THE INNER PLANETS

Star-land: Being Talks With Young People About the Wonders of the Heavens by Robert S. Ball is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE INNER PLANETS

MERCURY, VENUS, AND MARS.

We can hardly think of either the sun or the moon as a world in the sense in which our earth is a world, but there are some bodies called planets which seem more like worlds, and it is about them that we are now going to talk. Besides our Earth there are seven planets of considerable size, and a whole host of insignificant little ones. These planets are like ours in a good many respects. One of them, Venus, is about the same size as this earth; but the two others, Mercury and Mars, are very much smaller. There are also some planets very much larger than any of these, namely, Jupiter, Saturn, Uranus, and Neptune. We shall in this lecture chiefly discuss three bodies, namely, Mercury, Venus, and Mars, which, with the earth, form the group of “inner” planets.

The planets are all members of the great family dependent on the sun. Venus and the earth may be considered the pair of twins, alike in size and weight.135 Mercury and Mars are the babies of the system. The big brothers are Jupiter and Saturn. All the planets revolve round the sun, and derive their light and their heat from his beams. We should like to get a little closer to some of our fellow-planets and learn their actual geography. Unfortunately, even under the most favorable circumstances, they are a very long way off. They are many millions of miles distant, and are always at least a hundred times as far as the moon. But far as the planets may be, astronomers have been familiar with their existence for ages past. I can give you a curious proof of this. You remember how we said the first and the second days of the week were called after the sun and the moon, Sun-day and Moon-day, or Monday, respectively. Let us see about the other days. Tuesday is not quite so obvious, but translate it into French and we have at once Mardi; this word means nothing but Mars’ day, and our Tuesday means exactly the same. Wednesday is also readily interpreted by the French word Mercredi, or Mercury’s day, while Venus corresponds to Friday. Jupiter’s day is Thursday, while Saturn’s day is naturally Saturday. The familiar names of the days of the week are thus associated with the seven moving celestial bodies which have been known for uncounted ages.

HOW TO MAKE A DRAWING OF OUR SYSTEM.

I want every one who reads this book to make a little drawing of the sun and the planets. The apparatus that you will need is a pair of compasses; any sort of136 compasses that will carry a bit of pencil will do. You must also get a little scale that has inches and parts of inches divided upon it; any carpenter’s rule will answer. The drawing is intended to give a notion of the true sizes and positions of the fine family of which the earth is one member. The figure I have given (Fig. 46) is not on so large a scale as that which I ask you to use, and which I shall here mention. Try and do the work neatly, and then pin up your little drawings where you will be able to see them every day until you are quite familiar with the notion of what we mean by our solar system.

Fig. 46.—The Orbits of the Four Inner Planets.

137First open the compasses one inch, and then describe a circle, and mark a dot on this as “Mercury,” in neat letters, and also write on the circle “88 days.” At the centre you are to show the “Sun.” This circle gives the track followed by Mercury in its journey round the sun in the period of 88 days. Next open your compasses to 1¾ in., which you must do accurately by the scale. The circle drawn with this radius shows the relative size of the path of Venus, and to indicate the periodic time, you should mark it, “225 days.” The next circle you have to draw is a very interesting one. The compass is to be opened 2½ in. this time, and the path that it makes is to be marked “365 days.” This shows the high road along which we ourselves journey every year, along which we are, indeed, journeying at this moment. If you wanted to obtain from your figure any notions of the true dimensions of the system, the path of the earth will be the most convenient means of doing so. The earth is 93,000,000 miles from the sun, and our drawing shows its orbit as a circle of 2½ in. radius. It follows that each inch on our little scale will correspond to about 37,000,000 miles. As, therefore, the radius of the orbit of Mercury has been taken to be one inch, it follows that the distance of Mercury from the sun is about 37,000,000 miles.

We have, however, still one more circle to draw before we complete this little sketch. The compass must now open to four inches, and a circle which represents the orbit of Mars is then to be drawn. We mark on this “687 days,” and the inner part of the solar system is then fully represented. You see, this138 diagram shows how our earth is in every sense a planet. It happens that one of the four planets revolves outside the earth’s path, while there are two inside. By marking the days on the circles which show the periods of the planets, you perceive that the further a planet is from the sun, the longer is the time that it takes to go round. Perhaps you will not be surprised at this, for the length of the journey is, of course, greater in the greater orbits; but this consideration will not entirely explain the augmentation of the time of revolution. The further a planet is from the sun, the more slowly does it actually move, and therefore, for a double reason, the larger orbit will take a longer time. From London to Brighton is a much longer journey than from London to Greenwich, and, therefore, the journey by rail to Brighton will, of course, be a longer one than by rail to Greenwich. But suppose that you compared the railway journey to Greenwich with the journey, not by rail, but by coach, to Brighton, here the comparative slowness of the coach would form another reason besides the greater length of the journey for making the Brighton trip a much more tedious one than that to Greenwich. Mars may be likened to the coach which has to go all the way to Brighton, while Mercury may be likened to the train which flies along over the very short journey to Greenwich.

Fig. 47.—Comparative Sizes of the Planets.

We can easily show from our little sketch that Mercury must be moving more quickly than Mars, for the radii of the two circles are respectively one inch and four inches, and therefore the path of Mars must be four times as long as the orbit of Mercury. If139 Mars moved as fast as Mercury, he would, of course, require only four times as many days to complete his large path as Mercury takes for his small path; but four times 88 is 352, and, consequently, Mars ought to140 get round in 352 days if he moved as fast as Mercury does. As a matter of fact, Mars requires nearly twice that number of days; indeed, no less than 687, and hence we infer that the average speed of Mars cannot be much more than half that of Mercury.

Fig. 48.—Phases of an Inferior Planet.

To appreciate duly the position of the earth with regard to its brothers and sisters in the sun’s family it will be necessary to use your compasses in drawing another little sketch, by which the sizes of the four bodies themselves shall be fairly represented. Remember that the last drawing showed nothing whatever about the sizes of the bodies; it merely exhibited the dimensions of the paths in which they moved. As Mercury is the smallest globe of the four, we shall open the compasses half an inch and describe a circle to represent it. The earth and Venus are so nearly141 the same size (though the earth is a trifle the larger) that it is not necessary to attempt to exhibit the difference between them, so we shall represent both bodies by circles, each 1¼ inches in radius. Mars, like Mercury, is one of the globes smaller than the earth, and the circle that represents it will have a radius of ¾ of an inch. You should draw these figures neatly, and by a little shading make them look like globes. It would be better still if you were to make actual models, taking care, of course, to give each of them the exact size. A comparative view of the principal planets is shown in Fig. 47.

THE PLANET MERCURY.

Quicksilver is a bright and pretty metal, and, unlike every other metal, it is a liquid under ordinary circumstances. If you spill quicksilver, it is a difficult task to gather the liquid up again. It breaks into little drops, and you cannot easily lift them with your fingers; they slip away and escape your grasp. Quicksilver will run easily through a hole so small that water would hardly pass, and it is so heavy that an iron nail or a bunch of keys will float upon it. Now, this heavy, bright, nimble metal is known by another name besides quicksilver; a chemist would call it mercury, and the astronomers use exactly the same word to denote a pretty, bright, nimble, and heavy planet which seems to try to elude our vision. Though Mercury is so hard to see, yet it was discovered so long ago, that all record is lost of who the discoverer was.

142You must take special pains if you want to see the planet Mercury, for during the greater part of the year it is not to be seen at all. Every now and then a glimpse is to be had, but you must be on the alert to look out just after sunset, or you must be up very early in the morning so as to see it just before sunrise. Mercury is always found to be in attendance on the sun, so that you must search for him near the sun; that is, low down in the west in the evenings, or low down in the east in the mornings. To ascertain the proper time of the year at which to look for him you must refer to the almanac.

We have seen how Mercury revolves in a path inside that of Venus, and it is therefore nearer to the sun. Indeed, Mercury is so close to the sun that it is generally overpowered by his brilliance and cannot be seen at all. Like every other planet, Mercury is lighted by the sun’s rays, and shows phases in the telescope just as the moon does (Fig. 48). In this figure the different apparent sizes of the planet at different parts of its path are shown. Of course the nearer Mercury is to the earth the larger does it seem.

If we can only see Mercury so rarely, and if even then it is a very long way off, does it not seem strange that we can tell how heavy it is? Even if we had a pair of scales big enough to hold a planet, what, it may be asked, would be the use of the scales when the body to be weighed was about a hundred millions of miles away? Of course the weighing of a planet must be conducted in some manner totally different143 from the kind of weighing that we ordinarily use. Astronomers have, however, various methods for weighing these big globes, even though they can never touch them. We do not, of course, want to know how many pounds, or how many millions of tons they contain; there is but little use in trying to express the weight in that way. It gives no conception of a planet’s true importance. One world must be compared with another world, and we therefore estimate the weights of the other worlds by comparing them with that of our own. We accordingly have to consider Mercury placed beside the earth, and to see which of the two bodies is the bigger and the heavier, or what is the proportion between them. It so happens that Mercury, viewed as a world, is a very small body. It is a good deal less in size than our earth, and it is not nearly so massive. To show you how we found out the mass of Mercury I shall venture on a little story. It will explain one of the strange devices that astronomers have to use when they want to weigh a distant body in space.

There was once, and there is still, a little comet which flits about the sky; we shall call it after the name of its discoverer, Encke. There are sometimes splendid comets which everybody can see—we will talk about these afterwards—but Encke is not such a one. It is very faint and delicate, but astronomers are interested in it, and they always look out for it with their telescopes; indeed, they could not see the poor little thing without them. Encke goes for long journeys through space—so far that it becomes quite144 invisible, and remains out of sight for two or three years. All this time it is tearing along at a tremendous speed. If you were to take a ride on the comet, it would whirl you along far more swiftly than if you were sitting on a cannon-ball. When the comet has reached the end of its journey, then it turns round and returns by a different road, until at last it comes near enough to show itself. Astronomers give it all the welcome they can, but it won’t remain; sometimes it will hardly stay long enough for us to observe that it has come at all, and sometimes it is so thin and worn after all its wanderings that we are hardly able to see it. The comet never takes any rest; even during its brief visit to us it is scampering along all the time, and then again it darts off, gradually to sink into the depths of space, whither even our best telescopes cannot follow it. No more is there to be seen of Encke for another three years, when again it will come back for a while. Encke is like the cuckoo, which only comes for a brief visit every spring, and even then is often not heard by many who dearly love his welcome note; but Encke is a greater stranger than the cuckoo, for the comet never repeats his visit of a few weeks more than once in three years; and he is then so shy that usually very few catch a glimpse of him.

