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{{Use dmy dates|date=October 2019}}
[[File:EpicEarth-Globespin-tilt-23.4.gif|thumb|Earth's rotation imaged by [[Deep Space Climate Observatory]], showing axis
tilt]] '''Earth's rotation''' or '''Earth's spin''' is the [[rotation]] of planet [[Earth]] around its own [[Rotation around a fixed axis|axis]], as well as changes in the [[orientation (geometry)|orientation]] of the rotation axis in space. Earth rotates [[east]]ward, in [[prograde motion]]. As viewed from the northern [[polar star]] [[Polaris]], Earth turns [[counterclockwise]].
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In 499 CE, the [[Indian astronomy|Indian astronomer]] [[Aryabhata]] suggested that the spherical Earth rotates about its axis daily, and that the apparent movement of the stars is a relative motion caused by the rotation of Earth. He provided the following analogy: "Just as a man in a boat going in one direction sees the stationary things on the bank as moving in the opposite direction, in the same way to a man at [[Lanka]] the fixed stars appear to be going westward."<ref>{{Cite web |url=http://www.new1.dli.ernet.in/data1/upload/insa/INSA_1/20005b61_51.pdf |title=Archived copy |access-date=8 December 2013 |archive-url=https://web.archive.org/web/20131213060326/http://www.new1.dli.ernet.in/data1/upload/insa/INSA_1/20005b61_51.pdf |archive-date=13 December 2013 |url-status=dead }}</ref><ref>{{cite book|author=Kim Plofker|title=Mathematics in India|page=[https://books.google.com/books?id=DHvThPNp9yMC&pg=PA71 71]|year=2009|publisher=Princeton University Press|isbn=978-0-691-12067-6|title-link=Mathematics in India (book)}}</ref>
In the 10th century, some [[Astronomy in the medieval Islamic world|Muslim astronomers]] accepted that Earth rotates around its axis.<ref>{{Cite journal| volume = 108| issue = 67| pages = 762| author = Alessandro Bausani| title = Cosmology and Religion in Islam| journal = [[Scientia (Italian journal)|Scientia/Rivista di Scienza]]| date = 1973}}</ref> According to [[al-Biruni]], [[al-Sijzi]] (d. c. 1020) invented an [[astrolabe]] called ''al-zūraqī'' based on the idea believed by some of his contemporaries "that the motion we see is due to the Earth's movement and not to that of the sky."<ref name=young/><ref>{{Cite book| publisher = SUNY Press| isbn = 9781438414195| last = Nasr| first = Seyyed Hossein| title = An Introduction to Islamic Cosmological Doctrines| date = 1 January 1993|page=135}}</ref> The prevalence of this view is further confirmed by a reference from the 13th century which states: "According to the geometers [or engineers] (''muhandisīn''), the Earth is in constant circular motion, and what appears to be the motion of the heavens is actually due to the motion of the Earth and not the stars."<ref name=young>{{Cite book| publisher = Cambridge University Press| isbn = 9780521028875| editor-last1 = Young| editor-first1 = M. J. L.| title = Religion, Learning and Science in the 'Abbasid Period| date = 2 November 2006|page=413|url=https://books.google.com/books?id=cJuDafHpk3oC}}</ref> Treatises were written to discuss its possibility, either as refutations or expressing doubts about Ptolemy's arguments against it.<ref>{{cite encyclopedia | editor = Thomas Hockey | last = Ragep | first = Sally P. | title=Ibn Sīnā: Abū ʿAlī
In medieval Europe, [[Thomas Aquinas]] accepted Aristotle's view<ref>{{cite book|first=Thomas|last=Aquinas|title=Commentaria in libros Aristotelis De caelo et Mundo|at=Lib II, cap XIV}} trans in {{cite book|title=A Source Book in Medieval Science|editor-first=Edward|editor-last=Grant|year=1974|publisher=[[Harvard University Press]]}} pages 496–500</ref> and so, reluctantly, did [[John Buridan]]<ref>{{cite book|title=Quaestiones super libris quattuo De Caelo et mundo|first=John|last=Buridan|year=1942|pages=226–232}} in {{harvnb|Grant|1974|pp=500–503}}</ref> and [[Nicole Oresme]]<ref>{{cite book|title=Le livre du ciel et du monde|first=Nicole|last=Oresme|pages=519–539}} in {{harvnb|Grant|1974|pp=503–510}}</ref> in the fourteenth century. Not until [[Nicolaus Copernicus]] in 1543 adopted a [[Heliocentrism|heliocentric]] world system did the contemporary understanding of Earth's rotation begin to be established. Copernicus pointed out that if the movement of Earth is violent, then the movement of the stars must be very much more so. He acknowledged the contribution of the Pythagoreans and pointed to examples of relative motion. For Copernicus this was the first step in establishing the simpler pattern of planets circling a central Sun.<ref>{{cite book|first=Nicolas|last=Copernicus|title=On the Revolutions of the Heavenly Spheres|at=Book I, Chap 5–8}}</ref>
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===Empirical tests===
Earth's rotation implies that the [[equatorial bulge|Equator bulges]] and the [[geographical pole]]s are flattened.
