170 



ELEMENTARY SCIENCE 



which we live does not "warm up " to its warmest until after 

 the longest day, or "cool off" to its coldest until after the 

 shortest day. What are some of these reasons? If you re- 

 member what has been said of the relations of heat- to soil 

 and rock and water, you can solve this problem for yourself. 



Solar System. It is easy to see why the seasons re- 

 sult from changes in the length of day, but you cannot 



understand why the 

 days change in length 

 unless you under- 

 stand the solar system. 

 The solar system is 

 composed of the sun 

 and all the bodies 

 which revolve about 

 it, chief of which are 

 the eight planets. Of 

 these, the earth is one. 

 All of the planets, ex- 

 cept the earth, have 

 names that were given 

 them by the early astronomers in honor of ancient deities. 

 In the order of their nearness to the sun, the planets are: 

 Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, 

 Neptune (see Fig. 60). 



The planets revoke about the sun, following their orbits, 

 and also rotate on their axes. In the case of earth, one 

 complete rotation takes twenty-four hours, while a com- 

 plete revolution takes three hundred and sixty-five days. 

 Evidently, it is the earth's rotation which gives us day and 

 night, while its revolution determines the length of the 



FIG. 60. The solar system 



