Rotation period (astronomy)

The first one corresponds to the sidereal rotation period (or sidereal day), i.e., the time that the object takes to complete a full rotation around its axis relative to the background stars (inertial space).

For solid objects, such as rocky planets and asteroids, the rotation period is a single value.

For gaseous or fluid bodies, such as stars and giant planets, the period of rotation varies from the object's equator to its pole due to a phenomenon called differential rotation.

For objects that are not spherically symmetrical, the rotation period is, in general, not fixed, even in the absence of gravitational or tidal forces.

For example, Hyperion, a moon of Saturn, exhibits this behaviour, and its rotation period is described as chaotic.