248 ELEMENTS OF GENERAL SCIENCE 



are as much as a hundred miles in diameter and two or 

 three miles deep. There are also mountains of spirelike 

 character and of great height. 



The explanation of the lunar craters is not a simple 

 matter, and astronomers are not entirely in agreement on 

 the question. Their great size, the absence of any water on 

 the surface of the moon, and the absence of distinct lava 

 flows make it doubtful whether they are closely analogous 

 to the volcanic craters which are now found on the surface 

 of the earth. It has been suggested that they have been 

 depressed by impacts of great meteorites from the outside, 

 but this is not supported by adequate evidence. The moon 

 seems to have preserved for us, because of its lack of an 

 atmosphere and a water covering, a record of what the earth 

 probably went through at an early stage of its development. 



254. Climate of the moon. The moon revolves around the 

 earth, and the earth and moon revolve around the sun at a 

 distance four hundred times their distance from one another. 

 Consequently the average distance of the moon from the sun 

 is about the same as that of the earth, and, other things 

 being equal, its climate would be about the same as that of 

 the earth. There are two important factors, however, which 

 make it entirely different. The first of these is that it turns 

 on its axis very slowly; in fact, it rotates in such a way 

 that the same face of it is always turned toward the earth, 

 and its day is twenty-nine and one half of our days. If 

 there were no other difference, the daily change in tem- 

 perature would be enormously greater than on the earth. 

 If the earth's day were equal to that of the moon, the 

 temperature would probably fall below the freezing point 

 every night, even at the equator, and it would ascend to- 

 unusual heights during the day. 



The second reason why the climatic changes of the moon 

 are different from those of the earth is that it has no atmos- 

 pheric covering. The sun's rays are never cut off by clouds 



