inthe summer; and in the Southern Hemisphere just the reverse is the case, far more stars to the north can be seen in the winter than in thesummer. But always, whether it is winter or summer, there is one fixedpoint in each hemisphere round which all the other stars seem to swing, and this is the point immediately over the North or the South Poles. There is, luckily, a bright star almost at the point at which the NorthPole would seem to strike the sky were it infinitely lengthened. This isnot one of the brightest stars in the sky, but quite bright enough toserve the purpose, and if we stand with our faces towards it, we can besure we are looking due north. How can we discover this star forourselves in the sky? Go out on any starlight night when the sky isclear, and see if you can find a very conspicuous set of seven starscalled the Great Bear. I shall not describe the Great Bear, becauseevery child ought to know it already, and if they don't, they can askthe first grown-up person they meet, and they will certainly be told. (See map. ) [Illustration: CONSTELLATIONS NEAR THE POLE STAR. ] Having found the Great Bear, you have only to draw an imaginary linebetween the two last stars forming the square on the side away from thetail, and carry it on about three times as far as the distance betweenthose two stars, and you will come straight to the Pole Star. The twostars in the Great Bear which help one to find it are called thePointers, because they point to it. The Great Bear is one of the constellations known from the oldest times;it is also sometimes called Charles's Wain, the Dipper, or the Plough. It is always easily seen in England, and seems to swing round the PoleStar as if held by an invisible rope tied to the Pointers. Besides theGreat Bear there is, not far from it, the Little Bear, which is reallyvery like it, only smaller and harder to find. The Pole Star is the laststar in its tail; from it two small stars lead away parallel to theGreat Bear, and they bring the eye to a small pair which form one sideof a square just like that in the Great Bear. But the whole of theLittle Bear is turned the opposite way from the Great Bear, and the tailpoints in the opposite direction. And when you come to think of it, it is very ridiculous to have called these groups Bears at all, or totalk about tails, for bears have no tails! So it would have been betterto have called them foxes or dogs, or almost any other animal ratherthan bears. Now, if you look at the sky on the opposite side of the Pole Star fromthe Great Bear, you will see a clearly marked capital W made up of fiveor six bright stars. This is called Cassiopeia, or the Lady's Chair. In looking at Cassiopeia you cannot help noticing that there is a zoneor broad band of very many stars, some exceedingly small, whichapparently runs right across the sky like a ragged hoop, and Cassiopeiaseems to be set in or on it. This band is called the Milky Way, andcrosses not only our northern sky, but the southern sky too, thus makinga broad girdle round the whole universe. It is very wonderful, and noone has yet been able to explain it. The belt is not uniform and even, but it is here and there broken up into streamers and chips, having thesame appearance as a piece of ribbon which has been snipped about byscissors in pure mischief; or it may be compared to a great river brokenup into many channels by rocks and obstacles in its course. The Milky Way is mainly made up of thousands and thousands of smallstars, and many more are revealed by the telescope; but, as we see inCassiopeia, there are large bright stars in it too, though, of course, these may be infinitely nearer to us, and may only appear to us to be inthe Milky Way because they are between us and it. Now, besides the few constellations that I have mentioned, there arenumbers of others, some of which are difficult to discover, as theycontain no bright stars. But there are certain constellations whichevery one should know, because in them may be found some of thebrightest stars, those of the first magnitude. Magnitude means size, andit is really absurd for us to say a star is of the first magnitudesimply because it appears to us to be large, for, as I have explainedalready, a small star comparatively near to us might appear larger thana greater one further away. But the word 'magnitude' was used when menreally thought stars were large or small according to their appearance, and so it is used to this day. They called the biggest and brightestfirst magnitude stars. Of these there are not many, only some twenty, inall the sky. The next brightest--about the brightness of the Pole Starand the stars in the Great Bear--are of the second magnitude, and soon, each magnitude containing stars less and less bright. When we cometo stars of the sixth magnitude we have reached the limit of our sight, for seventh magnitude stars can only be seen with a telescope. Now thatwe understand what is meant by the magnitude, we can go back to theconstellations and try to find some more. If you draw an imaginary line across the two stars forming the backboneof the Bear, starting from the end nearest the tail, and continue itonward for a good distance, you will come to a very bright star calledCapella, which you will know, because near it are three little ones in atriangle. Now, Capella means a goat, so the small ones are called thekids. In winter Capella gets high up into the sky, and then there is tobe seen below her a little cluster called the Pleiades. There is nothingelse like this in the whole sky. It is formed of six stars, as itappears to persons of ordinary sight, and these stars are of the sixthmagnitude, the lowest that can be seen by the naked eye. But thoughsmall, they are set so close together, and appear so brilliant, twinkling like diamonds, that they are one of the most noticeableobjects in the heavens. A legend tells that there were once seven starsin the Pleiades clearly visible, and that one has now disappeared. Thisis sometimes spoken of as 'the lost Pleiad, ' but there does not seem tobe any foundation for the story. In old days people attached particularholiness or luck to the number seven, and possibly, when they found thatthere were only six stars in this wonderful group, they invented thestory about the seventh. As the Pleiades rise, a beautiful reddish star of the first magnituderises beneath them. It is called Aldebaran, and it, as well as thePleiades, forms a part of the constellation of Taurus the bull. InEngland we can see in winter below Aldebaran the whole of theconstellation of Orion, one of the finest of all the constellations, both for the number of the bright stars it contains and for the extentof the sky it covers. Four bright stars at wide distances enclose anirregular four-sided space in which are set three others close togetherand slanting downwards. Below these, again, are another three which seemto fall from them, but are not so bright. The figure of Orion as drawnin the old representations of the constellations is a very magnificentone. The three bright stars form his belt, and the three smaller onesthe hilt of his sword hanging from it. [Illustration: ORION AND HIS NEIGHBOURS. ] If you draw an imaginary line through the stars forming the belt andprolong it downwards slantingly, you will see, in the very height ofwinter, the brightest star in all the sky, either in the Northern orSouthern Hemisphere. This is Sirius, who stands in a class quite byhimself, for he is many times brighter than any other first magnitudestar. He never rises very high above the horizon here, but on crisp, frosty nights may be seen gleaming like a big diamond between theleafless twigs and boughs of the rime-encrusted trees. Sirius is the DogStar, and it is perhaps fortunate that, as he is placed, he can be seensometimes in the southern and sometimes in the northern skies, so thatmany more people have a chance of looking at his wonderful brilliancy, than if he had been placed near the Pole star. In speaking of thesupreme brightness of Sirius among the stars, we must remember thatVenus and Jupiter, which outrival him, are not stars, but planets, andthat they are much nearer to us. Sirius is so distant that the measuresfor parallax make hardly any impression on him, but, by repeatedexperiments, it has now been proved that light takes more than eightyears to travel from him to us. So that, if you are eight years old, youare looking at Sirius as he was when you were a baby! Not far from the Pleiades, to the left as you face them, are to