In his ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]'', Newton predicted this [[flattening]] would amount to one part in 230, and pointed to the [[pendulum]] measurements taken by [[Jean Richer|Richer]] in 1673 as corroboration of the change in [[gravity]],<ref>{{cite book |first=Isaac |last=Newton |title=Newton's Principia |page=412 |year=1846 |translator=A. Motte |url=https://archive.org/stream/100878576#page/412|publisher=New-York : Published by Daniel Adee }}</ref> but [[History of geodesy#
However, measurements by [[Maupertuis]] and the [[French Geodesic Mission]] in the 1730s established the [[figure of the Earth|oblateness of Earth]], thus confirming the positions of both Newton and [[Copernicus]].<ref>{{cite book |title=The Newton Wars and the Beginning of the French Enlightenment |first=J. B. |last=Shank |year=2008 |publisher=[[University of Chicago Press]] |pages=324, 355 |url=https://books.google.com/books?id=BBusxgu8-AAC&pg=PA234|isbn=9780226749471 }}</ref>
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=== {{anchor|ephemeris}} Mean solar day===
{{main|Solar time#Mean solar time}}
The average of the true solar day during the course of an entire year is the ''mean solar day'', which contains 86,400 mean solar seconds. Currently, each of these seconds is slightly longer than an [[SI]] second because Earth's mean solar day is now slightly longer than it was during the 19th century due to [[tidal friction]]. The average length of the mean solar day since the introduction of the leap second in 1972 has been about 0 to 2 ms longer than 86,400 SI seconds.<ref>{{cite web|url=http://hpiers.obspm.fr/eoppc/eop/eopc04_05/eopc04.62-now|title= INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE : EARTH ORIENTATION PARAMETERS : EOP (IERS) 05 C04|website=Hpiers.obspm.fr|access-date=22 September 2018}}</ref><ref>{{cite web|url=http://iopscience.iop.org/1538-3881/136/5/1906/pdf/1538-3881_136_5_1906.pdf |title=Physical basis of leap seconds|website=Iopscience.iop.org|access-date=22 September 2018}}</ref><ref>[http://tycho.usno.navy.mil/leapsec.html Leap seconds] {{webarchive |url=https://web.archive.org/web/20150312003149/http://tycho.usno.navy.mil/leapsec.html |date=12 March 2015 }}</ref> Random fluctuations due to core-mantle coupling have an amplitude of about 5 ms.<ref>{{cite web|url=http://www.ien.it/luc/cesio/itu/gambis.pdf|title=Prediction of Universal Time and LOD Variations|website=Ien.it|access-date=22 September 2018|archive-date=28 February 2008|archive-url=https://web.archive.org/web/20080228160903/http://www.ien.it/luc/cesio/itu/gambis.pdf|url-status=dead}}</ref><ref>R. Hide et al., [http://www.gps.caltech.edu/~clay/PDF/Hide1993.pdf "Topographic core-mantle coupling and fluctuations in the Earth's rotation"] 1993.</ref> The mean solar second between 1750 and 1892 was chosen in 1895 by [[Simon Newcomb]] as the independent unit of time in his [[Newcomb's Tables of the Sun|Tables of the Sun]]. These tables were used to calculate the world's [[ephemeris|ephemerides]] between 1900 and 1983, so this second became known as the [[ephemeris second]]. In 1967 the SI second was made equal to the ephemeris second.<ref>[http://tycho.usno.navy.mil/leapsec.html Leap seconds by USNO] {{webarchive |url=https://web.archive.org/web/20150312003149/http://tycho.usno.navy.mil/leapsec.html |date=12 March 2015 }}</ref>
The [[apparent solar time]] is a measure of Earth's rotation and the difference between it and the mean solar time is known as the [[equation of time]].
