488 Condition of the Moon's Surface. [October, 



position does not change much in three minutes. But then 

 a great change will have taken place in the moon's diurnal 

 course, simply because the moon's position with respect to 

 the equator, at any given phase, varies as the sun's does 

 (sometimes more, sometimes less, according to the position 

 of the nodes of the moon's orbit, but always to a con- 

 siderable degree), since the inclination of the moon's orbit to 

 the equator is never less than 18°; accordingly a long time 

 elapses before there is a close approach to identity in the 

 lunar and terrestrial conditions under which a lunar region 

 is observed. 



And it is to be noted that, when so far as the moon's 

 motion in her orbit is concerned there would otherwise be 

 a close approach to identity in the conditions, the continual 

 change in the inclination of the orbit causes a marked 

 difference in the elevation at which the moon is seen above 

 the horizon. 



If we add to these considerations the fact that the moon has 

 seasons, though they are not very marked, and that the sun's 

 elevation at lunar noon thus varies through an arc of about 

 3 , we see that a very long interval must elapse before there 

 is any very near approach to the conditions, lunar as well 

 as terrestrial, under which any lunar region is observed. 



As yet we have taken no account whatever of the lunar 

 librations. These occasion a distinct class of differences. 

 The varying solar elevation affects the actual aspect of any 

 lunar region as it would be seen from one and the same 

 standpoint ; and varying lunar elevations, by causing the moon 

 to be observed under different conditions of terrestrial atmo- 

 sphere, must manifestly produce varying effects. But the 

 lunar librations correspond to an actual change of place on 

 the observer's part. 



This would not be the place to give a full account of the 

 lunar librations. In fact, the subject would require much 

 more space than is here available. But there are certain 

 considerations which bear in a very important manner on 

 the question of change in the lunar surface. 



Let it be noticed, that the point which is at the centre of 

 the moon's disc when there is no. libration is carried by the 

 librations so as to occupy in turn every part of a lunar area 

 appreciably rectangular, some 15J wide in lunar longitude 

 and some 13^° wide in lunar latitude. Thus the lunar 

 region occupied by this point is viewed in every direction 

 corresponding to these limits. We see it square when it is 

 in its mean position, we see it tilted j\° on either side of its 

 mean position in longitude, and 6|° on either side of this 



