![EpicEarth-Globespin-tilt-23](https://upload.wikimedia.org/wikipedia/commons/a/a0/EpicEarth-Globespin-tilt-23.4.gif)
Earth's rotation or Earth's spin is the
rotation
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
of planet
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
around its own
axis
An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to:
Mathematics
* Axis of rotation: see rotation around a fixed axis
* Axis (mathematics), a designator for a Cartesian-coordinat ...
, as well as changes in the
orientation
Orientation may refer to:
Positioning in physical space
* Map orientation, the relationship between directions on a map and compass directions
* Orientation (housing), the position of a building with respect to the sun, a concept in building de ...
of the rotation axis in space. Earth rotates
east
East or Orient is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth.
Etymology
As in other languages, the word is formed from the fac ...
ward, in
prograde motion. As viewed from the northern
polar star
A pole star or polar star is a star, preferably bright, nearly aligned with the axis of a rotating astronomical body.
Currently, Earth's pole stars are Polaris (Alpha Ursae Minoris), a bright magnitude-2 star aligned approximately with its ...
Polaris, Earth turns
counterclockwise
Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite ...
.
The
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is called the True North Pole to distinguish from the Mag ...
, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the
Northern Hemisphere
The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
where Earth's axis of rotation meets its surface. This point is distinct from Earth's
North Magnetic Pole
The north magnetic pole, also known as the magnetic north pole, is a point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downward (in other words, if a magnetic compass needle is allowed t ...
. The
South Pole
The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...
is the other point where Earth's axis of rotation intersects its surface, in
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
.
Earth rotates once in about 24 hours with respect to the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, but once every 23 hours, 56 minutes and 4 seconds with respect to other distant stars (
see below). Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the
tidal effects the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
has on Earth's rotation.
Atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s show that a modern day is longer by about 1.7
millisecond
A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds.
A unit of 10 milliseconds may be called ...
s than a century ago,
slowly increasing the rate at which
UTC is adjusted by
leap second
A leap second is a one- second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observ ...
s. Analysis of historical astronomical records shows a slowing trend; the
length of a day increased about 2.3 milliseconds per century since the
8th century BCE
The 8th century BCE started the first day of 800 BC and ended the last day of 701 BC. The 8th century BC is a period of great change for several historically significant civilizations. In Egypt, the Twenty-third Dynasty of Egypt, 23rd and Twent ...
.
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. Because of that trend, engineers worldwide are discussing a 'negative leap second' and other possible timekeeping measures.
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 it spins faster, like a spinning ice skater drawing their arms in, causing them to spin faster.
History
![Earth Rotation (Nepal, Himalayas)](https://upload.wikimedia.org/wikipedia/commons/4/41/Earth_Rotation_%28Nepal%2C_Himalayas%29.jpg)
Among the ancient
Greeks
The Greeks or Hellenes (; el, Έλληνες, ''Éllines'' ) are an ethnic group and nation indigenous to the Eastern Mediterranean and the Black Sea regions, namely Greece, Cyprus, Albania, Italy, Turkey, Egypt, and, to a lesser extent, oth ...
, several of the
Pythagorean
Pythagorean, meaning of or pertaining to the ancient Ionian mathematician, philosopher, and music theorist Pythagoras, may refer to:
Philosophy
* Pythagoreanism, the esoteric and metaphysical beliefs purported to have been held by Pythagoras
* Ne ...
school believed in the rotation of Earth rather than the apparent diurnal rotation of the heavens. Perhaps the first was
Philolaus
Philolaus (; grc, Φιλόλαος, ''Philólaos''; ) was a Greek Pythagorean and pre-Socratic philosopher. He was born in a Greek colony in Italy and migrated to Greece. Philolaus has been called one of three most prominent figures in the Pyt ...
(470–385 BCE), though his system was complicated, including a
counter-earth
The Counter-Earth is a hypothetical body of the Solar System that orbits on the other side of the solar system from Earth.
A Counter-Earth or ''Antichthon'' ( el, Ἀντίχθων) was hypothesized by the pre-Socratic Greek philosopher Philol ...
rotating daily about a central fire.
