View source for Earth's rotation

{{Short description|Rotation of Earth around its axis}}
{{for|Earth's orbit around the sun|Earth's orbit{{!}}Earth's revolution}}
{{Redirect|Earth's rotation period|the duration of daylight and night|Daytime}}
{{Use dmy dates|date=October 2019}}

[[File:EpicEarth-Globespin-tilt-23.4.gif|thumb|Earth's rotation imaged by [[Deep Space Climate Observatory]], showing [[Axial tilt|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]].

The [[North Pole]], also known as the Geographic North Pole or Terrestrial North Pole, is the point in the [[Northern Hemisphere]] where Earth's axis of rotation meets its surface. This point is distinct from Earth's [[north magnetic pole]]. The [[South Pole]] is the other point where Earth's axis of rotation intersects its surface, in [[Antarctica]].

Earth rotates once in about 24 hours with respect to the [[Sun]], but once every 23 hours, 56 minutes and 4 seconds with respect to other distant stars ([[#Stellar and sidereal day|see below]]). Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the [[tidal acceleration|tidal effects]] the [[Moon]] has on Earth's rotation. [[Atomic clock]]s show that the modern day is longer by about 1.7 [[millisecond]]s than a century ago,<ref name="McCarthySeidelmann2009">{{cite book|author1=Dennis D. McCarthy|author2=Kenneth P. Seidelmann|title=Time: From Earth Rotation to Atomic Physics|url=https://books.google.com/books?id=fjFVW3-LGigC|date=18 September 2009|publisher=John Wiley & Sons|isbn=978-3-527-62795-0|page=232}}</ref> slowly increasing the rate at which [[Coordinated Universal Time|UTC]] is adjusted by [[leap second]]s. Analysis of historical astronomical records shows a slowing trend; the [[Day length fluctuations|length of a day]] increased by about 2.3 milliseconds per century since the [[8th century BC|8th century BCE]].<ref>{{cite journal |last=Stephenson |first=F. Richard |title=Historical eclipses and Earth's rotation |journal=Astronomy & Geophysics |year=2003 |volume=44 |number=2 |pages=2.22–2.27 |doi=10.1046/j.1468-4004.2003.44222.x |bibcode=2003A&G....44b..22S  }}</ref>

Scientists reported that in 2020 Earth had started spinning faster, after consistently spinning slower than 86,400 seconds per day in the decades before. On June 29, 2022, Earth's spin was completed in 1.59 milliseconds under 24 hours, setting a new record.<ref>{{Cite web |author1=Robert Lea |date=2022-08-03 |title=Earth sets record for the shortest day |url=https://www.space.com/earth-rotation-record-shortest-day |access-date=2022-08-08 |website=Space.com |language=en}}</ref> Because of that trend, engineers worldwide are discussing a 'negative leap second' and other possible timekeeping measures.<ref>{{cite news |last=Knapton |first=Sarah |title=The Earth is spinning faster now than at any time in the past half century |url=https://www.telegraph.co.uk/news/2021/01/04/earth-spinning-faster-now-time-past-half-century/ |access-date=11 February 2021 |work=The Telegraph |date=4 January 2021}}</ref>

This increase in speed is thought to be due to various factors, including the complex motion of its molten core, oceans, and atmosphere, the effect of celestial bodies such as the Moon, and possibly climate change, which is causing the ice at Earth's poles to melt. The masses of ice account for the [[Earth's shape]] being that of an [[oblate spheroid]], bulging around the equator. When these masses are reduced, the poles rebound from the loss of weight, and Earth becomes more spherical, which has the effect of bringing mass closer to its centre of gravity. [[Conservation of angular momentum]] dictates that a mass distributed more closely around its centre of gravity spins faster.<ref>{{cite news |last=Pappas |first=Stephanie |title=Humans Contribute to Earth's Wobble, Scientists Say |url=https://www.scientificamerican.com/article/humans-contribute-to-earth-rsquo-s-wobble-scientists-say/ |access-date=12 August 2022 |work=Scientific American |date=25 September 2018}}</ref>