befound two bright stars nearly the same size; these are the HeavenlyTwins, or Gemini. Returning now to the Great Bear, we find, if we draw a line through themiddle and last stars of his tail, and carry it on for a littledistance, we come fairly near to a cluster of stars in the form of ahorseshoe; there is only one fairly bright one in it, and some of theothers are quite small, but yet the horseshoe is distinct and verybeautiful to look at. This is the Northern Crown. The very bright starnot far from it is another first-class star called Arcturus. To the left of the Northern Crown lies Hercules, which is only mentionedbecause near it is the point to which the sun with all his systemappears at present to be speeding. For other fascinating constellations, such as Leo or the Lion, Andromedaand Perseus, and the three bright stars by which we recognize Aquila theEagle, you must wait awhile, unless you can get some one to point themout. Those which you have noted already are enough to lead you on to searchfor more. Perhaps some of you who live in towns and can see only a little strip ofsky from the nursery or schoolroom windows have already found thischapter dull, and if so you may skip the rest of it and go on to thenext. For the others, however, there is one more thing to know beforeleaving the subject, and that is the names of the string ofconstellations forming what is called the Zodiac. You may have heard therhyme: 'The Ram, the Bull, the Heavenly Twins, And next the Crab, the Lion shines, The Virgin and the Scales; The Scorpion, Archer, and He-goat, The Man that holds the watering-pot, The Fish with glittering tails. ' This puts in a form easy to remember the signs of the constellationswhich lie in the Zodiac, an imaginary belt across the whole heavens. Itis very difficult to explain the Zodiac, but I must try. Imagine for amoment the earth moving round its orbit with the sun in the middle. Now, as the earth moves the sun will be seen continually against a differentbackground--that is to say, he will appear to us to move not only acrossour sky in a day by reason of our rotation, but also along the sky, changing his position among the stars by reason of our revolution. Youwill say at once that we cannot see the stars when the sun is there, andno more we can. But the stars are there all the same, and every monththe sun seems to have moved on into a new constellation, according toastronomers' reckoning. If you count up the names of the constellationsin the rhyme, you will find that there are just twelve, one for eachmonth, and at the end of the year the sun has come round to the firstone again. The first one is Aries the Ram, and the sun is seen projectedor thrown against that part of the sky where Aries is, in April, when webegin spring; this is the first month to astronomers, and not January, as you might suppose. Perhaps you will learn to recognize all theconstellations in the Zodiac one day; a few of them, such as the Bulland the Heavenly Twins, you know already if you have followed thischapter. CHAPTER XII WHAT THE STARS ARE MADE OF How can we possibly tell what the stars are made of? If we think of thevast oceans of space lying between them and us, and realize that we cannever cross those oceans, for in them there is no air, it would seem tobe a hopeless task to find out anything about the stars at all. But eventhough we cannot traverse space ourselves, there is a messenger thatcan, a messenger that needs no air to sustain him, that moves moreswiftly than our feeble minds can comprehend, and this messenger bringsus tidings of the stars--his name is Light. Light tells us manymarvellous things, and not the least marvellous is the news he gives usof the workings of another force, the force of gravitation. In some waysgravitation is perhaps more wonderful than light, for though lightspeeds across airless space, it is stopped at once by any opaquesubstance--that is to say, any substance not transparent, as you knowvery well by your own shadows, which are caused by your bodies stoppingthe light of the sun. Light striking on one side of the earth does notpenetrate through to the other, whereas gravitation does. You remember, of course, what the force of gravitation is, for we read about that veryearly in this book. It is a mysterious attraction existing between allmatter. Every atom pulls every other atom towards itself, more or lessstrongly according to distance. Now, solid matter itself makes nodifference to the force of gravitation, which acts through it as thoughit were not there. The sun is pulling the earth toward itself, and itpulls the atoms on the far side of the earth just as strongly as itwould if there were nothing lying between it and them. Therefore, unlikelight, gravitation takes no heed of obstacles in the way, but acts inspite of them. The gravitation of the earth holds you down just thesame, though you are on the upper floor of a house, with many layers ofwood and plaster between you and it. It cannot pull you down, for thefloor holds you up, but it is gravitation that keeps your feet on theground all the same. A clever man made up a story about some one whoinvented a kind of stuff which stopped the force of gravitation goingthrough it, just as a solid body stops light; when this stuff was made, of course, it went right away off into space, carrying with it anyonewho stood on it, as there was nothing to hold it to the earth! That wasonly a story, and it is not likely anyone could invent such stuff, butit serves to make clear the working of gravitation. These two tirelessforces, light and gravitation, run throughout the whole universe, andcarry messages of tremendous importance for those who have minds tograsp them. Without light we could know nothing of these distant worlds, and without understanding the laws of gravity we should not be able tointerpret much that light tells us. To begin with light, what can we learn from it? We turn at once to ourown great light-giver, the sun, to whom we owe not only all life, butalso all the colour and beauty on earth. It is well known to men ofscience that colour lies in the light itself, and not in any particularobject. That brilliant blue cloak of yours is not blue of itself, butbecause of the light that falls on it. If you cannot believe this, gointo a room lighted only by gas, and hey, presto! the colour is changedas if it were a conjuring trick. You cannot tell now by looking at thecloak whether it is blue or green! Therefore you must admit that as thecolour changes with the change of light it must be due to light, and notto any quality belonging to the material of the cloak. But, you mayprotest, if the colour is solely due to light, and light falls oneverything alike, why are there so many colours? That is a very fairquestion. If the light that comes from the sun were of only onecolour--say blue or red--then everything would be blue or red all theworld over. Some doors in houses are made with a strip of red or blueglass running down the sides. If you have one in your house like that, go and look through it, and you will see an astonishing world made up ofdifferent tones of the same colour. Everything is red or blue, accordingto the colour of the glass, and the only difference in the appearance ofobjects lies in the different shades, whether things are light or dark. This is a world as it might appear if the sun's rays were only blue oronly red. But the sun's light is not of one colour only, fortunately forus; it is of all the colours mixed together, which, seen in a mass, makethe effect of white light. Now, objects on earth are only either seen bythe reflected light of the sun or by some artificial light. They have nolight of their own. Put them in the dark and they do not shine at all;you cannot see them. It is the sun's light striking on them that makesthem visible. But all objects do not reflect the light equally, and thisis because they have the power of absorbing some of the rays that strikeon them and not giving them back at all, and only those rays that aregiven back show to the eye. A white thing gives back all the rays, andso looks white, for we have the whole of the sun's light returned to usagain. But how about a blue thing? It absorbs all the rays except theblue, so that the blue rays are the only ones that come back or reboundfrom it again to meet our eyes, and this makes us see the object blue;and this is the case with all the other colours. A red object retainsall rays except the red, which it sends back to us; a yellow objectgives back only the