TheNorth Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in theNorthern Hemisphere where Earth's axis of rotation meets its surface. This point is distinct from Earth'snorth magnetic pole. TheSouth Pole is the other point where Earth's axis of rotation intersects its surface, inAntarctica.
Earth rotates once in about 24 hours with respect to theSun, 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 thetidal effects theMoon has on Earth's rotation.Atomic clocks show that the modern day is longer by about 1.7milliseconds than a century ago,[1] slowly increasing the rate at whichUTC is adjusted byleap seconds. Analysis of historical astronomical records shows a slowing trend; thelength of a day increased by about 2.3 milliseconds per century since the8th century BCE.[2]
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.[3] Because of that trend, engineers worldwide are discussing a 'negative leap second' and other possible timekeeping measures.[4]
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 theEarth's shape being that of anoblate 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.[5] the Earth's rotation speed is approximately 1,670 km/h at the equator, but it decreases with latitude, becoming zero at the poles. This is because the circumference of the Earth is widest at the equator and shrinks towards the poles. The speed at any given latitude can be calculated by multiplying the equatorial speed by the cosine of the latitude: at the equator, the speed is about 1,670 kilometres per hour (1,040 mph), calculated by dividing the Earth's equatorial circumference (approximately 40,075 kilometres (24,901 mi)) by the length of a sidereal day (approximately 23.93 hours). At mid-latitudes the speed decreases - for example, at 45 degrees latitude, the speed is roughly 1,180 kilometres per hour (730 mph) - (1670 × cos (45°)). At the poles the speed is effectively zero, as there is no distance to cover in a full rotation.
Among the ancientGreeks, several of thePythagorean school believed in the rotation of Earth rather than the apparent diurnal rotation of the heavens. Perhaps the first wasPhilolaus (470–385 BCE), though his system was complicated, including acounter-earth rotating daily about a central fire.[6]
A more conventional picture was supported byHicetas,Heraclides andEcphantus 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 theSun'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 theidea of a sphere of fixed stars that rotated about Earth.[7] This was accepted by most of those who came after, in particularClaudius Ptolemy (2nd century CE), who thought Earth would be devastated by gales if it rotated.[8]
In 499 CE, theIndian astronomerAryabhata 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 atLanka the fixed stars appear to be going westward."[9][10]
In the 10th century, someMuslim astronomers accepted that the Earth rotates around its axis.[11] According toal-Biruni,al-Sijzi (d. c. 1020) invented anastrolabe calledal-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."[12][13] 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."[12] Treatises were written to discuss its possibility, either as refutations or expressing doubts about Ptolemy's arguments against it.[14] At theMaragha andSamarkand observatories, Earth's rotation was discussed byTusi (born 1201) andQushji (born 1403); the arguments and evidence they used resemble those used by Copernicus.[15]
In medieval Europe,Thomas Aquinas accepted Aristotle's view[16] and so, reluctantly, didJohn Buridan[17] andNicole Oresme[18] in the fourteenth century. Not untilNicolaus Copernicus in 1543 adopted aheliocentric 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.[19]
Tycho Brahe, who produced accurate observations on whichKepler based hislaws of planetary motion, used Copernicus's work as the basis of asystem assuming a stationary Earth. In 1600,William Gilbert strongly supported Earth's rotation in his treatise on Earth's magnetism[20] and thereby influenced many of his contemporaries.[21]: 208 Those like Gilbert who did not openly support or reject the motion of Earth about the Sun are called "semi-Copernicans".[21]: 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;[22] such deflections would later be called theCoriolis effect. However, the contributions of Kepler,Galileo, andNewton gathered support for the theory of the rotation of the Earth.
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 theCoriolis effect, falling bodies veer slightly eastward from the vertical plumb line below their point of release, and projectiles veer right in theNorthern Hemisphere (and left in theSouthern) 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 ofcyclone rotation in the Northern and Southern hemispheres (anticlockwise andclockwise, 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 of8.2 meters, but definitive results were obtained later, in the late 18th and early 19th centuries, byGiovanni Battista Guglielmini inBologna,Johann Friedrich Benzenberg inHamburg andFerdinand Reich inFreiberg, using taller towers and carefully released weights.[n 1] 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 theFoucault pendulum first built by physicistLéon Foucault in 1851, which consisted of a lead-filled brass sphere suspended67 m from the top of thePanthéon 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 about11 degreesclockwise per hour. Foucault pendulums now swing inmuseums worldwide.
Earth'srotation period relative to the Sun (solar noon to solar noon) is itstrue solar day orapparent solar day.[26] It depends on Earth's orbital motion and is thus affected by changes in theeccentricity andinclination 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.[n 2] The true solar day tends to be longer nearperihelion when the Sun apparently moves along theecliptic through a greater angle than usual, taking about10 seconds longer to do so. Conversely, it is about10 seconds shorter nearaphelion. It is about20 seconds longer near asolstice when the projection of the Sun's apparent motion along the ecliptic onto thecelestial equator causes the Sun to move through a greater angle than usual. Conversely, near anequinox the projection onto the equator is shorter by about20 seconds. Currently, the perihelion and solstice effects combine to lengthen the true solar day near22 December by30 mean solar seconds, but the solstice effect is partially cancelled by the aphelion effect near19 June when it is only13 seconds longer. The effects of the equinoxes shorten it near26 March and16 September by18 seconds and21 seconds, respectively.[27][28]
The average of the true solar day during the course of an entire year is themean solar day, which contains 86,400 mean solar seconds. Currently, each of these seconds is slightly longer than anSI second because Earth's mean solar day is now slightly longer than it was during the 19th century due totidal 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.[29][30][31] Random fluctuations due to core-mantle coupling have an amplitude of about 5 ms.[32][33] The mean solar second between 1750 and 1892 was chosen in 1895 bySimon Newcomb as the independent unit of time in hisTables of the Sun. These tables were used to calculate the world'sephemerides between 1900 and 1983, so this second became known as theephemeris second. In 1967 the SI second was made equal to the ephemeris second.[34]
Theapparent solar time is a measure of Earth's rotation and the difference between it and the mean solar time is known as theequation of time.