==History==
[[File:Earth Rotation (Nepal, Himalayas).jpg|thumb|This [[long-exposure photo]] of the [[Northern celestial hemisphere|northern]] [[night sky]] above the [[Nepal]]i [[Himalayas]] shows the [[star trail|apparent paths]] of the stars as Earth rotates.]]
Among the ancient [[Greeks]], several of the [[Pythagoreanism|Pythagorean]] school believed in the rotation of Earth rather than the apparent diurnal rotation of the heavens. Perhaps the first was [[Philolaus]] (470–385 BCE), though his system was complicated, including a [[Counter-Earth|counter-earth]] rotating daily about a central fire.<ref>Pseudo-Plutarchus, Placita philosophorum (874d-911c), Stephanus page 896, section A, line 5 Ἡρακλείδης ὁ Ποντικὸς καὶ Ἔκφαντος ὁ Πυθαγόρειος κινοῦσι μὲν τὴν γῆν, οὐ μήν γε μεταβατικῶς, ἀλλὰ τρεπτικῶς τροχοῦ δίκην ἐνηξονισμένην, ἀπὸ δυσμῶν ἐπ' ἀνατολὰς περὶ τὸ ἴδιον αὐτῆς κέντρον; Plutarchus Biogr., Phil., Numa, Chapter 11, section 1, line 5, Νομᾶς δὲ λέγεται καὶ τὸ τῆς Ἑστίας ἱερὸν ἐγκύκλιον περιβαλέσθαι τῷ ἀσβέστῳ πυρὶ φρουράν, ἀπομιμούμενος οὐ τὸ σχῆμα τῆς γῆς ὡς Ἑστίας οὔσης, ἀλλὰ τοῦ σύμπαντος κόσμου, οὗ μέσον οἱ Πυθαγορικοὶ τὸ πῦρ ἱδρῦσθαι νομίζουσι, καὶ τοῦτο Ἑστίαν καλοῦσι καὶ μονάδα· τὴν δὲ γῆν οὔτε ἀκίνητον οὔτε ἐν μέσῳ τῆς περιφορᾶς οὖσαν, ἀλλὰ κύκλῳ περὶ τὸ πῦρ αἰωρουμένην οὐ τῶν τιμιωτάτων οὐδὲ τῶν πρώτων τοῦ κόσμου μορίων ὑπάρχειν. {{cite journal|first=George Bosworth|last=Burch|title=The Counter-Earth|journal=Osiris|volume=11|year=1954|pages=267–294|jstor=301675|doi=10.1086/368583|s2cid=144330867}}</ref>

A more conventional picture was supported by [[Hicetas]], [[Heraclides Ponticus|Heraclides]] and [[Ecphantus]] in the fourth century BCE who assumed that Earth rotated but did not suggest that Earth revolved about the Sun. In the third century BCE, [[Aristarchus of Samos]] suggested the [[Heliocentrism|Sun's central place]].

However, [[Aristotle]] in the fourth century BCE criticized the ideas of Philolaus as being based on theory rather than observation. He established the [[idea]] of a sphere of fixed stars that rotated about Earth.<ref>{{cite book|author=Aristotle|title=Of the Heavens|at=Book II, Ch 13. 1.|url=http://classics.mit.edu/Aristotle/heavens.2.ii.html}}</ref> This was accepted by most of those who came after, in particular [[Claudius Ptolemy]] (2nd century CE), who thought Earth would be devastated by gales if it rotated.<ref>{{cite book|title=Almagest Book I, Chapter 8|author=Ptolemy}}</ref>

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 the 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 the 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 al-Ḥusayn ibn ʿAbdallāh ibn Sīnā | encyclopedia = The [[Biographical Encyclopedia of Astronomers]] | publisher = Springer | year = 2007 | location = New York | pages = 570–2 | url=http://islamsci.mcgill.ca/RASI/BEA/Ibn_Sina_BEA.htm | isbn=978-0-387-31022-0|display-editors=etal}} ([http://islamsci.mcgill.ca/RASI/BEA/Ibn_Sina_BEA.pdf PDF version])</ref> At the [[Maragheh observatory|Maragha]] and [[Ulugh Beg Observatory|Samarkand observatories]], Earth's rotation was discussed by [[Nasir al-Din al-Tusi|Tusi]] (born 1201) and [[Ali Qushji|Qushji]] (born 1403); the arguments and evidence they used resemble those used by Copernicus.<ref name=Ragep>{{Citation |last=Ragep |first=F. Jamil |year=2001a |title=Tusi and Copernicus: The Earth's Motion in Context |journal=Science in Context |volume=14 |issue=1–2 |pages=145–163 |doi=10.1017/s0269889701000060|s2cid=145372613 }}</ref>

