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. Typically, the stated rotation period for a giant planet (such as Jupiter, Saturn, Uranus, Neptune) is its internal rotation period, as determined from the rotation of the planet's
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. As a result of this, the
moment of inertia of the object around the rotation axis can 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). For example,
Hyperion, a moon of
Saturn, exhibits this behaviour, and its rotation period is described as
chaotic. ==Rotation period of selected objects==