CHAPTER XI.

MORE ABOUT THE SOLAR SYSTEM.

We have now reached a point where it ought not to be difficult for us to picture to ourselves, with something of vividness, the general outlines of the Solar System. Awhile ago this Solar System was a very simple matter in the eyes of astronomers. There was the great sun fixed in the center, with seven planets circling round him--seven of course, it was said, since seven was the perfect number--and a few moons keeping pace with some of the planets, and an occasional comet, and a vast amount of black, empty space. But astronomers now begin to understand better the wonderful richness of the system as a whole; the immense variety of the bodies contained in it; the perpetual rush and stir and whirl of life in every part. Certainly there is no such thing as dull stagnation throughout the family.

First, we have the great, blazing, central sun; not a sun at rest, as regards the stars, but practically at rest as regards his own system, of which he is always head and center. Then come the four smaller planets, rapidly whirling round him, all journeying in the same direction, and all having their oval pathways lying on nearly the same flat plane in space. Then the broad belt of busy little planetoids. Then the four giant planets,--Jupiter nearly five times as far as our earth from the sun; Saturn nearly twice as far as Jupiter; Uranus nearly twice as far as Saturn; Neptune as far from Uranus as Uranus from Saturn,--all keeping on very nearly the same level as the four inner planets.

[Illustration: POSITION OF PLANETS INFERIOR TO JUPITER--SHOWING THE ZONE OF THE ASTEROIDS.]

And between and about these principal members of the system, with their accompanying moons, we have thousands of comets flashing hither and thither, with long, radiant trains; and myriads of meteorites, gathered often into dense, vast herds or families, but also scattered thickly throughout every part of the system, each tiny ball reflecting the sun’s rays with its little glimmer of light.

Broad reaches of black and empty space! Where are they? Perhaps nowhere. We are very apt, in our ignorance, to imagine that where we see nothing, there must of necessity be nothing. But, for aught we know, the whole Solar System, not to speak of sky-depths lying beyond, may be bright with reflecting bodies great and small, from the mighty Jupiter down to the fine diamond-dust of countless meteorites. In this earth of ours we find no emptiness. Closer and closer examination with the microscope only shows tinier and yet tinier wonders of form and life, each perfect in finish. Not of form only, but of _life_. How about that matter as regards the Solar System? Is our little world the one only spot in God’s great universe which teems with life? Are all other worlds mere barren, empty wastes? Surely not. We may safely conclude that life of one kind or another has been, is, or will be, upon our brother and sister worlds.

The same reasoning may be used for the distant stars--those millions of suns lying beyond reach of man’s unassisted eyes. Are they formed in vain? Do their beams pour uselessly into space, carrying light, warmth, and life-giving power to nothing? Surely around many of them, as around our sun, must journey worlds; and not only worlds, but worlds containing life.

We shall have to speak of this matter again as we go on. Whether men and women like ourselves could live on the other planets is another question. In some cases it looks doubtful, in some it would seem to be impossible. But the endless variety of life on this earth--life on land, life in the air, life in water, life underground, life in tropical heat, life in arctic cold--forbids us in anywise to be positive as to what may or may not be.

If no animals that we know could exist there, animals that we do not know might be found instead. Any one of the planets may, and very likely does, abound with life. Nay, the very meteorites themselves, before they catch fire and burn in our air, _may_ be the homes of tiny animalcula, altogether different from anything we see on earth. We can not fancy life without air, but neither could we fancy life in water, if we had not seen and known it to be possible.

I have spoken of the probable _brightness_ of the Solar System as a whole. We are so apt to think of things merely as we see them with our short sight, that it is well sometimes to try to realize them as they actually are. Picture to yourself the great central sun pouring out in every direction his burning rays of light. A goodly abundance of them fall on our earth, yet the whole amount of light and heat received over the whole surface of this world is only the two-thousand-millionth part of the enormous amount which he lavishly pours into space. How much of that whole is wasted? None, though God gives his gifts with a kingly profusion which knows no bounds. Each ray has its own work to do. Millions of rays are needed for the lighting and nourishing and warming of our companion-planets, while others are caught up by passing comets, and myriads flash upon swift, tiny meteorites. Of the rays not so used, many pass onwards into the vast depths beyond our system, and dwindle down into dim, starlike shining till they reach the far-off brother-stars of our sun.

Have they work to do there? We can not tell. We do not know how far the sun’s influence reaches. As head and center, he reigns only in his own system. As a star among stars, a peer among his equals, he may, for aught we can tell, have other work to do.

In an early chapter, mention was made of the earth’s three motions, two only being explained. First, she spins ceaselessly upon her axis. So does the sun, and so do the planets. Secondly, she travels ceaselessly round and round the sun in her fixed orbit. So does each one of the planets. Thirdly, she journeys ceaselessly onward through space with the sun. So also do the rest of the planets. These last two movements, thought of together, make the earth’s pathway rather perplexing at first sight. We talk of her orbit being an ellipse or oval; but how can it be an ellipse, if she is always advancing in one direction?

The truth is, the earth’s orbit is and is not an ellipse. As regards her yearly journey round the sun, roughly speaking, we may call it an ellipse. As regards her movement in space, it certainly is not an ellipse.

Think of the Solar System, with the orbits of all the planets, as lying _nearly flat_--in the manner that hoops might be laid upon a table, one within another. The asteroids, comets, and meteorites do not keep to the same level; but their light weight makes the matter of small importance.

Having imagined the sun thus in the center of a large table--a small ball, with several tiny balls traveling round him on the table at different distances--suppose the sun to rise slowly upwards, not directly up, but in a sharp slant, the whole body of planets continuing to travel round, and at the same time rising steadily with him.

