# Sidereal time

**Sidereal time**is a measure of the position of theEarth in its rotation around its axis, or time measured by the apparentdiurnal motion of thevernal equinox , which is very close to, but not identical to, the motion of stars. They differ by theprecession of the vernal equinox inright ascension relative to the stars.

Earth's**sidereal day**also differs from itsrotation period relative to the background stars by the amount of precession in right ascension during one day (8.4 ms). [*Seidelmann, p. 48.*] ItsJ2000 mean value is 23^{h}56^{m}4.090530833^{s}.Aoki, S., B. Guinot, G. H. Kaplan, H. Kinoshita, D. D. McCarthy and P. K. Seidelmann: " [*http://adsabs.harvard.edu/abs/1982A&A...105..359A The new definition of Universal Time*] ". "Astronomy and Astrophysics"**105**(2), 359-361, 1982.]**Definition**Sidereal time is defined as the

hour angle of the vernal equinox. When the meridian of the vernalequinox is directly overhead, local sidereal time is 00:00. Greenwich Sidereal Time is the hour angle of the vernal equinox at theprime meridian at Greenwich, England; local values differ according tolongitude . When one moves eastward 15° in longitude, sidereal time is larger by one hour (note that it wraps around at 24 hours). Unlike computing local solar time, differences are counted to the accuracy of measurement, not just in whole hours. Greenwich Sidereal Time andUT1 differ from each other by a constant rate (GST = 1.00273790935 × UT1). [*Seidelmann, pp. 52 and 698.*] Sidereal time is used at astronomical observatories because sidereal time makes it very easy to work out which astronomical objects will be observable at a given time. Objects are located in the night sky usingright ascension anddeclination relative to the celestial equator (analogous to longitude andlatitude on Earth), and when sidereal time is equal to an object's right ascension, the object will be at its highest point in the sky, or "culmination", at which time it is best placed for observation, as atmospheric extinction is minimised.**idereal time and solar time**Solar time is measured by the apparent diurnal motion of the sun, and local noon in solar time is defined as the moment when the sun is at its highest point in the sky (exactly due south or north depending on the observer's latitude and the season). The average time taken for the sun to return to its highest point is 24 hours.During the time needed by the Earth to complete a rotation around its axis (a sidereal day), the Earth moves a short distance (around 1°) along its orbit around the sun. Therefore, after a sidereal day, the Earth still needs to rotate a small extra angular distance before the sun reaches its highest point. A solar day is, therefore, around 4 minutes longer than a sidereal day.

The stars, however, are so far away that the Earth's movement along its orbit makes a generally negligible difference to their apparent direction (see, however,

parallax ), and so they return to their highest point in a sidereal day. A sidereal day is around 4 minutes shorter than a mean solar day.Another way to see this difference is to notice that, relative to the stars, the Sun appears to move around the Earth once per year. Therefore, there is one less

solar day per year than there are sidereal days. This makes a sidereal day approximately Fraction|365.24|366.24 times the length of the 24-hour solar day, giving approximately 23 hours, 56 minutes, 4.1 seconds (86,164.1 seconds).**Precession effects**The Earth rotation is not simply a simple rotation around an axis that would always remain parallel to itself. The Earth's rotation axis itself rotates about a second axis, orthogonal to the Earth orbit, taking about 25,800 years to perform a complete rotation. This phenomenon is called the

precession of the equinoxes. Because of this precession, the stars appear to move around the Earth in a manner more complicated than a simple constant rotation.For this reason, to simplify the description of Earth orientation in astronomy and

geodesy , it is conventional to describe Earth rotation relative to a frame which is itself precessing slowly. In this reference frame, Earth rotation is close to constant, but the stars appear to rotate slowly with a period of about 25,800 years. It is also in this reference frame that thetropical year , the year related to the Earth's seasons, represents one orbit of the Earth around the sun. The precise definition of a sidereal day is the time taken for one rotation of the Earth in this precessing reference frame.**Exact duration and its variation**A mean sidereal day is about 23 h 56 m 4.1 s in length. However, due to variations in the rotation rate of the Earth the rate of an ideal sidereal clock deviates from any simple multiple of a civil clock. In practice, the difference is kept track of by the difference

