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An effect in the orbits of celestial objects due to a commensurability in their orbital periods. If the ratio of their orbital periods is close to a small fraction, such as ⅓, ⅔, or ¾, the bodies are said to be in resonant orbits. Because of this, the bodies' mutual perturbations repeat at nearly the same points in their orbits, thereby building up to produce large oscillations, or even driving one of the bodies into another orbit. The Kirkwood gaps in the asteroid belt are places where asteroids would be in resonance with Jupiter. In Jupiter's satellite system, the mean motions of Io, Europa, and Ganymede are in resonance with each other, such that the three satellites can never line up on the same side of Jupiter. Other pairs of satellites such as Mimas and Tethys, Dione and Enceladus, and Titan and Hyperion are in resonance, the effects being to stabilize their orbits and produce in them oscillations of large amplitude and long period.

Subjects: Astronomy and Astrophysics.

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