In astronomy, the rotation period or spin periodof a celestial object (e.g., star, gas giant, planet, moon, asteroid) 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 (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.
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.[ citation needed ] 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.
|Celestial objects||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)
|25d 9h 7m 11.6s|
|~28 days (equatorial)|
|Mercury||58.6462 days||58d 15h 30m 30s||176 days|
|Venus||−243.0226 days||−243d 0h 33m||−116.75 days|
|Earth||0.99726968 days||0d 23h 56m 4.0910s||1.00 days (24h 00m 00s)|
|Moon||27.321661 days ||27d 7h 43m 11.5s||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||1d 0h 37m 22.663s||1.02749125 days|
|Ceres||0.37809 days||0d 9h 4m 27.0s||0.37818 days|
0.4135344 days (deep interior )
0.41007 days (equatorial)
0.4136994 days (high latitude)
|0d 9h 55m 30s |
0d 9h 55m 29.37s
0d 9h 50m 30s
0d 9h 55m 43.63s
|0.41358 d (9 h 55 m 33 s) (average)|
−0.00091 days (average, deep interior )
0.44401 days (deep interior )
0.4264 days (equatorial)
0.44335 days (high latitude)
|10h 33m 38s+ 1m 52s|
− 1m 19s
0d 10h 39m 22.4s
0d 10h 13m 59s
0d 10h 38m 25.4s
|0.43930 d (10 h 32 m 36 s)|
|Uranus||−0.71833 days||−0d 17h 14m 24s||−0.71832 d (−17 h 14 m 23 s)|
|Neptune||0.67125 days||0d 16h 6m 36s||0.67125 d (16 h 6 m 36 s)|
|Pluto||−6.38718 days ||–6d 9h 17m 32s||−6.38680 d (–6d 9h 17m 0s)|
|Haumea||0.1631458 ±0.0000042 days||0d 3h 56m 43.80 ±0.36s||0.1631461 ±0.0000042 days|
|Makemake||0.9511083 ±0.0000042 days||22h 49m 35.76 ±0.36s||0.9511164 ±0.0000042 days|
|Eris||~1.08 days||25h ~54m||~1.08 days|
* See Solar rotation for more detail.
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, initially detected in 1988, was confirmed in 2003. As of 1 August 2023, there are 5,484 confirmed exoplanets in 4,047 planetary systems, with 875 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and also much more about exoplanets, including composition, environmental conditions and potential for life.
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationship is found between a pair of objects. The physical principle behind orbital resonance is similar in concept to pushing a child on a swing, whereby the orbit and the swing both have a natural frequency, and the body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be self-correcting and thus stable. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Neptune and Pluto. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance between bodies with similar orbital radii causes large planetary system bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion. The Solar System has at least eight planets: the terrestrial planets Mercury, Venus, Earth and Mars, and the giant planets Jupiter, Saturn, Uranus and Neptune. These planets each rotate around an axis tilted with respect to its orbital pole. All planets of the Solar System other than Mercury possess a considerable atmosphere, and some share such features as ice caps, seasons, volcanism, hurricanes, tectonics, and even hydrology. Apart from Venus and Mars, the Solar System planets generate magnetic fields, and all except Venus and Mercury have natural satellites. The giant planets bear planetary rings, the most prominent being those of Saturn.
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked body possesses synchronous rotation, the object takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth, although there is some variability because the Moon's orbit is not perfectly circular. Usually, only the satellite is tidally locked to the larger body. However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon, as well as for Eris and Dysnomia. Alternative names for the tidal locking process are gravitational locking, captured rotation, and spin–orbit locking.
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Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star Polaris, Earth turns counterclockwise.
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In astronomy, a co-orbital configuration is a configuration of two or more astronomical objects orbiting at the same, or very similar, distance from their primary, i.e. they are in a 1:1 mean-motion resonance..
Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object. It may also describe other motions such as precession or nutation of an object's rotational axis. Prograde or direct motion is more normal motion in the same direction as the primary rotates. However, "retrograde" and "prograde" can also refer to an object other than the primary if so described. The direction of rotation is determined by an inertial frame of reference, such as distant fixed stars.
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