Commensurability (astronomy)

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,^[1] the 3:4 commensurability between the orbital periods of the Saturnian satellites Titan and Hyperion,^[2] the orbital periods associated with the Kirkwood gaps in the asteroid belt relative to that of Jupiter,^[3] and the 2:1 commensurability between Gliese 876 b and Gliese 876 c.

Commensurabilities are normally the result of an orbital resonance, rather than being due to coincidence.

See also

• Harmonic
• Ratio