THE MOON", PLANETS, AND COMETS 253 



IMPORTANT FACTS CONCERNING THE PLANETS 



NOTE. The moon (not a planet) has a diameter of 2163 miles. The sun 

 (a star) has a diameter of 866,540 miles. 



257. Mercury and Venus. The planets nearest the sun are 

 Mercury and Venus. The diameter of Mercury is a little less 

 than 3000 miles and, like the moon, its mass is so small that 

 it cannot retain an atmosphere. Since it is much nearer the 

 sun than the earth, it gets proportionately more light and 

 heat in fact nearly seven times as much per unit of area as 

 we receive. This fact, associated with the fact that it has no 

 atmosphere, means that the day side of Mercury is intensely 

 heated. Some vague markings which have been observed 

 upon its surface seem to indicate that it always presents the 

 same face toward the sun, just as the moon always presents 

 the same face toward the earth. If this is true one of its 

 sides is perpetually burned by the intense rays of the sun, 

 while the other is in perpetual night and, consequently, 

 very cold. 



The planet Venus has a diameter of . a little more than 

 7700 miles and is therefore nearly as large as the earth. 

 Observations show that it has an atmosphere comparable in 

 amount to that which surrounds the earth; for example, 

 when Venus passes between the sun and the earth it is seen 

 to be surrounded by an atmospheric ring which is illuminated 

 by the rays of the sun. Venus is in many respects similar 

 to the earth, and the possibility of its being inhabited by 



