Timekeeping system on Earth relative to the celestial sphere
This article is about the astronomical time system. For the novel, see Sidereal Time.
Picture of a poster clarifying the difference between a sidereal day and the more conventional solar dayAnimation showing the difference between a sidereal day and a solar day
Sidereal time ("sidereal" pronounced /saɪˈdɪəriəl,sə-/sy-DEER-ee-əl, sə-) is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".[1]
Viewed from the same location, a star seen at one position in the sky will be seen at the same position on another night at the same time of day (or night), if the day is defined as a sidereal day (also known as the sidereal rotation period). This is similar to how the time kept by a sundial (Solar time) 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 due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earth's rotation about its polar axis: solar time is reckoned according to the position of the Sun in the sky while sidereal time is based approximately on the position of the fixed stars on the theoretical celestial sphere.
More exactly, sidereal time is the angle, measured along the celestial equator, from the observer's meridian to the great circle that passes through the March equinox (the northern hemisphere's vernal equinox) and both celestial poles, and is usually expressed in hours, minutes, and seconds. (In the context of sidereal time, "March equinox" or "equinox" or "first point of Aries" is currently a direction, from the center of the Earth along the line formed by the intersection of the Earth's equator and the Earth's orbit around the Sun, toward the constellation Pisces-- during ancient times it was toward the constellation Aries). [2] Common time on a typical clock (using mean Solar time) measures a slightly longer cycle, affected not only by Earth's axial rotation but also by Earth's orbit around the Sun.
The March equinox itself precesses slowly westward relative to the fixed stars, completing one revolution in about 25,800 years, so the misnamed "sidereal" day ("sidereal" is derived from the Latin sidus meaning "star") is 0.0084 second shorter than the stellar day, Earth's actual period of rotation relative to the fixed stars.[3]
The slightly longer stellar period is measured as the Earth rotation angle (ERA), formerly the stellar angle.[4] An increase of 360° in the ERA is a full rotation of the Earth.
A sidereal day on Earth is approximately 86164.0905 seconds (23 h 56 min 4.0905 s or 23.9344696 h).
(Seconds are defined as per International System of Units and are not to be confused with ephemeris seconds.)
Each day, the sidereal time at any given place and time will be about four minutes shorter than local civil time (which is based on solar time), so that for a complete year the number of sidereal "days" is one more than the number of solar days.
^NIST n.d. A more precise definition is given below.
Siderealtime ("sidereal" pronounced /saɪˈdɪəriəl, sə-/ sy-DEER-ee-əl, sə-) is a system of timekeeping used especially by astronomers. Using sidereal...
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the local siderealtime, α object {\displaystyle \alpha _{\text{object}}} is the object's right ascension, GST is Greenwich siderealtime and λ observer...
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star it is orbiting, and is the basis of solar time. The synodic day is distinguished from the sidereal day, which is one complete rotation in relation...
equation of time for that date. A third method did not use the equation of time; instead, it used stellar observations to give siderealtime, exploiting...
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orbits the Sun once a year, the siderealtime at any given place and time will gain about four minutes against local civil time, every 24 hours, until, after...
time of a solar day, while reoccurring every sidereal day. The remaining small changes in the culmination period time from sidereal year to sidereal year...
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value of its turning number. The paradox is related to siderealtime: a sidereal day is the time Earth takes to rotate for a distant star to return to...
Southern Hemisphere. In astronomy, the March equinox is the zero point of siderealtime and, consequently, the right ascension and ecliptic longitude. It also...
intersects the celestial equator. Right ascension is usually measured in sidereal hours, minutes and seconds instead of degrees, a result of the method of...
calendar of this type is called a sidereal solar calendar. The mean calendar year of such a calendar approximates the sidereal year. Leaping from one lunation...
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astronomers to use the stars to measure siderealtime, which provided an accurate method for setting clocks. The equation of time was engraved on sundials so that...
The small golden star shows the position of the vernal equinox, and siderealtime can be read on the scale with golden Roman numerals. The zodiac is on...
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