In the days of the Apollo story most men still thought the earth was the centre of the universe; that the sun, moon, and stars moved around it. But Pythagoras, one of the Greek philosophers, had formed a general notion of the truth that the earth is only one planet in a great system. Then, along in the Sixteenth Century, came Copernicus, and by mathematical calculation--he was a fine hand at figures--began to find out things that showed the wise old Greek had made a happy guess. Then Galileo, Kepler, Newton, and others, each working on different parts of the problem, finally settled the question. They found that there are just worlds of worlds, and that ours is only one of them. * * * * * About the time of the American Revolution a great French mathematician, Laplace, worked out a story of the origin of the earth which is, briefly, this: What we know now as the solar system--the sun with its attendant worlds--was once a single big ball of fiery gas, a nebula. As this nebula cooled it shrank, and as it shrank it whirled faster because it had a smaller track in which to turn, and with an equal amount of force would, of course, get around oftener. The faster it whirled the more the outside of it tended to fly off, as water flies off a whirling grindstone or as a stone flies from a sling. This centrifugal or "fly-away" force was greatest at the sun's equator, and it threw off big rings. Afterward, around some centre of greater density in these rings, the gaseous particles in the rest of the ring gathered, so forming spheres. Then some of the spheres themselves threw off rings in the same way which became what are called satellites. The moon, which is our satellite, Laplace supposed to have originated in this way. The ring which Saturn still wears he thought would some day become a satellite. [Illustration: _By permission of the Mount Wilson Observatory_ WATCHING THE MAKING OF WORLDS At first you won't see anything very striking about this picture, perhaps; but doesn't it give you something of a thrill to be told that you are here looking not only at the making of a _world_, but of worlds of worlds? A whole solar system! In the course of unthinkable time that big, round ball in the center will be the sun, and what appear to be little knots wrapped close around it--they are really far from each other and from the sun--will become rounded worlds like ours. They will be forced into roundness by their own gravity, pulling toward their centers. They don't look any farther apart than the strands in a little sister's braided hair, do they? But remember how small this picture is compared with what it represents. What here show as little dark lines, separating the embryo worlds, are in reality vast spaces, like those you see between the stars at night--millions and millions and millions of miles! ]