An astronomer and a mathematician were great friends, and they used to help each other in their work. The astronomer watched Encke’s comet, noted exactly where it was, on each night it was visible, and then told the mathematician all he had seen. Provided with this information the mathematician sharpens his145 pencil, sits down at his desk, and begins to work long columns of figures, until at length he discovers how to make a time table which shall set forth the wanderings of Encke. He is able to verify the accuracy of his table in a very unmistakable way by venturing upon prophecies. The mathematician predicts to the astronomer the very day and the very hour at which the comet will reappear. He even indicates the very part of the heavens to which the telescope must be directed, in order to greet the wanderer on his return. When the time comes the astronomer finds that his friend has been a true prophet; there is the comet on the expected day, and in the expected constellation.

This happens again and again, so that the mathematician, with his pencil and his figures, marks stage by stage the progress of Encke through the years of his invisible voyage. At each moment he knows where the comet is situated, though utterly unable to see it.

The joint labors of the two friends having thus discovered law and order in the movements of the comet, you may judge of their dismay when on one occasion Encke disappointed them. He appeared, it is true, but then he was a little late, and he was also not in the spot where he was expected. There was nearly being a serious difference between the two friends. The astronomer accused the mathematician of having made mistakes in his figures, the mathematician retorted that the astronomer must have made some blunder in his observations. A quarrel was imminent, when finally it was suggested to interrogate Encke himself, and see whether he could offer any explanation. The mathematician146 employed peculiar methods that I could not explain, so I shall transform his processes into a dialogue between himself and the offending comet.

“You are late,” said he to the comet. “You have not turned up at the time I expected you, nor are you exactly in the right place; nor, indeed, for that matter, are you now moving exactly as you ought to do. In fact, you are entirely out of order, and what explanation have you to give of this irregularity?”

You see the questioner felt quite confident that there must have been some cause at work that he did not know of. Mathematicians have one great privilege; they are the only people in the world who never make any mistakes. If they knew accurately all the various influences that were at work on the comet, they could, by working out the figures, have found exactly where the comet would be placed. If the comet was not there, it is inevitable that there must have been something or other acting upon the comet, of which the mathematician was in ignorance.

The comet, like every other transgressor, immediately began to make excuses, and to shuffle off the blame on somebody else. “I was,” said Encke, “going quietly on my rounds as usual. I was following out stage by stage the track that you know so well, and I would certainly have completed my journey and have arrived here in good time and in the spot where you expected me had I been let alone, but unfortunately I was not let alone. In the course of my long travels—but at a time when you could not have seen me—I had the misfortune to come very close to a planet, of147 which I dare say you have heard—it is called Mercury. I did not want to interfere with Mercury; I was only anxious to hurry past and keep on my journey, but he was meddlesome, and began to pull me about, and I had a great deal of trouble to get free from him, but at last I did shake him off. I kept my pace as well as I could afterwards, but I could not make up the lost time, and consequently I am here a little late. I know I am not just where I ought to be, nor am I now moving quite as you expect me to do; the fact is, I have not yet quite recovered from the bad treatment I have experienced.”

The astronomer and the mathematician proceeded to test this story. They found out what Mercury was doing; they knew where he was at the time, and they ascertained that what the comet had said was true, and that it had come very close indeed to the planet. The astronomer was quite satisfied, and was proposing to turn to some other matter, when the mathematician said:—

“Tarry a moment, my friend. It is the part of a wise man to extract special benefit from mishaps and disasters. Let us see whether the tribulations of poor Encke cannot be made to afford some very valuable information. We expected to find Encke here. Well, he is not here—he is there, a little way off. Let us measure the distance between the place where Encke is, and the place where he ought to have been.”

This the astronomer did. “Well,” he said, “what will this tell you? It merely expresses the amount of delinquency on the part of Encke.”

148“No doubt,” said the mathematician, “that is so; but we must remember that the delinquency, as you call it, was caused by Mercury. The bigger and the heavier Mercury was, the greater would be his power of doing mischief, the more would he have troubled poor Encke, and the larger would be the derangement of the comet in consequence of the unfortunate incident. We have measured how much Encke has actually been led astray. Had Mercury been heavier than he is, that distance would have been larger; and if Mercury had been lighter than he is, you would not, of course, have found so large an error in the comet.”

We may illustrate what is meant in this way. A steamer sails from Liverpool to New York, and in favorable circumstances the voyage across the Atlantic should be accomplished within a week. But supposing that in the middle of the ocean a storm is encountered, by which the ship is driven from her course. She will, of course, be delayed, and her voyage will be lengthened. A trifling storm, perhaps, she will not mind, but a heavy storm might delay her six hours; a still greater storm might keep her back half a day; while cases are not infrequent in which the delay has amounted to one day, or two days, or even more.

The delay which the ship has experienced may be taken as a measure of the vehemence of the storm. I am not supposing that her machinery has broken down; of course, that sometimes happens at sea, as do calamities of a far more tragic nature. I am merely supposing the ship to be exposed to very heavy weather, from which she emerges just as sound as she was when the storm149 began. In such cases as this we may reasonably measure the intensity of the storm by the number of hours’ delay to which the passengers were subjected. “The weather we had was much worse than the weather you had,” one traveller may say to another. “Our ship was two days late, while you escaped with a loss of one day.”

When the comet at last returned to the earth after a cruise of three years through space, the number of hours by which it was late expressed the vehemence of the storm it experienced. The only storm that the comet would have met with, at least in so far as our present object is concerned, was the trouble that it had with Mercury. The mass of Mercury was, therefore, involved in the delay of the comet. In fact, the delay was a measure of the mass of the planet. I do not attempt to describe to you all the long work through which the mathematician had to plod before he could ascertain the mass of Mercury. It was a very tedious and a very hard sum, but at last his calculations arrived at the answer, and showed that Mercury must be a light globe compared to the earth. In fact, it would take twenty-five globes, each equal to Mercury, to weigh as much as the earth.

I dare say you will think that this was a very long and roundabout way of weighing. Supposing, however, we had to weigh a mountain, or rather a body which was bigger than fifty thousand mountains, and which was also many millions of miles away, all sorts of expedients would have to be resorted to. I have told you one of them. If you feel any doubts as to the accuracy150 with which such weighings can be made, then I must tell you that there are many other methods, and that these all agree in giving concordant results.

Fig. 49.—Relative Weights of Mercury and the Earth.

We hardly know anything as to what the globe of Mercury may be like. We can see little or nothing of the nature of its surface. We only perceive the planet to be a ball, brightly lighted by the sun, and we cannot satisfactorily discern permanent features thereon, as we are able to do on some of the other planets.

THE PLANET VENUS.

You will have no difficulty in recognizing Venus, but you must choose the right time to look out for her. In the first place, you need never expect to see Venus151 very late at night. You should look for the planet in the evening, as soon as it is dark, towards the west, or in the morning, a little before sunrise, towards the east. I do not, however, say that you can always see Venus, either before sunrise or after sunset. In fact, for a large part of the year, this planet is not to be seen at all. You should therefore consult the almanac, and unless you find that Venus is stated to be an evening star or a morning star, you need not trouble to search for it. I may, however, tell you that Venus can never be an evening star and a morning star at the same time. If you can see it this evening after sundown, there is no use in getting up early in the morning to look out for it again. The planet will remain for several weeks a splendid object after sunset, and then will gradually disappear from the west, and in a couple of months later will be the morning star in the east. Venus requires a year and seven months to run through her changes, so that if you find her a bright evening star to-night, you may feel sure that she was a bright evening star a year and seven months ago, and that she will be a bright evening star in a year and seven months to come. Nor must you ever expect to see her right overhead; she is always to the west or to the east.

The splendor of Venus, when at her best, will prevent you at such times from mistaking this planet for an ordinary star. She is then more than twenty times as bright as any star in the heavens. The most conclusive proof of the unrivalled brightness of Venus is found in the fact that she can be recognized in broad daylight without a telescope. Even on the brightest152 June afternoons the lovely planet is sometimes to be discerned like a morsel of white cloud on the perfect azure of the sky.

Venus is so brilliant that perhaps you will hardly credit me when I tell you that she has no more light of her own than has a stone or a handful of earth, or a button. Is it possible that this is the case, you will say, for as we see the planet so exquisitely beautiful, how can she be merely a huge stone high up in the heavens? The fact is that Venus shines by light not her own, but by light which falls upon her from the sun. She is lighted up just as the moon, or just as our own earth is lighted. Her radiance merely arises from the sunbeams which fall upon her. It seems at first surprising that mere sunbeams on the planet can give her the brilliancy that is sometimes so attractive. Let me show you an illustration which will, I trust, convince you that sunbeams will be adequate even for the glory of Venus.

Here is a button. I hang it by a piece of fine thread, and when I dip it into the beam from the electric lamp, look at the brilliancy with which the mimic planet glitters. You cannot see the shape of the button; it is too small for that; you merely see it as a brilliant gem, radiating light all around. Therefore, we need not be surprised to learn that the brilliancy of the evening star is borrowed from the sun, and that if, while we are looking at the planet in the evening, the sun were to be suddenly extinguished, the planet would also vanish from view, though the stars would shine as before.

