142 REPORT — 1859. 



of more than |th of the moon's diameter, namely 0-0688 to the east, and the 

 same quantity to the west of the mean position. On account of perspective, 

 the effect of libration in longitude is much less apparent on the eastern and 

 western peripheral meridians, which shift towards the centre by a quantity 

 equal only to g4^th of the moon's diameter (0'004.8). 



The equator and its parallels, which at the period of mean Libration in 

 Latitude were straight lines, become more or less open ellipses under other 

 circumstances; the ratio between the conjugate and transverse axes of all 

 the parallels being constant for a given inclination of the lunar axis. At 

 a maximum libration in latitude the equator becomes an ellipse, whose con- 

 jugate axis is 0* 1 1 8 1 ; the transverse axis being equal to the diameter of the 

 moon considered as unity : so that a point in the centre of the equator is 

 shifted 0*059 of the diameter to the north or to the south by a maximum 

 northern or southern libration, and will move by the sum of these librations 

 to an apparent extent of ^th of the diameter of the lunar disc. The apparent 

 motion of the north and south poles towards the centre is on account of 

 perspective only yf^th of the diameter (0-0035). 



Libration in latitude also causes a change in the ellipses which delineate 

 the meridians, causing an inclination of their axes to the line joining the 

 poles, and also a change in the ratios of their transverse to their conjugate 

 axes. For example, the meridian distant 7° 55' from the centre (this being 

 the position of central meridian at a maximum libration in longitude) would 

 have its transverse axis inclined 0° 56'*3 to the pole, the conjugate axis being 

 no longer O'l 377 but 0-1368 of the transverse. The peripheral meridians 



