In astronomy, the rotation period of a celestial object is the time that the object takes to complete a single revolution around its axis of rotation relative to the background stars. It differs from the object's solar day, which may include an extra fractional rotation needed 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/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 is its internal rotation period, as determined from the rotation of the planet's magnetic field. For objects that are not sphericallysymmetrical, 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, 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. For example, 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 consists of 86,400 seconds of mean solar time, by definition. Each of these seconds is slightly longer than an SIsecond 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, is seconds of mean solar time Earth's rotation period relative to the precessing or moving meanvernal equinox, its sidereal day, is seconds of mean solar time Thus the sidereal day is shorter than the stellar day by about 8.4 ms. The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 and 1962–2005. Recently 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.
