Rotation period (astronomy) explained

In astronomy, the rotation period or spin period^[1] of a celestial object (e.g., star, 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 giant planets, 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 giant planet (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. 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)			Apparent rotational period viewed from Earth
Sun	25.379995 days (Carrington rotation) 35 days (high latitude)	25^d 9^h 7^m 11.6^s 35^d		~28 days (equatorial)^[2]
Mercury	58.6462 days^[3]	58^d 15^h 30^m 30^s	176 days^[4]
Venus	−243.0226 days^[5] ^[6]	−243^d 0^h 33^m	−116.75 days^[7]
Earth	0.99726968 days^[8]	0^d 23^h 56^m 4.0910^s	1.00 days (24^h 00^m 00^s)
Moon	27.321661 days^[9] (equal to sidereal orbital period due to spin-orbit locking, a sidereal lunar month)	27^d 7^h 43^m 11.5^s	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	1^d 0^h 37^m 22.663^s	1.02749125^[10] days
Ceres	0.37809 days^[11]	0^d 9^h 4^m 27.0^s	0.37818 days
Jupiter	0.41354 days(average) 0.4135344 days (deep interior^[12]) 0.41007 days (equatorial) 0.4136994 days (high latitude)	0^d 9^h 55^m 30^s 0^d 9^h 55^m 29.37^s 0^d 9^h 50^m 30^s 0^d 9^h 55^m 43.63^s	(9 h 55 m 33 s)^[13] (average)
Saturn	days (average, deep interior^[14]) 0.44401 days (deep interior) 0.4264 days (equatorial) 0.44335 days (high latitude)	 ^[15] ^[16] 0^d 10^h 39^m 22.4^s^[17] 0^d 10^h 13^m 59^s^[18] 0^d 10^h 38^m 25.4^s	(10 h 32 m 36 s)
Uranus	−0.71833 days	−0^d 17^h 14^m 24^s	(−17 h 14 m 23 s)
Neptune	0.67125 days	0^d 16^h 6^m 36^s	(16 h 6 m 36 s)
Pluto	−6.38718 days (synchronous with Charon)	–6^d 9^h 17^m 32^s	(–6^d 9^h 17^m 0^s)
Haumea	0.1631458 ±0.0000042 days^[19]	0^d 3^h 56^m 43.80 ±0.36^s	0.1631461 ±0.0000042 days
	0.9511083 ±0.0000042 days^[20]	22^h 49^m 35.76 ±0.36^s	0.9511164 ±0.0000042 days
	~1.08 days^[21]	25^h ~54^m	~1.08 days

• See Solar rotation for more detail.

See also

• Apparent retrograde motion
• Day length fluctuations
• Earth's rotation periods
• List of slow rotators (minor planets)
• List of fast rotators (minor planets)
• Retrograde motion
• Rotation (astronomy)
• Rotational speed
• Stellar rotation
• Synodic day

External links

• Book: Solar System Dynamics . Murray . Carl D. . Stanley F. . Dermott . amp . . 1999 . 531 . 0-521-57295-9. Note, the rotation periods for Mercury and Earth in this work may be inaccurate.

Notes and References

1. Web site: Period . COSMOS - The SAO Encyclopedia of Astronomy . 2023-08-03.
2. Book: Phillips, Kenneth J. H. . 1995 . Guide to the Sun . 78–79 . . 978-0-521-39788-9.
3. Book: Allen's Astrophysical Quantities . Clabon Walter Allen . Clabon Walter . Allen . Arthur N. . Cox . amp . . 2000 . 0-387-98746-0 . 296.
4. Web site: ESO . ESO . 2021-06-03.
5. 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. . Jao . Joseph S. . Snedeker . Lawrence G. . Ghigo . Frank D. . Bonsall . Amber . 3 . Spin state and moment of inertia of Venus . Nature Astronomy . 29 April 2021 . 5 . 7 . 676–683 . 10.1038/s41550-021-01339-7. 2103.01504 . 2021NatAs...5..676M . 232092194 .
7. Web site: How long is a day on Venus? . TE AWAMUTU SPACE CENTRE . 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. Book: Allen's Astrophysical Quantities . Clabon Walter Allen . Clabon Walter . Allen . Arthur N. . Cox . amp . . 2000 . 0-387-98746-0 . 308.
10. Web site: Allison . Michael . Schmunk . Robert . Mars24 Sunclock — Time on Mars . NASA GISS.
11. Chamberlain . Matthew A. . Sykes . Mark V. . Esquerdo . Gilbert A. . Ceres lightcurve analysis – Period determination . . 188 . 2 . 451–456 . 2007 . 10.1016/j.icarus.2006.11.025 . 2007Icar..188..451C .
12. Rotation period of the deep interior is that of the planet's magnetic field.
13. Web site: Rotation Period and Day Length . Seligman . Courtney . June 12, 2021.
14. Found through examination of Saturn's C Ring
15. News: McCartney . Gretchen . Wendel . JoAnna . Scientists Finally Know What Time It Is on Saturn . 18 January 2019 . . 18 January 2019 .
16. Mankovich, Christopher . etal . Cassini Ring Seismology as a Probe of Saturn's Interior. I. Rigid Rotation . 17 January 2019 . . 871 . 1 . 1 . 10.3847/1538-4357/aaf798 . 1805.10286 . 2019ApJ...871....1M . 67840660 . free .
17. Kaiser . M. L. . etal . Voyager Detection of Nonthermal Radio Emission from Saturn . Science. 1980 . 209 . 4462 . 1238–1240 . 10.1126/science.209.4462.1238 . 17811197 . 1980Sci...209.1238K . 2060/19800013712 . 44313317 . free .
18. Book: Visual Lunar and Planetary Astronomy . Saturn . Paul . Abel . 2013 . The Patrick Moore Practical Astronomy Series . 149–171 . Springer . New York, NY . 10.1007/978-1-4614-7019-9_8. 978-1-4614-7018-2 .
19. High-Precision Photometry of Extreme KBO 2003 EL61 . Pedro . Lacerda . David . Jewitt . Nuno . Peixinho . amp . 2008-04-02 . . 135 . 5 . ((1,749–1,756)) . 2008-09-22 . 10.1088/0004-6256/135/5/1749 . 2008AJ....135.1749L. 0801.4124. 115712870 .
20. Long-term photometric monitoring of the dwarf planet (136472) Makemake . 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 . Astronomy & Astrophysics . 625 . A46 . 10.1051/0004-6361/201935274 . 1904.03679 . 2019-04-09. 2019A&A...625A..46H . 102350991 .
21. Web site: December 14, 2019, solution date. JPL Small-Body Database Browser: 136199 Eris (2003 UB313). February 20, 2020. https://web.archive.org/web/20160412115622/http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=136199. April 12, 2016. live.