Thus we explain the appearance of Venus. The evening star is a beautiful, luminous point, but it has153 no shape which can be discerned with the unaided eye. When, however, the telescope is turned towards Venus we have the delightful spectacle of a tiny moon, which goes through its phases just as does our own satellite. When first seen as an evening star Venus will often be like the moon at the quarter, and then it will pass to the crescent shape. Then the crescent becomes gradually thinner, and next will follow a brief period of invisibility before the appearance of Venus as the morning star. It seems at first a little strange that Venus when brightest should not be full like the moon, which in similar circumstances is, of course, a complete circle of light. The planet, however, has a very marked crescent-shaped form in these circumstances. But at this time the planet is so near us that the gain of brilliancy from the diminution of distance more than compensates for the small part of the illuminated side which is turned towards us.

You ought all to try to get some one to show you Venus through a telescope. A very large instrument is not necessary, and I feel sure you will be delighted to see the beautiful moon-shaped planet. You will then have no difficulty in understanding how the brightness of the planet has come from the sun. The changes in the crescent merely depend upon the proportion of the illuminated side which is turned towards us. Were Venus itself a sunlike body we should, of course, see no crescent, but only a bright circle of light.

In Fig. 50 you will notice an imaginary picture of a young astronomer surveying Venus with a telescope. I have not, as is obvious, attempted to show the different154 objects in their proper proportions. The sun is supposed to have set, so that his beams do not reach the astronomer. Night has begun at his observatory; but the sunbeams fall on Venus, and light her up on that side turned towards the sun. A part of this lighted side is, of course, seen by the telescope which the astronomer is using, and thus the planet seems to him like a crescent of light.

Fig. 50.—To show that Venus shines by Sunlight.THE TRANSIT OF VENUS.

We might naturally think from Fig. 46 that Venus must pass at every revolution directly between the earth and the sun; and therefore it might appear that what is called the transit of Venus across the sun ought155 to occur every time between the appearance of the planet as the evening star and the next following appearance as the morning star. No doubt on each of these occasions Venus seems to approach the sun closely; but the orbits of Venus and the Earth do not lie quite in the same plane, and hence the planet usually passes just over or just under the sun, so that it is a very rare event indeed for her to come right in front of the sun. But this does sometimes happen. It happened, for instance, in the year 1874, and again in the year 1882; but, alas! I cannot hold out to you the prospect of ever seeing another such spectacle. There will be no further occurrence of the transit of Venus until the year 2004, though there will be another eight years later, in 2012.

It seems rather odd that one transit of Venus should be followed by another after an interval of eight years, and that then a period of much more than a century should have to elapse before there will be a repetition of a similar pair. This is in consequence of a curious relation between the motion of Venus and the motion of the Earth, which I must endeavor to explain with the help of a little illustration.

Let us suppose a clock with ordinary numbers round the dial, but so arranged that the slowly moving short hand requires 365.26 days to complete one revolution round the dial, while the more rapidly moving long hand revolves in 224.70 days. The short hand will then go round once in a year, and the long hand once during the revolution of Venus. Let us suppose that both hands start together from XII, then in 224.70156 days the long hand is round to XII again, but the short hand will have only advanced to about VII, and by the time it reaches XII the long hand will have completed a large part of a second circuit. It happens that the two numbers 224.70 and 365.26 are very nearly in the ratio of 8 to 13. In fact, if the numbers had only been 224.8 and 365.3 respectively, they would be exactly in the proportion of 8 to 13. It, therefore, follows that eight revolutions of the short hand must occupy very nearly the same time as thirteen revolutions of the long hand. After eight years the short hand will of course be found again at XII; and at the same moment the long hand will also be back at XII, after completing thirteen revolutions.

We can now understand why the transits, when they do occur, generally arrive in pairs at an interval of eight years. Suppose that at a certain time Venus happens to interpose itself directly between the earth and the sun, then, when eight years have elapsed, the earth is, of course, restored for the eighth time since the first transit to the same place, and Venus has returned to almost the same spot for the thirteenth time. The two bodies are practically in the same condition as they were at first, and, therefore, Venus again intervenes, and the planet is beheld as a black spot on the sun’s surface. We must not push this argument too far; the relation between the two periods of revolution, though nearly, is not exactly 8 to 13. The consequence is that when another eight years have elapsed, the planet passes a little above the sun or a little below the sun, and thus a third occurrence of the transit is avoided for more157 than a century. The next transit will take place at the opposite side of the path.

We were fortunate enough to be able to see the transit of Venus in 1882 from Great Britain. Perhaps I should say a part of the transit, for the sun had set long before the planet had finished its journey across the disk. Venus looked like a small round black spot, stealing in on the bright surface of the sun and gradually advancing along the short chord that formed its track.

Fig. 51.—Venus in Transit across the Sun.

An immense deal of trouble was taken in 1882, as well as in 1874, to observe this rare occurrence. Expeditions were sent to various places over the earth where the circumstances were favorable. Indeed, I do not suppose that there was ever any other celestial event158 about which so much interest was created. The reason why the event attracted so much attention was not solely on account of its beauty or its singularity; it was because the transit of Venus affords us a valuable means of learning the distance of the sun. When observations of the transit of Venus made at opposite sides of the earth are brought together, we are enabled to calculate from them the distance of Venus, and knowing that, we can find the distance of the sun and the distances and the sizes of the planets. This is very valuable information; but you would have to read some rather hard books on astronomy if you wanted to understand clearly how it is that the transit of Venus tells us all these wonderful things. I may, however, say that the principle of the method is really the same as that mentioned on pp. 19–25. When you remember that not we ourselves, nor our children, and hardly our grandchildren, will ever be able to see another transit of Venus, you will, perhaps, not be surprised that we tried to make the most of such transits as have occurred in our time.

VENUS AS A WORLD.

Though Venus exhibits such pretty crescents in the telescope, yet I must say that in other respects a view of the planet is rather disappointing. Venus is adorned by such a very bright dress of sunbeams that we can see but little more than those sunbeams, and we can hardly make out anything of the actual nature of the planet itself. We can sometimes discern faint marks159 upon the globe, but it is impossible even to make a conjecture of what the Venus country is like. This is greatly to be regretted, for Venus approaches comparatively close to the earth, and is a world so like our own in size and other circumstances that we feel a legitimate curiosity to learn something more about her.

But the marks on the planet, though very faint, are still sufficiently definite to have enabled some sharp-sighted astronomers to answer a question of much interest. They have made it plain that in one most important respect Venus is very unlike our Earth. Our globe, of course, rotates on its axis once each day, but Venus requires no less than 225 days to complete each rotation. In fact, this planet rotates in such a fashion that she always keeps the same face to the sun. The inhabitants of Venus will therefore find that it is perennial day on one side of this globe and everlasting night on the other.

Venus is one of the few globes which might conceivably be the abode of beings not very widely different from ourselves. In one condition especially—namely, that of weight—she resembles the earth so closely that those bodies which we actually possess would probably be adapted, so far as strength is concerned, for a residence on the sister planet. Our present muscles would not be unnecessarily strong, as they would be on the moon, nor should we find them too weak, as they would certainly prove to be were we placed on one of the very heavy bodies of our system. Nor need the temperature of Venus be regarded as presenting any insuperable difficulties. She is, of course, nearer to the sun than160 we are, but then climate depends on other conditions besides nearness to the sun, so that the question as to whether Venus would be too hot for our abode could not be readily decided. The composition of the atmosphere surrounding the planet would be the most material point in deciding whether terrestrial beings could live there. I think it to be in the highest degree unlikely that the atmosphere of Venus should chance to suit us in the requisite particulars, and therefore I think there is not much likelihood that Venus is inhabited by any men, women, or children resembling those on this earth.

THE PLANET MARS AND HIS MOVEMENTS.

The path of the earth lies between the orbits of the planets Venus and Mars. It is natural for us to endeavor to learn what we can about our neighbors. We ought to know something, at all events, as to the people who live next door to us on each side. I have, however, already said that we cannot observe very much upon Venus. The case is very different with respect to Mars. He is a planet which we are fortunately enabled to study minutely, and he is full of interest when we examine him through a good telescope.

The right season for observing Mars must, of course, be awaited, as he is not always visible. Such seasons recur about every two years, and then for months together Mars will be a brilliant object in the skies every night. Nor has Mars necessarily to be sought in the early morn or immediately after sunset, in the161 manner we have already described for Venus and Mercury. At the time Mars is at his best he comes into the highest position at midnight, and he can generally be seen for hours before, and be followed for hours subsequently. You may, however, find some difficulty in recognizing him. You probably would not at first be able to distinguish Mars from a fixed star. No doubt this planet is a ruddy object, but some stars are also ruddy, and this is at the best a very insecure characteristic for identification. I cannot give you any more general directions, except that you should get your papa to point out Mars to you the next time it is visible. It is just conceivable that papa himself might not know how to find Mars. If so, the sooner he gets a set of star maps and begins to teach himself and to teach you, the better it will be for you both.

Mars, though apparently so like a star, differs in some essential points from any star in the sky. The stars proper are all fixed in the constellations, and they never change their relative positions. The groups which form the Great Bear or the Belt of Orion do not alter, they are just the same now as they were centuries ago. But the case is very different with a planet such as Mars. The very word planet means a wanderer, and it is justly applied, because Mars, instead of staying permanently in any one constellation, goes constantly roaming from one group to the other. He is a very restless body; sometimes he pays his respects to the heavenly Twins, and is found near Castor and Pollux in Gemini, then he goes off and has a brief sojourn with the Bull, but it looks as if that fierce animal got tired162 of his company and hunted him off to the Lion. His quarters then become still more critical. Sometimes it looks as if he desired to seek for peace beneath the waters, and so he visits Aquarius, while at other times he is found in dangerous proximity to the claws of the Crab.

Mars cannot even make up his mind to run steadily round the heavens in one direction; sometimes he will bolt off rapidly, then pause for a while, and turn back again; then the original impulse will return, and he will resume his journey in the direction he at first intended. It is no wonder that I am not able to give you very explicit directions as to how you may secure a sight of a truant whose wanderings are apparently so uncertain. Yet there is a definite order underlying all his movements. Astronomers, who make it their business to study the movements of Mars, can follow him on his way; they know exactly where he is now, and where he will be every night for years and years to come. The people who make the almanacs come to the astronomers and get hints from them as to what Mars intends to do, so that the almanacs announce the positions in which the planet will be found with as much regularity as if he was in the habit of behaving with the orderly propriety of the sun or the moon.

