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Equatorial bulge

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175: 554:, ran slower than their exact counterparts in Paris. Measurements of the acceleration due to gravity at the equator must also take into account the planet's rotation. Any object that is stationary with respect to the surface of the Earth is actually following a circular trajectory, circumnavigating the Earth's axis. Pulling an object into such a circular trajectory requires a force. The acceleration that is required to circumnavigate the Earth's axis along the equator at one revolution per 43: 217:
of the equatorial diameter. If Earth were scaled down to a globe with an equatorial diameter of 1 metre (3.3 ft), that difference would be only 3 mm (0.12 in). While too small to notice visually, that difference is still more than twice the largest deviations of the actual surface from
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As long as there is no equilibrium there can be violent convection, and as long as there is violent convection friction can convert kinetic energy to heat, draining rotational kinetic energy from the system. When the equilibrium state has been reached then large scale conversion of kinetic energy to
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At the poles, the gravitational acceleration is 9.8322 m/s. The difference of 0.0178 m/s between the gravitational acceleration at the poles and the true gravitational acceleration at the Equator is because objects located on the Equator are about 21 km (13 mi) further away from
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The Earth's rotation rate is still slowing down, though gradually, by about two thousandths of a second per rotation every 100 years. Estimates of how fast the Earth was rotating in the past vary, because it is not known exactly how the moon was formed. Estimates of the Earth's rotation 500 million
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In summary, there are two contributions to the fact that the effective gravitational acceleration is less strong at the equator than at the poles. About 70% of the difference is contributed by the fact that objects circumnavigate the Earth's axis, and about 30% is due to the non-spherical shape of
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on contraction keeps driving the increase in rotational kinetic energy. As the contraction proceeds, the rotation rate keeps going up, hence the required force for further contraction keeps going up. There is a point where the increase of rotational kinetic energy on further contraction would be
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Fixed to the vertical rod is a spring metal band. When stationary the spring metal band is circular in shape. The top of the metal band can slide along the vertical rod. When spun, the spring-metal band bulges at its equator and flattens at its poles in analogy with the
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is 0.0339 m/s. Providing this acceleration decreases the effective gravitational acceleration. At the Equator, the effective gravitational acceleration is 9.7805 m/s. This means that the true gravitational acceleration at the Equator must be 9.8144 m/s
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Something analogous to this occurs in planet formation. Matter first coalesces into a slowly rotating disk-shaped distribution, and collisions and friction convert kinetic energy to heat, which allows the disk to self-gravitate into a very oblate spheroid.
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A way for one to get a feel for the type of equilibrium involved is to imagine someone seated in a spinning swivel chair and holding a weight in each hand; if the individual pulls the weights inward towards them,
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for the equilibrium configuration of a self-gravitating spheroid, composed of uniform density incompressible fluid, rotating steadily about some fixed axis, for a small amount of flattening, is approximated by:
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The diagram illustrates that on all latitudes the effective gravitational acceleration is decreased by the requirement of providing a centripetal force; the decreasing effect is strongest on the Equator.
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Such perturbations, which were earlier used to map the Earth's gravitational field from space, may play a relevant disturbing role when satellites are used to make tests of
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has a rather slight equatorial bulge; its equatorial diameter is about 43 km (27 mi) greater than its polar diameter, with a difference of about
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Renzetti, G. (2014). "Satellite orbital precessions caused by the first odd zonal J3 multipole of a non-spherical body arbitrarily oriented in space".
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is being done and their rotational kinetic energy increases. The increase in rotation rate is so strong that at the faster rotation rate the required
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The Earth's rate of rotation is slowing down mainly because of tidal interactions with the Moon and the Sun. Since the solid parts of the Earth are
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Renzetti, G. (2013). "Satellite Orbital Precessions Caused by the Octupolar Mass Moment of a Non-Spherical Body Arbitrarily Oriented in Space".
1260:{\displaystyle J_{2}={\frac {2\varepsilon _{\mathrm {E} }}{3}}-{\frac {{R_{\mathrm {E} }}^{3}{\omega _{\mathrm {E} }}^{2}}{3GM_{\mathrm {E} }}}} 275:, Chimborazo is not as high above sea level as Everest is. Similarly the lowest point on Earth, measured from the center and outwards, is the 519:
The resultant force provides required centripetal force. Without this centripetal force frictionless objects would slide towards the equator.
