Part 10

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 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 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.

We must first take another lesson in drawing, and 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.

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 tacks (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 string 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.

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 around 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 with 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 was 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 are 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 pay 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.