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 the Earth is violent, then the stars' movement must be 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>

[[Tycho Brahe]], who produced accurate observations on which [[Johannes Kepler|Kepler]] based his [[Kepler's laws of planetary motion|laws of planetary motion]], used Copernicus's work as the basis of a [[Tychonic system|system]] assuming a stationary Earth. In 1600, [[William Gilbert (physician)|William Gilbert]] strongly supported Earth's rotation in his treatise on Earth's magnetism<ref>{{cite book|first=William|last=Gilbert|title=De Magnete, On the Magnet and Magnetic Bodies, and on the Great Magnet the Earth|pages=313–347|url=https://archive.org/stream/williamgilbertof00gilb#page/316 |publisher=New York, J. Wiley & sons|year=1893}}</ref> and thereby influenced many of his contemporaries.<ref name = "Russell1972">{{cite book|title=The Reception of Copernicus' Heliocentric Theory|editor= J. Dobrzycki|first=John L|last=Russell| chapter=Copernican System in Great Britain|year= 1972|publisher= Springer|isbn= 9789027703118|chapter-url=https://books.google.com/books?id=tAWp34_GYLAC&pg=PA208}}</ref>{{rp|208}} Those like Gilbert who did not openly support or reject the motion of Earth about the Sun are called "semi-Copernicans".<ref name = "Russell1972"/>{{rp|221}} A century after Copernicus, [[Riccioli]] disputed the model of a rotating Earth due to the lack of then-observable eastward deflections in falling bodies;<ref>[[Almagestum novum]], chapter nine, cited in {{cite journal|title=126 arguments concerning the motion of the earth. GIOVANNI BATTISTA RICCIOLI in his 1651 ALMAGESTUM NOVUM|first=Christopher M.|last=Graney|year=2012|journal=Journal for the History of Astronomy|at=volume 43, pages 215–226|arxiv = 1103.2057}}</ref> such deflections would later be called the [[Coriolis effect]]. However, the contributions of Kepler, [[Galileo]], and [[Isaac Newton|Newton]] gathered support for the theory of the rotation of the Earth.

===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#European calculations|initial measurements]] of [[meridian arc|meridian lengths]] by [[Jean Picard|Picard]] and [[Jacques Cassini|Cassini]] at the end of the 17th century suggested the opposite.
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>

In Earth's rotating frame of reference, a freely moving body follows an apparent path that deviates from the one it would follow in a fixed frame of reference. Because of the [[Coriolis effect]], falling bodies veer slightly eastward from the vertical plumb line below their point of release, and projectiles veer right in the [[Northern Hemisphere]] (and left in the [[Southern Hemisphere|Southern]]) from the direction in which they are shot. The Coriolis effect is mainly observable at a meteorological scale, where it is responsible for the opposite directions of [[cyclone]] rotation in the Northern and Southern hemispheres (anticlockwise and [[clockwise]], respectively).

[[Robert Hooke]], following a suggestion from Newton in 1679, tried unsuccessfully to verify the predicted eastward deviation of a body dropped from a height of {{nowrap|8.2 meters}}, but definitive results were obtained later, in the late 18th and early 19th centuries, by [[Giovanni Battista Guglielmini]] in [[Bologna]], [[Johann Friedrich Benzenberg]] in [[Hamburg]] and [[Ferdinand Reich]] in [[Freiberg, Saxony|Freiberg]], using taller towers and carefully released weights.<ref group=n>See [[:de:Fallexperimente zum Nachweis der Erdrotation|Fallexperimente zum Nachweis der Erdrotation]] (German Wikipedia article).</ref> A ball dropped from a height of 158.5&nbsp;m departed by 27.4&nbsp;mm from the vertical compared with a calculated value of 28.1&nbsp;mm.

