If you are holding something in your hand and you let it go, what happens? It falls to the ground, of course. Now, why should it do so? You will say: 'How could it do anything else?' But that is only because you are hampered by custom. Try to shake yourself free, and think, Why should it go down instead of up or any other way? The first man who was clever enough to find some sort of an answer to this question was the great philosopher Sir Isaac Newton, though he was not quite the first to be puzzled by it. After years of study he discovered that every thing attracts every other thing in proportion to their masses (which is what you know as weight) and their distance from each other. In more scientific language, we should say every body instead of every thing, for the word body does not only mean a living body, but every lump or mass of matter in the universe. The earth is a body in this We know that nothing here on earth continues to move for ever; everything has to be kept going. Anything left to itself has a tendency to stop. Why is this? This is because here in the world there is something that fights against the moving thing and tries to stop it, whether it be sent along the ground or thrown up in the air. You know what friction is, of course. If you rub your hands along any rough substance you will quickly feel it, THE EARTH AND MOON HANGING IN SPACE THE EARTH AND MOON HANGING IN SPACE Gravitation affects everything connected with the earth; even our air is held to the earth by gravitation. It grows thinner and thinner as we get further away from the earth. At the top of a high mountain the air is so thin that men have difficulty in breathing, and at a certain height they could not breathe at all. As they cannot breathe in very fine air, it is impossible for them to tell by personal experiment exactly where the air ends; but they have tried to find out in other ways, and though different men have come to different conclusions on the subject, it is safe to say that at about two hundred miles above the earth there is nothing that could be called air. Thus we can now picture our spinning earth clothed in a garment of air that clings closely about her, and grows thinner and thinner until it melts away altogether, for there is no air in space. Now in the beginning God made the world, and set it off by a first impulse. We know nothing about the details, though further on you shall hear what is generally supposed to have taken place; we only know that, at some remote age, this world, probably very different from what it is now, together with the other planets, was sent spinning off into space on its age-long journey. These planets were not sent off at random, but must have had some particular connection with each other and with the sun, for they all belong to one system or family, and act and react on each other. Now, if they had been at rest and not in movement, they would have fallen right into the sun, drawn by the force of gravitation; then they would have been burned up, and there would have been an end of them. But the first force had imparted to them the impulse to go on in a straight line, so when the sun pulled the result was a movement between the two: the planets did not continue to move in a straight line, neither did they fall on to the sun, but they went on a course between the two—that is, a circle—for the sun never let them get right away from him, but compelled them to move in circles round him. There is a very common instance of this kind of thing which we can Of course, every planet gives a pull at every other planet too, but these pulls are so small compared with that of the sun that we need not at present notice them. Then we come to another point. We said that every body pulled every other body in proportion to their weights and their distance. Now, gravity acts much more strongly when things are near together than when they are far away from each other; so that if a smaller body is near to another somewhat larger than itself, it is pulled by it much more strongly than by a very much larger one at a considerably greater distance. We have an instance of this in the case of the earth and moon: as the earth responds to the pull of the sun, so the moon responds to the pull of the earth. The moon is so comparatively near to the earth that the earth-pull forces her to keep on going round and round, instead of leaving her free to circle round the sun by herself; and yet if you think of it the moon does go round the sun too. Recall that game we had when the sun was in the middle, and the two smaller girls, representing the earth and moon, went round it. The moon-child turned round the earth-child, but all the while the earth-child was going round the sun, so |