PHYSICAL ASTRONOMY. 253 



tion already mentioned by the name of the E- 

 vection, 129. 



When the transverse axis of the Moon's orbit is di- 

 rected to the Sun, or lies in the line of the syzy- 

 gies, because the force, taken in syzygiesirom the 

 gravitation of the Moon, is, caeteris paribiis, pro- 

 portional to her distance from the Earth ; it is 

 greatest when the Moon is in the apogee, and least 

 when the Moon is in the perigee ; the greatest 

 possible diminution is therefore made from the 

 quantity that is already the least, and the least di- 

 minution from that which is already the greatest ; 

 their difference therefore is augmented, and the 

 motion at the apogee made slower than usual, and 

 at the perigee faster. When the line of the apsides 

 is in the quadratures, the contrary happens ; the 

 gravitation at the apogee is most augmented, and 

 at the perigee least augmented: the difference 

 therefore is diminished. 



In the former case, the equation of the centre appears 

 to be increased, and in the second it appears to be 

 diminished. The quantity is (1 .20 / .36 // ) mul- 

 tiplied into the sine of twice the Moon's angular 

 distance from the Sun, minus the mean anomaly of 

 the Moon. MAYER adds another term to the 

 evection, equal to -f- 26" multiplied into the sine 

 of double the preceding argument. Theoria Lu- 

 na, Lond. 1767, 50. 



254. The 