The most celebrated test of Earth's rotation is the [[Foucault pendulum]] first built by physicist [[Léon Foucault]] in 1851, which consisted of a lead-filled brass sphere suspended {{nowrap|67 m}} from the top of the [[Panthéon (Paris)|Panthéon]] in Paris. Because of Earth's rotation under the swinging pendulum, the pendulum's plane of oscillation appears to rotate at a rate depending on latitude. At the latitude of Paris, the predicted and observed shift was about {{nowrap|11 degrees}} [[clockwise]] per hour. Foucault pendulums now swing in [[List of Foucault pendulums|museums worldwide]].

== Periods {{anchor|Period}} ==
{{main|Day#Definitions}}
[[File:Starry Spin-up.jpg|thumb|Starry circles arc around the south celestial pole, seen overhead at [[ESO]]'s [[La Silla Observatory]].<ref>{{cite web|title=Starry Spin-up|url=http://www.eso.org/public/images/potw1534a/|access-date=24 August 2015}}</ref>]]

===True solar day===
{{Main|Solar time}}
Earth's [[rotation period]] relative to the Sun ([[solar noon]] to solar noon) is its ''true solar day'' or ''apparent solar day''.<ref>{{Cite web |title=What Is Solar Noon? |url=https://www.timeanddate.com/astronomy/solar-noon.html |access-date=2022-07-15 |website=timeanddate.com |language=en}}</ref> It depends on Earth's orbital motion and is thus affected by changes in the [[orbital eccentricity|eccentricity]] and [[inclination]] of Earth's orbit. Both vary over thousands of years, so the annual variation of the true solar day also varies. Generally, it is longer than the mean solar day during two periods of the year and shorter during another two.{{refn|When Earth's eccentricity exceeds 0.047 and perihelion is at an appropriate equinox or solstice, only one period with one peak balances another period that has two peaks.<ref name=Meeus/>|group=n}} The true solar day tends to be longer near [[apsis|perihelion]] when the Sun apparently moves along the [[ecliptic]] through a greater angle than usual, taking about {{nowrap|10 seconds}} longer to do so. Conversely, it is about {{nowrap|10 seconds}} shorter near [[apsis|aphelion]]. It is about {{nowrap|20 seconds}} longer near a [[solstice]] when the projection of the Sun's apparent motion along the ecliptic onto the [[celestial equator]] causes the Sun to move through a greater angle than usual. Conversely, near an [[equinox]] the projection onto the equator is shorter by about {{nowrap|20 seconds}}. Currently, the perihelion and solstice effects combine to lengthen the true solar day near {{nowrap|22 December}} by {{nowrap|30 mean}} solar seconds, but the solstice effect is partially cancelled by the aphelion effect near {{nowrap|19 June}} when it is only {{nowrap|13 seconds}} longer. The effects of the equinoxes shorten it near {{nowrap|26 March}} and {{nowrap|16 September}} by {{nowrap|18 seconds}} and {{nowrap|21 seconds}}, respectively.<ref name="Meeus">{{cite book|author1=Jean Meeus|author2=J. M. A. Danby|title=Mathematical Astronomy Morsels|url=https://books.google.com/books?id=rXbvAAAAMAAJ|date=January 1997|publisher=Willmann-Bell|isbn=978-0-943396-51-4|pages=345–346}}</ref><ref>{{cite web|url=http://www.pierpaoloricci.it/dati/giornosolarevero_eng.htm|title=pierpaoloricci.it/dati/giorno solare vero VERSIONE EN|first=Pierpaolo|last=Ricci|website=Pierpaoloricci.it|access-date=22 September 2018}}</ref>

=== {{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]].

