Commensurability (astronomy)

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Commensurability is the property of two orbiting objects, such as planets, satellites, or asteroids, whose orbital periods are in a rational proportion.

Examples include the 2:3 commensurability between the orbital periods of Neptune and Pluto, the 3:4 commensurability between the orbital periods of the Saturnian satellites Titan and Hyperion, the orbital periods associated with the Kirkwood gaps in the asteroid belt relative to that of Jupiter, and the 2:1 commensurability between the extrasolar planets Gliese 876b and Gliese 876c.

Commensurabilities are believed to result in most cases from orbital resonance effects, rather than being due to coincidence.

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