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A variation in the movement of a celestial object in its orbit about another which cannot be accounted for by their mutual gravitational attraction. Inequalities usually arise because of the perturbing forces of one or more other massive objects in the system. For example, the so-called great inequality in the orbital movements of the planets Jupiter and Saturn about the Sun is an oscillation in their heliocentric longitude with a period of some 900 years caused by their mutual perturbations and also by the nearly 2 : 5 resonance in their mean motions. See also Lunar Inequality.

Subjects: Astronomy and Astrophysics.

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