===Stellar and sidereal day{{anchor|Stellar day}}===
[[File:Sidereal day (prograde).svg|thumb|On a [[direct motion|prograde]] planet like Earth, the ''stellar day'' is shorter than the [[solar time|solar day]]. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360 degrees and the distant star is overhead again but the Sun is not (1→2 = one stellar day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).]]
Earth's rotation period relative to the [[International Celestial Reference Frame]], called its ''stellar day'' by the [[International Earth Rotation and Reference Systems Service]] (IERS), is {{nowrap|86 164.098 903 691}} seconds of mean solar time (UT1) {{nowrap|(23{{smallsup|h}} 56{{smallsup|m}} {{gaps|4.098|903|691}}{{smallsup|s}}}}, {{gaps|0.997|269|663|237|16|u=mean solar days}}).<ref name=IERS>{{cite web|url=http://hpiers.obspm.fr/eop-pc/models/constants.html|title=USEFUL CONSTANTS|website=Hpiers.obspm.fr|access-date=22 September 2018}}</ref>{{refn|Aoki, the ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".<ref>Aoki, ''et al''., "[http://adsabs.harvard.edu/abs/1982A&A...105..359A The new definition of Universal Time]", ''Astronomy and Astrophysics'' '''105''' (1982) 359–361.</ref>|group=n}} Earth's rotation period relative to the [[axial precession|precessing]] mean vernal [[equinox (celestial coordinates)|equinox]], named ''[[sidereal day]]'', is {{gaps|86|164.090|530|832|88|u=seconds}} of mean solar time (UT1) {{nowrap|(23{{smallsup|h}} 56{{smallsup|m}} {{gaps|4.090|530|832|88}}{{smallsup|s}}}}, {{gaps|0.997|269|566|329|08|u=mean solar days}}).<ref name=IERS/> Thus, the sidereal day is shorter than the stellar day by about {{nowrap|8.4 ms}}.<ref>{{cite book |title=Explanatory Supplement to the Astronomical Almanac |editor-last=Seidelmann |editor-first=P. Kenneth |location=Mill Valley, California |publisher=University Science Books |year=1992 |page=48 |isbn=978-0-935702-68-2}}</ref>

Both the stellar day and the sidereal day are shorter than the mean solar day by about {{nowrap|3 minutes}} {{nowrap|56 seconds}}. This is a result of the Earth turning 1 additional rotation, relative to the celestial reference frame, as it orbits the Sun (so 366.24 rotations/y). The mean solar day in SI seconds is available from the IERS for the periods {{nowrap|1623–2005}}<ref>[http://hpiers.obspm.fr/eop-pc/earthor/ut1lod/lod-1623.html IERS Excess of the duration of the day to 86,400s … since 1623] {{webarchive|url=https://web.archive.org/web/20081003083543/http://hpiers.obspm.fr/eop-pc/earthor/ut1lod/lod-1623.html |date=3 October 2008 }} Graph at end.</ref> and {{nowrap|1962–2005}}.<ref>{{cite web|url=http://hpiers.obspm.fr/eop-pc/earthor/ut1lod/figure3.html|archive-url=https://web.archive.org/web/20070813203913/http://hpiers.obspm.fr/eop-pc/earthor/ut1lod/figure3.html|url-status=dead|archive-date=13 August 2007|title=Excess to 86400s of the duration day, 1995–1997|date=13 August 2007|access-date=22 September 2018}}</ref>

Recently (1999–2010) the average annual length of the mean solar day in excess of 86,400 SI seconds has varied between {{nowrap|0.25 ms}} and {{nowrap|1 ms}}, which must be added to both the stellar and sidereal days given in mean solar time above to obtain their lengths in SI seconds (see [[Fluctuations in the length of day]]).

===Angular speed{{anchor|Speed}}===
[[File:Earth_rotation_tangential_speed.svg|thumb|Plot of latitude versus tangential speed. The dashed line shows the [[Kennedy Space Center]] example. The dot-dash line denotes typical airliner [[cruise speed]].]]
{{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&nbsp;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&nbsp;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&nbsp;km/h = 1,470.2&nbsp;km/h. Latitude is a placement consideration for [[spaceport]]s.

{{comparison_of_Earth_farthest_points.svg}}
The peak of the [[Cayambe (volcano)|Cayambe volcano]] is the point of [[Earth]]'s surface farthest from its axis; thus, it rotates the fastest as Earth spins.<ref>{{cite web |last=Klenke |first=Paul |title=Distance to the Center of the Earth |url=https://www.summitpost.org/distance-to-the-center-of-the-earth/849764 |publisher=Summit Post |access-date=4 July 2018}}</ref>

== Changes ==
[[File:AxialTiltObliquity.png|thumb|Earth's [[axial tilt]] is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing.]]

