The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) is as its sidereal rotation period i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the background stars, measured in sidereal time.
This type of rotation period differs from the object's synodic rotation period (a solar day), measured in solar time, which may differ by a fractional or multiple rotation to accommodate the portion of the object'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 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.
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.
Earth
Earth's rotation period relative to the Sun (its mean solar day) consists of 86,400 seconds of mean solar time, by definition.
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 deceleration.
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  seconds of mean solar time (UT1)  Earth's rotation period relative to the precessing or moving mean vernal equinox, its sidereal day, is  seconds of mean solar time (UT1)  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.
, United States Naval Observatory University Science Books, 1992, p.48, .
The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 Graph at end.
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
See also
Apparent retrograde motion
List of slow rotators (minor planets)
List of fast rotators (minor planets)
Retrograde motion
Rotational speed
Synodic day
References
External links
Note, the rotation periods for Mercury and Earth in this work may be inaccurate.