A more conventional picture was supported by
Hicetas
Hicetas ( grc, Ἱκέτας or ; c. 400 – c. 335 BC) was a Greek philosopher of the Pythagorean School. He was born in Syracuse. Like his fellow Pythagorean Ecphantus and the Academic Heraclides Ponticus, he believed that the daily movemen ...
,
Heraclides and
Ecphantus
Ecphantus or Ecphantos ( grc, Ἔκφαντος) or Ephantus () is a shadowy Greek pre-Socratic philosopher. He may not have actually existed. He is identified as a Pythagorean of the 4th century BCE, and as a supporter of the heliocentric theo ...
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
Aristarchus of Samos (; grc-gre, Ἀρίσταρχος ὁ Σάμιος, ''Aristarkhos ho Samios''; ) was an ancient Greek astronomer
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or ...
suggested the
Sun's central place.
However,
Aristotle
Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of phil ...
in the fourth century BCE criticized the ideas of Philolaus as being based on theory rather than observation. He established the
idea
In common usage and in philosophy, ideas are the results of thought. Also in philosophy, ideas can also be mental representational images of some object. Many philosophers have considered ideas to be a fundamental ontological category of being ...
of a sphere of fixed stars that rotated about Earth. This was accepted by most of those who came after, in particular
Claudius Ptolemy
Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importa ...
(2nd century CE), who thought Earth would be devastated by gales if it rotated.
In 499 CE, the
Indian astronomer Aryabhata
Aryabhata ( ISO: ) or Aryabhata I (476–550 CE) was an Indian mathematician and astronomer of the classical age of Indian mathematics and Indian astronomy. He flourished in the Gupta Era and produced works such as the ''Aryabhatiya'' (which ...
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
Lanka (, ) is the name given in Hindu epics to the island fortress capital of the legendary asura king Ravana in the epics of the ''Ramayana'' and the ''Mahabharata''. The fortress was situated on a plateau between three mountain peaks known ...
the fixed stars appear to be going westward."
In the 10th century, some
Muslim astronomers
Islamic astronomy comprises the Astronomy, astronomical developments made in the Islamic world, particularly during the Islamic Golden Age (9th–13th centuries), and mostly written in the Arabic language. These developments mostly took place in ...
accepted that Earth rotates around its axis. According to
al-Biruni
Abu Rayhan Muhammad ibn Ahmad al-Biruni (973 – after 1050) commonly known as al-Biruni, was a Khwarazmian Iranian in scholar and polymath during the Islamic Golden Age. He has been called variously the "founder of Indology", "Father of Co ...
,
Abu Sa'id al-Sijzi
Abu Sa'id Ahmed ibn Mohammed ibn Abd al-Jalil al-Sijzi (c. 945 - c. 1020, also known as al-Sinjari and al-Sijazi; fa, ابوسعید سجزی; Al-Sijzi is short for "Al-Sijistani") was an Iranian Muslim astronomer, mathematician, and astrolo ...
(d. circa 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."
The prevalence of this view is further confirmed by a reference from the 13th century which states: "According to the geometers
r engineers
R, or r, is the eighteenth letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''ar'' (pronounced ), plural ''ars'', or in Irelan ...
(''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."
Treatises were written to discuss its possibility, either as refutations or expressing doubts about Ptolemy's arguments against it. At the
Maragha
Maragheh ( fa, مراغه, Marāgheh or ''Marāgha''; az, ماراغا ) is a city and capital of Maragheh County, East Azerbaijan Province, Iran. Maragheh is on the bank of the river Sufi Chay. The population consists mostly of Iranian Azerba ...
and
Samarkand observatories, Earth's rotation was discussed by
Tusi
''Tusi'', often translated as "headmen" or "chieftains", were hereditary tribal leaders recognized as imperial officials by the Yuan, Ming, and Qing dynasties of China, and the Later Lê and Nguyễn dynasties of Vietnam. They ruled certain e ...
(b. 1201) and
Qushji (b. 1403); the arguments and evidence they used resemble those used by Copernicus.
