Lunar Theory

Lunar Theory explains the moon's motions in the heavens by mathematical reasoning, founded on the law of gravitation. If the earth were the onlv body which attracted the moon the theory would be very simple, but the attractive force of the sun causes many irregularities in the moon's path. The sun's attraction on the moon varies with the position of earth and moon in the earth's orbit, and with the position of the moon in her own orbit, so that the moon is a little behind or in front of the place she would occupy, if her path were a true ellipse with the earth in one focus. This same attractive force causes the moon's orbit to slightly alter its inclination to the ecliptic - the average inclination being about 5°. The moon's orbit, as a whole, moves round the earth every nine years, and her nodes (q.v.) move backwards on the ecliptic about 19J° every year. It will thus be seen that the sun's attractive force is alone sufficient to make the Lunar Theory extremely difficult and complex; but a further small inequality is due to the fact that the earth is not a perfect sphere. Venus also exerts a slight influence.