THE MOON, PLANETS, AND COMETS 249 



or even diminished by the partial screen of an atmosphere, 

 and consequently the temperature rises very high in the long 

 interval between sunrise and sunset, which is nearly fifteen of 

 our days. After the sun is set, the other extreme of tem- 

 perature soon begins to develop. There is no atmosphere to 

 hold in the heat which has been accumulated during the day. 

 The temperature falls in the course of two or three hours 

 from a maximum reached in the daytime, which is supposed 

 to be near the boiling point, down to what is probably near 

 freezing point. In the course of one of our days it descends 

 probably very far below our zero and remains there the rest 

 of the night. 



As a further consequence of the absence of both water and 

 atmosphere on the moon, and of the extremes of temperature, 

 it follows there can be no life upon it. It is believed by 

 astronomers that the conditions on the moon have never 

 been essentially more favorable for life than they are at 

 present, and that they will not become more favorable in 

 the future unless in some way its mass increases so that it 

 may hold an atmosphere around it by the power of its gravi- 

 tation. The moon is a dead world, which has pursued its 

 course around the earth and, with the earth, has revolved 

 around the sun through long ages. It illuminates the 

 earth part of the time with its feeble rays and produces 

 the tides, which rush in and out twice a day upon the 

 shores of the oceans. 



255. Eclipses. The earth casts a shadow, which extends, of 

 course, in the direction opposite to the sun. When the moon 

 passes into this shadow it is eclipsed, for then the direct 

 light of the sun is cut off. The moon would be eclipsed 

 every month at full moon if its orbit were not somewhat 

 inclined to that of the earth so that ordinarily it passes 

 above or below the earth's shadow. 



The moon too casts a shadow, which now and then 

 strikes on the earth and produces, in a limited region, an 



