270 



Lunar Perspective. 



disc by which, the alternate visibility and concealment of the 

 polar regions are rendered evident. 



While the change of latitude produces an oscillation of the 

 spots from N". to S. and vice versa, accompanied by alterations 

 of form, the change of velocity of motion occasions a similar 

 oscillation from W. to E. and the reverse. Popularly speak- 

 ing, the moon always presents the same face to the earth, but 

 as in one part of her orbit, that which is nearest to the earth, 

 her motion is quickest, and in the opposite, that which is 

 farthest, her motion is slowest, it is clear that unless the point 

 on her surface cut by the line joining the centres of the earth 

 and moon could accommodate itself to this varying velocity it 

 must sometimes be to the W. and at others to the E. of a given 

 point. Let the ellipse j9 cac (Fig. 2) represent the moon's orbit ; 

 let o be the given point which will occupy the centre of the appa- 



^ Tig. 2. 

 rent disc at the nearest (perigee) and farthest (apogee) distance 

 of the moon from the earth ; let e o p and e o a represent the 

 line joining the centres of the earth and moon in each position, 

 and let o' and o" represent the points on the surface of the 

 moon cut by the lines e o' c and e o" c in two positions of the 

 moon, one intermediate between apogee and perigee, the other 

 intermediate between perigee and apogee. When the moon 

 is beginning to move quicker than the mean motion, the point 

 o is W. of the centre o of the apparent disc W. o o' E, but as 

 she comes up to perigee the point o is gradually transferred 

 towards the E., and occupies the centre of the apparent disc, at 

 the time of perigee when the motion is quickest ; but while it 