UTC –UT1 , which is measured by radio telescopes and kept on file and available to the public at theIERS and at theUnited States Naval Observatory .Given a tropical year of 365.242190402 days from Simon et al. [

*Simon, J. L., P. Bretagnon, J. Chapront, M. Chapront-Touzé, G Francou and J. Laskar: " [*] this gives a sidereal day of 86,400 × Fraction|365.242190402|366.242190402, or 86,164.09053 seconds.*http://adsabs.harvard.edu/abs/1994A%26A...282..663S Numerical expressions for precession formulas and mean elements for the moon and the planets*] ". "Astronomy and Astrophysics"**282**, 663-683, 1994.Because this is the period of rotation in a precessing reference frame, it is not directly related to the mean rotation rate of the Earth in an inertial frame, which is given by ω=2π/T where T is the slightly longer stellar day given by Aoki et al. as 86,164.09890369732 seconds. This can be calculated by noting that ω is the magnitude of the vector sum of the rotations leading to the sidereal day and the precession of that rotation vector. In fact, the period of the Earth's rotation varies on hourly to interannual timescales by around a

millisecond , [*Hide, R., and J. O. Dickey: "Earth's variable rotation". "Science"*] together with a secular increase in length of day of about 2.3 milliseconds per century which mostly results from slowing of the Earth's rotation by tidal friction. [**253**(1991) 629-637.*Stephenson, F.R. "Historical eclipses and Earth's rotation". Cambridge University Press, 1997, 557pp.*]**ee also***

Earth rotation

*Sidereal month **References***P. Kenneth Seidelmann, ed., "Explanatory supplement to the Astronomical Almanac", (Mill Valley, Cal.: University Science Books, 1992)

**External links*** [

*http://tycho.usno.navy.mil/sidereal.html Web based Sidereal time calculator*]

* For more details, see the [*http://docs.kde.org/en/3.1/kdeedu/kstars/ai-sidereal.html article on sidereal time*] from Jason Harris' Astroinfo.

* [*http://sourceforge.net/projects/solarclock SolarClock*] , open source, freeware application to convert between gregorian/julian date and to/from sidereal time.

*Wikimedia Foundation.
2010.*

### Look at other dictionaries:

**sidereal time**— n. 1. time measured by the sidereal day: it is divided into the sidereal hour (1/ 24 of a sidereal day), the sidereal minute (1/ 60 of a sidereal hour), and the sidereal second (1/ 60 of a sidereal minute) 2. the hour angle of the vernal equinox … English World dictionary**Sidereal time**— Time Time, n.; pl. {Times}. [OE. time, AS. t[=i]ma, akin to t[=i]d time, and to Icel. t[=i]mi, Dan. time an hour, Sw. timme. [root]58. See {Tide}, n.] 1. Duration, considered independently of any system of measurement or any employment of terms… … The Collaborative International Dictionary of English**sidereal time**— Astron. time measured by the diurnal motion of stars. A sidereal day is about four minutes shorter than a solar day, with hours, minutes, and seconds all proportionally shorter. [1805 15] * * * ▪ astronomy time as measured by the apparent… … Universalium**sidereal time**— The sidereal time at any place is the hour angle of the vernal equinox at that place … Aviation dictionary**sidereal time**— noun measured by the diurnal motion of stars • Hypernyms: ↑cosmic time • Hyponyms: ↑sidereal day, ↑day, ↑sidereal year, ↑sidereal hour, ↑sidereal month … Useful english dictionary**sidereal time**— noun Date: 1764 1. time based on the sidereal day 2. the hour angle of the vernal equinox at a place … New Collegiate Dictionary**sidereal time**— side′real time′ n. astron. time measured by the diurnal motion of stars • Etymology: 1805–15 … From formal English to slang**sidereal time**— noun Time measured by the apparent diurnal motion of the vernal equinox; approximated by that based on the rotation of the Earth relative to the stars … Wiktionary**sidereal time**— noun Astronomy time reckoned from the motion of the earth (or a planet) relative to the distant stars (rather than with respect to the sun) … English new terms dictionary**Greenwich sidereal time**— The sidereal time elapsed since the transit of the Greenwich Meridian by the first point of the Aries … Aviation dictionary