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{{see also|Earth rotation angle}}
The [[angular speed]] of Earth's rotation in inertial space is {{gaps|(7.292|115|0}} ± {{gaps|0.000|000|1)| e=-5 | u=[[radians]] per SI second}}.<ref name=IERS/><ref group=n>It can be established that SI seconds apply to this value by following the citation in "USEFUL CONSTANTS" to E. Groten [https://www.gfy.ku.dk/~iag/HB2000/part4/groten.htm "Parameters of Common Relevance of Astronomy, Geodesy, and Geodynamics"] {{Webarchive|url=https://web.archive.org/web/20190321122405/https://www.gfy.ku.dk/~iag/HB2000/part4/groten.htm |date=21 March 2019 }} which states units are SI units, except for an instance not relevant to this value.</ref> Multiplying by (180°/π radians) × (86,400 seconds/day) yields {{gaps|360.985|6|u=°/day}}, indicating that Earth rotates more than 360 degrees relative to the fixed stars in one solar day. Earth's movement along its nearly circular orbit while it is rotating once around its axis requires that Earth rotate slightly more than once relative to the fixed stars before the mean Sun can pass overhead again, even though it rotates only once (360°) relative to the mean Sun.<ref group=n>In astronomy, unlike geometry, 360° means returning to the same point in some cyclical time scale, either one mean solar day or one sidereal day for rotation on Earth's axis, or one sidereal year or one mean tropical year or even one mean [[Julian year (astronomy)|Julian year]] containing exactly {{nowrap|365.25 days}} for revolution around the Sun.</ref> Multiplying the value in rad/s by Earth's equatorial radius of {{nowrap|6,378,137 m}} ([[WGS84]] ellipsoid) (factors of 2π radians needed by both cancel) yields an equatorial speed of {{convert|465.10|m/s|kph}}.<ref>Arthur N. Cox, ed., ''[https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA244 Allen's Astrophysical Quantities]'' p.244.</ref> Some sources state that Earth's equatorial speed is slightly less, or {{nowrap|1,669.8 km/h}}.<ref>Michael E. Bakich, ''[https://archive.org/details/cambridgeplaneta00baki/page/50 The Cambridge planetary handbook]'', p.50.</ref> This is obtained by dividing Earth's equatorial circumference by {{nowrap|24 hours}}. However, the use of the solar day is incorrect; it must be the [[sidereal day]], so the corresponding time unit must be a sidereal hour. This is confirmed by multiplying by the number of sidereal days in one mean solar day, {{nowrap|1.002 737 909 350 795}},<ref name=IERS/> which yields the equatorial speed in mean solar hours given above of 1,674.4 km/h.
The tangential speed of Earth's rotation at a point on Earth can be approximated by multiplying the speed at the equator by the cosine of the latitude.<ref>{{cite web|author=Butterworth|author2=Palmer|name-list-style=amp|title=Speed of the turning of the Earth|url=http://teacherlink.ed.usu.edu/tlnasa/reference/imaginedvd/files/imagine/docs/ask_astro/answers/970401c.html|work=Ask an Astrophysicist|publisher=NASA Goddard Spaceflight Center|access-date=3 February 2019|archive-date=8 January 2019|archive-url=https://web.archive.org/web/20190108205010/http://teacherlink.ed.usu.edu/tlnasa/reference/imaginedvd/files/imagine/docs/ask_astro/answers/970401c.html|url-status=dead}}</ref> For example, the [[Kennedy Space Center]] is located at latitude 28.59° N, which yields a speed of: cos(28.59°) × 1,674.4 km/h = 1,470.2 km/h. Latitude is a placement consideration for [[spaceport]]s.
{{comparison_of_Earth_farthest_points.svg}}
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The primary monitoring of Earth's rotation is performed by [[very-long-baseline interferometry]] coordinated with the [[Global Positioning System]], [[satellite laser ranging]], and other [[satellite geodesy]] techniques. This provides an absolute reference for the determination of [[universal time]], [[precession]] and [[nutation]].<ref>{{cite web|url=http://hpiers.obspm.fr/eop-pc/techniques/techniques.html|title=Permanent monitoring|website=Hpiers.obspm.fr|access-date=22 September 2018}}</ref>
The absolute value of Earth rotation including [[UT1]] and [[nutation]] can be determined using space geodetic observations, such as very-long-baseline interferometry and [[lunar laser ranging]], whereas their derivatives, denoted as length-of-day excess and nutation rates can be derived from satellite observations, such as [[GPS]], [[GLONASS]], [[Galileo (satellite navigation)|Galileo]]<ref>{{cite journal |last1=Zajdel |first1=Radosław |last2=Sośnica |first2=Krzysztof |last3=Bury |first3=Grzegorz |last4=Dach |first4=Rolf |last5=Prange |first5=Lars |title=System-specific systematic errors in earth rotation parameters derived from GPS, GLONASS, and Galileo |journal=GPS Solutions |date=July 2020 |volume=24 |issue=3 |pages=74 |doi=10.1007/s10291-020-00989-w|doi-access=free |bibcode=2020GPSS...24...74Z }}</ref> and satellite laser ranging to geodetic satellites.<ref>{{cite journal |last1=Sośnica |first1=K. |last2=Bury |first2=G. |last3=Zajdel |first3=R. |title=Contribution of
===Ancient observations===
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== See also ==
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* [[Allais effect]]
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== External links ==
*[http://www.usno.navy.mil/USNO/earth-orientation USNO Earth Orientation] {{Webarchive|url=https://web.archive.org/web/20110514132451/http://www.usno.navy.mil/USNO/earth-orientation |date=14 May 2011 }} new site, being populated
*[https://web.archive.org/web/20130802032453/http://maia.usno.navy.mil/ USNO IERS] old site, to be abandoned
*[http://hpiers.obspm.fr/eop-pc/ IERS Earth Orientation Center: Earth rotation data and interactive analysis]
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