THE SEASONS AND THE SOLAR SYSTEM 171 



year. Let us remember, then, that as the earth makes its 

 annual revolution about the sun, it spins as it goes, and 

 each spin is what we call a day. Since the sun "rises" 

 in the east and "sets" in the west, the direction of this 

 spinning is evidently from west to east. At night we 

 are in the shadow of the earth, but we go on spuming toward 

 the east, till presently the sun peeps over the eastern hori- 

 zon, and we say it is day. 



FIG. 61. Diagram of the earth's orbit. 



The fact just suggested, namely, that earth and sun are 

 not always the same distance apart, seems to give us a 

 clew to the solution of the problem we are studying, the 

 problem of variations in the amount of heat received on 

 the earth. Consider then the shape of the orbit of the 

 earth (see Fig. 61). It is not a circle. It is a kind of curve 

 called an ellipse, and when the earth, following this path, is 

 farthest from the sun it is three million miles farther away 

 than when it is nearest. However, the difference of dis- 

 tance is so small as compared with the total distance that 

 evidently it can have but small effect on the amount of 

 heat received on the earth. 



