Rotation period

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'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. [ [ 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). [ 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.] 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 [ [ IERS Excess of the duration of the day to 86400s … since 1623] Graph at end.] and 1962–2005. [ [ 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
*Prograde and retrograde motion


External links

*cite book |title= [,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.

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • rotation period — noun rotational period …   Wiktionary

  • Rotation-powered pulsar — is one of the major classes of pulsars. A Rotation powered pulsar is a rapidly rotating neutron star, whose electromagnetic radiation is observed in regularly spaced intervals, or pulses . It differs from other types of pulsars in that the source …   Wikipedia

  • Rotation in office — Rotation in office, or term limits, dates back to the American Revolution, and prior to that to the democracies and republics of antiquity. The council of 500 in ancient Athens rotated its entire membership annually, as did the ephorate in… …   Wikipedia

  • Rotation — This article is about movement of a physical body. For other uses, see Rotation (disambiguation). A polyhedron resembling a sphere rotating around an axis. A rotation is a circular movement of an object around a center (or point) of rotation. A… …   Wikipedia

  • Rotation around a fixed axis — Rotational motion can occur around more than one axis at once, and can involve phenomena such as wobbling and precession. Rotation around a fixed axis is a special case of rotational motion, which does not involve those phenomena. The kinematics… …   Wikipedia

  • Rotation number — This article is about the rotation number, which is sometimes called the map winding number or simply winding number. There is another meaning for winding number, which appears in complex analysis. In mathematics, the rotation number is an… …   Wikipedia

  • rotation — ro|ta|tion [rəuˈteıʃən US rou ] n 1.) [U] when something turns with a circular movement around a central point rotation of ▪ the rotation of the Earth on its axis rotation about/around ▪ the planet s rotation around the sun 2.) one complete… …   Dictionary of contemporary English

  • period — /pear ee euhd/, n. 1. a rather large interval of time that is meaningful in the life of a person, in history, etc., because of its particular characteristics: a period of illness; a period of great profitability for a company; a period of social… …   Universalium

  • rotation — 1. Turning or movement of a body around its axis. 2. A recurrence in regular order of certain events, such as the symptoms of a periodic disease. 3. In medical education, a period of time on a particular service or specialty …   Medical dictionary

  • rotation — /rəυ teɪʃ(ə)n/ noun the act of taking turns ♦ to fill the post of chairman by rotation to let each member of the group act as chairman for a period then give the post to another member ♦ two directors retire by rotation two directors retire… …   Dictionary of banking and finance

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.