By carefully considering this double movement, you will see that the real motion of the earth--as also of each of the planets--is not a going round on a flat surface to the same point from which she started, but is a corkscrew-like winding round and round upwards through space. Yet as regards the central sun, the shape of the orbit comes very near being an ellipse, if calculated simply by the earth’s distance from him at each point in turn of her pathway through the year.

An illustration may help to explain this. On the deck of a moving vessel, you see a little boy walking steadily round and round the mast. Now is that child moving in a circle, or is he not? Yes, he is. No, he is not. He walks in a circle as regards the position of the mast, which remains always the center of his pathway. But his movement _in space_ is never a circle, since he constantly advances, and does not once return to his starting-point. You see how the two facts are possible side by side. Being carried forward by the ship, with no effort of his own, the forward motion does not interfere with the circling motion. Each is performed independently of the other.

It is the same with the earth and the planets. The sun, by force of his mighty attraction, bears them along wherever he goes--no exertion on their part, so to speak, being needed. That motion does not in the least interfere with their steady circling round the sun.

Just as--to use another illustration--the earth, turning on her axis, bears through space a man standing on the Equator at the rate of one thousand miles an hour. But this uniform movement, unfelt by himself, does not prevent his walking backwards, or forwards, or in circles, as much as he will.

So, also, a bird in the air is unconsciously borne along with the atmosphere, yet his freedom to wheel in circles for any length of time is untouched.

A few words about the orbits of the planets. I have more than once remarked that these pathways are, in shape, not circles, but ellipses. A circle is a line drawn in the shape of a ring, every part of which is at exactly the same distance from the center-point or focus. But an ellipse, instead of being, like a circle, perfectly round, is oval in shape; and instead of having only one focus, it has two foci, neither being exactly in the center. Foci is the plural word for focus. If an ellipse is only slightly oval--or slightly _elliptical_--the two foci are near together. The more oval or _eccentric_ the ellipse, the farther apart are the two foci.

You may draw a circle in this manner. Lay a sheet of white paper on a board, and fix a nail through the paper into the board. Then pass a loop of thread--say an inch or an inch and a half in length--round the nail, and also round a pencil, which you hold. Trace a line with the pencil, keeping the loop tight, so that the distance of your line from the nail will be always equal, and when it joins you have a circle. The nail in the center is the focus of the circle.

To draw an ellipse, you must fix _two_ nails. Let them be about half an inch apart; pass a loop over both of them, and again placing a pencil point within the loop, again trace a line carefully all round, keeping the thread drawn tight. This time an oval instead of a circle will appear. By putting the nails nearer together or farther apart, you may vary as you will the shape of the ellipse.

In the orbits of the earth and the planets, all of which are ellipses in shape, the sun is not placed in the exact center, but in one of the two foci, the second being empty. So at one time of the year the planet is nearer to the sun than at another time. Our earth is no less than three millions of miles nearer in winter than she is in summer--speaking of the winter and summer of the northern hemisphere. Three millions of miles is so tiny a piece out of ninety-three millions of miles, that it makes little or no difference in our feelings of heat or cold.

The orbits of the comets are ellipses also, but ellipses often so enormously lengthened out, that the two foci are almost--if one may so speak--at the two _ends_ of the oval. To draw a good comet orbit, you must fix the two nails on your paper some five or six inches apart, with a loop of thread just large enough to slip over them both, and to allow the pencil to pass round them. When your ellipse is drawn, you must picture the sun in the place of one of the two nails, and you will see how, in their pathways, the comets at one time pass very near the sun, and at another time travel very far away from him.

[Illustration: COMPARATIVE SIZE OF THE PLANETARY WORLDS.]

It is generally found in families, not only that the parent or head of the family has great influence over all the members, but that each member has influence over each other member. Brother influences brother, and sister influences sister.

This, too, we find in the Solar System. Not only does the sun, by his powerful attraction, bind the whole family together, but each member of the family attracts each other member. True, the force of the sun’s attraction is overpowering in amount compared with others. The sun attracts the planets, and the planets attract the sun; but their feeble pulling is quite lost in the display of his tremendous strength.

Among themselves we see the power more plainly. The earth attracts the moon, keeping her in constant close attendance; and the moon attracts the earth, causing a slight movement on her part, and also causing the tides of the sea. Each planet has more or less power to hinder or help forward his nearest brother-planet. For instance, when Jupiter on his orbit draws near the slower Saturn on his orbit, Saturn’s attraction pulls him on, and makes him move faster than usual; but as soon as he gets ahead of Saturn then the same attraction pulls him back, and makes him go more slowly than usual. Jupiter has the same influence over Saturn; and so also have Saturn and Uranus over one another, or Uranus and Neptune.

In early days astronomers were often greatly puzzled by these quickened and slackened movements, which could not be explained. Now the “perturbations” of the planets, as they are called, are understood and allowed for in all calculations. Indeed, it is by means of this very attraction that Neptune was discovered, and the planets have actually been weighed. What a wonderful difference we find in this picture of the Solar System, as we now know it to be, from the old-world notion of our earth as the center of the universe!

When we think of all the planets, and of the magnificent sun; when we pass onward in imagination through space, and find our sun himself merely one twinkling star amid the myriads of twinkling stars scattered broadcast through the heavens, while planets and comets have sunk to nothing in the far distance,--then indeed we begin to realize the unutterable might of God’s power! Why, our earth and all that it contains may be regarded as but one grain of dust in the wide universe.