'Once upon a time,' long, long ago, the earth was not a compact, round, hard body such as she is now, but much larger and softer, and as she rotated a fragment broke off from her; it did not go right away from her, but still went on circling round with the motion it had inherited from her. As the ages passed on both the earth and this fragment, which had been very hot, cooled down, and in cooling became smaller, so that the distance between them was greater than it had been before they shrank. And there were other causes also that tended to thrust the two further from each other. Yet, compared with the other heavenly bodies, they are still near, and by looking up into the sky at night you can generally see this mighty fragment, which is a quarter the diameter of the earth--that is to say, a quarter the width of the earth measured from side to side through the middle. It is--as, of course, you have guessed--the moon. The moon is the nearest body to us in all space, and so vast is the distance that separates us from the stars that we speak as if she were not very far off, yet compared with the size of the earth the space lying between us and her is very great. If you went right round the world at the thickest part--that is to say, in the region of the Equator--and when you arrived at your starting-point went off once again, and so on until you had been round ten times, you would only then have travelled about as far as from the earth to the moon! The earth is not the only planet which has a moon, or as it is called, a satellite, in attendance. Some of the larger planets have several, but there is not one to compare with our moon. Which would you prefer if you had the choice, three or four small moons, some of them not much larger than a very big bright star, or an interesting large body like our own moon? I know which I should say. 'You say that the moon broke off from the earth, so perhaps there may be some people living on her,' I hear someone exclaim.