=== In rotational axis ===
{{main|Earth's rotation axis}}

Earth's rotation axis moves with respect to the fixed stars ([[inertial space]]); the components of this motion are [[precession]] and [[nutation]]. It also moves with respect to Earth's crust; this is called [[polar motion]].

Precession is a rotation of Earth's rotation axis, caused primarily by external torques from the gravity of the Sun, Moon and other bodies. The polar motion is primarily due to free core [[nutation]] and the [[Chandler wobble]].

=== In rotational speed ===
{{Main|Day length fluctuations|ΔT (timekeeping)}}

==== Tidal interactions ====
Over millions of years, Earth's rotation has been slowed significantly by ''[[tidal acceleration]]'' through gravitational interactions with the Moon. Thus [[angular momentum]] is slowly transferred to the Moon at a rate proportional to <math>r^{-6}</math>, where <math>r</math> is the orbital radius of the Moon. This process has gradually increased the length of the day to its current value, and resulted in the Moon being ''[[tidally locked]]'' with Earth.

This gradual rotational deceleration is empirically documented by estimates of day lengths obtained from observations of ''[[tidal rhythmites]]'' and ''[[stromatolite]]s''; a compilation of these measurements<ref name=":0">{{Cite journal|last=Williams|first=George E.|date=1 February 2000|title=Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit|journal=Reviews of Geophysics|language=en|volume=38|issue=1|pages=37–59|doi=10.1029/1999RG900016|issn=1944-9208|bibcode=2000RvGeo..38...37W|s2cid=51948507|doi-access=free}}</ref> found that the length of the day has increased steadily from about 21 hours at 600 Myr ago<ref name=Zahnle/> to the current 24-hour value. By counting the microscopic lamina that form at higher tides, tidal frequencies (and thus day lengths) can be estimated, much like counting tree rings, though these estimates can be increasingly unreliable at older ages.<ref>{{Cite book|chapter=Periodic Growth Features in Fossil Organisms and the Length of the Day and Month|last=Scrutton|first=C. T.|date=1 January 1978|publisher=Springer Berlin Heidelberg|isbn=9783540090465|editor-last=Brosche|editor-first=Professor Dr Peter|pages=154–196|language=en|doi=10.1007/978-3-642-67097-8_12|editor-last2=Sündermann|editor-first2=Professor Dr Jürgen|title = Tidal Friction and the Earth's Rotation}}</ref>

==== Resonant stabilization ====

[[File:Simulated evolution of Earth's day length over time.png|thumb|A simulated history of Earth's day length, depicting a resonant-stabilizing event throughout the Precambrian era<ref name="Bartlett 5716–5724">{{Cite journal|last1=Bartlett|first1=Benjamin C.|last2=Stevenson|first2=David J.|date=1 January 2016|title=Analysis of a Precambrian resonance-stabilized day length|journal=Geophysical Research Letters|volume=43|issue=11|language=en|pages=5716–5724|doi=10.1002/2016GL068912|issn=1944-8007|arxiv=1502.01421|bibcode=2016GeoRL..43.5716B|s2cid=36308735}}</ref>]]

The current rate of tidal deceleration is anomalously high, implying Earth's rotational velocity must have decreased more slowly in the past. Empirical data<ref name=":0"/> tentatively shows a sharp increase in rotational deceleration about 600 Myr ago. Some models suggest that Earth maintained a constant day length of 21 hours throughout much of the [[Precambrian]].<ref name=Zahnle>{{Cite journal|last1=Zahnle|first1=K.|last2=Walker|first2=J. C.|date=1 January 1987|title=A constant daylength during the Precambrian era?|journal=Precambrian Research|volume=37|issue=2|pages=95–105|issn=0301-9268|pmid=11542096|doi=10.1016/0301-9268(87)90073-8|bibcode=1987PreR...37...95Z|citeseerx=10.1.1.1020.8947}}</ref> This day length corresponds to the semidiurnal [[Resonance|resonant period]] of the thermally driven [[atmospheric tide]]; at this day length, the decelerative lunar torque could have been canceled by an accelerative torque from the atmospheric tide, resulting in no net torque and a constant rotational period. This stabilizing effect could have been broken by a sudden change in global temperature. Recent computational simulations support this hypothesis and suggest the [[Marinoan glaciation|Marinoan]] or [[Sturtian glaciation]]s broke this stable configuration about 600 Myr ago; the simulated results agree quite closely with existing paleorotational data.<ref name="Bartlett 5716–5724"/>