yellow rays, and so on. What an extraordinary andmysterious fact! Imagine a brilliant flower-garden in autumn. Here wehave tall yellow sunflowers with velvety brown centres, clustering pinkand crimson hollyhocks, deep red and bright yellow peonies, slenderfairy-like Japanese anemones, great bunches of mauve Michaelmas daisies, and countless others, and mingled with all these are many shades ofgreen. Yet it is the light of the sun alone that falling on all thesevaried objects, makes that glorious blaze of colour; it seemsincredible. It may be difficult to believe, but it is true beyond alldoubt. Each delicate velvety petal has some quality in it which causesit to absorb certain of the sun's rays and send back the others, and itscolour is determined by those it sends back. Well then how infinitely varied must be the colours hidden in the sun'slight, colours which, mixed all together, make white light! Yes, this isso, for all colours that we know are to be found there. In fact, thecolours that make up sunlight are the colours to be seen in the rainbow, and they run in the same order. Have you ever looked carefully at arainbow? If not, do so at the next chance. You will see it begins bybeing dark blue at one end, and passes through all colours until it getsto red at the other. We cannot see a rainbow every day just when we want to, but we can seeminiature rainbows which contain just the same colours as the real onesin a number of things any time the sun shines. For instance, in thecut-glass edge of an inkstand or a decanter, or in one of thoseold-fashioned hanging pieces of cut-glass that dangle from thechandelier or candle-brackets. Of course you have often seen thesecolours reflected on the wall, and tried to get them to shine upon yourface. Or you have caught sight of a brilliant patch of colour on thewall and looked around to see what caused it, finally tracing it to somethick edge of shining glass standing in the sunlight. Now, the cut-glassedge shows these colours to you because it breaks up the light thatfalls upon it into the colours it is made of, and lets each one come outseparately, so that they form a band of bright colours instead of justone ray of white light. This is perhaps a little difficult to understand, but I will try toexplain. When a ray of white light falls on such a piece of glass, whichis known as a prism, it goes in as white light at one side, but thethree-cornered shape of the glass breaks it up into the colours it ismade of, and each colour comes out separately at the other side--namely, from blue to red--like a little rainbow, and instead of one ray of whitelight, we have a broad band of all the colours that light is made of. Who would ever have thought a pretty plaything like this could have toldus what we so much wanted to know--namely, what the sun and the starsare made of? It seems too marvellous to be true, yet true it is that forages and ages light has been carrying its silent messages to our eyes, and only recently men have learnt to interpret them. It is as if sometelegraph operator had been going steadily on, click, click, click, foryears and years, and no one had noticed him until someone learnt thecode of dot and dash in which he worked, and then all at once what hewas saying became clear. The chief instrument in translating the messagethat the light brings is simply a prism, a three-cornered wedge ofglass, just the same as those hanging lustres belonging to thechandeliers. When a piece of glass like this is fixed in a telescope insuch a way that the sun's rays fall on it, then there is thrown on to apiece of paper or any other suitable background a broad coloured band oflovely light like a little rainbow, and this is called the sun'sspectrum, and the instrument by which it is seen is called aspectroscope. But this in itself could tell us little; the message itbrings lies in the fact that when it has passed through the telescope, so that it is magnified, it is crossed by hundreds of minute blacklines, not placed evenly at all, but scattered up and down. There may betwo so close together that they look like one, and then three far apart, and then some more at different distances. When this remarkableappearance was examined carefully it was found that in sunlight thelines that appeared were always exactly the same, in the same places, and this seemed so curious that men began to seek for an explanation. Someone thought of an experiment which might teach us something aboutthe matter, and instead of letting sunlight fall on the prism, he madean artificial light by burning some stuff called sodium, and thenallowed the band of coloured light to pass through the telescope; whenhe examined the spectrum that resulted, he found that, though numbers oflines to be found in the sun's spectrum were missing, there were a fewlines here exactly matching a few of the lines in the sun's spectrum;and this could not be the result of chance only, for the lines are somathematically exact, and are in themselves so peculiarly distributed, that it could only mean that they were due to the same cause. What couldthis signify, then, but that away up there in the sun, among otherthings, stuff called sodium, very well known to chemists on earth, isburning? After this many other substances were heated white-hot so as togive out light, in order to discover if the lines to be seen in theirspectra were also to be found in the sun's spectrum. One of these wasiron, and, astonishing to say, all the many little thread-like linesthat appeared in its spectrum were reproduced to a hair's-breadth, amongothers, in the sun's spectrum. So we have found out beyond allpossibility of doubt some of the materials of which the sun is made. Weknow that iron, sodium, hydrogen, and numerous other substances andelements, are all burning away there in a terrific furnace, to which anyfurnace we have on earth is but as the flicker of a match. It was not, of course, much use applying this method to the planets, forwe know that the light which comes from them to us is only reflectedsunlight, and this, indeed, was proved by means of the spectroscope. Butthe stars shine by their own light, and this opened up a wide field forinquiry. The difficulty was, of course, to get the light of one starseparated from all the rest, because the light of one star is very faintand feeble to cast a spectrum at all. Yet by infinite patiencedifficulties were overcome. One star alone was allowed to throw itslight into the telescope; the light passed through a prism, and showed afaint band of many colours, with the expected little black lines cuttingacross it more or less thickly. Examinations have thus been made ofhundreds of stars. In the course of them some substances as yet unknownto us on earth have been encountered, and in some stars oneelement--hydrogen--is much stronger than in others; but, on the whole, speaking broadly, it has been satisfactorily shown that the stars aremade on the same principles as our own sun, so that the reasoning ofastronomers which had argued them to be suns was proved. [Illustration: THE SPECTRUM OF THE SUN AND SIRIUS. ] We have here in the picture the spectrum of the sun and the spectrum ofArcturus. You can see that the lines which appear in the band of lightbelonging to Sirius are also in the band of light belonging to the sun, together with many others. This means that the substances flaming outand sending us light from the far away star are also giving out lightfrom our own sun, and that the sun and Sirius both contain the sameelements in their compositions. This, indeed, seems enough for the spectroscope to have accomplished; ithas interpreted for us the message light brings from the stars, so thatwe know beyond all possibility of mistake that these glowing, twinklingpoints of light are brilliant suns in a state of intense heat, and thatin them are burning elements with which we ourselves are quite familiar. But when the spectroscope had done that, its work was not finished, forit has not only told us what the stars are made of, but another thingwhich we could never have known without it--namely, if they are movingtoward us or going away from us. CHAPTER XIII RESTLESS STARS You remember we have already remarked upon the difficulty of telling howfar one star lies behind another, as we do not know their sizes. It is, to take another similar case, easy enough to tell if a star moves to oneside or the other, but very difficult by ordinary observation to tell ifit is advancing toward us or running away from us, for the only means