The rotation period of an astronomical object is the time that 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.
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 giants, 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. 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). Hyperion, a satellite of Saturn, exhibits this behaviour, and its rotation period is described as chaotic.
Earth's rotation period relative to the Sun (its mean solar day) is 86,400 seconds of mean solar time. 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 acceleration. 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, misnamed 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.
Rotation period of selected objects 
|Celestial Objects||Rotation period|
|Sun||25.379995 days (equatorial)
35 days (high latitude)
|25d 9h 7m 11.6s
|Mercury||58.6462 days||58d 15h 30m 30s|
|Venus||–243.0187 days||–243d 0h 26m|
|Earth||0.99726968 days||0d 23h 56m 4.0910s|
(synchronous toward Earth)
|27d 7h 43m 11.5s
|Mars||1.02595675 days||1d 0h 37m 22.663s|
|Ceres||0.37809 days||0d 9h 4m 27.0s|
|Jupiter||0.4135344 days (deep interior)
0.41007 days (equatorial)
0.41369942 days (high latitude)
|0d 9h 55m 29.37s
0d 9h 50m 30s
0d 9h 55m 43.63s
|Saturn||0.44403 days (deep interior)
0.426 days (equatorial)
0.443 days (high latitude)
|0d 10h 39m 24s
0d 10h 14m
0d 10h 38m
|Uranus||–0.71833 days||–0d 17h 14m 24s|
|Neptune||0.67125 days||0d 16h 6m 36s|
(synchronous with Charon)
|–6d 9h 17m 32s
|Haumea||0.163145 days||0d 3h 54m 56s|
See also 
- Leap seconds by USNO
- IERS EOP Useful constants
- 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 and Astrophysics 105 (1982) 359–361.
- Explanatory Supplement to the Astronomical Almanac, ed. P. Kenneth Seidelmann, Mill Valley, Cal., University Science Books, 1992, p.48, ISBN 0-935702-68-7.
- IERS Excess of the duration of the day to 86400s … since 1623 Graph at end.
- IERS Variations in the duration of the day 1962–2005
- Rotation and pole position for the Sun and planets Rotation period in days is 360° divided by the coefficient of d.
- PDF (215KB) pp7–8
- Clabon Walter Allen and Arthur N. Cox (2000). Allen's Astrophysical Quantities. Springer. p. 296. ISBN 0-387-98746-0.
- This rotation is negative because the pole which points north of the ecliptic rotates in the opposite direction to most other planets.
- 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.
- Clabon Walter Allen and Arthur N. Cox (2000). Allen's Astrophysical Quantities. Springer. p. 308. ISBN 0-387-98746-0.
- 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.
- Pedro Lacerda, David Jewitt and Nuno Peixinho (2008-04-02). "High-Precision Photometry of Extreme KBO 2003 EL61". The Astronomical Journal 135 (5): 1749–1756. arXiv:0801.4124. Bibcode:2008AJ....135.1749L. doi:10.1088/0004-6256/135/5/1749. Retrieved 2008-09-22.