We must not lay all the blame on Mars for the eccentricities of his movements. Our earth is to a very large extent responsible. What we think to be Mars’ vagaries are often to be explained by the fact that we ourselves on the earth are rapidly shifting about and altering our point of view.

Fig. 52.—How the Tree seems to move about.

I was driving down a pretty country road with a little girl three years old beside me, when I was addressed with the little remark, “Look at the tree going about in the field.” Now, you or I, with our longer experience of the world around us, know that it is not the custom of trees to take themselves up and walk about the fields. But this was what this little girl saw, or rather what she thought she saw; and very often what we do see is something very different from what we think we see. We think we see Mars performing all these extraordinary movements, as the little girl thought she saw the tree moving about. But just as that little girl, when she grew to be a big girl, found that what she thought was a tree walking across the field must really have some quite different explanation, so we, too, find that what Mars seems to do is one thing, and what Mars actually does is quite another thing.

Let us see what the little girl noticed. She was164 looking at the tree, and first she saw it on one side of the house, and then she saw it on the opposite side (Fig. 52). If it had been a cow instead of a tree, of course the natural supposition would have been that the cow had walked. Our little friend may, perhaps, have thought it unusual for a tree to walk, but still she saw the undoubted fact that the tree had shifted to the other side of the house, and therefore, perhaps, remembering what the cow could do, she said the tree had moved.

Fig. 53.—A Specimen of the Track of Mars.

The little girl did not stop to reflect that she herself had entirely changed her position, and hence arose the surprising phenomenon of a tree that could move about. You will understand this, at once, from the two positions of the car here shown. In the first position, as the girl looks at the tree, the dotted line shows the direction of her glance, and the other dotted line shows how the apparent places of the tree and the house have altered. It is her change of place that has accomplished the transformation. Observe also that the tree appeared to her to move in the direction opposite to that in which she is going.

Mars generally appears to move round among the stars from west to east. In fact, if we were viewing165 him from the sun he would always seem to move in this manner. But at certain seasons our earth is moving very fast past Mars, and this will make him appear to move in the opposite direction. This apparent motion is sometimes so much in excess of his real motion, that it may give us an entirely incorrect idea of what the planet is actually doing.

Thus, notwithstanding that Mars is moving one way, he may appear to us who dwell on the earth to be going in the opposite way. This illusion only happens for a short time, just when we are passing Mars, as we do every two years. The effect on the planet is to make the path he pursues at this time something like that shown in Fig. 53. The planet is nearest to us at the time he is moving in this loop. He is then to be seen at his best in the telescope, so that it is especially interesting to watch Mars through this critical part of his career.

I want to show you how to make a little calculation which will explain the law by which the seasons when we can see Mars best will follow each other. The period he requires for a voyage round the sun is not quite two years, for that would be 730 days, and Mars only takes 687 days for his journey. It is, however, true that 1-15/17 years is very nearly the period of Mars. Hence, every 32 years Mars will complete 17 rounds. From this we shall be able to see how long it will take after the earth once passes Mars before they pass again. I shall suppose there is a circular course, around which two boys start together to run a race. One of these boys is such a good runner that he will get quite round in 17 minutes;166 but the other boy can hardly run more than half as quickly, for he will require 32 minutes to complete one circle. Here then is the question. Suppose the two boys to start together: how long will it be before the faster runner gains one complete circuit on the other? By the time the good runner (A) has completed one circuit, the bad runner (B) has only got a little more than halfway. When A has completed his second circuit, he has, of course, run for twice 17 minutes—that is, for 34 minutes. This is two minutes longer than the time B requires to get round once; therefore B is only ahead by a distance which A could cover in about one minute; but B will have advanced during this minute a distance for which A will require another half-minute, during which B covers a distance for which A will need a further quarter, and so on. But all these intervals—one minute, half a minute, a quarter of a minute, one-eighth, one-sixteenth, and so on—added together amount to two minutes, and hence it follows that B will not be overtaken until about two minutes after A has completed his second round—that is, in 36 minutes altogether.

We can pass from this illustration to the case of the planet Mars and the earth. The orbit of the earth is traversed in a year, and therefore, after the earth has once passed Mars, which is then, as astronomers would say, in opposition, about two years and the eighth of a year—that is, two years and six or seven weeks—will elapse before Mars is again favorably placed. You will thus see that we need not expect to observe Mars under the best conditions every year. Besides, the distance167 of the planet from the earth at opposition varies so greatly that some oppositions are more favorable than others.

The time has come when I must tell you something about the shapes of the paths in which the earth and the other planets perform their great journeys round the sun. Perhaps you will think that I am going to contradict some of the things that I have told you before. I have often represented the orbits of the planets as circles, and now I am going to tell you that this is not correct. The fact is that the paths are nearly circles; but, still, there is some departure from the exact circular shape. Mars, in particular, moves in a path which is more different from a circle than the path of the earth, and consequently it is appropriate to introduce this subject when we are engaged about Mars.

Fig. 54.—How to draw an Ellipse.

We must first take another lesson in drawing, and168 the appliances I want you to use for the purpose are very simple. You must have a smooth board and some tacks or drawing-pins, besides paper, pencil, and twine.

Fig. 55.—Specimens of Ellipses.

We first lay a sheet of paper on the board, and then put in two tacks through the paper and into the board. It does not much matter where we put them in. Next we take a piece of twine and tie the two ends together so as to form a loop, which we pass round the two tacks169 (Fig. 54). In the loop I place the pencil, and then you see I move it round, taking care to keep the twine stretched. Thus I produce a pretty curve, which we call the ellipse. I must ask all of you to practise this experiment. Try with different lengths of string, and try using different distances between the tacks. Here are some sketches of two shapes of ellipse and a parabola (Fig. 55). Elliptic curves can be made almost circles by putting the two tacks close together, or they can be made very long in comparison with their width. They are all pretty and graceful figures, and are often useful for ornamental work. The ellipse is a pretty shape for beds of flowers in a grass-plot.

The importance of the ellipse to astronomers is greater than that of any other geometrical figure. In fact, all the planets, as they perform their long and unceasing journeys round the sun, move in ellipses; and though it is true that these ellipses are very nearly circles, yet the difference is quite appreciable.

It is also important to observe that the sun is not in the centre of the ellipse which the planet describes. The sun is nearer to one end than to the other. And the actual position of the sun must be particularly noted. Suppose that some mighty giant were preparing to draw an exact path for the earth, or for Mars, of course he would want to have millions of miles of string for producing a big enough curve, and one of the nails that he used would have to be driven right into the sun. The following is the astronomer’s more accurate method of stating the facts. He calls each of the points represented by the tacks around which the string170 is looped a focus of the ellipse; the two points together are said to be the foci; and as the planet is describing its orbit, the position of the sun will lie exactly at one of the foci.

The ellipse is a curve that nature is very fond of reproducing. From an electric light, a brilliant beam will diverge. If you hold a globe in the beam, and let the shadow fall on a sheet of paper, it forms an ellipse. If you hold the sheet squarely, the shadow is a circle; but as you incline it, you obtain a beautiful oval, and by gradually altering the position, you can get a greatly elongated curve. Indeed, you can thus produce an ellipse of almost any form. The electric light is not indispensable for this purpose; any ordinary bright lamp with a small flame will answer, and by taking different sized balls and putting them in various positions, you can make many ellipses, great and small.

THE DISCOVERIES MADE BY TYCHO AND KEPLER.

It was by the observations of a celebrated old astronomer, named Tycho Brahe, that the true shape of a planet’s path came to be afterwards determined. Tycho lived in days before telescopes were invented. He had few of the excellent contrivances for measuring which we have in our observatories. We shall take a look at this fine old astronomer, as he sits amid his curious astronomical machines.

Fig. 56.—Tycho Brahe in his Observatory.

He lived on an island near Copenhagen, and he has given us a picture of himself (Fig. 56), as he is seated with his quaint apparatus, and his assistants around172 him, busily engaged in observing the heavens. You see the walls of his observatory are decorated with pictures; and one of the great Danish hounds which the King of Denmark had presented to him lies asleep at his feet. I do not think we should now encourage big dogs in the observatory at night. Nor do modern astronomers put on their velvet robes of state, as Tycho was said to have done when he entered into the presence of the stars, as, by so doing, he showed his respect for the heavens. Astronomers, nowadays, rather prefer to wear some comfortable coat which shall keep out the cold, no matter what may be its appearance from the picturesque point of view. In this wonderful contrivance, you see Tycho Brahe did not use any actual telescope. He observed through a small opening in the wall, and lest there should be any mistake as to what is going on, you see he is pointing towards it, and giving his three assistants their instructions. The most important work is being done by the man on the right. He is engaged in making the actual observation. But he has no aid from magnifying lenses. All he can do is to slide a pointer up or down till it is just in line with the planet or star as he sees it through the hole opposite.

On the circle a number of marks have been engraved, and there are numbers placed opposite to the marks; it is by these that the position of the object is to be ascertained. If the object is high, then the pointer will be low; and if the object is low, then the pointer will be high. The observer calls out the position when he has found it, and there, you see, is a man ready with173 writing materials to take down the observation. Notice also the other astronomer who is looking at the clock. He gives the time, which must also be recorded accurately. In fact, the entire process of finding the place of a heavenly body consists in two observations—one from the circle and the other from the clock; so that though Tycho had no telescope to aid his vision, yet the principle on which his work was done was the same as that which we use in our observatories at this moment.

You may think that such a concern would hardly be capable of producing much reliable work. However, Tycho compensated in a great degree for the imperfection of his instrument by the skill with which he used it. He had a noble determination to do his very best. Perseverance will accomplish wonders even with very imperfect means. A great astronomer has said that a skilful observer ought to be able to make valuable measurements with a common cart-wheel!

