ON MAPPING THE SURFACE OF THE MOON. 



229 



The distortion on the orthographical projection arises from the greater 

 foreshortening of objects near the limb, as seen from the earth, than the 

 true orthographical projection will represent. 



20. During the passage of the moon from perigee, at which point her 

 motion is quickest, to apogee, where it is slowest, the motion in her orbit 

 is shiver from day to day, while her motion in rotation continues uniform ; 

 the consequence is, that while passing from perigee to mean distance the 

 first meridian is transferred eastwardly (see fig. 7), which is inverted, where 

 E" represents the earth, W P E o the moon's equator when she is in perigee, 

 being its intersection with the first meridian, W o' E' the segment of the 



Fig. 7. 



moon's equator presented to the earth at a given distance from perigee, c o'. 

 a radius from the moon's centre to the first meridian, the angle E"co'= 

 the quantity gained by the axial over the orbital motion = the difference 

 between .the moon's true and mean longitudes nearly =librati on in longi- 

 tude, by which the western portions come into view, and the first meridian 



Pig. 8 