In medieval Europe,
Thomas Aquinas
Thomas Aquinas, OP (; it, Tommaso d'Aquino, lit=Thomas of Aquino; 1225 – 7 March 1274) was an Italian Dominican friar and priest who was an influential philosopher, theologian and jurist in the tradition of scholasticism; he is known wi ...
accepted Aristotle's view and so, reluctantly, did
John Buridan and
Nicole Oresme
Nicole Oresme (; c. 1320–1325 – 11 July 1382), also known as Nicolas Oresme, Nicholas Oresme, or Nicolas d'Oresme, was a French philosopher of the later Middle Ages. He wrote influential works on economics, mathematics, physics, astrology an ...
in the fourteenth century. Not until
Nicolaus Copernicus
Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic canon, who formulated ...
in 1543 adopted a
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.
Tycho Brahe
Tycho Brahe ( ; born Tyge Ottesen Brahe; generally called Tycho (14 December 154624 October 1601) was a Danish astronomer, known for his comprehensive astronomical observations, generally considered to be the most accurate of his time. He was k ...
, who produced accurate observations on which
Kepler
Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...
based his
laws of planetary motion, used Copernicus's work as the basis of a
system
A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment (systems), environment, is described by its boundaries, ...
assuming a stationary Earth. In 1600,
William Gilbert strongly supported Earth's rotation in his treatise on Earth's magnetism and thereby influenced many of his contemporaries.
Those like Gilbert who did not openly support or reject the motion of Earth about the Sun are called "semi-Copernicans".
A century after Copernicus,
Riccioli
Giovanni Battista Riccioli, SJ (17 April 1598 – 25 June 1671) was an Italian astronomer and a Catholic priest in the Jesuit order. He is known, among other things, for his experiments with pendulums and with falling bodies, for his discussion ...
disputed the model of a rotating Earth due to the lack of then-observable eastward deflections in falling bodies; such deflections would later be called the
Coriolis effect
In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
. However, the contributions of Kepler,
Galileo and
Newton gathered support for the theory of the rotation of Earth.
Empirical tests
Earth's rotation implies that the
Equator bulges and the
geographical pole
A geographical pole or geographic pole is either of the two points on Earth where its axis of rotation intersects its surface. The North Pole lies in the Arctic Ocean while the South Pole is in Antarctica. North and South poles are also define ...
s are flattened. In his ''
Principia'', Newton predicted this
flattening
Flattening is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution ( spheroid) respectively. Other terms used are ellipticity, or oblateness. The usual notation for flattening i ...
would occur in the ratio of 1:230, and pointed to the
pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward th ...
measurements taken by
Richer in 1673 as corroboration of the change in
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
, but
initial measurements of
meridian lengths by
Picard and
Cassini at the end of the 17th century suggested the opposite. However, measurements by
Maupertuis
Pierre Louis Moreau de Maupertuis (; ; 1698 – 27 July 1759) was a French mathematician, philosopher and man of letters. He became the Director of the Académie des Sciences, and the first President of the Prussian Academy of Science, at the ...
and the
French Geodesic Mission
The French Geodesic Mission to the Equator (french: Expédition géodésique française en Équateur, also called the French Geodesic Mission to Peru and the Spanish-French Geodesic Mission) was an 18th-century expedition to what is now Ecuador c ...
in the 1730s established the
oblateness of Earth, thus confirming the positions of both Newton and
Copernicus
Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic canon, who formulated ...
.
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
In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
, falling bodies veer slightly eastward from the vertical plumb line below their point of release, and projectiles veer right in the
Northern Hemisphere
The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
(and left in the
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
Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite ...
, respectively).
Hooke, following a suggestion from Newton in 1679, tried unsuccessfully to verify the predicted eastward deviation of a body dropped from a height of , but definitive results were obtained later, in the late 18th and early 19th century, by
Giovanni Battista Guglielmini
Giovanni Battista Guglielmini (; 16 August 1763 – 15 December 1817) was an Italian physicist. Guglielmini's experiments take place in the list of experiments by scientists ( Hooke, Guglielmini, Benzenberg, Reich, Foucault) to demonstrate the ...
in
Bologna
Bologna (, , ; egl, label= Emilian, Bulåggna ; lat, Bononia) is the capital and largest city of the Emilia-Romagna region in Northern Italy. It is the seventh most populous city in Italy with about 400,000 inhabitants and 150 different nat ...
,
Johann Friedrich Benzenberg
Johann Friedrich Benzenberg (5 May 1777 – 7 June 1846) was a German astronomer, geologist, and physicist.