It was with instruments on the principle of that which I have here shown that Tycho made his celebrated observations of Mars. Week after week, month after month, year after year, did the patient old astronomer track the planet through his capricious wanderings.

Before we try to explain anything, it is of course necessary to ascertain, with all available accuracy, what the thing actually is. Therefore, when we seek to explain the irregular movements of a planet, the first thing to be done is to make a careful examination of the nature of those irregularities. And this was174 what Tycho strove to do with the best means at his disposal.

The full benefit of Tycho’s work was realized by Kepler when he commenced to search out the kind of figure in which Mars was moving. First he tried various circles, and then he sought, by placing the centre in different positions, to see whether it would not be possible to account thus for the irregularities of the wayward planet. It would not do; the movement was not circular. This was thought very strange in those days, for the circle was regarded as the only perfect curve, and it was considered quite impossible for a planet to have any motion except it were the most perfect. There was, however, no help for it; so Kepler sagaciously tried the ellipse, which he considered to be the most perfect curve next to the circle. He continued his long calculations, until at last he succeeded in finding one particular ellipse, placed in one particular position, which would just explain the strange wanderings of our erratic neighbor. It was not alone that the motion of the planet traced out an ellipse; it was further discovered that the sun lies at one of the foci of the curve. If the sun were anywhere else, the motion of the planet would have been different from that which Tycho had found it to be.

You must know that this discovery is one of the very greatest that have ever been made in the whole extent of human knowledge. After it had been proved that the orbit of Mars was elliptic, it became plain that the same path must be traced by every planet. There are very big planets, and there are small ones; there are175 planets which move in very large orbits, and there are planets whose paths are comparatively small. In all cases the high road which the planet follows is invariably an ellipse, and the sun is invariably to be found situated at the focus. It is surely interesting to find that these beautiful ellipses which we can draw so simply with a piece of twine and a pencil should be also the very same figures which our great earth and all the other bodies which revolve around the sun are ever compelled to follow.

Kepler also made another great discovery in connection with the same subject. If the planet moved in a circle with the sun in the centre, then there would be very good reason to expect that it would always move at the same speed, for there would be no reason why it should go faster at one place than at another. In fact, the planet would then be revolving always at the same distance from the sun, and every part of its path would be exactly like every other part. But when we consider that the motion is performed in an ellipse, so that the planet is curving round more rapidly at the extremities of its path than in the other parts where the curvature is less perceptible, we have no reason to expect that the speed shall remain the same all round.

We know that the engine-driver of a railway train always has to slacken speed when he is going round a sharp curve. If he did not do so, his train would be very likely to run off the line, and a dreadful accident would follow. The engine-driver is well aware that the conditions of pace are dependent on the curvature of his line. The planet finds that it, too, must pay176 attention to the curves; but the extraordinary point is that the planet acts exactly in the opposite way to the engine-driver. The planet puts on its highest pace at one of the most critical curves in the whole journey. There are two specially sharp curves in the planet’s path. These are, of course, the two extremities of the ellipse which it follows. The cautious engine-driver would, of course, creep round these with equal care, and no doubt the planet goes slowly enough about that end of the ellipse which is farthest from the sun. There its pace is slower than anywhere else; but from that moment onwards the planet steadily applies itself to getting up more and more speed. As it traverses the comparatively straight portion of the celestial road, the pace is ever accelerating until the sharp curve near the sun is being approached; then the velocity gets more and more alarming, until at last, in utter defiance of all rules of engine-driving, the planet rushes round one of the worst parts of the orbit at the highest possible speed. And yet no accident happens, though the planet has no nicely laid lines to keep it on the track.

If lines are necessary to save a railway train from destruction, how can we possibly escape when we have no similar assistance to keep us from flying away from the sun and off into infinite space? Kepler has taught us to measure the changes in the speed of the body with precision. He has shown that the planet must, at every point of its long journey, possess exactly the right speed; otherwise everything would go wrong. I dare say you have seen, at different points along a line of railway, boards put up here and there, with notices177 like, “Ten miles an hour.” These words are, of course, an intimation to the engine-driver that he is not to vary from the speed thus stated. Kepler has given us a law which is equivalent to a large number of caution boards, fixed all round the planet’s path, indicating the safe speed for the journey at every stage. It is fortunate for us that the planet is careful to observe these regulations. If the earth were to leave her track, the consequences would be far worse than those of the most frightful railway accident that ever happened. Whichever side we took would be almost equally disastrous. If we went inwards we should plunge into the sun, and if we went outwards we should be frozen by cold.

We owe our safety to the care with which the speed of the earth is prescribed. When near the sun, the earth is pulled inwards with exceptionally strong attraction. We are often told that when a strong temptation seizes us, the wisest thing that we can do is to run away as hard as possible. This is just what the laws of dynamics cause the earth to do at this critical time. She puts on her very best pace, and only slackens when she has got well away from the danger.

The peril that we are exposed to when the earth is at the other end of the orbit is of an opposite character. We are then a long way from the sun, and the pull which it can exercise upon the earth is correspondingly lessened. Care is then required lest we should escape altogether from the sun’s warmth and his guidance. We must therefore give time to the sun to exercise his power, so as to enable the earth to be recalled; accordingly178 we move as slowly as possible until the sun conquers the earth’s disposition to fly off, and we begin to return.

You may remember that when we were speaking about the moon, I showed you how a body might revolve around the earth in a circle under the influence of an attraction towards the earth’s centre. So long as the path is really a circle, then the power with which the earth is drawing the body remains the same. In a precisely similar way, a body could revolve around the sun in a circle, in which case also the attraction of the sun will remain the same all round. But now we have a very much more difficult case to consider. If the body does not always remain at the same distance, the power of the sun will not be the same at the different places. Whenever the object is near the sun, the attraction will be greater than when it is farther off. For example, when the distance between the two bodies is doubled, then the pull is reduced to the fourth part of what it was before.

THE DISCOVERIES MADE BY NEWTON.

I have now some great discoveries to talk to you about, which were made by Sir Isaac Newton. He was not an astronomer who looked much through a telescope, though he made many remarkable experiments. He used to sit in his study and think, and then he used to draw figures with his pencil, and make long calculations. At last he was able to give answers to the questions: What is the reason why the planet moves179 in an ellipse? Why should it move in this curve rather than in any other? Why should this ellipse be so placed that the sun lies at one of the foci?

If the planet had run uniformly round its course, Newton would have found his task an impossible one. But I have already explained that the motion is not uniform. I described how the planet hurried along with extra speed at certain parts of its path; how it lingered at other parts; how, in fact, it never preserved the same rate for even a single minute during the whole journey. Kepler had shown how to make a time-table for the whole journey. In fact, just as a captain on a long voyage keeps a record of each day’s run, and shows how to-day he makes 170 miles, and to-morrow perhaps 200, and the next day 210, while the day after he may fall back to 120, so Kepler gave rules by which the log of a planet in its voyage round the sun might be so faithfully kept that every day’s run would be accurately recorded.

When Newton commenced his work, one of the first questions he had to consider was the following: Suppose that a great globe like a planet, or a small globe like a marble, or an irregular body like an ordinary stone, were to be thrown into space, and were then to be left to follow its course without any force whatever acting upon it, where would it go to?

You may say, at once, that a body under such circumstances will presently fall down to the ground; and so, of course, it will, if it be near the earth. I am not, however, talking of anything near the earth; I want you to imagine a body far off in the depths of180 space, among the stars. Such a body need not necessarily fall down here, for you see the moon does not fall, and the sun does not.

If you were at a great distance from our globe and from all other large globes—so far, indeed, that their attractions were imperceptible—you could try the experiment that I wish now to describe. Throw a stone as hard as ever you can, and what will happen? Of course, when you do it down here, it moves in a pretty curve through the air, and tumbles to the ground; but away in open space, what will the stone do? There will be no such motion as up or down, as we ordinarily understand it; for though the earth, no doubt, will lie in one particular direction at a great distance, yet there will be other bodies just as large in other directions; and there is no reason why the stone should move towards one of these rather than to another; in fact, if they are all far enough, as the stars are from us, their attractions will be quite inappreciable. There is, therefore, not the slightest reason why the stone should swerve to one side more than to another. There is no more reason why it should turn to the right than why it should turn to the left. Nor could you throw the stone so as to make it follow a curved path. You can, of course, make it describe a curve while it remains in your hand, but the moment the stone has left your hand, it proceeds on its journey by a law over which you have no control. As the direction cannot be changed towards one side more than towards the other, the stone must simply follow a straight line from the very moment when it is released from your hand.

181The speed with which the stone is started will also not change. You might at first think that it would gradually abate, and ultimately cease. No doubt a stone thrown along the road will behave in this way, but that is because the stone rubs against the ground. If you throw a stone across a sheet of ice, then it will run a very long distance before it stops, and all the time it will be moving in a straight line. In this case there is but little loss by rubbing against the ice, because it is so smooth. Thus we see that if the path be exceedingly smooth, the body will run a long way before it stops. Think of the distance a railway train will run if, while travelling at full speed along a level line, the steam is turned off.

These illustrations all show that if you let a body alone, after having once started it, and do not try to pull it this way or that way, and do not make it rub against things, that body will move on continually in a straight line, and will keep up a uniform speed. We can apply this reasoning to a stone out in space. It would certainly move in a straight line, and would go on and on forever, without losing any of its pace.

I need hardly tell you that no one has ever been able to try this experiment. In the first place, we reside upon the surface of the earth, and we have no means of ascending into those elevated regions where the stone is supposed to be projected. There is also another difficulty which we cannot entirely avoid, and that arises from the resistance of the air. All movements down here are impeded because the body has to force its way through the air; and in doing so it invariably loses182 some of its speed. Out in open space there is, of course, no air, and no loss of speed can therefore arise from this cause.

Fig. 57.—The Humming-top.