wehave of judging is if it gets larger or smaller, and at that enormousdistance the fact whether it advances or recedes makes no difference inits size. Now, the spectroscope has changed all this, and we can tellquite as certainly if a star is coming toward us as we can if it movesto one side. I will try to explain this. You know, perhaps, that soundis caused by vibration in the air. The noise, whatever it is, jars theair and the vibrations strike on our ears. It is rather the same thingas the result of throwing a stone into a pond: from the centre of thesplash little wavelets run out in ever-widening circles; so through theair run ever-widening vibrations from every sound. The more vibrationsthere are in a second the shriller is the note they make. In a high notethe air-vibrations follow one another fast, pouring into one's ear at aterrific speed, so that the apparatus in the ear which receives themitself vibrates fiercely and records a high note, while a lower notebrings fewer and slower vibrations in a second, and the ear is not somuch disturbed. Have you ever noticed that if a railway engine issweeping-toward you and screaming all the time, its note seems to getshriller and shriller? That is because the engine, in advancing, sendsthe vibrations out nearer to you, so more of them come in a second, andthus they are crowded up closer together, and are higher and higher. Now, light is also caused by waves, but they are not the same as soundwaves. Light travels without air, whereas sound we know cannot travelwithout air, and is ever so much slower, and altogether a grosser, clumsier thing than light. But yet the waves or rays which make lightcorrespond in some ways to the vibrations of sound. What corresponds tothe treble on the piano is the blue end of the spectrum in light, andthe bass is the red end. Now, when we are looking at the spectrum ofany body which is advancing swiftly toward us, something of the sameeffect is observed as in the case of the shrieking engine. Take any starand imagine that that star is hastening toward us at a pace of threehundred miles a second, which is not at all an unusual rate for a star;then, if we examine the band of light, the spectrum, of such a star, weshall observe an extraordinary fact--all these little lines we havespoken of are shoved up toward the treble or blue end of the spectrum. They still remain just the same distances from each other, and are intwos and threes or single, so that the whole set of lines is unalteredas a set, but everyone of them is shifted a tiny fraction up toward theblue end of the spectrum, just a little displaced. Now if, instead ofadvancing toward us, this same star had been rushing away from us at asimilar pace, all these lines would have been moved a tiny bit towardthe red or bass end of the spectrum. This is known to be certainly true, so that by means of the spectroscope we can tell that some of thesegreat sun-stars are advancing toward us and some receding from us, according to whether the multitudes of little lines in the spectrum areshifted slightly to the blue or the red end. You remember that it has been surmised that the pace the sun moves withhis system is about twelve miles a second. This seems fast enough to us, who think that one mile a minute is good time for an express train, butit is slow compared with the pace of many of the stars. As I have said, some are travelling at a rate of between two hundred and three hundredmiles a second; and it is due to the spectroscope that we know not onlywhether a star is advancing toward us or receding from us, but alsowhether the pace is great or not; it even tells us what the pace is, upto about half a mile a second, which is very marvellous. It is a curiousfact that many of the small stars show greater movement than the largeones, which mayor may not mean that they are nearer to us. It may be taken as established that there is no such thing as absoluterest in the universe: everything, stars and nebulę alike, are movingsomewhere; in an infinite variety of directions, with an infinitevariety of speed they hasten this way and that. It would be impossiblefor any to remain still, for even supposing it had been so 'in thebeginning, ' the vast forces at work in the universe would not let itremain so. Out of space would come the persistent call of gravitation:atoms would cry silently to atoms. There could be no perfect equalityof pull on all sides; from one side or another the pull would be thestronger. Slowly the inert mass would obey and begin falling toward it;it might be an inch at a time, but with rapid increase, until at last italso was hastening some whither in this universe which appears to us tobe infinite. It must be remembered that these stars, even when moving at an enormouspace, do not change their places in the sky when regarded by ordinaryobservers. It would take thousands of years for any of theconstellations to appear at all different from what they are now, eventhough the stars that compose them are moving in different directionswith a great velocity, for a space of many millions of miles, at thedistance of most of the stars, would be but as the breadth of a finehair as seen by us on earth. So thousands of years ago men looked up atthe Great Bear, and saw it apparently the same as we see it now; yet forall that length of time the stars composing it have been rushing in thisdirection and that at an enormous speed, but do not appear to us on theearth to alter their positions in regard to each other. I know ofnothing that gives one a more overwhelming sense of the mightiness ofthe universe and the smallness of ourselves than this fact. From age toage men look on changeless heavens, yet this apparently stable universeis fuller of flux and reflux than is the restless ocean itself, and thevery wavelets on the sea are not more numerous nor more restless thanthe stars that bestrew the sky. CHAPTER XIV THE COLOURS OF THE STARS Has it ever occurred to you that the stars are not all of the samecolour? It is true that, just glancing at them casually, you might saythey are all white; but if you examine them more carefully you cannothelp seeing that some shine with a steely blue light, while others arereddish or yellowish. These colours are not easy to distinguish with thenaked eye, and might not attract any attention at all unless they werepointed out; yet when attention is drawn to the fact, it is impossibleto deny the redness of some, such as Aldebaran. But though we may admitthis, we might add that the colours are so very faint and inconspicuous, that they might be, after all, only the result of imagination. To prove that the colours are constant and real we must use a telescope, and then we need have no further doubt of their reality, for instead ofdisappearing, the colours of some stars stand out quite vividly beyondthe possibility of mistake. Red stars are a bright red, and they are themost easily seen of all, though the other colours, blue and yellow andgreen, are seen very decidedly by some people. The red stars have beendescribed by various observers as resembling 'a drop of blood on a blackfield, ' 'most magnificent copper-red, ' 'most intense blood-red, ' and'glowing like a live coal out of the darkness of space. ' Some people seethem as a shining red, like that of a glowing cloud at sunset. Thereforethere can be no doubt that the colours are genuine enough, and aretelling us some message. This message we are able to read, for we havebegun to understand the language the stars speak to us by their lightsince the invention of the spectroscope. The spectroscope tells us thatthese colours indicate different stages in the development of the stars, or differences of constitution--that is to say, in the elements of whichthey are made. Our own sun is a yellow star, and other yellow stars areakin to him; while red and blue and green stars contain differentelements, or elements in different proportions. Stars do not always remain the same colours for an indefinite time; onestar may change slowly from yellow to white, and another from red toyellow; and there are instances of notable changes, such as that of thebrilliant white Sirius, who was stated in old times by many differentobservers to be a red star. All this makes us think, and year by yearthought leads us on to knowledge, and knowledge about these distant sunsincreases. But though we know a good deal now, there are still manyquestions we should like to ask which we cannot expect to have answeredfor a long time yet, if ever. The star colours have