In astronomy, a rotation period is the time an astronomical object takes to complete one revolution around its rotation axis. For solid objects, such as rocky planets and asteroids, the rotation period is a single value. For gaseous/fluid bodies, such as stars and gas giant planets, the period of rotation varies from the equator to the poles, called differential rotation. Typically, the stated rotation period for a gas giant (ie, Jupiter) is the internal rotation period, as determined from the rotation of the magnetic field.

For objects that are not spherically symmetrical, the rotation period is in general not fixed, even in the absence of gravitational or tidal forces. This is because, although the rotation axis is fixed in space (by the conservation of angular momentum), it is not necessarily fixed in the body of the object itself. The moment of inertia of the object around the rotation axis can therefore vary, and hence the rate of rotation can vary (because the product of the moment of inertia and the rate of rotation is equal to the angular momentum, which is fixed). Hyperion, a satellite of Saturn, exhibits this behaviour, and its rotation period is described as chaotic.

Rotation period of selected objects

See also

• Synchronous rotation
• Prograde and retrograde motion

Astronomy

Период на въртене | Període de rotació | Période de rotation | Period rotacije | Periodo di rotazione | Rotasjonsperiode | Rotačná perióda | 自轉週期