On aprograde planet like Earth, thestellar day is shorter than thesolar 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).
Both the stellar day and the sidereal day are shorter than the mean solar day by about3 minutes56 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 periods1623–2005[38] and1962–2005.[39]
Recently (1999–2010) the average annual length of the mean solar day in excess of 86,400 SI seconds has varied between0.25 ms and1 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 (seeFluctuations in the length of day).
Plot of latitude versus tangential speed. The dashed line shows theKennedy Space Center example. The dot-dash line denotes typical airlinercruise speed.
Theangular speed of Earth's rotation in inertial space is(7.2921150 ±0.0000001)×10^−5radians per SI second.[35][n 4] Multiplying by (180°/π radians) × (86,400 seconds/day) yields360.9856 °/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.[n 5] Multiplying the value in rad/s by Earth's equatorial radius of6,378,137 m (WGS84 ellipsoid) (factors of 2π radians needed by both cancel) yields an equatorial speed of 465.10 metres per second (1,674.4 km/h).[40] Some sources state that Earth's equatorial speed is slightly less, or1,669.8 km/h.[41] This is obtained by dividing Earth's equatorial circumference by24 hours. However, the use of the solar day is incorrect; it must be thesidereal 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,1.002 737 909 350 795,[35] 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.[42] For example, theKennedy 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 forspaceports.
WhileEverest is Earth's highest elevation (green) andMauna Kea is tallest from its base (orange),Cayambe is farthest from Earth's axis (pink) andChimborazo is farthest from Earth's centre (blue). Not to scale
The peak of theCayambe volcano is the point ofEarth's surface farthest from its axis; thus, it rotates the fastest as Earth spins.[43]
Earth's rotation axis moves with respect to the fixed stars (inertial space); the components of this motion areprecession andnutation. It also moves with respect to Earth's crust; this is calledpolar 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 corenutation and theChandler wobble.
Over millions of years, Earth's rotation has been slowed significantly bytidal acceleration through gravitational interactions with the Moon. Thusangular 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 beingtidally locked with Earth.
This gradual rotational deceleration is empirically documented by estimates of day lengths obtained from observations oftidal rhythmites andstromatolites; a compilation of these measurements[44] found that the length of the day has increased steadily from about 21 hours at 600 Myr ago[45] 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.[46]
A simulated history of Earth's day length, depicting a resonant-stabilizing event throughout the Precambrian era[47]
The current rate of tidal deceleration is anomalously high, implying Earth's rotational velocity must have decreased more slowly in the past. Empirical data[44] 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 thePrecambrian.[45] This day length corresponds to the semidiurnalresonant period of the thermally drivenatmospheric 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 theMarinoan orSturtian glaciations broke this stable configuration about 600 Myr ago; the simulated results agree quite closely with existing paleo-rotational data.[47]
The length of the day can also be influenced by man-made structures. For example,NASA scientists calculated that the water stored in theThree Gorges Dam has increased the length of Earth's day by 0.06 microseconds due to the shift in mass.[50]
There are recorded observations ofsolar andlunar eclipses byBabylonian andChinese astronomers beginning in the 8th century BCE, as well as from themedieval Islamic world[54] 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.[55][56]
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.[57]
However, if thegiant-impact hypothesis for the origin of the Moon is correct, this primordial rotation rate would have been reset by theTheia 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.[59] Tidal effects would then have slowed this rate to its modern value.
^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.[27]
^Aoki, the ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".[36]
^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 meanJulian year containing exactly365.25 days for revolution around the Sun.
^Aquinas, Thomas.Commentaria in libros Aristotelis De caelo et Mundo. Lib II, cap XIV. trans inGrant, Edward, ed. (1974).A Source Book in Medieval Science.Harvard University Press.Bibcode:1974sbms.book.....G. pages 496–500
^Buridan, John (1942).Quaestiones super libris quattuo De Caelo et mundo. pp. 226–232. inGrant 1974, pp. 500–503
^Oresme, Nicole.Le livre du ciel et du monde. pp. 519–539. inGrant 1974, pp. 503–510
^Copernicus, Nicolas.On the Revolutions of the Heavenly Spheres. Book I, Chap 5–8.
^Almagestum novum, chapter nine, cited inGraney, Christopher M. (2012). "126 arguments concerning the motion of the earth. GIOVANNI BATTISTA RICCIOLI in his 1651 ALMAGESTUM NOVUM".Journal for the History of Astronomy. volume 43, pages 215–226.arXiv:1103.2057.
^Newton, Isaac (1846).Newton's Principia. Translated by A. Motte. New-York: Published by Daniel Adee. p. 412.
^Seidelmann, P. Kenneth, ed. (1992).Explanatory Supplement to the Astronomical Almanac. Mill Valley, California: University Science Books. p. 48.ISBN978-0-935702-68-2.
^Scrutton, C. T. (1 January 1978). "Periodic Growth Features in Fossil Organisms and the Length of the Day and Month". In Brosche, Professor Dr Peter; Sündermann, Professor Dr Jürgen (eds.).Tidal Friction and the Earth's Rotation. Springer Berlin Heidelberg. pp. 154–196.doi:10.1007/978-3-642-67097-8_12.ISBN978-3-540-09046-5.
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