CHAPTER X 

 ORIGIN AND DEVELOPMENT OF THE EARTH 



The earth and the planets. It is a matter of common 

 knowledge that the earth is a ball which revolves about the 

 sun, another but much larger body of similar shape. 

 Seven other planets more or less like the earth also wheel 

 about the central sun (Fig. 314). Among them are the 

 familiar stars, Jupiter and Venus. Four of the planets are 

 larger than the earth, three are smaller. Two are nearer the 

 sun, while five are farther away from it. The moon is a much 



FIG. 314. Diagram of the solar system. The size of the sun and particu- 

 larly the planets is enormously exaggerated as compared to the breadth 

 of the orbits. 



smaller body, which is controlled by the gravity of the 

 earth and therefore revolves about it. Most of the other 

 planets have similar moons (satellites), Saturn has nine of 

 them. In addition to its motion around the sun, each of the 

 planets rotates on its own axis, much as a top spins while 

 gliding about on the floor. It is a significant fact that all of 

 the planets move in the same general direction, and the nearly 

 circular paths which they follow, year after year, lie so nearly 

 in the same plane that the whole system may be compared 

 in shape to a disk. As we now know it, this solar system 



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