WHY THE EARTH MOVES ROUND THE SUN.
Papa—Lucy.
Papa. You remember, Lucy, that I explained to you some time ago what was the cause that things fell to the ground.
Lucy. O yes; it was because the ground drew them to it.
Pa. True. That is a consequence of the universal law in nature, that bodies attract each other in proportion to their bulk. So a very small thing in the neighbourhood of a very large one, always tends to go to it, if not prevented by some or other power. Well—you know I told you that the sun was a ball a vast many times bigger than the ball we inhabit, called the earth; upon which you properly asked, how then it happened that the earth did not fall into the sun.
Lu. And why does it not?
Pa. That I am going to explain to you. You have seen your brother whirl round an ivory ball tied to the end of a string, which he held in his hand.
Lu. Yes; and I have done it myself, too.
Pa. Well, then—you felt that the ball was continually pulling, as if it tried to make its escape?
Lu. Yes; and one my brother was swinging did make its escape, and flew through the sash.
Pa. It did so. That was a lesson, in the centrifugal motion, or that power by which a body thus whirled, continually endeavours to fly off from the centre round which it moves. This is owing to the force or impulse you give it at setting out, as if you were going to throw it away from you. The string by which you hold it, on the contrary, is the power which keeps the ball toward the centre, called the centripetal power. Thus you see there are two powers acting upon the ball at the same time, one to make it fly off, the other to hold it in; and the consequence is, that it moves directly according to neither, but between both; that is, round and round. This it continues to do while you swing it properly; but if the string breaks or slips off, away flies the ball; on the other hand, if you cease to give it the whirling force, it falls toward your hand.
Lu. I understand all this.
Pa. I will give you another instance of this double force acting at the same time. Do not you remember seeing some curious feats of horsemanship?
Lu. Yes.
Pa. One of them was, that a man standing with one leg upon the saddle, and riding full speed, threw up balls into the air, and catched them as they fell.
Lu. I remember it very well.
Pa. Perhaps you would have expected these balls to have fallen behind him, as he was going at such a rate?
Lu. So I did.
Pa. But you saw that they fell into his hand as directly as if he had been standing quite still. That was because at the instant he threw them up, they received the motion of the horse straight forward as well as the upright motion that he gave them, so that they made a slanting line through the air, and came down in the same place they would have reached if he had held them in his hand all the while.
Lu. That is very curious, indeed!
Pa. In the same manner you may have observed, in riding in a carriage, that if you throw anything out of the window, it falls directly opposite, just as if the carriage was standing still, and is not left behind you.
Lu. I will try that the next time I ride in one.
Pa. You are then to imagine the sun to be a mighty mass of matter, many thousand times bigger than our earth, placed in the centre, quiet and unmoved. You are to conceive our earth, as soon as created, launched with vast force in a straight line, as if it were a bowl on a green. It would have flown off in this line for ever, through the boundless regions of space, had it not at the same instant received a pull from the sun by its attraction. By the wonderful skill of the Creator, these two forces were made exactly to counterbalance each other; so that just as much as the earth, from the original motion given to it, tends to fly forward, just so much the sun draws it to the centre; and the consequence is, that it takes a course between the two, which is a circle round and round the sun.
Lu. But if the earth was set a rolling like a bowl upon a green, I should think it would stop of itself, as the bowl does.
Pa. The bowl stops because it is continually rubbing against the ground, which checks its motion, but the ball of the earth moves in empty space, where there is nothing to stop it.
Lu. But if I throw a ball through the air, it will not go on for ever, but it will come down to the ground.
Pa. That is because the force with which you can throw it is much less than the force by which it is drawn to the earth. But there is another reason, too, which is the resistance of the air. This space all round us and over us is not empty space; it is quite full of a thin transparent fluid called air.
Lu. Is it?
Pa. Yes. If you move your hand quickly through it, you will find something resisting you, though in a slight degree. And the wind, you well know, is capable of pressing against anything with almost irresistible force; and yet wind is nothing but a quantity of air put into violent motion. Everything, then, that moves through the air is continually obliged to push some of this fluid out of the way, by which means it is constantly losing part of its motion.
Lu. Then the earth would do the same?
Pa. No; for it moves in empty space.
Lu. What, does it not move through the air?
Pa. The earth does not move through the air, but carries the air along with it. All the air is contained in what is called the atmosphere, which you may compare to a kind of mist or fog clinging all round to the ball of the earth, and reaching a certain distance above it, which has been calculated at above forty-five miles.
Lu. That is above the clouds, then.
Pa. Yes: all the clouds are within the atmosphere, for they are supported by the air. Well—this atmosphere rolls about along with the earth, as if it were a part of it, and moves with it through the sky, which is a vast field of empty space. In this immense space are all the stars and planets, which have also their several motions. There is nothing to stop them, and therefore they continually go on, by means of the force that the Creator has originally impressed upon them.
Lu. Do not some of the stars move round the sun, as well as our earth?
Pa. Yes; those that are called planets. These are all subject to the same laws of motion with our earth. They are attracted by the sun as their centre, and form, along with the earth, that assemblage of worlds, which is called the solar system.
Lu. Is the moon one of them?
Pa. The moon is called a secondary planet, because its immediate connexion is with our earth, round which it rolls, as we do round the sun. It, however, accompanies our earth on its journey round the sun. But I will tell you more about its motion, and about the other planets and stars another time. It is enough at present, if you thoroughly understand what I have been describing.
Lu. I think I do.