There are, however, several actual experiments by which we can assure ourselves of the general truth. Set a humming-top spinning (Fig. 57); it gradually comes to rest, partly because of the rubbing of its point on the table, and partly because it has to force its way through the air. In fact, the hum of the top that you hear is only produced at the expense of its motion. Supposing I use a much heavier top; if I set it spinning it will keep up for many minutes, because its weight gives it a better store of power wherewith to overcome the resistance of the air. I remember hearing a story about Professor Clerk-Maxwell. He had, when at Cambridge, invented one of these large and heavy tops, which would spin for a long time. One evening the top was left spinning on a plate in his room when his friends took their departure, and no doubt it came to rest in due time. Early the next morning, Professor Maxwell, hearing the same friends coming up to his rooms again, jumped out of bed, set the top spinning, and then got back to bed, and pretended to be asleep. He thus astounded his friends, who, of course, imagined that the top must have been spinning all the night long!

183If we spin a top under the receiver of an air pump (Fig. 58), it will keep up its motion for a very much longer time after the air has been exhausted than it would in ordinary circumstances. Such experiments prove that the motion of a body will not of itself naturally die out, and that if we could only keep away the interfering forces altogether, the motion would continue indefinitely with unabated speed. What I have been endeavoring to illustrate is called the first law of motion. It is written thus:—

“Every body continues in its state of rest or of uniform motion in a straight line, except in so far as it may be compelled by impressed forces to change that state.”

Fig. 58.—To illustrate the First Law of Motion.

I would recommend you to learn this by heart. I can assure you it is quite as well worth knowing as184 those rules in the Latin Grammar with which many of you, I have no doubt, are acquainted. The best proof of the first law of motion is derived, not from any experiments, but from astronomy. We make many calculations about the movements of the sun, the moon, the stars, and then we venture on predictions, and we find those predictions verified. Thus we had a transit of Venus across the sun in 1882, and every astronomer knew that this was going to occur, and many went to the ends of the earth so that they might see it favorably. Their anticipations were realized; they always are. Astronomers make no mistakes in these matters. They know that there will be another transit of Venus in the year 2004, but not sooner. The calculations by which these accurate prophecies are made involve this first law of motion; and as we find that such prophecies are always fulfilled, we know that the first law of motion must be true also.

Newton knew that if a planet were merely left alone in space, it would continue to move on forever in a straight line. But Kepler had shown that the planet did not move in a straight line, but that it described an ellipse. One conclusion was obvious. There must be some force acting upon the planet which pulls it away from the straight line it would otherwise pursue. We may, for the sake of illustration, imagine this force to be applied by a rope attached to the planet so that at every moment it is dragged by some unseen hand. To find the direction this rope must have, we take the law of Kepler, which explains the rules according to which the planet varies its speed. I cannot enter into the185 question fully, as it would be too difficult for us to discuss now. I should have to talk a great deal more about mathematics than would be convenient just at present; but I think you can all understand the result to which Newton was led. He showed that the rope must always be directed towards the sun. In other words, suppose that there was no sun, but that in the place which it occupied there was a strong enough giant constantly pulling away at the planet, then we should find that the speed of the planet would alter just in the way it actually does. Thus we learn that some force must reside in the sun by which the planet is drawn, and this force is exerted, although there is no visible bond between the sun and the planet.

There is another fact to be learned about the sun’s attraction, and this time we obtain it by knowing the shape of the curve followed by the planet. The laws by which the planet’s speed is regulated prove that the force emanates from the sun. We shall now learn much more when we take into account that the path of the planet is an ellipse, of which the sun lies at the focus. Nothing has been said as yet regarding the magnitude of the pull which is being exerted by the sun. Is that pull to be always the same, or is it to be greater at some times than at other times? Newton showed that no ellipse other than a circle could be described, if the pull from the sun were always the same. Its magnitude must be continually changed, and the nearer the planet lies to the sun, the more vehement is the pull it receives. Newton laid down the exact law by which the force on the planet at any186 one place in its path could be compared with the force at any other position. Let us suppose that the planet is in a certain position, and that it then passes into a second position, which is twice as far from the sun. The pull upon the planet at the shorter distance is not only greater than the pull at the longer distance, but it is actually four times as much. Stating this result a little more generally, we assert, in the language of astronomers, that the attraction varies inversely as the square of the distance. If this law were departed from, then I do not say that it would be impossible for the planet to revolve around the sun in some fashion, but the motion would not be performed in an ellipse described around the sun in the focus.

You see how very instructive are the laws which Kepler discovered. From the first of them we were able to infer that the sun attracts the planets; from the second, we have learned how the magnitude of the attracting force varies.

The true importance of these great discoveries will be manifest when we compare them with what we have already learned with regard to the movements of the moon. As the moon revolves around the earth it is held by the earth’s attraction, and the moon follows a path which, though nearly a circle, is really an ellipse. This orbit is described around the earth just as the earth describes its path around the sun. That law by which a stone falls to the ground in consequence of the earth’s attraction is merely an illustration of a great general principle. Every body in the whole universe attracts every other body.

187Think of two weights lying on the table. They no doubt attract each other, but the force is an extremely small one—so small, indeed, that you could not measure it by any ordinary appliance. One or both of the attracting masses must be enormously big if their mutual gravitation is to be readily appreciable. The attraction of the earth on a stone is a considerable force, because the earth is so large, even though the stone may be small. Imagine a pair of colossal solid iron cannon-balls, each 53 yards in diameter, and weighing about 417,000 tons. Suppose these two globes were placed a mile apart, the pull of one of them on the other by gravitation would be just a pound weight. Notwithstanding the size of these masses, the hand of a child could prevent any motion of one ball by the attraction of the other. If, however, they were quite free to move, and there was absolutely no friction, the balls would begin to draw together; at first they would creep so slowly that the motion would hardly be noticed. The pace would no doubt continue to improve slowly, but still not less than three or four days must elapse before they will have come together.

By the kindness of Professor Dewar, I am enabled to exhibit a contrivance with which we can illustrate the motion of a planet around the sun. Here is a long wire suspended from the roof of this theatre, and attached to its lower end is an iron ball, made hollow for the sake of lightness. When I draw the ball aside, it swings to and fro with the regularity of a great pendulum. But when I place a powerful magnet in its neighborhood (Fig. 59), you see that as soon as the188 ball gets near the magnet it is violently drawn to one side, and follows a curved path. This magnet may be taken to represent the sun, while the ball is like our earth, or any other planet, which would move in a straight line were it not for the attraction of the sun which draws the body aside.

Fig. 59.—The Effect of Attraction.THE GEOGRAPHY OF MARS.

We will now say something with respect to the geography of our fellow-planet, a subject which seems all the more interesting because Mars is so like the earth in many respects. We require a fairly good telescope for the purpose of seeing him well, but when such an instrument is directed to the planet, a beautiful189 picture of another world is unfolded (Fig. 60). There are many things visible on his surface, but we must always remember that even with our most powerful telescopes the planet still appears a long way off.

Fig. 60.—Views of Mars.

Fig. 61.—Mars.

(By Douglass, Lowell Observatory.)

In the most favorable circumstances, Mars is at least one hundred times as far from us as the moon. But we know that an object on the moon must be as large191 as St. Paul’s Cathedral if it is to be visible in our telescopes. An object on Mars must be, therefore, at least one hundred times as broad and one hundred times as long as St. Paul’s Cathedral if it is to be discernible by astronomers on our earth. We can, therefore, only expect to see the general features of our fellow-planet. Were we looking at our earth from a similar distance, and with equally good telescopes, the continents and oceans, and the larger seas and islands, would all be large enough to be conspicuous. It is, however, doubtful whether they could ever be properly revealed through the serious impediment to vision which our atmosphere would offer.

It fortunately happens that the surface of Mars is only obscured by clouds to a very trifling extent, and we are thus able to see a panorama of our neighboring globe laid before us. Mars is not nearly so large as our earth, the diameters of the two bodies being nearly as two to one. It follows that the number of acres on the planet is only a quarter of the number of acres on the earth. Careful telescopic scrutiny shows that the chief features which we see on Mars are of a permanent character. In this respect Mars is much more like the moon than the sun. The latter presents to us merely glowing vapors, with hardly more permanence than is possessed by the clouds in our own sky. On the other hand, the entire absence of clouds from the moon enables us to see the permanent features on its surface. Most of the visible features on Mars are also invariable; though occasionally it would seem that the climate produces some changes in its appearance.

Fig. 62.—The South Pole of Mars, September, 1877 (Green).

We first notice that there are differently colored parts on Mars. The darkish or bluish regions are usually spoken of as seas or oceans; though we should be going beyond our strict knowledge were we to assert that water is actually found there. Look at the horn-shaped object in the centre of the lower picture in Fig. 60. We call it the Kaiser Sea, and it is so strongly marked that even in a small telescope it can be often seen. You must not, however, always expect to notice this feature when you look at the planet through a telescope, for it turns round and round. We can make a globe representing Mars. On this are to be depicted this great sea and the other characteristic objects. But as we turn the globe around, the opposite side of the planet is brought into view, and other features are revealed like those represented in the upper figure. Mars requires 24 hours 37 minutes 22.7 seconds to complete a single rotation. It is somewhat remarkable that this only differs from the earth’s period of rotation by a little more than half an hour.

193Mars contains what we call continents as well as oceans, and we also find there lakes and seas and straits. These objects are indicated in the drawings that are here represented. But the most striking features which the planet displays are the marvellous white regions, which are seen both at its North Pole and at its South Pole (Fig. 62). If we were able to soar aloft above our earth and take a bird’s-eye view of our own polar regions, we should see a white cap at the middle of the arctic circle. This appearance would be produced by the eternal ice and snow. It would increase during the long, dark winter, and be somewhat reduced by melting during the continuously bright summer. Though we cannot thus see our earth, yet we can sometimes observe one Pole of Mars and sometimes the other, and we find each of these Poles crowned with a dense white cap, which increases during the severity of its winter, and which declines again with the warmth of the ensuing summer.