some meanings which we cannot even guess; we canonly notice the facts regarding them. For instance, blue stars are neverknown to be solitary--they always have a companion, but why this shouldbe so passes our comprehension. What is it in the constitution of a bluestar which holds or attracts another? Whatever it may be, it isestablished by repeated instances that blue stars do not stand alone. Inthe constellation of Cygnus there are two stars, a blue and a yellowone, which are near enough to each other to be seen in the sametelescope at the same time, and yet in reality are separated by analmost incredible number of billions of miles. But as we know that ablue star is never seen alone, and that it has often as its companion ayellowish or reddish star, it is probable that these two, situated at anenormous distance from one another, are yet in some mysterious waydependent on each other, and are not merely seen together because theyhappen to fall in the same field of view. Many double stars show most beautifully contrasted colours: among themare pairs of yellow and rose-red, golden and azure, orange and purple, orange and lilac, copper-colour and blue, apple-green and cherry-red, and so on. In the Southern Hemisphere there is a cluster containing somany stars of brilliant colours that Sir John Herschel named it 'theJewelled Cluster. ' I expect most of you have seen an advertisement of Pear's Soap, in whichyou are asked to stare at some red letters, and then look away to somewhite surface, such as a ceiling, when you will see the same letters ingreen. This is because green is the complementary or contrasting colourto red, and the same thing is the case with blue and yellow. When anyone colour of either of these pairs is seen, it tends to make the otherappear by reaction, and if the eye gazed hard at blue instead of red, itwould next see yellow, and not green. Now, many people to whom thiscurious fact is known argue that perhaps the colours of the doublestars are not real, but the effect of contrast only; for instance, theysay a red star near a companion white one would tend to make thecompanion appear green, and so, of course, it would. But this does notaccount for the star colours, which are really inherent in the starsthemselves, as may be proved by cutting off the light of one star, andlooking only at the other, when its colour still appears unchanged. Another argument equally strong against the contrast theory is that thecolours of stars in pairs are by no means always those which wouldappear if the effect was only due to complementary colours. It is notalways blue and yellow or red and green pairs that we see, though theseare frequent, but many others of various kinds, such as copper and blue, and ruddy and blue. We have therefore come to the conclusion that there are in thisastonishing universe numbers of gloriously coloured suns, some of whichapparently lie close together. What follows? Why, we want to know, ofcourse, if these stars are really pairs connected with each other, or ifthey only appear so by being in the same line of sight, though one isinfinitely more distant than the other. And that question also has beenanswered. There are now known thousands of cases in which stars, hitherto regarded as single, have been separated into two, or even more, by the use of a telescope. Of these thousands, some hundreds have beencarefully investigated, and the result is that, though there areundoubtedly some in which the connexion is merely accidental, yet in byfar the greater number of cases the two stars thus seen together havereally some connexion which binds them to one another; they aredependent on one another. This has been made known to us by the workingof the wonderful law of gravitation, which is obeyed throughout thewhole universe. We know that by the operation of this law two mightysuns will be drawn toward each other with a certain pull, just as surelyas we know that a stone let loose from the hand will fall upon theearth; so by noting the effect of two mighty suns upon each other manyfacts about them may be found out. By the most minute and carefulmeasurements, by the use of the spectroscope, and by every resourceknown to science, astronomers have, indeed, actually found out with anear approach to exactness how far some of these great suns lie fromeach other, and how large they are in comparison with one another. The very first double star ever discovered was one which you havealready seen, the middle one in the tail of the Great Bear. If you lookat it you will be delighted to find that you can see a wee star close toit, and you will think you are looking at an example of a double starwith your very own eyes; but you will be wrong, for that wee star isseparated by untold distances from the large one to which it seems sonear. In fact, any stars which can be seen to be separate by the nakedeye must lie immeasurably far apart, however tiny seems the spacebetween them. Such stars may possibly have some connexion with eachother, but, at any rate in this case, such a connexion has not beenproved. No, the larger star itself is made up of two others, which canonly be seen apart in a telescope. Since this discovery double starshave been plentifully found in every part of the sky. The average spacebetween such double stars as seen from our earth is--what do you think?It is the width of a single hair held up thirty-six feet from our eyes!This could not, of course, be seen without the use of a telescope oropera-glasses. It serves to give some impression of star distances whenwe think that the millions and millions of miles lying between thosestars have shrunk to that hair's-breadth seen from our point of view. Twin stars circle together round a common centre of gravity, and arebound by the laws of gravitation just as the planets are. Our sun is asolitary star, with no companion, and therefore such a state of thingsseems to us to be incredible. Fancy two gigantic suns, one topaz-yellowand the other azure-blue, circling around in endless movement! Where insuch a system would there be room for the planets? How could planetsexist under the pull of two suns in opposite directions? Still morewonders are unfolded as the inquiry proceeds. Certain irregularities inthe motions of some of these twin systems led astronomers to infer thatthey were acted upon by another body, though this other body was notdiscernible. In fact, though they could not see it, they knew it must bethere, just as Adams and Leverrier knew of the existence of Neptune, before ever they had seen him, by the irregularities in the movements ofUranus. As the results showed, it was there, and was comparable in sizeto the twin suns it influenced, and yet they could not see it. So theyconcluded this third body must be dark, not light-giving like itscompanions. We are thus led to the strange conclusion that some of thesesystems are very complicated, and are formed not only of shining suns, but of huge dark bodies which cannot be called suns. What are they, then? Can they be immense planets? Is it possible that life may thereexist? No fairy tale could stir the imagination so powerfully as thethought of such systems including a planetary body as large or largerthan its sun or suns. If indeed life exists there, what a varied scenemust be presented day by day! At one time both suns mingling theirflashing rays may be together in the sky; at another time only oneappears, a yellow or blue sun, as the case may be. The surface of suchplanets must undergo weird transformations, the foliage showing one daygreen, the next yellow, and the next blue; shadows of azure and orangewill alternate! But fascinating as such thoughts are, we can get nofurther along that path. To turn from fancy to facts, we find that telescope and spectroscopehave supplied us with quite enough matter for wonder without callingupon imagination. We have discovered that many of the stars which seemto shine with a pure single light are double, and many more consist notonly of two stars, but of several, some of which may be dark bodies. ThePole Star was long known to be double, and is now discovered to have athird member in its system. These multiple systems vary from oneanother in almost every case. Some are made up of a mighty star and acomparatively small one; others are composed of stars equal inlight-giving power--twin suns. Some progress swiftly round their orbits, some go slowly; indeed, so slowly that during the century they have beenunder observation only the very faintest sign of movement has