would be about 13 days of continued light alternately with 13 days of con- 

 tinued darkness ; the analogy, then, which prevails among the planets with 

 regard to days and nights, and which forms a main argument in favor of the 

 conclusion that they are inhabited globes like the earth, does not ho'i good in 

 the case of the moon. 



Although as a general proposition it be true that the same hemisphere of the i 

 moon is always turned toward the earth, yet there are small variations at the / 

 edge called librations, which it is necessary to notice. The axis of the moon is < 

 not exactly perpendicular to its orbit, but is inclined at a small angle. By rea- \ 

 son of this inclination, the northern and southern poles of the moon lean al- 

 ternately in a slight -degree to and from the earth. 



When the north pole leans toward the earth, we see a little more of that re- 

 gion, and a little less when it leans the contrary way. This variation in the 

 northern and southern regions of the moon visible to us, is called the libration 

 in latitude. 



In order that in a strict sense the same hemisphere should be continually 

 turned toward the earth, the time of rotation of the earth upon its axis must not 

 only be equal the time of rotation in its orbit, which in fact it is, but its 

 angular velocity on its axis in every part of its course, must be exactly equal to 

 its angular velocity on its orbit. Now it happens that while its angular ve- 

 locity on its axis is rigorously uniform throughout the month, its angular ve- 

 locity in its orbit is subject to a slight variation ; the consequence of this is 

 that a little more of its eastern or western edge is seen at one time than at 

 another. This is called the libration in longitude. 



By the diurnal motion of the earth, we are carried with it round its axis ; the 

 stations from which we view the moon in the morning and the evening, or rather 

 when it rises, and when it sets, are then different according to the latitude of 

 the earth in which we are placed. By thus viewing it from different places, 

 we see it under slightly different aspects. This is another cause of a variation, 

 which we see in its eastern and western edges ; this is called the diurnal 

 libration. 



PHASES OF THE MOON. 



While the moon revolves round the earth, its illuminated hemisphere is al- 

 ways presented to the sun ; it therefore takes various positions in reference to 

 the earth. In the annexed diagram the effects of this are exhibited. Let S repre- 

 sent the sun, and T the earth ; when the moon is at A, between the sun and the 

 earth, its illuminated hemisphere being turned toward the sun, its dark hemi- 

 sphere will be presented toward the earth ; it will therefore be invisible. In 

 this position the moon is said to be in conjunction. When it moves to the po- 

 sition B, the enlightened hemisphere being still presented to the sun, a small 

 portion of it only is turned to the earth, and it appears as a thin crescent, as 

 represented at b. When the moon takes the position of C. at right angles to 

 the sun, it is said to be in quadrature ; one half of the enlightened hemisphere 

 only is then presented to the earth, and the moon appears halved, as represented 

 at c. When it arrives at the position D, the greater part of the enlightened 

 portion is turned to the earth, and it is gibbous, appearing as represented at d. 

 When the moon comes in opposition to the sun, as seen at E, the enlightened 

 hemisphere is turned full toward the earth, and the moon will appear full, un- 

 less it be obscured by the earth's shadow, which rarely happens. In the same 

 manner it is shown that at F it is again gibbous ; at G it is halved, and at H 

 it is a crescent. 



When the moon is full, being in opposition to the sun, it will necessarily be 



