The Secondary Planets. 313 



since the creation ; whereas the satellites of the 

 other planets have only been known to astrono- 

 mers since the invention of telescopes. 



The moon completes her resolution in some- 

 what less than a month, during which period she 

 is once in conjunction with the sun, and once in 

 opposition. While the earth traverses not quite 

 a twelfth part of her orbit, that is, not the whole 

 of one of the signs of the zodiac, the moon com- 

 pletes her revolution or orbit round the earth. 



Since the moon has no other light than what 

 she receives from the sun, it follows that she can 

 never have more than one half of her surface 

 enlightened ; but it depends upon the relative 

 position of the spectator with regard to the sun, 

 whether more or less of the face of the moon 

 will appear enlightened. For, being of a globu- 

 lar figure, it depends upon this position what part 

 of her orb shall receive the rays of the sun in 

 such a manner as to reflect them back to the eye 

 of the spectator. These different appearances of 

 the moon are called her phases*. 



Thus, when the spectator is placed at T, be- 

 tween S 5 the sun, and moon, at L, (PL XXVIII. 

 fig. 119.) the whole side of the moon which is 

 opposed to him will be enlightened, and she is 



* These appearances will be pretty correctly represented 

 by moving an ivory ball suspended from a siring round 

 the flame of a candle, and observing in what manner the 

 light is reflected from different parts of its surface, accord- 

 ing to the position in which it is held. 



VOL. I. P 



