The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to 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. The other type of commonly used rotation period is the object's synodic rotation period (or solar day), measured in solar time, which may differ by a fraction of a rotation or more than one rotation to accommodate the portion of the object's orbital period 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.
This section may be too technical for most readers to understand.(May 2015)
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 86164.098 903 691 seconds of mean solar time (UT1) (23h 56m 4.098 903 691s). Earth's rotation period relative to the precessing or moving mean vernal equinox, its sidereal day , is 86164.090 530 832 88 seconds of mean solar time (UT1) (23h 56m 4.090 530 832 88s). 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 (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.
|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 14m 00s
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.
The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems.
A planet is a large astronomical body that is not a star or stellar remnant. There are competing scientific definitions of a 'planet'. In the dynamicist definition adopted by the International Astronomical Union (IAU), a planet is a non-stellar body that is massive enough to be rounded by its own gravity, that directly orbits a star, and that has cleared its orbital zone of competing objects. The IAU has also declared that there are eight planets in the Solar System, independently of the formal definition. In the geological definition used by most planetologists, a planet is a geologically active sub-stellar body, possibly a satellite. In addition to the eight Solar planets accepted by the IAU, these include dwarf planets such as Eris and Pluto and planetary-mass moons.
Tau Ceti, Latinized from τ Ceti, is a single star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under 12 light-years from the Solar System, it is a relatively nearby star and the closest solitary G-class star. The star appears stable, with little stellar variation, and is metal-deficient.
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A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.
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The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars.
A synodic day is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time.
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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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This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.
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HD 215152 is the Henry Draper Catalogue designation for a star in the zodiac constellation of Aquarius. It has an apparent visual magnitude of 8.13, meaning it is too faint to be seen with the naked eye. Parallax measurements made by the Hipparcos and Gaia spacecraft provide distance estimates of around 70 light years. The star has a relatively high proper motion, moving across the sky at an estimated 0.328 arc seconds per year along a position angle of 205°.