Rotation period
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.
{| border=1 cellspacing=0 cellpadding=2|
Sun|24.66 days (equator), about 35 days near the poles
|Mercury|58.6462 d (58 d 15.5088 h)
|Venus|243.0185 d
|Earth|0.997 268 d (23.9344 h) (86 164 s)
|Moon|27.321 661 d (synchronous)
|Mars|1.025 957 d (24.622 962 h)
|Jupiter|0.413 538 021 d (9 h 55 min 29.685 s)
|Saturn|0.444 009 259 2 d (10 h 39 min 22.400 00 s)
|Uranus|0.718 333 333 d (17 h 14 min 24.000 00 s)
|Neptune|0.671 250 00 d (16 h 6 min 36.000 00 s)
|Pluto|6.387 d (6 d 9 h 17.6 min)
