272 Lunar Perspective. 



to the illustration of lunar perspective, it is essential to change 

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

 joining the eye of the observer and the moon's centre. It is 

 clear that in this, as in the former case, the point on the moon's 

 surface cut by this line will be the centre of the apparent disc, 

 and here it may be proper to remark that the numerical ex- 

 pressions of the values of the libration of the apparent disc in 

 latitude and longitude have reference only to the centre of the 

 apparent disc, as seen from a given point on the earth's surface 

 at a given time. These expressions are simply the latitude and 

 longitude of this centre — thus, if the libration in latitude be 

 equal to 3° N. and that in longitude to 2° W., the meaning is 

 that the centre of the apparent disc is 3° N. of the equator, and 

 2° W. of the first meridian — the equator being S. of its normal 

 position and the first meridian E. 



Returning to the consideration of perspective. The smallest 

 amount of foreshortening takes place at the centre of the disc, 

 and as the observer is looking upon a globe, the foreshortening 

 rapidly increases towards the margin in every direction. Lunar 

 objects are of all conceivable shapes and sizes, from the exten- 

 sive Maria to the smallest discernible hillocks and pits. They 

 will, however, undergo apparent changes of form and position, 

 in accordance with well recognized laws. Most objects of any 

 size will — as they are found near to or removed from the 

 margin — be presented to the eye under elliptical forms, gene- 

 rally of great irregularity, all the shorter axes being directed 

 towards the centre, and all the longer axes being portions of 

 curves more or less concentric with the margin. In conse- 

 quence of the inequality of the moon's motions, above-mentioned, 

 the approach and recess of the spots towards and from the 

 margin are very irregular, bringing prominently into view at 

 some seasons certain features, and at others concealing them ; 

 thus it arises that no two drawings of a lunar object at different 

 epochs, by the same observer, or by two observers, even at the 

 same epoch, agree fully in detail. It is true that an experienced 

 eye will detect manifest errors, but with the most assiduous 

 attention capable of producing the most scrupulous accuracy, 

 differences must occur as the results of differing perspective. 

 This is very evident from an inspection of a collection of photo- 

 graphs taken at different states of libration. The well-known 

 elliptical spot the Mare Crisium is seen, when the moon is 

 moving from apogee to perigee, to approach somewhat closely 

 to the margin, the surface in the direction of the shorter axis 

 being much contracted, and in a particular position the N. 

 boundary appears as a straight line of mountains. During the 

 period when the moon moves from perigee to apogee the Mare 

 Crisium attains its greatest distance from the margin, the finely 