beendetected; and in other systems, which we are bound to suppose double, the stars are so slow in their movements that no progress seems to havebeen made at all. The star we know as the nearest to us in the heavens, Alpha Centauri, iscomposed of two very bright partners, which take about eighty-sevenyears to traverse their orbit. They sometimes come as near to each otheras Saturn is to the sun. In the case of Sirius astronomers found outthat he had a companion by reason of his irregularities of movementbefore they discovered that companion, which is apparently a very smallstar, only to be discerned with good telescopes. But here, again, itwould be unwise to judge only by what we see. Though the star appearssmall, we know by the influence it exercises on Sirius that it is verynearly the same size as he is. Thus we judge that it is poor inlight-giving property; in fact, its shining power is much less than thatof its companion, though its size is so nearly equal. This is notwonderful, for Sirius's marvellous light-giving power is one of thewonders of the universe; he shines as brilliantly as twenty-nine orthirty of our suns! In some cases the dark body which we cannot see may even be larger thanthe shining one, through which alone we can know anything of it. Here wehave a new idea, a hint that in some of these systems there may be amighty earth with a smaller sun going round it, as men imagined our sunwent around the earth before the real truth was found out. So we see that, when we speak of the stars as suns comparable with oursun, we cannot think of them all as being exactly on the same model. There are endless varieties in the systems; there are solitary suns likeours which may have a number of small planets going round them, as inthe solar system; but there are also double suns going round each other, suns with mighty dark bodies revolving round them which may be planets, and huge dark bodies with small suns too. Every increase of knowledgeopens up new wonders, and the world in which we live is but one kind ofworld amid an infinite number. In this chapter we have learnt an altogether new fact--the fact that thehosts of heaven comprise not only those shining stars we are accustomedto see, but also dark bodies equally massive, and probably equallynumerous, which we cannot see. In fact, the regions of space may bestrewn with such dark bodies, and we could have no possible means ofdiscovering them unless they were near enough to some shining body toexert an influence upon it. It is not with his eyes alone, or with hissenses, man knows of the existence of these great worlds, but oftensolely by the use of the powers of his mind. CHAPTER XV TEMPORARY AND VARIABLE STARS It is a clear night, nearly all the world is asleep, when an astronomercrosses his lawn on his way to his observatory to spend the dark hoursin making investigations into profound space. His brilliant mind, following the rays of light which shoot from the furthest star, willtraverse immeasurable distances, while the body is forgotten. Justbefore entering the observatory he pauses and looks up; his eye catchessight of something that arrests him, and he stops involuntarily. Yet anystranger standing beside him, and gazing where he gazes, would seenothing unusual. There is no fiery comet with its tail stretching acrossfrom zenith to horizon, no flaming meteor dashing across the darkenedsky. But that there is something unusual to be seen is evident, for theastronomer breathes quickly, and after another earnest scrutiny of theobject which has attracted him, he rushes into the observatory, searchesfor a star-chart, and examines attentively that part of the sky atwhich he has been gazing. He runs his finger over the chart: here andthere are the well-known stars that mark that constellation, but here?In that part there is no star marked, yet he knows, for his own eyeshave told him but a few moments ago, that here there is actually blazinga star, not large, perhaps, but clear enough to be seen without atelescope--a star, maybe, which no eye but his has yet observed! He hurries to his telescope, and adjusts it so as to bring the strangerinto the field of view. A new star! Whence has it come? What does itmean? By the next day at the latest the news has flown over the wires, and allthe scientific world is aware that a new star has been detected where nostar ever was seen before. Hundreds of telescopes are turned on to it;its spectrum is noted, and it stands revealed as being in a state ofconflagration, having blazed up from obscurity to conspicuousness. Nightafter night its brilliance grows, until it ranks with the brighteststars in heaven, and then it dies down and grows dim, graduallysinking--sinking into the obscurity from whence it emerged so briefly, and its place in the sky knows it no more. It may be there still, butso infinitely faint and far away that no power at our command can revealit to us. And the amazing part of it is that this huge disaster, thismighty conflagration, is not actually happening as it is seen, but hashappened many hundreds of years ago, though the message brought by thelight carrier has but reached us now. There have not been a great many such outbursts recorded, though manymay have taken place unrecorded, for even in these days, when trainedobservers are ceaselessly watching the sky, 'new' stars are not alwaysnoticed at once. In 1892 a new star appeared, and shone for two monthsbefore anyone noticed it. This particular one never rose to any verybrilliant size. I twas situated in the constellation of Auriga, and wasnoticed on February 1. It remained fairly bright until March 6, when itbegan to die down; but it has now sunk so low that it can only be seenin the very largest telescopes. Photography has been most useful in recording these stars, for when oneis noticed it has sometimes been found that it has been recorded on aphotographic plate taken some time previously, and this shows us howlong it has been visible. More and more photography becomes the usefulhandmaid of astronomers, for the photographic prepared plate is moresensitive to rays of light than the human eye, and, what is more usefulstill, such plates retain the rays that fall upon them, and fix theimpression. Also on a plate these rays are cumulative--that is to say, if a very faint star shines continuously on a plate, the longer theplate is exposed, within certain limits, the clearer will the image ofthat star become, for the light rays fall one on the top of the other, and tend to enforce each other, and so emphasize the impression, whereaswith our eyes it is not the same thing at all, for if we do not see anobject clearly because it is too faint, we do not see it any better, however much we may stare at the place where it ought to be. This isbecause each light ray that reaches our eye makes its own impression, and passes on; they do not become heaped on each other, as they do on aphotographic plate. One variable star in Perseus, discovered in 1901, rose to suchbrilliancy that for one night it was queen of the Northern Hemisphere, outshining all the other first-class stars. It rose into prominence with wonderful quickness, and sank equally fast. At its height it outshone our sun eight thousand times! This star wasso far from us that it was reckoned its light must take about threehundred years to reach us, consequently the great conflagration, orwhatever caused the outburst, must have taken place in the reign ofJames I. , though, as it was only seen here in 1901, it was called thenew star of the new century. When these new stars die down they sometimes continue to shine faintlyfor a long time, so that they are visible with a telescope, but in othercases they may die out altogether. We know very little about them, andhave but small opportunity for observing them, and so it is not safe tohazard any theories to account for their peculiarities. At first mensupposed that the great flame was made by a violent collision betweentwo bodies coming together with great velocity so that both flared up, but this speculation has been shown by the spectroscope to beimprobable, and now it is supposed by some people that two starsjourneying through space may pass through a nebulous region, and thusmay flare up, and such a theory is backed up by the fact that a verygreat number of such stars do seem to be mixed up in some strange waywith a nebulous haze. All these new stars that we have been discussing so far have onlyblazed up once and then died down, but there is another class of starsquite as