A departure in the Moon's orbital motion from its behaviour according to Kepler's laws. It results not only from the perturbing presence of the Sun and the planets, but also from tidal forces and the non-spherical shapes of the Earth and Moon. Lunar theory takes all these factors into account to produce a mathematical description of the Moon's past, present, and future geocentric positions and velocities over thousands of years. Such a description contains not only terms describing the elliptical orbit of the Moon, but also hundreds of periodic terms—*inequalities—due to those other factors. Among those factors are evection, variation (1), and the annual equation.
Subjects: Astronomy and Astrophysics.