SKCT. v.] LUNAH THEORY. 39 



SECTION V. 



Lunar Theory Periodic Perturbations of the Moon Equation of Centre 

 Evection Variation Annual Equation Direct and Indirect Action of 

 Planets The Moon's Action on the Earth disturbs her own Motion 

 Excentricity and Inclination of Lunar Orbit invariable Acceleration 

 Secular Variation in Nodes and Perigee Motion of Nodes and Perigee 

 inseparably connected with the Acceleration Nutation of Lunar Orbit 

 Form and Internal Structure of the Earth determined from it Lunar, 

 Solar, and Planetary Eclipses Occultations and Lunar Distances Mean 

 Distance of the Sun from the Earth obtained from Lunar Theory 

 Absolute Distances of the Planets, how found. 



OUR constant companion, the moon,next claims ourattention. 

 Several circumstances concur to render her motions the most 

 interesting, and at the same time the most difficult to investi- 

 gate, of all the bodies of our system. In the solar system, 

 planet troubles planet ; but, in the lunar theory, the sun is the 

 great disturbing cause, his vast distance being compensated 

 by his enormous magnitude, so that the motions of the moon 

 are more irregular than those of the planets ; and, on account 

 of the great ellipticity of her orbit, and the size of the sun, the 

 approximations to her motions are tedious and difficult, be- 

 yond what those unaccustomed to such investigations could 

 imagine. The average distance of the moon from the centre 

 of the earth is only 237,360 miles, so that her motion among 

 the stars is perceptible in a few hours. She completes a cir- 

 cuit of the heavens in 27 d 7 h 43 m 4 S '7, moving in an orbit 

 whose excentricity is about 12,985 miles. The moon is about 

 four hundred times nearer to the earth than the sun. The 

 proximity of the moon to the earth keeps them together. 

 For so great is the attraction of the sun, that, if the moon were 

 farther from the earth, she would leave it altogether, and 

 would revolve as an independent planet about the sun. 