==== Global events ====

[[File:Deviation of day length from SI day.svg|thumb|Deviation of day length from SI-based day]]

Some recent large-scale events, such as the [[2004 Indian Ocean earthquake]], have caused the length of a day to shorten by 3 microseconds by reducing Earth's [[moment of inertia]].<ref>[http://www.nature.com/news/2004/041229/full/news041229-6.html Sumatran earthquake sped up Earth's rotation], ''Nature'', 30 December 2004.</ref> [[Post-glacial rebound]], ongoing since the last [[ice age]], is also changing the distribution of Earth's mass, thus affecting the moment of inertia of Earth and, by the [[conservation of angular momentum]], Earth's rotation period.<ref name="wu-peltier-1984">{{cite journal |last1=Wu |first1=P. |last2=Peltier |first2=W.R. |year=1984 |title=Pleistocene deglaciation and the earth's rotation: a new analysis |journal=Geophysical Journal of the Royal Astronomical Society |volume=76 |pages=753–792 |doi=10.1111/j.1365-246X.1984.tb01920.x |issue=3|bibcode = 1984GeoJ...76..753W |doi-access=free }}</ref>

The length of the day can also be influenced by man-made structures. For example, [[NASA]] scientists calculated that the water stored in the [[Three Gorges Dam]] has increased the length of Earth's day by 0.06 microseconds due to the shift in mass.<ref>{{cite web|title=NASA Details Earthquake Effects on the Earth|url=https://www.jpl.nasa.gov/news/news.php?feature=716|website=NASA/JPL|access-date=22 March 2019}}</ref>

===Measurement===
{{see also|Universal time#Measurement}}

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 Multi-GNSS Constellation to SLR-Derived Terrestrial Reference Frame |journal=Geophysical Research Letters |date=16 March 2018 |volume=45 |issue=5 |pages=2339–2348 |doi=10.1002/2017GL076850|bibcode=2018GeoRL..45.2339S |s2cid=134160047 }}</ref>

===Ancient observations===
There are recorded observations of [[solar eclipse|solar]] and [[lunar eclipse]]s by [[Babylonian astronomy|Babylonian]] and [[Chinese astronomers]] beginning in the 8th century BCE, as well as from the [[astronomy in the medieval Islamic world|medieval Islamic world]]<ref>{{Citation |title=Solar and lunar eclipses recorded in medieval Arab chronicles |date=1997-06-05 |url=http://dx.doi.org/10.1017/cbo9780511525186.012 |work=Historical Eclipses and Earth's Rotation |pages=431–455 |publisher=Cambridge University Press |doi=10.1017/cbo9780511525186.012 |isbn=9780521461948 |access-date=2022-07-15|url-access=subscription }}</ref> and elsewhere. These observations can be used to determine changes in Earth's rotation over the last 27 centuries, since the length of the day is a critical parameter in the calculation of the place and time of eclipses. A change in day length of milliseconds per century shows up as a change of hours and thousands of kilometers in eclipse observations. The ancient data are consistent with a shorter day, meaning Earth was turning faster throughout the past.<ref>{{cite journal|author=Sid Perkins|date=6 December 2016|title=Ancient eclipses show Earth's rotation is slowing|journal=Science|doi=10.1126/science.aal0469|url=https://www.science.org/content/article/ancient-eclipses-show-earth-s-rotation-slowing|url-access=subscription|doi-access=free}}</ref><ref>{{cite journal |author1=FR Stephenson |author2=LV Morrison |author3=CY Hohonkerk |title= Measurement of the Earth's rotation: 720 BC to AD 2015 |journal= Proceedings of the Royal Society A |date=7 December 2016 |doi= 10.1098/rspa.2016.0404 |pmid=28119545 |volume= 472 |issue= 2196 |pages=20160404 |bibcode=2016RSPSA.47260404S |pmc=5247521 }}</ref>

