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Rotation period
From Wikipedia, the free encyclopedia
The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars. It differs from the planet's solar day, which includes an extra fractional rotation needed to accommodate the portion of the planet's orbital period during one day. Earth's rotation period differs from its sidereal day by the amount of precession in right ascension during one day (8.4 ms).[1]
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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 (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. 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.
| Planet | Rotation period[2][3] |
|---|---|
| Sun | 25.379995 days (25 days 9 hours 7 minutes 11.6 seconds) at equator about 35 days near the poles |
| Mercury | 58.646225199 days (58 days 15 hours 30 minutes 33.8572 seconds) |
| Venus | 243.018483986 days (243 days 0 hours 26 minutes 37.0164 seconds) |
| Earth | 0.9972696323 days (23 hours 56 minutes 4.0962 seconds) |
| Earth's Moon (synchronous) |
27.321613297 days (27 days 7 hours 43 minutes 7.3889 seconds) |
| Mars | 1.0259567565 days (24 hours 37 minutes 22.66376 seconds) |
| Jupiter | 0.413538021 days (9 hours 55 minutes 29.685 seconds) |
| Saturn | 0.444009259 days (10 hours 39 minutes 22.4 seconds) |
| Uranus | 0.718333333 days (17 hours 14 minutes 24 seconds) |
| Neptune | 0.671250000 days (16 hours 6 minutes 36 seconds) |
| Pluto | 6.387246004 days (6 days 9 hours 17 minutes 38.0547 seconds) |
- ^ Explanatory Supplement to the Astronomical Almanac, ed. P. Kenneth Seidelmann, Mill Valley, Cal., University Science Books, 1992, p.48, ISBN 0-935702-68-7.
- ^ Rotation and pole position for the Sun and planets Rotation period in days is 360° divided by the coefficient of d. In most cases, source precision is 0.0000001° or 0.0000001/360 ≈ 3×10−10 days, so most table entries are rounded to 1×10−9 days.
- ^ Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000PDF (215KB) pp7–8