Mercury, Venus, and Mars—How to make a Drawing of our System—The Planet Mercury—The Planet Venus—The Transit of Venus—Venus as a World—The Planet Mars and his Movements—The Ellipse—The Discoveries made by Tycho and Kepler—The Discoveries made by Newton—The Geography of Mars—The Satellites of Mars—How the Telescope aids in Viewing Faint Objects—The Asteroids, or Small Planets.
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. 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.
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 of 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.
First 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, this 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.
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. If 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 to 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.
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 nearly 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.
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.
You 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 different 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 quite 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 his 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 mathematician 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, of 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.”
“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 storm 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 accuracy 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.
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.
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 Venus 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 brightest 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 has 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 different 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.
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 ought 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 224.70 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 more 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.
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 event 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.
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 marks 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 than 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 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 the 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 tired 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.
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 was 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.
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 viewing 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; 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 distance 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.