# Rotation period

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Rotation period

The rotation period of an astronomical object is the time 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 giant planets, 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. 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

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. [ [http://tycho.usno.navy.mil/leapsec.html Leap seconds by USNO] ]

Earth's rotation period relative to the fixed stars, called its "stellar day" by the International Earth Rotation and Reference Systems Service (IERS), is nowrap|86164.098 903 691 seconds of mean solar time (UT1) nowrap|(23smallsup|h 56smallsup|m 4.098 903 691smallsup|s). [http://hpiers.obspm.fr/eop-pc/models/constants.html 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.", " [http://adsabs.harvard.edu/abs/1982A&A...105..359A The new definition of Universal Time] ", "Astronomy and Astrophysics" 105 (1982) 359–361.] Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its "sidereal day", is nowrap|86164.090 530 832 88 seconds of mean solar time (UT1) nowrap|(23smallsup|h 56smallsup|m 4.090 530 832 88smallsup|s). Thus the sidereal day is shorter than the stellar day by about 8.4 ms. ["Explanatory Supplement to the Astronomical Almanac", ed. P. Kenneth Seidelmann, Mill Valley, Cal., University Science Books, 1992, p.48, ISBN 0-935702-68-7.] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 [ [http://hpiers.obspm.fr/eop-pc/earthor/ut1lod/lod-1623.html IERS Excess of the duration of the day to 86400s … since 1623] Graph at end.] and 1962–2005. [ [http://web.archive.org/web/20070813203913/http://hpiers.obspm.fr/eop-pc/earthor/ut1lod/figure3.html IERS Variations in the duration of the day 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

ee also

*Poles of astronomical bodies

References

*cite book |title= [http://books.google.com/books?id=aU6vcy5L8GAC&pg=PP1&lr=&as_brr=3&sig=ACfU3U1bZQ8_-tZrvTNYL76lNZ0m6vZrzw#PPA531,M1 Solar System Dynamics] |author=Murray, Carl D. and Stanley F. Dermott |publisher=Cambridge UP |year=1999 |pages=531 |isbn=0-521-57295-9 Rotation periods of Mercury and Earth are wrong.

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