Biography
Benzenberg was born near Elberfeld, Germany on 5 May 1777 to Heinrich Benzenberg and Johanna Elisabeth. He married Charlotte Pla ...
in
Hamburg
(male), (female) en, Hamburger(s),
Hamburgian(s)
, timezone1 = Central (CET)
, utc_offset1 = +1
, timezone1_DST = Central (CEST)
, utc_offset1_DST = +2
, postal ...
and
Ferdinand Reich
Ferdinand Reich (19 February 1799 – 27 April 1882) was a German chemist who co-discovered indium in 1863 with Hieronymous Theodor Richter.
Reich was born in Bernburg and died in Freiberg. He was color blind, or could only see in whites a ...
in
Freiberg
Freiberg is a university and former mining town in Saxony, Germany. It is a so-called ''Große Kreisstadt'' (large county town) and the administrative centre of Mittelsachsen district.
Its historic town centre has been placed under heritage c ...
, using taller towers and carefully released weights.
[See Fallexperimente zum Nachweis der Erdrotation (German Wikipedia article).] A ball dropped from a height of 158.5 m departed by 27.4 mm from the vertical compared with a calculated value of 28.1 mm.
The most celebrated test of Earth's rotation is the
Foucault pendulum
The Foucault pendulum or Foucault's pendulum is a simple device named after French physicist Léon Foucault, conceived as an experiment to demonstrate the Earth's rotation. A long and heavy pendulum suspended from the high roof above a circular ...
first built by physicist
Léon Foucault
Jean Bernard Léon Foucault (, ; ; 18 September 1819 – 11 February 1868) was a French physicist best known for his demonstration of the Foucault pendulum, a device demonstrating the effect of Earth's rotation. He also made an early measurement ...
in 1851, which consisted of a lead-filled brass sphere suspended from the top of the
Panthéon
The Panthéon (, from the Classical Greek word , , ' empleto all the gods') is a monument in the 5th arrondissement of Paris, France. It stands in the Latin Quarter, atop the , in the centre of the , which was named after it. The edifice was b ...
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
clockwise
Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite ...
per hour. Foucault pendulums now swing in
museums around the world.
Periods
True solar day
Earth's
rotation period relative to the Sun (
solar noon
Noon (or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for meridiem, literally 12:00 noon), 12 p.m. (for post meridiem, literally "after noon"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 ( military time).
Sola ...
to solar noon) is its ''true solar day'' or ''apparent solar day''. It depends on Earth's orbital motion and is thus affected by changes in the
eccentricity
Eccentricity or eccentric may refer to:
* Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal"
Mathematics, science and technology Mathematics
* Off-Centre (geometry), center, in geometry
* Eccentricity (g ...
and
inclination
Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...
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. The true solar day tends to be longer near
perihelion
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any elli ...
when the Sun apparently moves along the
ecliptic
The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
through a greater angle than usual, taking about longer to do so. Conversely, it is about shorter near
aphelion
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ell ...
. It is about longer near a
solstice
A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many countr ...
when the projection of the Sun's apparent motion along the ecliptic onto the
celestial equator
The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract proj ...
causes the Sun to move through a greater angle than usual. Conversely, near an
equinox
A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
the projection onto the equator is shorter by about . Currently, the perihelion and solstice effects combine to lengthen the true solar day near by solar seconds, but the solstice effect is partially cancelled by the aphelion effect near when it is only longer. The effects of the equinoxes shorten it near and by and , respectively.
Mean solar day
The average of the true solar day during the course of an entire year is the ''mean solar day'', which contains . 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
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away f ...
. 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 . Random fluctuations due to core-mantle coupling have an amplitude of about 5 ms. The mean solar second between 1750 and 1892 was chosen in 1895 by
Simon Newcomb
Simon Newcomb (March 12, 1835 – July 11, 1909) was a Canadian–American astronomer, applied mathematician, and autodidactic polymath. He served as Professor of Mathematics in the United States Navy and at Johns Hopkins University. Born in N ...
as the independent unit of time in his
Tables of the Sun. These tables were used to calculate the world's
ephemerides
In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly ...
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.
The
apparent solar time
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time (sundial ti ...
is a measure of Earth's rotation and the difference between it and the mean solar time is known as the
equation of time
In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in F ...
