1 INTRODUCTION TO ASTRONOMY. 



that she turns but once upon her axis while she performs a revolution 

 round the earth ; so that the inhabitants of the moon have but one day 

 and one night in the course of a lunar month. Since we always see the 

 same hemisphere of the moon, the inhabitants of that hemisphere alone 

 can perceive the earth. One half of the moon, therefore, enjoys our light 

 every night, while the other half has constantly nights of darkness ; and 

 we appear to the inhabitants of the moon under all the changes or phases 

 which the moon exhibits to us. 



Fig. 15. 



Fig. 16. 



These phases require some explanation. Injig. 16, let us say that S 

 represents the sun, E the earth, and ABCD, &c. the moon in different parts 

 of her orbit. When the moon is at A, her dark side being turned towards 

 the earth, we shall not see her ; but her disappearance is of very short 

 duration, and as she advances in her orbit we perceive her under the form 

 of a new moon : when she has gone through one-sixth of her orbit at B, 

 one quarter of her enlightened hemisphere will be turned towards the 



