34 DISTURBING ACTION OF THE SUN. SECT. V. 



ereases or diminishes her gravity to the earth. The 

 second, acting in the direction of a tangent to her orbit, 

 disturbs her motion in longitude ; and the .third, acting 

 perpendicularly to the plane of her orbit, disturbs her 

 motion in latitude that is, it brings her nearer or re- 

 moves her farther from the plane of the ecliptic than 

 she would otherwise be. The periodic perturbations' 

 in the moon arising from these forces, are perfectly sim- 

 ilar to the periodic perturbations of the planets. But 

 they are much greater and more numerous ; because 

 the sun is so large, that many inequalities which are 

 quite insensible in the motions of the planets, are of 

 great magnitude in those of the moon. Among the in- 

 numerable periodic inequalities to which the moon's 

 motion in longitude is liable, the most remarkable are, 

 the Equation of the Center, which is the difference be- 

 tween the moon's mean and true longitude, the Evec- 

 tion, the Variation, and the Annual Equation. The 

 disturbing force which acts in the line joining the moon 

 and earth produces the Evection : it diminishes the ec- 

 centricity of the lunar orbit in conjunction and opposi- 

 tion, thereby making it more circular, and augments it 

 in quadrature, which consequently renders it more ellip- 

 tical. The period of this inequality is less than thirty- 

 two days. Were the increase and diminution always 

 the same, the Evection would only depend upon the 

 distance of the moon from the sun ; but its absolute 

 value also varies with her distance from the perigee 

 (N. 102) of her orbit. Ancient astronomers, who ob- 

 served the moon solely with a view to the prediction of 

 eclipses, which can only happen in conjunction and oppo- 

 sition, where the eccentricity is diminished by the Evec- 

 tion, assigned too small a value to the ellipticity of her 

 orbit (N. 193). The Evection was discovered by Ptole- 

 my from observation, about A.D. 140. The variation 

 produced by the tangential disturbing force, which is 

 at its maximum when the moon is 45 distant from the 

 sun, vanishes when that distance amounts to a quadrant, 

 and also when the moon is in conjunction and opposi- 

 tion ; consequently, that inequality never could have 

 been discovered from the eclipses : its period is half a 

 lunar month (N. 104). The Annual Equation depends 



