. CHAPTER II 



THE EARTH'S NEAREST NEIGHBORS: THE MOON AND 

 THE PLANETS 



22. The Earth, a Planet. It has been known for five 

 hundred years that the earth moves around the sun once in 

 about 365 days. It has since been learned that the earth 

 is nearly spherical; that its orbit is nearly a circle; that it 

 is kept in place by an attracting force, called gravitation; 

 and that there could be no life on the earth if it were not 

 for the light and heat which it receives from the sun. Be- 

 sides the earth, there are seven other bodies of which 

 nearly all these facts are true. These bodies are called 

 planets. 



23. The Motion of the Planets. The star Regulus 

 changes its position in the sky, but it is always at the end 

 of the handle of the sickle in Leo. Similar statements may 

 be made of all the stars we have studied. They do not 

 change their position with relation to other stars. Among 

 the stars, however, are seen a few bright bodies that shine 

 with a steadier light and do not keep .the same places in 

 the constellations. At the end of a month, for instance, 

 we may see that they have moved away from or nearer to 

 aome star in the same part of the sky. These are the planets 

 (from a Greek word meaning "wanderer"). The brightest 

 of the planets are Venus, Jupiter, Mars, and Saturn. It 

 is very seldom that all four are seen at the same time in 

 any evening, but two are often in the sky together, and 

 sometimes three. The three other planets Mercury, 

 Uranus, and Neptune cannot be seen well without a 

 telescope. 



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