Sketches of Mars have been made by many astronomers; among them we may mention Mr. Green, who made a beautiful series of pictures at Madeira in 1877. These may be supplemented by the drawings of Mr. Knobel in 1884, when the opposite Pole of the planet was turned to view. The drawings show the polar snows, and there seem to be some elevated districts in his arctic regions which retain a little patch of snow after the main body of the ice cap has shrunk within its summer limits. An interesting case of this kind is shown in Fig. 62, which has been copied from one of Mr. Green’s drawings.

It has lately been surmised that the continents on194 Mars are occasionally inundated by floods of water. There are also indications of clouds hanging over the Martian lands, but the inhabitants of that planet, in this respect, escape much better than we do. A certain amount of atmosphere always surrounds Mars, though it is much less copious than that we have here. As to the composition of this atmosphere we know nothing. For anything we can tell, it might be a gas so poisonous that a single inspiration would be fatal to us; or if it contained oxygen in much larger proportion than our air does, it might be fatal from the mere excitement to our circulation which an over-supply of stimulant would produce. I do not think it the least likely that our existence could be supported on Mars, even if we could get there. We also require certain conditions of climate, which would probably be totally different from those we should find on Mars.

Many remarkable observations of Mars have been lately made by Mr. Percival Lowell. It seems very doubtful how far our former division of continents and oceans on Mars can be maintained. Mr. Lowell has paid special attention to a wonderful system of lines on the planet’s surface to which the name of “canals” has been given, which often show such a degree of regularity as would almost suggest the idea that they had been laid down by intelligent guidance.

THE SATELLITES OF MARS.

When Mars appeared in his full splendor in 1877, he was for the first time honored with the notice of195 instruments capable of doing him justice. I do not, however, mean that in former apparitions he was not also carefully observed, but a great improvement had recently taken place in telescopes, and it was thus under specially favorable auspices that his return was welcomed in 1877. This year will be always celebrated in astronomical history for a beautiful discovery made by Professor Asaph Hall, the illustrious astronomer at Washington.

Before I can explain what this discovery was, I must have a little talk about moons, or satellites as they are often called. You know that we have one moon, which is constantly revolving round the earth, and accompanies the earth in its long voyage round the sun. But the earth is only a planet, and there are many other planets which are worlds like ours. It is natural to compare these worlds, and as we have one moon, why should not the other planets also have moons? If there are children in one house in a square, why should there not be children in the other houses? We find that some of the other planets have satellites, but they do not seem to be distributed very regularly. In fact, they are almost as capriciously allotted as the children would be in eight houses that you might take at random.

Fig. 63.—Mars and his Two Satellites.

In Number One there lives an old bachelor, and in Number Two a single lady. These are Mercury and Venus, and of course there are no children in either of these houses. Number Three is inhabited by old mother Earth, and she has got a fine big son, called the Moon. Number Four is a nice little house inhabited by Mars.196 There are to be found a pair of little twins, and nimble creatures they are too. Number Five is a great mansion. A very big man lives here, called Jupiter, with four robust sons and daughters that everybody knows. I fancy they must go to many dancing parties, for every night they may be seen whirling round and round. For three hundred years these four moons have been known to astronomers, but in 1892 there was an addition to the family in the shape of a tiny moon which had never been seen up to that time. Number Six is also a fine big house, though not quite so big as Number Five, but larger than any of the others. It is inhabited by Saturn,197 and contains the biggest family of all. Up till the other day eight sons and daughters were known to live here, but they are not nearly so sturdy as Jupiter’s children; in fact, the young Saturns do not make much display, and some of them are so delicate that they are hardly ever seen. In this household, too, a new member has recently appeared. For fifty years the family was known to consist of these eight sons and daughters, but in August, 1898, when they were being photographed in a group, it was discovered that a ninth moon had been added. Number Seven is also a fine large house; but Uranus, who lives there, is such a recluse that unless you carefully keep your eye on his house, you will hardly ever catch a glimpse of him. There are four children in that house, I believe, but we hardly know them. They move in circles of their own, and apparently have seen a good deal of trouble. Only one more house is to be mentioned, and that is Number Eight, inhabited by Neptune. It contains one child, but we are hardly on visiting terms with this household, and we know next to nothing about it.

Before 1877, Mars appeared to be in the same condition as Venus or Mercury—that is, devoid of the dignity of attendants. There was, however, good reason for thinking that there might be some satellites to Mars, only that we had not seen them. You see that, as Number Three had one child, and Numbers Five, Six, Seven, and Eight had each one, or more than one, it seemed hard that poor Number Four should have none at all. It was, however, certain that if there were any satellites to Mars, they must be comparatively small things; for198 if Mars had even one considerable moon, it must have been discovered long ago.

On the memorable occasion in 1877, Professor Hall discovered that the ruddy planet Mars was attended, not alone by one moon, but by two. Their behavior was most extraordinary. It appeared to him at first almost as if one of these little moons was playing at hide-and-seek. Sometimes it would peep out at one side of the planet, and sometimes at the other side. I have here a picture (Fig. 63) which shows how these moons of Mars revolve. That is the globe of the planet himself in the middle, and he is turning round steadily in a period which is nearly the same as our day. But the remarkable point is that the inner of the moons of Mars runs round the planet in 7 hours 39 minutes. It would seem very strange in our sky if we had a little moon which rose in the west instead of in the east, and which galloped right across the heavens three times every day—and this is what Mars has. The outer moon takes a more leisurely journey, for he requires 30 hours 18 minutes to complete a circuit. If for no other reason than to see these wonderful moons, it would be very interesting to visit Mars.

The satellites of this planet are in contrast to our moon. In the first place, our moon takes 27 days to go round the earth, and is comparatively a long way off. The moons of Mars are much nearer to their planet, and they go round much more quickly. There is also another difference. The moons of Mars are much smaller bodies than our moon. If we represent Mars by a good-sized football, his moons, on the same scale,199 would be hardly so big as the smallest-sized grains of shot. Does it not speak well for the power of telescopes in these modern days that objects so small as the satellites of Mars should be seen at all? You remember, of course, that neither Mars himself nor his moons have any light of their own. They shine solely in consequence of the sunlight which falls upon them. They are merely lighted like the earth itself, or like the moon. The difficulty about observing the satellites is all the greater because they are seen in the telescope close to such a brilliant body as Mars. The glare from the bright planet is such that when we want to see faint objects like the satellites we have to hide Mars, so as to get a comparatively dark space in which to search.

Now that they know exactly what to look for, a good many astronomers have observed the satellites of Mars. A superb telescope is nevertheless required. And, in fact, you could not find a better test for the excellence of an instrument than to try if it will show these delicate objects. But do not imagine that merely having a good telescope and a clear sky is all that is requisite for making astronomical discoveries. You might just as well say that by putting a first-rate cricket-bat in any man’s hands will ensure his making a grand score. Every boy knows that the bat does not make the cricketer, and I can assure him that neither will the telescope make the astronomer. In both cases, no doubt, there is some element of luck. But of this you may be certain: that as it is the man that makes the score, and not the bat, so it is the astronomer that makes the discovery, and not his telescope.

200Deimos and Phobos were the names of the two personages, according to Homer, whose duty it was to attend on the god Mars, and to yoke his steeds. A conclave of classical scholars and astronomers appropriately decided that Deimos and Phobos must be the names of the two satellites to the planet which bears the name of Mars.

HOW THE TELESCOPE AIDS IN VIEWING FAINT OBJECTS.

We have been hitherto talking about large planets, which, if not as big as our earth, are at least as big as our moon. But now we have to say a few words about a number of little planets, many of them being so very small that a million rolled together would not form a globe so big as this earth. These little objects you cannot see with your unaided eye, and even with a telescope they only look like very small stars.

I have often been asked why it is that a telescope enables us to see objects, both faint and small, which our unaided eyes fail to show. Perhaps this will be a good opportunity to say a few words on the subject. I think we can explain the utility of the telescope by examining our own eyes. The eye undergoes a remarkable transformation when its owner passes from darkness into a brilliantly lighted room (Fig. 64). Here you see two views of an eye, and you notice the great difference between them. They are not intended to be the eyes of two different people, or the two eyes of the same person; they are merely two conditions of the same eye.201 They are intended to illustrate two different states of the eye of a collier. The right shows his eye when he is above ground in bright daylight; the left is his eye when he has gone down the coal-pit to his useful work in the dark regions below. I remember when I went down a coal-pit I was lowered down a long shaft, and when the bottom was reached a safety lamp was handed to me. The gloom was such, that at first I found some little difficulty in guiding my steps, but the capable guide beside me said in an encouraging voice, “You will be all right, sir, in a few moments, for you will get your pit-eyes.” I did get my “pit-eyes,” as he promised, and was able to see my way along sufficiently to enjoy the wonderful sights that are met with in the depths below.

Fig. 64.

The change that came over my eyes is that which these two pictures illustrate: the black, round spot in the centre is an opening covered with a transparent window, by which light enters the eye; the black spot is called the pupil, and nature has provided a beautiful contrivance by which the pupil can get larger or smaller, so as to make vision agreeable. When there is a great deal of light we limit the amount that enters by contracting202 the pupil so as to make the opening smaller. Thus the picture with the small pupil represents the state of the collier’s eye when he is above ground in bright sunlight. When he descends into the pit, where the light is very scanty, then he wants to grasp as much of it as ever he can, and consequently his pupil enlarges so as to make a wider opening, and this is what he calls getting his “pit-eyes.”

But you need not go down a coal-mine to see the use of the iris—for so that pretty membrane is called which surrounds the pupil. Every time you pass from light into darkness the same thing can be perceived. When we turn down the lights in a room, so that we are in comparative darkness, our pupils gradually expand. As soon as the lights are turned up again, then our pupils begin to contract. Other animals have the same contrivance in their eyes. You may notice in the Zoölogical Gardens how quickly the pupil of the lion contracts when he raises his eyes to the light. The power of rapidly changing the pupil might be of service to a beast of prey. Imagine him crouching in a dense shade to wait for his dinner; then of course the pupil will be large from deficiency of light; but when he springs out suddenly on his victim, in bright light, it would surely be of advantage to him to be able at once to see clearly. Accordingly his pupil adjusts itself to the altered conditions with a rapidity that might not be necessary for creatures of less predaceous habits.

