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Sidereal time
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{{Short description|Timekeeping system on Earth relative to the celestial sphere}} {{About|the astronomical time system|the novel|Christopher Meredith#Publications{{!}}Sidereal Time}} {{Use dmy dates|date=March 2018}} [[File:Sidereal Day poster (simple).png|thumb|upright=1.35|A sidereal day is 1 Earth rotation relative to the stars; a [[solar day]] is 1 Earth rotation relative to the Sun. The Earth rotates 366 times per 'normal' 365-day year relative to the stars, so Earth's sidereal day is 4 minutes shorter than Earth's solar day. (Angle exaggerated for visibility.)]] [[File:Sidereal Day versus Solar Day.webm|thumb|Animation comparing a sidereal day to a [[Synodic day|solar day]]]] '''Sidereal time''' ("sidereal" pronounced {{IPAc-en|s|aɪ|ˈ|d|ɪər|i|əl|,_|s|ə|-}} {{respell|sy|DEER|ee|əl|,_|sə|-}}) is a system of [[timekeeping]] used especially by [[astronomer]]s. Using sidereal time and the [[celestial coordinate system]], it is easy to locate the positions of [[astronomical object|celestial objects]] in the [[night sky]]. Sidereal time is a "time scale that is based on [[Earth's rotation|Earth's rate of rotation]] measured relative to the [[fixed stars]]".<ref>{{harvnb|NIST|n.d.}} A more precise definition is given below.</ref> {{anchor|Sidereal day}} A '''sidereal day''' (also known as the '''sidereal rotation period''') represents the time for one rotation about the planet axis relative to the stars.<ref>{{Cite web |title=Sidereal day |url=https://www.britannica.com/science/sidereal-day |access-date=2025-04-30 |website=www.britannica.com |language=en}}</ref> 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. 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 (astronomy)|meridian]] to the [[great circle]] that passes through the [[March equinox]] (the northern hemisphere's vernal equinox) and both [[celestial pole]]s, 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.){{Sfn|Urban|Seidelmann|2013|loc= "Glossary" s.v. hour angle, hour circle, sidereal time}} Common time on a typical clock (using [[Solar time#Mean solar time|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 [[axial precession|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 seconds shorter than the [[stellar day]], Earth's actual period of rotation relative to the fixed stars.{{Sfn|Urban|Seidelmann|2013|p=78}} The slightly longer stellar period is measured as the [[Earth rotation angle]] (ERA), formerly the stellar angle.{{Sfn|IERS|2013}} An increase of 360° in the ERA is a full rotation of the Earth. {{anchor|Sidereal day}}A sidereal day on Earth is approximately 86164.0905 [[second]]s (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 second]]s.) 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.
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