=== Cyclic variability ===
Around every 25–30 years Earth's rotation slows temporarily by a few milliseconds per day, usually lasting around five years. 2017 was the fourth consecutive year that Earth's rotation has slowed. The cause of this variability has not yet been determined.<ref>{{cite news |last=Nace |first=Trevor |title=Earth's Rotation Is Mysteriously Slowing Down: Experts Predict Uptick In 2018 Earthquakes |url=https://www.forbes.com/sites/trevornace/2017/11/20/earths-rotation-is-mysteriously-slowing-down-experts-predict-uptick-in-2018-earthquakes/#610e73cf6f24 |access-date=18 October 2019 |work=Forbes |language=en}}</ref>

== Origin ==
[[File:The_Mysterious_Case_of_the_Disappearing_Dust.jpg|right|thumb|An artist's rendering of the [[protoplanetary disk]]]]
Earth's original rotation was a vestige of the original [[angular momentum]] of the cloud of [[dust]], [[Rock (geology)|rocks]] and [[gas]] that coalesced to form the [[Solar System]]. This [[primordial cloud]] was composed of [[hydrogen]] and [[helium]] produced in the [[Big Bang]], as well as heavier [[chemical element|elements]] [[Coronal mass ejection|ejected]] by [[supernova]]s. As this [[interstellar dust]] is heterogeneous, any asymmetry during gravitational accretion resulted in the angular momentum of the eventual planet.<ref>{{cite web| url = http://curious.astro.cornell.edu/56-our-solar-system/planets-and-dwarf-planets/general-questions/218-why-do-planets-rotate-intermediate| title = Why do planets rotate?| work = Ask an Astronomer}}</ref>

However, if the [[giant-impact hypothesis]] for the origin of the Moon is correct, this primordial rotation rate would have been reset by the [[Theia (planet)|Theia]] impact 4.5&nbsp;billion years ago. Regardless of the speed and tilt of Earth's rotation before the impact, it would have experienced a day some five hours long after the impact.<ref name=areps15>{{cite journal | last=Stevenson | first=D. J. | title=Origin of the moon–The collision hypothesis | journal=Annual Review of Earth and Planetary Sciences | date=1987 | volume=15 | issue=1 | pages=271–315 | bibcode=1987AREPS..15..271S | doi=10.1146/annurev.ea.15.050187.001415 }}</ref> Tidal effects would then have slowed this rate to its modern value.

== See also ==
<!-- Use the {{wikibooks|Geodesy|Earth's rotation}} template when [[wikibooks:Geodesy]] has something on this subject...-->
{{div col|colwidth=22em}}
* [[Allais effect]]
* [[Diurnal cycle]]
* [[Earth's orbit]]
* [[Earth orientation parameters]]
* [[Formation and evolution of the Solar System]]
* [[Geodesic]] (in mathematics)
* [[Geodesics in general relativity]]
* [[Geodesy]]
* [[History of Earth]]
* [[History of geodesy]]
* [[Inner core super-rotation]]
* [[Nychthemeron]]
* [[Rossby wave]]
* [[Spherical Earth]]
* [[World Geodetic System]]
{{div col end}}

== Notes ==
<references group=n/>

== References ==
{{reflist}}

== 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]
*[http://www.iers.org/ International Earth Rotation and Reference Systems Service (IERS)]
*[http://curious.astro.cornell.edu/126-observational-astronomy/timekeeping/clocks/767-if-the-earth-s-rotation-period-is-less-than-24-hours-why-don-t-our-clocks-fall-out-of-sync-with-the-sun-intermediate?highlight=WyJlYXJ0aCdzIiwiZWFydGgiLCInZWFydGgnIiwiJ2VhcnRoJy4iLCJlYXJ0aCciLCJyb3RhdGlvbiIsImVhcnRoJ3Mgcm90YXRpb24iXQ== If the Earth's rotation period is less than 24 hours, why don't our clocks fall out of sync with the Sun?]

{{Earth}}
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{{DEFAULTSORT:Earth's Rotation}}
[[Category:Dynamics of the Solar System]]
[[Category:Earth|Rotation]]
[[Category:Rotation]]