These changes of the pupil explain how the telescope aids our eyes when we want to discern any faint objects, like the little planets. Such bodies are not visible to203 the unaided eyes, because our pupils are not large enough to grasp sufficient light for the purpose. Even when they are opened to the utmost, we want something that shall enable them to open wider still. We must therefore borrow assistance from some device which shall have an effect equivalent to an enlargement of the pupil beyond the limits that nature has actually assigned to it. What we want is something like a funnel which shall transform a large beam of rays into a small one. I may explain what I mean by the following illustration: Suppose that it is raining heavily, and that you want to fill a bucket with water. If you merely put the bucket out in the middle of a field, it will never be filled; but bring it to where the rain-shoot from a house-top is running down, and then your bucket will be running over in a few moments. The reason, of course, is that the broad top of the house has caught a vast number of drops and brought them together in the narrow shoot, and so the bucket is filled. In the same way the telescope gathers the rays of light that fall on the object glass, and condenses them into a small beam which can enter the eye. We thus have what is nearly equivalent to an eye with a pupil as big as the object glass. Thus the effect of a grand telescope amounts to a practical increase of the pupil from the size of a threepenny-piece up to that of a dinner-plate, or even much larger still.

THE ASTEROIDS OR SMALL PLANETS.

An asteroid is like a tiny star, and in fact the two bodies are very often mistaken. If we could get close204 to the objects, we should see a wide difference between them. We should find the asteroid to be a dark planet like our earth, lighted only by the rays from the sun. The star, small and faint though it may seem, is itself a bright sun, at such a vast distance that it is only visible as a small point. The star is millions of times as far from us as the planet, and utterly different in every respect.

It is a curious fact that the planets should happen to resemble the stars so closely. We can find an analogous fact in quite another part of nature. In visiting a good entomological collection, you will be shown some of the wonderful leaf-like insects. These creatures have wings, exactly formed to imitate leaves of trees, with the stalks and veins completely represented. When one of these insects lies at rest, with its wings folded, among a number of leaves, it would be almost impossible to penetrate the disguise. This mimicry is no doubt an ingenious artifice to deceive the birds or other enemies that want to eat the insect. There is, however, one test which the cunning bird could apply: the leaves do not move about of their own accord, but the leaf-insects do. If therefore the bird will only have the patience to wait, he will see a pair of the seeming leaves move, and then the deception will be to him a deception no longer, and he will gobble up the poor insect.

In our attempts to discover the planets we experience just the same difficulties as the insect-eating bird. Wide as is the true difference between a planet and a star, there is yet such a seeming resemblance between205 them that we are often puzzled to know which is which. The planets imitate the stars so successfully, that when one of them is presented to us among myriads of stars it is impossible for us to detect the planet by its appearance. But we can be cunning—we can steadily watch, and the moment we find one of these star-like points beginning to creep about we can pounce upon it. We know by its movements that it is only disguised as a star, but that it is really one of the planets.

It is not always easy to discover the asteroids even by this principle, for unfortunately these bodies move very slowly. If you have a planet in the field of view, it will creep along so gradually that an hour or more must have elapsed before it has shifted its position with respect to the neighboring stars to any appreciable extent. The search for such little planets is therefore a tedious one, but there are two methods of conducting it: the new one, which has only recently come into use, and the old one. I shall speak of the old one first.

Although the body’s motion is so slow, yet when sufficient time is allowed, the planet will not only move away from the stars close by, but will even journey round the entire heavens. The surest way of making the discovery is to study a small part of the heavens now and to examine the same locality again months or years afterwards. Memory will not suffice for this purpose. No one could recollect all the stars he saw with sufficient distinctness to be confident that the field of the telescope on the second occasion contained either more or fewer stars than it did on the first. The only206 way of doing this work is to draw a map of the stars very carefully. This is a tedious business, for the stars are so numerous that even in a small part of the heavens there will be many thousands of stars visible in the telescope. All of these will have to be entered faithfully in their true places on the map. When this has been done the map must be laid aside for a season, and then it is brought out again and compared with the sky. No doubt the great majority of the objects will be found just as they were before. These are the stars, the distant suns, and our concern is not at present with them. Sometimes it will happen that an object marked on the first map has left a vacant place on the second. This, however, does not help us much, for, whatever the object was, it has vanished into obscurity, and a new planet could hardly be discovered in this way. But sometimes it will happen that there is a small point of light seen in the second map which has no corresponding point in the first. Then, indeed, the expectation of the astronomer is aroused; he may be on the brink of a discovery. Of course he watches accurately the little stranger. It might be some star that had been accidentally overlooked when forming the map, or it might possibly be a star that has become bright in the interval. But here is a ready test: is the body moving? He looks at it very carefully, and notes its position with respect to the adjacent stars. In an hour or two his suspicions may be confirmed; if the object be in motion, then it is really a planet. A few further observations, made on subsequent days, will show the path of the body. And the astronomer has only to assure himself207 that the object is not one of the planets that have been already found before he announces his discovery.

The new method of searching for small planets, which has only come into use in recent years, is a very beautiful one, and renders the process of making such discoveries much more easy than the older method which I have just described.

We can take photographs of the heavenly bodies by adjusting a sensitive plate in the telescope so that the images of the objects we desire to see shall fall upon it. The method will apply to very small stars, if by excellent clockwork and careful guiding we can keep the telescope constantly pointing to the same spot until the stars have had time to imprint their little images. Thus we obtain a map of the heavens, made in a thoroughly accurate manner. Indeed, the delicacy of photography for this purpose is so great, that the plates show many stars which cannot be seen with even the greatest of telescopes. Suppose that a little planet happened to lie among the stars which are being photographed. All the time that the plate is being exposed the wanderer is, of course, creeping along, and after an hour (exposures even longer are often used), it may have moved through a distance sufficient to ensure its detection. The plate will, therefore, show the stars as points, but the planet will betray its presence by producing a streak.

The asteroids now known number between 400 and 500. Out of this host a few afford some information to the astronomer, but the majority of them are objects possessing individually only the slightest interest. No208 small planet is worth looking at as a telescopic picture. We should consider that asteroid to be a large one which possessed a surface altogether as great as England or France. Many of these planets have a superficial extent not so large as some of our great counties. A globe which was just big enough to be covered by Yorkshire—if you could imagine that large county neatly folded round it—would make a very respectable minor planet.

We know hardly anything of the nature of these small worlds, but it is certain that any living beings they could support must have a totally different nature from the creatures that we know on this earth. We can easily prove this by making a calculation. I shall suppose a small planet one hundred miles wide, its diameter being, therefore, the one-eightieth part of the diameter of the earth. If we were landed on such a globe, we should be far more puzzled by the extraordinary lightness of everything than we should be in the similar case of the moon to which I referred (p. 124). If we suppose the planet to be constructed of materials which had the same density as those of which the earth is made, then every weight would be reduced to the eightieth part of what it is here.

There would be one curious consequence of residence on such a globe. We have heard of attempts to make flying machines, or to provide a man with wings by which he shall soar aloft like the birds. All such contrivances have hitherto failed. It may be possible to make a pair of wings by which a man can fly down, but it is quite another matter when he tries to fly up209 again. Suppose, however, we were living on a small planet, it would be perfectly easy to fly, for as our bodies would only seem to weigh a couple of pounds, we ought to be able to flap a pair of wings strong enough to overcome so trivial a force. I should, however, add that this is on the supposition that the atmosphere has the same density as our own.

Life on these small planets would indeed be extraordinary. Let us take, for example, Flora, and see how a game of lawn tennis on that body would be managed. The very slightest blow of the racket would drive the ball a prodigious distance before it could touch the ground; indeed, unless the courts were about half a mile long, it would be impossible to serve any ball that was not a fault. Nor is there any great exertion necessary for playing lawn tennis on Flora, even though the courts are several hundred acres in extent. As a young lady ran to meet the ball and return it, each of her steps might cover a hundred yards or so without extra effort; and should she have the misfortune to get a fall, her descent to the ground would be as gentle as if she was seeking repose on a bed of the softest swan’s-down.

These little planets cluster together in a certain part of our system. Inside are the four inner planets, of which we have already spoken; outside are the four outer planets, of which we have soon to speak. Between these two groups there was a vacant space. It seemed unreasonable that where there was room for planets, planets should not be found. Accordingly the search was made, and these objects were discovered.210 Even at the present day, more and more are being constantly added to the list.

Up till quite recently all the small planets which had been discovered confined themselves to the space lying between the paths of the major planets Mars and Jupiter. This invariable rule was, however, departed from in the case of one of these bodies which was discovered in August, 1898. This little body, which was known for some time by the provisional appellation of D Q, and which has now been definitely christened Eros, is an exception to this rule. It travels at an average distance from the sun actually less than that of Mars, and at the nearest point can come within 15,000,000 miles of the earth.

We occasionally get information from these little bodies; for in their revolutions through the solar system, they sometimes pick up scraps of useful knowledge, which we can elicit from them by careful examination. For example, one of the most important problems in the whole of astronomy is to determine the sun’s distance. I have already mentioned one of the ways of doing this, which is given by the transit of Venus. Astronomers never like to rely on a single method; we are therefore glad to discover any other means of solving the same problem. This it is which the little planets will sometimes do for us. Juno on one occasion approached very close to the earth, and astronomers in various parts of the globe observed her at the same time. When they compared their observations they measured the sun’s distance. But I am not going to trouble you now with a matter so difficult. Suffice it to say, that for this, as211 for all similar investigations, the observers were constrained to use the very same principle as that which we illustrated in Fig. 5.

Let me rather close this lecture with the remark that we have here been considering only the lesser members of the great family which circulate round the sun, and that we shall speak in our next lecture of the giant members of our system.