Rotation period

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In astronomy, a rotation period is the time an astronomical object takes to complete one revolution around its rotation axis.

1 Measuring rotation
2 Rotation period of selected objects
3 See also
4 Sources

[edit] Measuring rotation

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 due to a phenomenon 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 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. 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 is different from period of revolution, which is the amount of time it takes one object to complete one orbit around a second object. Thus, the earth has a rotation period of about 24 hours, and a period of revolution of about 365 days. Since the number of earth days in an earth year (rotations per revolution) is not a whole number (= about 364.24 days per year), leap years (and occasional leap seconds) are needed to reconcile the earth calendar. On the other hand, earth's moon has a rotation period that is exactly equal to its period of revolution around the earth since the same side of the moon always faces the earth; this is called synchronous rotation.

[edit] Rotation period of selected objects

Planet	Rotation Period
Sun	25 days 9 hours 7 minutes 13 seconds (25.38 days) (equator), about 35 days near the poles
Mercury	58 days 15.5088 hours (58.6462 days)
Venus	243.0185 days
Earth	0.997 270 days (23.934 47 h or 86 164 seconds)
Earth's Moon	27.321 661 days (synchronous)
Mars	24.622 962 hours (1.025 957 days)
Jupiter	9 hours 55 minutes 29.685 seconds (0.413 538 021 days)
Saturn	10 hours 39 minutes 22.4 seconds (0.444 009 259 2 days)
Uranus	97 hours 29 minutes 54 seconds
Neptune	16 hours 6 minutes 36 seconds (0.671 250 00 days)
Pluto	6 days 9 hours 17.6 minutes (6.387 days)