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, 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] (equal to sidereal orbital period due to spin-orbit locking, a sidereal lunar month) 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] (synchronous with Charon) –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.

## References

1. ^ "Period". COSMOS - The SAO Encyclopedia of Astronomy. Retrieved 2023-08-03.
2. ^ Phillips, Kenneth J. H. (1995). Guide to the Sun. Cambridge University Press. pp. 78–79. ISBN 978-0-521-39788-9.
3. Allen, Clabon Walter & Cox, Arthur N. (2000). Allen's Astrophysical Quantities. Springer. p. 296. ISBN 0-387-98746-0.
4. ^ "ESO". ESO. Retrieved 2021-06-03.
5. ^ a b c This rotation is negative because the pole which points north of the invariable plane rotates in the opposite direction to most other planets.
6. ^ Margot, Jean-Luc; Campbell, Donald B.; Giorgini, Jon D.; et al. (29 April 2021). "Spin state and moment of inertia of Venus". Nature Astronomy. 5 (7): 676–683. arXiv:2103.01504. Bibcode:2021NatAs...5..676M. doi:10.1038/s41550-021-01339-7. S2CID 232092194.
7. ^ "How long is a day on Venus?". TE AWAMUTU SPACE CENTRE. Retrieved 2021-06-03.
8. ^ 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.
9. ^ a b Allen, Clabon Walter & Cox, Arthur N. (2000). Allen's Astrophysical Quantities. Springer. p. 308. ISBN 0-387-98746-0.
10. ^ Allison, Michael; Schmunk, Robert. "Mars24 Sunclock — Time on Mars". NASA GISS.
11. ^ 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.
12. ^ a b Rotation period of the deep interior is that of the planet's magnetic field.
13. Seligman, Courtney. "Rotation Period and Day Length". Retrieved June 12, 2021.
14. ^ Found through examination of Saturn's C Ring
15. ^ McCartney, Gretchen; Wendel, JoAnna (18 January 2019). "Scientists Finally Know What Time It Is on Saturn". NASA. Retrieved 18 January 2019.
16. ^ 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.
17. ^ Kaiser, M. L.; et al. (1980). "Voyager Detection of Nonthermal Radio Emission from Saturn". Science. 209 (4462): 1238–1240. Bibcode:1980Sci...209.1238K. doi:10.1126/science.209.4462.1238. hdl:2060/19800013712. PMID 17811197. S2CID 44313317.
18. ^ a b Abel, Paul (2013). "Saturn". Visual Lunar and Planetary Astronomy. The Patrick Moore Practical Astronomy Series. New York, NY: Springer. doi:10.1007/978-1-4614-7019-9_8.
19. ^ 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.
20. ^ T. A. Hromakina; I. N. Belskaya; Yu. N. Krugly; V. G. Shevchenko; J. L. Ortiz; P. Santos-Sanz; R. Duffard; N. Morales; A. Thirouin; R. Ya. Inasaridze; V. R. Ayvazian; V. T. Zhuzhunadze; D. Perna; V. V. Rumyantsev; I. V. Reva; A. V. Serebryanskiy; A. V. Sergeyev; I. E. Molotov; V. A. Voropaev; S. F. Velichko (2019-04-09). "Long-term photometric monitoring of the dwarf planet (136472) Makemake". Astronomy & Astrophysics. 625: A46. arXiv:1904.03679. Bibcode:2019A&A...625A..46H. doi:10.1051/0004-6361/201935274. S2CID 102350991.
21. ^ "JPL Small-Body Database Browser: 136199 Eris (2003 UB313)" (December 14, 2019, solution date). Archived from the original on April 12, 2016. Retrieved February 20, 2020.