268 Lunar Perspective. 



goes apparent changes of shape as well as position — within 

 narrow limits — during the nioon's passage above the horizon ; 

 and, near the margin, objects that were invisible at the time of 

 the moon's rising, will be seen at the time of her setting. This 

 phenomenon is called the diurnal libration. 



The condition of Rhseticus being in the zenith of an 

 observer on the earth, depends on the moon's passage of either 

 the ascending or descending node, but as the passage of the 

 node may occur with any degree of the moon's declination 

 north or south, the observer who has had Rhasticus in his zenith 

 in one lunation will not have it in the next ; he may be either 

 N.or S. of the point on the earth's surface cut by the line joining 

 the centres of the earth and moon. In one case, the N. interior 

 slope will be foreshortened; and in the other, the S. interior 

 slope. 



This foreshortening in a N. and S. direction will be aug- 

 mented by the positions of observers on the earth's surface, in 

 proportion as they are removed from the earth's equator. In 

 high latitudes, and towards the poles, where the moon attains but 

 a low altitude at meridian passage, the greatest foreshortening 

 of lunar objects will be observed, especially in those regions 

 which are removed farthest from the eye, in consequence of the 

 inclination of the moon's equator to the plane of the earth's 

 equator. From this it follows, that no two observers on the 

 earth's surface will see any given lunar object of exactly the 

 same form, or in exactly the same position on the disc. 



The above-mentioned changes in form and position are 

 slight compared with those that result from the inclination of 

 the moon's orbit and axis to the ecliptic, combined with the 

 varying velocity of her motion in her orbit. Bearing in mind 

 that the line joining the centres of the earth and moon will cut 

 different points of the surfaces of both bodies at different times, 

 it is evident that a spot, such as Rhaeticus, will have an oscilla- 

 tory motion, or one allied to it, during every lunation, or interval 

 from one new moon to the succeeding. It is when the moon 

 is in either node that Rhoaticus will be seen on a line dividing 

 the apparent disc into two equal parts. As the moon moves 

 N. of the ecliptic Rhaaticus appears to move on the moon's disc 

 towards the N., and the regions in the neighbourhood of the 

 moon's south pole come into view, the result being that 

 Rhasticus, with all the objects in the moon's 1ST. hemisphere, 

 are more foreshortened than at the time of the passage of the 

 node, while those in the S. hemisphere are less, a few coming 

 into such positions that they may be viewed in the zenith at 

 meridian passage without foreshortening. 



As soon as the moon has attained her greatest N. latitude 

 she begins to return towards the ecliptic, and at the same time 



