# Sidereal time

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Sidereal time (pronounced /saɪˈdɪəri.əl/) is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky. From a given observation point, a star found at one location in the sky will be found at basically the same location at another night when observed at the same sidereal time. This is similar to how the time kept by a sundial can be used to find the location of the Sun. Just as the Sun and Moon appear to rise in the east and set in the west, so do the stars. Both solar time and sidereal time make use of the regularity of the Earth's rotation about its polar axis. The basic difference between the two is that solar time maintains orientation to the Sun while sidereal time maintains orientation to the stars in the night sky. The exact definition of sidereal time fixes it to the vernal equinox. Therefore sidereal time correlates closely to the inertial rotation of the Earth, whereas common time on a typical clock correlates to a slightly faster rotation, accounting not only for the Earth's axial rotation but also for the Earth's annual revolution around the Sun of roughly (slightly less than) 1 degree per day.

A sidereal day is approximately 23 hours, 56 minutes, 4.091 seconds (23.93447 hours or 0.99726957 SI days), corresponding to the time it takes for the Earth to complete one rotation relative to the vernal equinox. The vernal equinox itself precesses very slowly in a westward direction relative to the fixed stars, completing one revolution every 26,000 years approximately. As a consequence, the misnamed sidereal day, as "sidereal" is derived from the Latin sidus meaning "star", is some 0.008 seconds shorter than the Earth's period of rotation relative to the fixed stars. The longer true sidereal period is called a stellar day by the International Earth Rotation and Reference Systems Service (IERS). It is also referred to as the sidereal period of rotation.

The direction from the Earth to the Sun is constantly changing (because the Earth revolves around the Sun over the course of a year), but the directions from the Earth to the distant stars do not change nearly as much. Therefore the cycle of the apparent motion of the stars around the Earth has a period that is not quite the same as the 24-hour average length of the solar day.

Maps of the stars in the night sky usually make use of declination and right ascension as coordinates. These correspond to latitude and longitude respectively. While declination is measured in degrees, right ascension is measured in units of hours and minutes, because it was most natural to name locations in the sky in connection with the time when they crossed the meridian.

In the sky, the meridian is an imaginary line going from north to south that goes through the point directly overhead, or the zenith. The right ascension of any object currently crossing the meridian is equal to the current local (apparent) sidereal time, ignoring for present purposes that part of the circumpolar region north of the north celestial pole (for an observer in the northern hemisphere) or south of the south celestial pole (for an observer in the southern hemisphere) that is crossing the meridian the other way.

Because the Earth orbits the Sun once a year, the sidereal time at any one place at midnight will be about four minutes later each night, until, after a year has passed, one additional sidereal day has transpired compared to the number of solar days that have gone by.