# Rotation period (astronomy)

In astronomy, the rotation period or spin period of a celestial object (e.g., star, planet, moon, asteroid) has two definitions. The first one corresponds to the sidereal rotation period (or sidereal day), i.e., the time that the object takes to complete a full rotation around its axis relative to the background stars (inertial space). The other type of commonly used "rotation period" is the object's synodic rotation period (or solar day), 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 (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.

## Rotation period of selected objects

| Celestial objects | Rotation period with respect to distant stars, the sidereal rotation period (compared to Earth's mean Solar days) | Rotation period with respect to distant stars, the sidereal rotation period (compared to Earth's mean Solar days) | Synodic rotation period (mean Solar day) | Apparent rotational period viewed from Earth |
| Sun | 25.379995 days (Carrington rotation) 35 days (high latitude) | 25$^d$ 9$^h$ 7$^m$ 11.6$^s$ 35$^d$ | | ~28 days (equatorial) |
| Mercury | 58.6462 days | 58$^d$ 15$^h$ 30$^m$ 30$^s$ | 176 days | |
| Venus | −243.0226 days | −243$^d$ 0$^h$ 33$^m$ | −116.75 days | |
| Earth | 0.99726968 days | 0$^d$ 23$^h$ 56$^m$ 4.0910$^s$ | 1.00 days (24$^h$ 00$^m$ 00$^s$) | |
| Moon | 27.321661 days (equal to sidereal orbital period due to spin-orbit locking, a sidereal lunar month) | 27$^d$ 7$^h$ 43$^m$ 11.5$^s$ | 29.530588 days (equal to synodic orbital period, due to spin-orbit locking, a synodic lunar month) | none (due to spin-orbit locking) |
| Mars | 1.02595675 days | 1$^d$ 0$^h$ 37$^m$ 22.663$^s$ | 1.02749125 days | |
| Ceres | 0.37809 days | 0$^d$ 9$^h$ 4$^m$ 27.0$^s$ | 0.37818 days | |
| Jupiter | 0.41354 days(average) 0.4135344 days (deep interior) 0.41007 days (equatorial) 0.4136994 days (high latitude) | 0$^d$ 9$^h$ 55$^m$ 30$^s$ 0$^d$ 9$^h$ 55$^m$ 29.37$^s$ 0$^d$ 9$^h$ 50$^m$ 30$^s$ 0$^d$ 9$^h$ 55$^m$ 43.63$^s$ | 0.41358 d (9 h 55 m 33 s) (average) | |
| Saturn | 0.44002+0.00130 −0.00091 days (average, deep interior) 0.44401 days (deep interior) 0.4264 days (equatorial) 0.44335 days (high latitude) | 10$^h$ 33$^m$ 38$^s$ $^{+1m52s}$ $_{−1m19s}$  0$^d$ 10$^h$ 39$^m$ 22.4$^s$ 0$^d$ 10$^h$ 13$^m$ 59$^s$ 0$^d$ 10$^h$ 38$^m$ 25.4$^s$ | 0.43930 d (10 h 32 m 36 s) | |
| Uranus | −0.71833 days | −0$^d$ 17$^h$ 14$^m$ 24$^s$ | −0.71832 d (−17 h 14 m 23 s) | |
| Neptune | 0.67125 days | 0$^d$ 16$^h$ 6$^m$ 36$^s$ | 0.67125 d (16 h 6 m 36 s) | |
| Pluto | −6.38718 days (synchronous with Charon) | –6$^d$ 9$^h$ 17$^m$ 32$^s$ | −6.38680 d (–6$^d$ 9$^h$ 17$^m$ 0$^s$) | |
| Haumea | 0.1631458 ±0.0000042 days | 0$^d$ 3$^h$ 56$^m$ 43.80 ±0.36$^s$ | 0.1631461 ±0.0000042 days | |
| Makemake | 0.9511083 ±0.0000042 days | 22$^h$ 49$^m$ 35.76 ±0.36$^s$ | 0.9511164 ±0.0000042 days | |
| Eris | ~15.786 days | ~15$^d$ 18$^h$ 53$^m$ | ~15.7872 days | |