In astronomy, the rotation period or spin period[1] of a celestial object (e.g., star, gas giant, planet, moon, asteroid) has two definitions. The first one corresponds to the sidereal rotation period (or sidereal day), i.e., the time that the object takes to complete a full rotation around its axis relative to the background stars (inertial space). The other type of commonly used "rotation period" is the object's synodic rotation period (or solar day), which may differ, by a fraction of a rotation or more than one rotation, to accommodate the portion of the object's orbital period around a star or another body during one day.
Measuring rotation
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 gas giants, 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 gas giant (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.[citation needed] 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
| Celestial objects | Rotation period with respect to distant stars, the sidereal rotation period (compared to Earth's mean Solar days) | Synodic rotation period (mean Solar day) | Apparent rotational period viewed from Earth | |
|---|---|---|---|---|
| Sun* | 25.379995 days (Carrington rotation) 35 days (high latitude) | 25d 9h 7m 11.6s 35d | ~28 days (equatorial)[2] | |
| Mercury | 58.6462 days[3] | 58d 15h 30m 30s | 176 days[4] | |
| Venus | −243.0226 days[5][6] | −243d 0h 33m | −116.75 days[7] | |
| Earth | 0.99726968 days[3][8] | 0d 23h 56m 4.0910s | 1.00 days (24h 00m 00s) | |
| Moon | 27.321661 days[9] | 27d 7h 43m 11.5s | 29.530588 days[9] (equal to synodic orbital period, due to spin-orbit locking, a synodic lunar month) | none (due to spin-orbit locking) |
| Mars | 1.02595675 days[3] | 1d 0h 37m 22.663s | 1.02749125[10] days | |
| Ceres | 0.37809 days[11] | 0d 9h 4m 27.0s | 0.37818 days | |
| Jupiter | 0.41354 days(average) 0.4135344 days (deep interior[12]) 0.41007 days (equatorial) 0.4136994 days (high latitude) | 0d 9h 55m 30s[3] 0d 9h 55m 29.37s[3] 0d 9h 50m 30s[3] 0d 9h 55m 43.63s[3] | 0.41358 d (9 h 55 m 33 s)[13] (average) | |
| Saturn | 0.44002+0.00130 −0.00091 days (average, deep interior[14]) 0.44401 days (deep interior[12]) 0.4264 days (equatorial) 0.44335 days (high latitude) | 10h 33m 38s + 1m 52s − 1m 19s [15][16] 0d 10h 39m 22.4s[17] 0d 10h 13m 59s[18] 0d 10h 38m 25.4s[18] | 0.43930 d (10 h 32 m 36 s)[13] | |
| Uranus | −0.71833 days[3][5] | −0d 17h 14m 24s | −0.71832 d (−17 h 14 m 23 s)[13] | |
| Neptune | 0.67125 days[3] | 0d 16h 6m 36s | 0.67125 d (16 h 6 m 36 s)[13] | |
| Pluto | −6.38718 days[3][5] | –6d 9h 17m 32s | −6.38680 d (–6d 9h 17m 0s)[13] | |
| Haumea | 0.1631458 ±0.0000042 days[19] | 0d 3h 56m 43.80 ±0.36s | 0.1631461 ±0.0000042 days | |
| Makemake | 0.9511083 ±0.0000042 days[20] | 22h 49m 35.76 ±0.36s | 0.9511164 ±0.0000042 days | |
| Eris | ~1.08 days[21] | 25h ~54m | ~1.08 days | |
* See Solar rotation for more detail.
See also
References
External links
- Murray, Carl D. & Dermott, Stanley F. (1999). Solar System Dynamics. Cambridge University Press. p. 531. ISBN 0-521-57295-9.Note, the rotation periods for Mercury and Earth in this work may be inaccurate.
