Lunar Perspective. 271 



continues quicker than the mean, the point o is transferred 

 still further towards the E., arriving at its limit when the 

 moon's motion begins to be slower than the mean, after which 

 the motion of the point o is towards the W., occupying the 

 centre of the visible disc at the time of apogee, and proceeding 

 still further W. until the motion is beginning to be quicker 

 than the mean. It is to be noted that the regularity now 

 described is greatly interfered with by the libration in latitude. 



That which is true of one point on the surface is true of 

 every other, so that all objects partake of this change, moving 

 E. during the period the moon is moving quicker, and moving 

 W. while she is moving slower than her mean motion, and from 

 this it follows that during the period of her quicker motion 

 certain objects near her E. margin are concealed, while other 

 objects near her W. margin are brought into view. On the 

 other hand, while passing through the slower part of her orbit 

 objects near the E. margin come into view while those near the 

 W. are gradually concealed. These changes of position, which 

 are accompanied by changes of form arising from a greater or 

 less foreshortening, constitute the phenomenon called libration 

 in longitude. 



As general results of the libration in latitude and longitude 

 it may be briefly stated — 



First. That when the moon is in perigee or apogee at the 

 passage of either node the apparent disc is in a state of mean 

 libration, for the line joining the centres of the earth and moon 

 cuts the moon's equator in the point which is equidistant from 

 the W. and E. limits of change of position arising from libra- 

 tion in longitude. It is from the line at right-angles to the 

 equator (the first meridian) that the longitudes of lunar objects 

 are reckoned. In consequence of the inequality of the motion 

 of the nodes, and that of the line p o e o a joining the perigean 

 and apogean points (the line of the apsides), a state of mean 

 libration can only occur once in three years. 



Second. That when the moon has N. latitude all the objects 

 on her visible surface are 1ST. of their mean, or normal positions, 

 and that when she has S. latitude they are S. of these positions. 



Third. That while the' moon is moving from apogee to 

 perigee all objects on her visible surface are W. of their normal 

 positions, and that while she is moving from perigee to apogee 

 they are E. of these positions. 



It may from these data be easy, by means' of lunar maps, 

 and taking from the Nautical Almanack, for any given time, her 

 latitude, and the sides of her orbit, to form a tolerable idea of 

 the apparent surface as to the proximity to the margin, or 

 otherwise, of the most salient features. 



In applying the above-mentioned principles and phenomena 



