THE SUN AND HIS PEBBLE WORLDS
However the worlds of our solar system may have been made, when they were done there was the sun in the centre and his worlds travelling around him in their ordered orbits. Nearest the sun is Mercury. Then Venus, Earth, Mars, Jupiter, Saturn, Uranus; then, finally, Neptune nearly 3,000,000,000 miles away and with an orbit so big that Christmas comes only once in 60,000 years!
YOU CAN SEE THESE WORLDS IN THE MAKING
Owing to the more powerful telescopes of to-day, and the amount of exploring among the worlds that has been going on since the time of Laplace, several things have been discovered that have brought his theory into question. For one thing, many more nebulæ have been found in space than were known when Laplace worked out his great conception, and among them all not one has been found with a central mass surrounded by a ring. Moreover, our sharp-eyed telescopes show that Saturn's ring, which Laplace thought was a solid mass, is really made up of a great number of small satellites: baby worlds. The greater number of these nebulæ are like the ones you see in the illustration on [page 5]. They consist of very bright centres with spirals streaming out from opposite sides. Just take a look at the picture. Doesn't the shape of those spirals suggest that the central mass is whirling? And notice the little white lumps here and there. The thinner, veil-like portions of the mass, as well as the "lumps," are supposed to be made of particles of matter, but the lumps to be more condensed. All the particles, big and little, are known to be revolving about the central mass, much as the earth revolves about the sun. The little white lumps, or knots, in the filmy skein are supposed to be worlds in the making. Being larger than the other particles, they draw the smaller to them, according to the same law of gravitation which makes every unsupported thing on earth fall to the ground, because the earth is so much bigger than anything there is on it. Since these bright little lumps behave so much like the worlds we know as planets, and yet are relatively so small, they are called planetessimals, or "little planets." So Professor Chamberlin's idea of the origin of worlds is known as the "planetessimal theory."
HOW YOU CAN WATCH THE WORLD TURN ROUND
Timepieces, you know, are really machines for keeping track of the apparent movement of the sun. Here is a device, as simple as a sun-dial and much simpler than a clock, by which you can record the actual motion of the earth. Sprinkle the surface of the water in a bowl with chalk dust. On this, sift from a piece of paper powdered charcoal or pencil dust, so as to make a clean-cut band extending across the centre and over the edge of the bowl. In the course of several hours you will find that the black band has swept round from east to west, because the water has stood still while the bowl has been carried from west to east by the whirling world.
According to this theory the earth was once a mere baby world like those white lumps, and grew by gathering in its smaller neighbors from time to time by the power of gravitation. The larger it grew the more particles of solid matter it could draw to itself. Then it drew larger masses, for with increased mass came an increased pull of gravity. In the same way the earth is still growing, for it is thought that the shooting stars or meteors we see at night are little planets being gathered in.
II. How the Continents Came Up Out of the Sea
And before I got to be myself at all, while I was still only a part of the big pebble called the Earth, your geography and I lay at the bottom of the sea.