peculiar, and even more difficult to explain, and these arecalled variable stars. They get brighter and brighter up to a certainpoint, and then die down, only to become bright once more, and thesechanges occur with the utmost regularity, so that they are known and canbe predicted beforehand. This is even more unaccountable than a suddenand unrepeated outburst, for one can understand a great flare-up, butthat a star should flare and die down with regularity is almost beyondcomprehension. Clearly we must look further than before for anexplanation. Let us first examine the facts we know. Variable starsdiffer greatly from each other. Some are generally of a low magnitude, and only become bright for a short time, while others are bright most ofthe time and die down only for a short time. Others become very bright, then sink a little bit, but not so low as at first; then they becomebright again, and, lastly, go right down to the lowest point, and theykeep on always through this regular cycle of changes. Some go throughthe whole of these changes in three days, and others take much longer. The periods, as the intervals between the complete round of changes arecalled, vary, in fact, between three days and six hundred! It may seemimpossible that changes covering so long as six hundred days could beknown and followed, but there is nothing that the patience ofastronomers will not compass. One very well-known variable star you can see for yourselves, and as anounce of observation is worth a pound of hearsay, you might take alittle trouble to find it. Go out on any clear starlight night and look. Not very far from Cassiopeia (W. ), to the left as you face it, are threebright stars running down in a great curve. These are in theconstellation called Perseus, and a little to the right of the middleand lowest one is the only variable star we can see in the sky without atelescope. This is Algol. For the greater part of three days he is a bright star ofabout the second magnitude, then he begins to fade, and for four and ahalf hours grows steadily dimmer. At the dimmest he remains for abouttwenty minutes, and then rises again to his ordinary brightness in threeand a half hours. How can we explain this? You may possibly be able tosuggest a reason. What do you say to a dark body revolving round Algol, or, rather, revolving with him round a common centre of gravity? Ifsuch a thing were indeed true, and if such a body happened to passbetween us and Algol at each revolution, the light of Algol would be cutoff or eclipsed in proportion to the size of such a body. If the darkbody were the full size of Algol and passed right between him and us, itwould cut off all the light, but if it were not quite the same size, alittle would still be seen. And this is really the explanation of thestrange changes in the brightness of Algol, for such a dark body as weare imagining does in reality exist. It is a large dark body, verynearly as large as Algol himself, and if, as we may conjecture, it is amighty planet, we have the extraordinary example of a planet and its sunbeing nearly the same size. We have seen that the eclipse happens everythree days, and this means, of course, that the planetary body must goround its sun in that time, so as to return again to its positionbetween us and him, but the thing is difficult to believe. Why, thenearest of all our planets to the sun, the wee Mercury, takeseighty-seven days to complete its orbit, and here is a mighty bodyhastening round its sun in three! To do this in the time the large darkplanet must be very near to Algol; indeed, astronomers have calculatedthat the surfaces of the two bodies are not more than about two millionmiles apart, and this is a trifle when we consider that we ourselves aremore than forty-six times as far as that from the sun. At this distanceAlgol, as observed from the planet, will fill half the sky, and the heathe gives out must be something stupendous. Also the effects ofgravitation must be queer indeed, acting on two such huge bodies soclose together. If any beings live in such a strange world, the pullwhich draws them to their mighty sun must be very nearly equal to thepull which holds them to their own globe; the two together maycounteract each other, but the effect must be strange! From irregularities in the movements of Algol it has been judged thatthere may be also in the same system another dark body, but of itnothing has been definitely ascertained. But all variable stars need not necessarily be due to the light beingintercepted by a dark body. There are cases where two bright stars inrevolving round each other produce the same effect; for when seen sideby side the two stars give twice as much light as when one is hiddenbehind the other, and as they are seen alternately side by side and inline, they seem to alter regularly in lustre. CHAPTER XVI STAR CLUSTERS AND NEBULĘ Could you point out any star cluster in the sky? You could if you wouldonly think for a minute, for one has been mentioned already. This is thecluster known as the Pleiades, and it is so peculiar and so differentfrom anything else, that many people recognize the group and know whereto look for it even before they know the Great Bear, the favouriteconstellation in the northern sky, itself. The Pleiades is a real starcluster, and the chief stars in it are at such enormous distances fromone another that they can be seen separately by the eye unaided, whereasin most clusters the stars appear to be so close together that without atelescope they make a mere blur of brightness. For a long time it wassupposed that the stars composing the Pleiades could not really beconnected because of the great distances between them; for, as you know, even a hair's-breadth apparently between stars signifies in reality manymillions of miles. Light travelling from the Pleiades to us, at that incomprehensible paceof which you already know, takes a hundred and ninety years to reach us!At this incredibly remote distance lies the main part of the clusterfrom us; but it is more marvellous still that we have every reason tobelieve that the outlying stars of this cluster are as far from thecentral ones as the nearest star we know of, Alpha Centauri, is from us!Little wonder was it, then, that men hesitated to ascribe to thePleiades any real connection with each other, and supposed them to bemerely an assemblage of stars which seemed to us to lie together. With the unaided eye we see comparatively few stars in the Pleiades. Sixis the usual number to be counted, though people with very good sighthave made out fourteen. Viewed through the telescope, however, the scenechanges: into this part of space stars are crowded in astonishingprofusion; it is impossible to count them, and with every increase inthe power of the telescope still more are revealed. Well over a thousandin this small space seems no exaggerated estimate. Now, it is impossibleto say how many of these really belong to the group, and how many areseen there accidentally, but observations of the most prominent oneshave shown that they are all moving in exactly the same direction atthe same pace. It would be against probability to conceive that such athing could be the result of mere chance, considering the infinitevariety of star movements in general, and so we are bound to believethat this wonderful collection of stars is a real group, and not only anapparent one. So splendid are the great suns that illuminate this mighty system, thatat least fifty or sixty of them far surpass our own sun in brilliancy. Therefore when we look at that tiny sparkling group we must inimagination picture it as a vast cluster of mighty stars, all controlledand swayed by some dominant impulse, though separated by spaces enoughto make the brain reel in thinking of them. If these suns possess alsoattendant planets, what a galaxy of worlds, what a universe within auniverse is here! Other star clusters there are, not so conspicuous as the Pleiades, andmost of these can only be seen through a telescope, so we may bethankful that we have one example so splendid within our own vision. There are some clusters so far and faintly shining that they were atfirst thought to be nebulę, and not stars at all; but the telescopegradually revealed the fact that many of these are made up of stars, and so people began to think that all faint shining patches of nebulouslight were really star clusters, which would be resolved into stars ifonly we had better telescopes. Since the invention of the spectroscope, however, fresh light has been