The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) is as its sidereal rotation period the time that the object takes to complete a single revolution around its axis of rotation relative to the background stars, measured in sidereal time. This type of rotation period differs from the object's synodic rotation period (a solar day), measured in solar time, which may differ by a fractional or multiple rotation to accommodate the portion of the object's orbital period during one day.
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.
This section may be too technical for most readers to understand.(May 2015)
Earth's rotation period relative to the Sun (its mean solar day) consists of 86,400 seconds of mean solar time, by definition. Each of these seconds is slightly longer than an SI second because Earth's solar day is now slightly longer than it was during the 19th century, due to tidal deceleration. The mean solar second between 1750 and 1892 was chosen in 1895 by Simon Newcomb as the independent unit of time in his Tables of the Sun. These tables were used to calculate the world's ephemerides between 1900 and 1983, so this second became known as the ephemeris second. The SI second was made equal to the ephemeris second in 1967. 
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86164.098 903 691 seconds of mean solar time (UT1) (23h 56m 4.098 903 691s).   Earth's rotation period relative to the precessing or moving mean vernal equinox, its sidereal day, is 86164.090 530 832 88 seconds of mean solar time (UT1) (23h 56m 4.090 530 832 88s).  Thus the sidereal day is shorter than the stellar day by about 8.4 ms.  The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005  and 1962–2005.  Recently (1999–2005) the average annual length of the mean solar day in excess of 86400 SI seconds has varied between 0.3 ms and 1 ms, which must be added to both the stellar and sidereal days given in mean solar time above to obtain their lengths in SI seconds.
|Celestial objects||Rotation period with respect to distant stars, the sidereal period (compared to Earth days)||Synodic rotation period (solar day)||Apparent rotational period|
viewed from Earth
|Sun||25.379995 days (
35 days (high latitude)
|25d 9h 7m 11.6s
|~28 days at its equator |
|Mercury||58.6462 days ||58d 15h 30m 30s||176 days |
|Venus||−243.0226 days  ||−243d 0h 33m||−116.75 days |
|Earth||0.99726968 days  ||0d 23h 56m 4.0910s||1.00 days (24h 00m 00s)|
||27d 7h 43m 11.5s||29.530588 days  (equal to synodic orbital period, due to spin-orbit locking, a synodic lunar month)||none (due to spin-orbit locking)|
|Mars||1.02595675 days ||1d 0h 37m 22.663s||1.02749125  days|
|Ceres||0.37809 days ||0d 9h 4m 27.0s|
0.4135344 days (deep interior )
0.41007 days (equatorial)
0.4136994 days (high latitude)
|0d 9h 55m 30s
0d 9h 55m 29.37s 
0d 9h 50m 30s 
0d 9h 55m 43.63s 
|0.4135764 d (9 h 55 m 33 s) |
−0.00091 days (average, deep interior )
0.44401 days (deep interior )
0.4264 days (equatorial)
0.44335 days (high latitude)
|10h 33m 38s + 1m 52s
− 1m 19s  
0d 10h 39m 22.4s 
0d 10h 14m 00s 
0d 10h 38m 25.4s 
|0.43930 d (10 h 32 m 36 s) |
|Uranus||−0.71833 days  ||−0d 17h 14m 24s||−0.71832 d (−17 h 14 m 23 s) |
|Neptune||0.67125 days ||0d 16h 6m 36s||0.67125 d (16 h 6 m 36 s) |
||–6d 9h 17m 32s||−6.38680 d (–6d 9h 17m 0s) |
|Haumea||0.163145 days ||0d 3h 54m 56s|
- "Leap Seconds". United States Naval Observatory.
- "Useful Constants". Earth Orientation Parameters. International Earth Rotation and Reference Systems Service.
- Aoki, the ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time". Aoki, et al., " The new definition of Universal Time", Astronomy & Astrophysics 105 (1982) 359–361.
- Explanatory Supplement to the Astronomical Almanac, ed. P. Kenneth Seidelmann, Mill Valley, Cal., United States Naval Observatory University Science Books, 1992, p.48, ISBN 0-935702-68-7.
- "Excess of the duration of the day to 86400s ... since 1623". Earth Orientation Parameters. International Earth Rotation and Reference Systems Service. Archived from the original on 2008-10-03. Graph at end.
- "Variations in the duration of the day 1962–2005". Earth Orientation Parameters. International Earth Rotation and Reference Systems Service. Archived from the original on 2007-08-13.
- Phillips, Kenneth J. H. (1995). Guide to the Sun. Cambridge University Press. pp. 78–79. ISBN 978-0-521-39788-9.
- Allen, Clabon Walter & Cox, Arthur N. (2000). Allen's Astrophysical Quantities. Springer. p. 296. ISBN 0-387-98746-0.
- "ESO". ESO. Retrieved 2021-06-03.
- This rotation is negative because the pole which points north of the invariable plane rotates in the opposite direction to most other planets.
- Margot, Jean-Luc; Campbell, Donald B.; Giorgini, Jon D.; et al. (29 April 2021). "Spin state and moment of inertia of Venus". Nature Astronomy. arXiv: 2103.01504. Bibcode: 2021NatAs.tmp...74M. doi: 10.1038/s41550-021-01339-7. S2CID 232092194.
- "How long is a day on Venus?". TE AWAMUTU SPACE CENTRE. Retrieved 2021-06-03.
- Reference adds about 1 ms to Earth's stellar day given in mean solar time to account for the length of Earth's mean solar day in excess of 86400 SI seconds.
- Allen, Clabon Walter & Cox, Arthur N. (2000). Allen's Astrophysical Quantities. Springer. p. 308. ISBN 0-387-98746-0.
- Allison, Michael; Schmunk, Robert. "Mars24 Sunclock — Time on Mars". NASA GISS.
- Chamberlain, Matthew A.; Sykes, Mark V.; Esquerdo, Gilbert A. (2007). "Ceres lightcurve analysis – Period determination". Icarus. 188 (2): 451–456. Bibcode: 2007Icar..188..451C. doi: 10.1016/j.icarus.2006.11.025.
- Rotation period of the deep interior is that of the planet's magnetic field.
- Seligman, Courtney. "Rotation Period and Day Length". Retrieved June 12, 2021.
- Found through examination of Saturn's C Ring
- McCartney, Gretchen; Wendel, JoAnna (18 January 2019). "Scientists Finally Know What Time It Is on Saturn". NASA. Retrieved 18 January 2019.
- Mankovich, Christopher; et al. (17 January 2019). "Cassini Ring Seismology as a Probe of Saturn's Interior. I. Rigid Rotation". The Astrophysical Journal. 871 (1): 1. arXiv: 1805.10286. Bibcode: 2019ApJ...871....1M. doi: 10.3847/1538-4357/aaf798. S2CID 67840660.
- Kaiser, M. L.; et al. "Voyager Detection of Nonthermal Radio Emission from Saturn". Science.
- "Saturn § Orbit and Rotation". Wikipedia, The Free Encyclopedia.
- Lacerda, Pedro; Jewitt, David & Peixinho, Nuno (2008-04-02). "High-Precision Photometry of Extreme KBO 2003 EL61". The Astronomical Journal. 135 (5): 1,749–1,756. arXiv: 0801.4124. Bibcode: 2008AJ....135.1749L. doi: 10.1088/0004-6256/135/5/1749. S2CID 115712870. Retrieved 2008-09-22.