Lunar Perspective. 269 



the spots in the N. hemisphere commence their return to the 

 equator of the apparent disc,, becoming less and less fore- 

 shortened in their progress. After the moon has passed her 

 descending node certain of the N. objects are seen on the S. 

 part of the moon's disc ; and they, with all the spots in the 

 S. hemisphere, become more foreshortened until the greatest 

 S. latitude is attained. At this time the regions about the S. 

 pole, which were seen when the moon was in the opposite part 

 of her orbit, are concealed, and corresponding regions in the 

 neighbourhood of the moon's N. pole become visible. Upon 

 the moon's return towards the ecliptic, with a motion from S. 

 to N., all the objects on her surface partake of the same, the 

 N. polar regions are gradually concealed, while the S. polar are 

 as gradually brought into view. From these phenomena it 

 follows that during the period that the moon's latitude is 

 becoming more and more N., viz., from her greatest S. to her 

 greatest N. latitude, the whole of the objects on her surface, 

 visible to the earth, have a motion across her disc in the same 

 direction, i.e., from S. to N., some going out while others are 

 coming into view, and during the period that her latitude is 

 becoming more and more S., viz., from her greatest N. to her 

 greatest S. latitude, the same objects have a motion from N. to 

 S. This libratory motion is called the moon's libration in 

 latitude, which may be rendered more intelligible by the an- 

 nexed diagram (Fig. 1), in which .57 will represent the centre of the 



Fie. 1. 



earth, n s the moon's N. and S. poles, q q the moon's equator, 

 m E the line joining the centres at her greatest N. latitude, and 

 c e c the ecliptic ; the accented letters have the same significa- 

 tion when the moon is at her greatest S. latitude, the dotted 

 line in each case represents half the boundary of the visible 