thrown on the matter, for the spectrumwhich is shown by some of the nebulous patches is not the same as thatshown by stars, and we know that many of these strange appearances arenot made up of infinitely distant stars. We are talking here quite freely about nebulę because we have met onelong ago when we discussed the gradual evolution of our own system, andwe know quite well that a nebula is composed of luminous faintly-glowinggas of extreme fineness and thinness. We see in the sky at the presenttime what we may take to be object-lessons in our own history, for wesee nebulę of all sorts and sizes, and in some stars are mixed up, andin others stars are but dimly seen, so that it does not require a greatstretch of the imagination to picture these stars as being born, emerging from the swaddling bands of filmy webs that have enwrappedthem; and other nebulę seem to be gas only, thin and glowing, with nostars at all to be found in it. We still know very little about thesemysterious appearances, but the work of classifying and resolving themis going on apace. Nebulę are divided into several classes, but theeasiest distinction to remember is that between white nebulę and greennebulę. This is not to say that we can see some coloured green, but thatgreen appears in the spectrum of some of the nebulę, while the spectrumof a white nebula is more like that of a star. It is fortunate for us that in the sky we can see without a telescopeone instance of each of the several objects of interest that we havereferred to. We have been able to see one very vivid example of a variable star; wehave seen one very beautiful example of a star cluster; and it remainsto look for one very good example of a white nebula. Just as in finding Algol you were doing a little bit of practical work, proving something of which you had read, so by seeing this nebula youwill remember more about nebulę in general than by reading many chapterson the subject. This particular nebula is in Andromeda, and is not farfrom Algol; and it is not difficult to find. It is the only one that canbe well seen without a telescope, and was known to the ancients; it isbelieved to have been mentioned in a book of the tenth century! If you take an imaginary line down from the two left-hand stars ofCassiopeia, and follow it carefully, you will come before long to arather faint star, and close to it is the nebula. When you catch sight of it you will, perhaps, at first be disappointed, for all you will see is a soft blur of white, as if someone had laid adab of luminous paint on the sky with a finger; but as you gaze at itnight after night and realize its unchangeableness, realize also that itis a mass of glowing gas, an island in space, infinitely distant, unsupported and inexplicable, something of the wonder of it will creepover you. [Illustration: _Dr. Max Wolf. _ THE GREAT NEBULA IN ANDROMEDA. ] Thousands of telescopic nebulę are now known, and have been examined, and they are of all shapes. Roughly, they have been divided up intoseveral classes--those that seem to us to be round and those that arelong ovals, like this one in Andromeda; but these may, of course, beonly round ones seen edgewise by us; others are very irregular, andspread over an enormous part of the sky. The most remarkable of these isthat in Orion, and if you look very hard at the middle star in thesword-hilt of Orion, you may be able to make out a faint mistiness. This, when seen through a telescope, becomes a wonderful andfar-spreading nebula, with brighter and darker parts like gulfs init, and dark channels. It has been sometimes called the Fish-mouthNebula, from a fanciful idea as to its shape. Indeed, so extraordinarilyvaried are these curious structures, that they have been compared withnumbers of different objects. We have some like brushes, othersresembling fans, rings, spindles, keyholes; others like animals--a fish, a crab, an owl, and so on; but these suggestions are imaginative, andhave nothing to do with the real problem. In _The System of the Stars_Miss Clerke says: 'In regarding these singular structures we seem to seesurges and spray-flakes of a nebulous ocean, bewitched into suddenimmobility; or a rack of tempest-driven clouds hanging in the sky, momentarily awaiting the transforming violence of a fresh onset. Sometimes continents of pale light are separated by narrow straits ofcomparative darkness; elsewhere obscure spaces are hemmed in by luminousinlets and channels. ' One curious point about the Orion Nebula is that the star which seems tobe in the midst of it resolves itself under the telescope into not onebut six, of various sizes. Nebulę are in most cases too enormously remote from the earth for us tohave any possible means of computing the distance; but we may take itthat light must journey at least a thousand years to reach us from them, and in many cases much more. Therefore, if at the time of the NormanConquest a nebula had begun to grow dim and fade away, it would, for allintents and purposes, still be there for us, and for those that comeafter us for several generations, though all that existed of it inreality would be its pale image fleeting onward through space in alldirections in ever-widening circles. That nebulę do sometimes change we have evidence: there are cases inwhich some have grown indisputably brighter during the years they havebeen under observation, and some nebulę that have been recorded bycareful observers seem to have vanished. When we consider that thesestrange bodies fill many, many times the area of our whole solar systemto the outermost bounds of Neptune's orbit, it is difficult to imaginewhat force it is that acts on them to revive or quench their light. Thatthat light is not the direct result of heat has long been known; it isprobably some form of electric excitement causing luminosity, very muchas it is caused in the comets. Indeed, many people have been tempted tothink of the nebulę as the comets of the universe, and in some pointsthere are, no doubt, strong resemblances between the two. Both shine inthe same way, both are so faint and thin that stars can be seen throughthem; but the spectroscope shows us that to carry the idea too far wouldbe wrong, as there are many differences in constitution. We have seen that there are dark stars as well as light stars; if so, may there not be dark nebulę as well as light ones? It may very well beso. We have seen that there are reasons for supposing our own system tohave been at first a cool dark nebula rotating slowly. The heavens maybe full of such bodies, but we could not discern them. Their thinnesswould prevent their hiding any stars that happened to be behind them. Noevidence of their existence could possibly be brought to us by anychannel that we know. It is true that, besides the dark rifts in the bright nebulę, which maythemselves be caused by a darker and non-luminous gas, there are alsostrange rifts in the Milky Way, which at one time were conjectured to bedue to a dark body intervening between us and the starry background. This idea is now quite discarded; whatever may cause them, it is notthat. One of the most startling of these rifts is that called theCoal-Sack, in the Southern Hemisphere, and it occurs in a part of thesky otherwise so bright that it is the more noticeable. No possibleexplanation has yet been suggested to account for it. Thus it may be seen that, though much has been discovered, much remainsto be discovered. By the patient work of generations of astronomers wehave gained a clear idea of our own position in the universe. Here arewe on a small globe, swinging round a far mightier and a self-luminousglobe, in company with seven other planets, many of which, includingourselves, are attended by satellites or moons. Between the orbits ofthese planets is a ring or zone of tiny bodies, also going round thesun. Into this system flash every now and then strange luminousbodies--some coming but once, never to return; others returning againand again. Far out in space lies this island of a system, and beyond the gulfs ofspace are other suns, with other systems: some may be akin to ours andsome quite different. Strewn about at infinite distances are starclusters, nebulę, and other mysterious objects. The whole implies design, creation, and the working of a mightyintelligence; and yet there are small, weak creatures here on thislittle globe who refuse to believe in a God, or who, while acknowledgingHim, would believe themselves to know better than He. THE END BILLING AND SONS, LTD. , PRINTERS, GUILDFORD