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, which, despite its name, is not relative to the stars but is relative to the northward vernal equinox, which moves slowly across the celestial sphere. The component of this precession along the celestial equator during one day, and hence the difference between Earth's rotation period and its sidereal day is 8.4 ms.[1]
Contents |
[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 (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. 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). Hyperion, a satellite of Saturn, exhibits this behaviour, and its rotation period is described as chaotic.
[edit] Rotation period of selected objects
| Planet | Rotation period |
|---|---|
| Sun | 25.379995 days at equator, about 35 days near the poles[2][3] |
| Mercury | 1047.51 hours[4] |
| Venus | -5832.444 hours[4] |
| Earth | 23.93419 hours[4] |
| Moon | 655.72 hours (synchronous toward Earth)[4] |
| Mars | 24.622962 hours[4] |
| Jupiter | 9.92425 hours[4] |
| Saturn | 10.65622 hours[4] |
| Uranus | -17.24 hours[4] |
| Neptune | 16.11 hours[4] |
| Pluto | -153.29 hours[4] |
[edit] See also
[edit] References
- ^ 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.
- ^ Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000PDF (215KB) pp7–8
- ^ a b c d e f g h i j Murray, Carl D. and Stanley F. Dermott (1999). Solar System Dynamics. Cambridge UP. ISBN 0-521-57295-9.
