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inequality
- (math.) The statement that one quantity is less than or greater than another.
- (astron.) Any irregularity in an object's orbital motion that can't be explained as a result of the mutual gravitational attraction between the body and its primary. Inequalities in a planet's movement around the Sun, for example, are due mainly to perturbations by other planets.
The great inequality, also known as Laplace's period, is a slight rocking of both Jupiter and Saturn, as they trek around the ecliptic. Each oscillation takes some 840 to 960 years (the precise figure isn't known) and is mostly caused by the near 5:2 resonance of their orbits: Jupiter makes five complete trips around the Sun for every two (or thereabouts) of Saturn.
The lunar equality stems from several factors, which include the perturbing effects of the Sun and other planets, tidal forces, and the non-spherical shapes of Earth and the Moon.
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