Rotation period (astronomy)

In astronomy, the rotation period or spin period of a celestial object has two definitions. The first one corresponds to the sidereal rotation period, i.e., the time that the object takes to complete a full rotation around its axis relative to the background stars. The other type of commonly used "rotation period" is the object's synodic rotation period, 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 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, 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 moon of Saturn, exhibits this behaviour, and its rotation period is described as chaotic.

Rotation period of selected objects

Planet	Orbital period	Orbital period	Orbital velocity
Mercury	87.969	0.241	47.9 km/s
Venus	224.701	0.615	35.0 km/s
Earth	365.256	1.000	29.8 km/s
Mars	686.980	1.881	24.1 km/s
Jupiter	4,332.589	11.862	13.1 km/s
Saturn	10,759.22	29.457	9.7 km/s
Uranus	30,688.5	84.020	6.8 km/s
Neptune	60,182	164.8	5.4 km/s

''Orbital velocity values are mean orbital speeds along each planet's orbit around the Sun.''