THE EARTH AS A PLANET 



357 



There is no atmosphere, no water, no protection from the 

 intense solar heating. For two weeks at a time the sun 

 shines continuously on the same places. There are no 

 clouds in the sky to prevent radiation of this heat, and it must 

 become extremely cold through the two weeks of continuous 

 absence of sunshine that follows. A lunar landscape lit up 



FIG. 117. A portion of the moon's surface as seen in a telescope. Note 

 the shadows cast by mountains, and others within the craters of lunar vol- 

 canos now extinct. The sunlight is from the left side. 



by the sun's rays in such a way as to throw into shadow 

 reliefs the rugged mountainous surface of the moon is, how- 

 ever, a most interesting sight when viewed through a good 

 telescope. Photographs of the moon's surface when made by 

 aid of the telescope give a good idea of what the astronomer 

 beholds. 



Our moon has a diameter of about two thousand miles, or 

 one-fourth that of the earth's diameter. Its volume then is 

 approximately (J^) 3 or J^ 4 that of the earth. Its weight, 

 however, as calculated by astronomers is but J^o of that 

 of the earth, showing that the material of which the moon 



