Commensurability (astronomy)

This article is about the meaning of 'commensurable' and derived words in astronomy. For other senses, see commensurability.

In astronomy, two orbiting objects such as planets, satellites or asteroids are said to be in exhibit commensurability if their orbital periods are in rational proportion (see also commensurability (mathematics)).

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

Commensurabilities are believed to be the result of orbital resonance effects.