38 FIRST YEAR COURSE IN GENERAL SCIENCE 



see only the thin edges of the rings. Saturn has ten 

 moons, some of which are very small. 



31. The Moon. Except the sun, the moon is the 

 most important to the earth of all the heavenly bodies. It is 



our nearest neighbor, being less than a 

 quarter of a million miles away. It is 

 so familiar a sight that, except at the 

 time of new or full moon, we scarcely 

 remark upon it. The moon is the only 

 satellite of the earth. Its diameter is 

 about 2,000 miles, one quarter of the 

 earth's diameter. No other planet has 

 1914 a satellite so large in proportion to its 

 own size. 



Like the satellites of the other 

 planets, the moon revolves in a nearly 

 circular orbit. It takes almost twenty- 

 eight days to go once around the earth 

 and therefore makes about thirteen 

 revolutions a year. While the moon 

 makes one journey around its orbit, it 

 rotates only once upon its axis. Because 

 the moon's period of revolution is the same as its period of 

 rotation, it always turns the same side toward the earth. 

 As the diagram shows (Fig. 11), when the moon has gone one 

 quarter of the distance around the earth, it has turned upon 

 its axis one quarter of the way. Thus, though each portion 

 of the moon is in a different position with relation to the 

 sun, the same side of the moon is always turned toward 

 the earth, around which it is revolving. No one ever saw 

 the other side of the moon. 



32. Changes in the Apparent Form of the Moon. 

 The moon, like the planets and the other satellites, is nearly 

 spherical in form, but it does not always appear so. At 

 some times it looks like a disk and at others a half-disk, and 



1907 



1909 



1911 



1913 



FIG. 10. SATURN 

 1. From which posi- 

 tion of Saturn should 

 we receive the most 

 light? Why? 2. How 

 many years are there 

 between the brightest 

 and the least bright 

 appearances of Saturn? 

 3. Does its brightness 

 change suddenly? 



