Lunar Perspective. 267 



from JEgilops ovata as a very likely solution of the question. 

 At the same time we would strongly recommend those who 

 have doubts on the point to repeat the experiments for their 

 own satisfaction, from which even now some fresh ideas might 

 be had or knowledge obtained. 



LUNAR PERSPECTIVE. 



BY W. E. BIRT, F.R.A.S. 



Every observer of the moon's disc is well acquainted with the 

 apparent changes which the features undergo from time to 

 time, especially those near the margin ; sometimes they appear 

 close to the edge of the disc, and at others they are removed 

 to a moderate distance from it, considerable alterations of form 

 accompanying these oscillations of position. As these changes, 

 both in position and form, depend upon the phenomenon known 

 as the moon's libration, it may not be uninteresting if we 

 attempt to explain the principles upon which they are effected. 



The fundamental idea to be apprehended is simply this. 

 At any given moment a line will join the centres of the earth 

 and moon, and as both bodies are globular, this line will cut 

 through a point in the surface of each. At the point on the 

 earth's surface cut by this line let an observer be placed, and 

 at the point on the moon's surface let there be a spot that can 

 have the earth in its zenith. Rhaeticus, which is situated on 

 the moon's equator, is such a spot, and it is clear that, in the 

 case suggested, Rhgeticus and the observer will be respectively 

 in each other's zenith, the crater occupying the middle of the 

 moon's disc as seen from the earth. From this it will neces- 

 sarily follow that the observer on the earth, looking through 

 his telescope, directed to the zenith, will see the crater Rhseticus 

 of nearly its true form, for the depth of the crater being very 

 minute, as compared with the distance of the observer, per- 

 spective will scarcely interfere. 



The perception of the true form of Rhseticus, with its 

 interior shelving sides, will, however, be but momentary, viz., 

 at the time of the moon's passing the meridian. While she is 

 E. of the meridian the observer will see the interior E. slope 

 wider than when looking directly into the crater at meridian 

 passage, the interior W. slope being foreshortened. After 

 meridian passage, the E. slope is foreshortened and the W. is 

 viewed more directly, and from these phenomena it may be 

 easily deduced that every object on the moon's surface under- 



