Mr. W. R. Birt on the Moon's Libration. 



307 



axis, which does not, however, retain its position on the appa- 

 rent disk, but revolves around the central point. 



" This ellipse opens out and undergoes changes of form pro- 

 portional to the interval elapsing from the epoch of mean libra- 

 tion until the epoch when the greatest excursion of libration in 

 longitude towards the east (of the point of intersection of the 

 equator and the first meridian) coincides with the passage of the 

 ascending node when the equator is represented as a straight 

 line across the apparent disk and the first meridian by the curve 

 c E m in fig. 10, where the libration of the centre of the apparent 



Fig. 10. 



disk is nothing in latitude, but west in longitude. When the 

 first meridian returns to its normal position, the equator is re- 

 presented by the curve E" N q (fig. 10), and the point of inter- 

 section is situated at o u (nearly) ; the libration of the centre in 

 this case is nothing in longitude but south in latitude. 



" At this epoch, intermediate between two of mean libration, 

 the path of the point of intersection of the equator and first me- 

 ridian may be represented by the four diagonals, of which o* o" 

 (fig. 10) is one, or, perhaps more correctly, by a wavy ellipse ; 



Y2 