.
Stellar and sidereal day
![Sidereal day (prograde)](https://upload.wikimedia.org/wikipedia/commons/8/8b/Sidereal_day_%28prograde%29.svg)
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 seconds of mean solar time (UT1) , ).
Earth's rotation period relative to the axial precession, precessing mean vernal equinox (celestial coordinates), equinox, named ''sidereal time, sidereal day'', is of mean solar time (UT1) , ).
[ Thus, the sidereal day is shorter than the stellar day by about .
Both the stellar day and the sidereal day are shorter than the mean solar day by about . This is a result of the Earth turning 1 additional rotation, relative to the celestial reference frame, as it orbits the Sun (so 366.25 rotations/y). The mean solar day in SI seconds is available from the IERS for the periods and .
Recently (1999–2010) the average annual length of the mean solar day in excess of has varied between and , 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
The angular speed of Earth's rotation in inertial space is ± .[It can be established that SI seconds apply to this value by following the citation in "USEFUL CONSTANTS" to E. Grote]
"Parameters of Common Relevance of Astronomy, Geodesy, and Geodynamics"
which states units are SI units, except for an instance not relevant to this value. Multiplying by (180°/π radians) × (86,400 seconds/day) yields , indicating that Earth rotates more than 360° 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.[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 for revolution around the Sun.] Multiplying the value in rad/s by Earth's equatorial radius of (WGS84 ellipsoid) (factors of 2π radians needed by both cancel) yields an equatorial speed of . Some sources state that Earth's equatorial speed is slightly less, or . This is obtained by dividing Earth's equatorial circumference by . 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, ,[ 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. For example, the Kennedy Space Center is located at latitude 28.59° N, which yields a speed of: cos(28.59°) × 1674.4 km/h = 1470.2 km/h. Latitude is a placement consideration for spaceports.
The peak of the Cayambe (volcano), Cayambe volcano is the point of ]Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's surface farthest from its axis; thus, it rotates the fastest as Earth spins.
Changes
In rotational 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
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, Moon and other bodies. The polar motion is primarily due to Nutation, free core nutation and the Chandler wobble.
In rotational speed
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 , where 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 Tidal locking, tidally locked with Earth.
This gradual rotational deceleration is empirically documented by estimates of day lengths obtained from observations of tidal rhythmites and stromatolites; a compilation of these measurements found that the length of the day has increased steadily from about 21 hours at 600 Myr ago[ 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.
]
Resonant stabilization
The current rate of tidal deceleration is anomalously high, implying Earth's rotational velocity must have decreased more slowly in the past. Empirical data 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. This day length corresponds to the semidiurnal Resonance frequency, 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 glaciations broke this stable configuration about 600 Myr ago; the simulated results agree quite closely with existing paleorotational data.
Global events
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. 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.
The length of the day can also be influenced by manmade 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.
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.
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 and Satellite laser ranging to geodetic satellites.
Ancient observations
There are recorded observations of solar eclipse, solar and lunar eclipses by Babylonian astronomy, Babylonian and Chinese astronomy, Chinese astronomers beginning in the 8th century BCE, as well as from astronomy in the medieval Islamic world, the medieval Islamic world 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.
Cyclic variability
Around every 25–30 years Earth's rotation slows temporarily by a few milliseconds per day, usually lasting around 5 years. 2017 was the fourth consecutive year that Earth's rotation has slowed. The cause of this variability has not yet been determined.
Origin
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 supernovas. As this interstellar dust is heterogeneous, any asymmetry during gravitational accretion resulted in the angular momentum of the eventual planet.
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 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. Tidal effects would then have slowed this rate to its modern value.
See also
* 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
* List of important publications in geology
* Nychthemeron
* Spherical Earth
* World Geodetic System
Notes
References
External links
USNO Earth Orientation
new site, being populated
USNO IERS
old site, to be abandoned
IERS Earth Orientation Center: Earth rotation data and interactive analysis
International Earth Rotation and Reference Systems Service (IERS)
If the Earth's rotation period is less than 24 hours, why don't our clocks fall out of sync with the Sun?
{{DEFAULTSORT:Earth's Rotation
Dynamics of the Solar System
Earth, Rotation
Rotation