The ratio between the
orbital periods of two bodies. Expressed as a
dimensionless ratio
> 1.
Alternately: The
inverse of the ratio the orbital
frequencies.
Whole number orbital resonances with large bodies are often unstable, owing to undue gravitational influences. In the
asteroid belt there are gaps corresponding to certain whole number orbital resonances with
Jupiter: i.e. 2:1, 3:1, 5:2, 7:2. The
Cassini gap appears in the rings of
Saturn, produced by an orbital resonances of 2:1 with the inner moon
Mimas.
As
Grzcyrgba points out, the nearly 2:1 orbital resonance between
Europa and
Io is responsible for the tremendous tidal forces that cause Io's
volcanism.