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orbital precessions. They depend on the orientation of the Earth's symmetry axis in the inertial space, and, in the general case, affect
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larger than the release of gravitational potential energy. The contraction process can only proceed up to that point, so it halts there.
17: 244:. This surface coincides with the mean water surface level in oceans, and is extrapolated over land by taking into account the local 291:. But since the ocean also flattens, like Earth and its atmosphere, Litke Deep is not as low below sea level as Challenger Deep is. 1624:
Iorio, L. (2011). "Perturbed stellar motions around the rotating black hole in Sgr A* for a generic orientation of its spin axis".
802:{\displaystyle f={\frac {a_{e}-a_{p}}{a}}={\frac {5}{4}}{\frac {\omega ^{2}a^{3}}{GM}}={\frac {15\pi }{4}}{\frac {1}{GT^{2}\rho }}} 1929: 107: 583:
The fact that the Earth's gravitational field slightly deviates from being spherically symmetrical also affects the orbits of
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because the much smaller relativistic effects are qualitatively indistinguishable from the oblateness-driven disturbances.
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Renzetti, G. (2012). "Are higher degree even zonals really harmful for the LARES/LAGEOS frame-dragging experiment?".
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In calculations, when a coordinate system is used that is co-rotating with the Earth, the vector of the notional
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heat ceases. In that sense the equilibrium state is the lowest state of energy that can be reached.
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axis of the coordinate system adopted is aligned along the Earth's symmetry axis, then only the
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of the Earth than at the poles, which corresponds to a smaller gravitational acceleration.
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causes a distortion from this spherical shape; a common measure of the distortion is the
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Real flattening is smaller due to mass concentration in the center of celestial bodies.
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points outward, and is just as large as the vector representing the centripetal force.
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As long as the proto-planet is still too oblate to be in equilibrium, the release of
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The forces at play in the case of a planet with an equatorial bulge due to rotation.
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is thus about 21 km (13 mi). An observer standing at sea level on either
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More precisely, Earth's surface is usually approximated by an ideal
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Outward bulge around a planet's equator due to its rotation
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Because of a planet's rotation around its own axis, the
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about the body's axis. A rotating body tends to form an
492: 1085: 1019: 954: 896: 542:. In the 17th century, following the invention of the 1148: 1115:{\displaystyle M\simeq {\tfrac {4}{3}}\pi \rho a^{3}} 1077: 1057: 1011: 991: 926: 868: 846: 820: 670: 649: 976:{\displaystyle a_{p}\approx a\,(1-{\tfrac {2f}{3}})} 913:{\displaystyle a_{e}\approx a\,(1+{\tfrac {f}{3}})} 396:is much closer to this standard ellipsoid than the 67:. Unsourced material may be challenged and removed. 1259: 1114: 1063: 1037: 997: 975: 912: 852: 826: 801: 655: 360: mi) to either pole, meaning a difference of 271:. But since the ocean also bulges, like Earth and 983:are respectively the equatorial and polar radius, 178:Comparison between an oblate spheroid and sphere. 1945: 559:(9.7805 + 0.0339 = 9.8144). 482:years ago are around 20 modern hours per "day". 462:is larger than with the starting rotation rate. 410:Hydrostatic equilibrium § Planetary geology 388: mi) between the diameters, and a relative 310:, as well as the "center of the Earth". In the 30:For the feature on some of Saturn's moons, see 546:, French scientists found that clocks sent to 578: 298:, for the purposes of defining precisely the 1912:UCSD David T.Sandwell – Gravity field (2002) 1038:{\displaystyle \omega ={\tfrac {2\pi }{T}}} 426:tends to contract a celestial body into a 1839: 1811: 1796: 1768: 1639: 943: 885: 127:Learn how and when to remove this message 1854: 1723: 1678: 502: 413: 404:The equilibrium as a balance of energies 173: 14: 1946: 1682:Journal of Astrophysics and Astronomy 1623: 1357:is the central body's rotation rate ( 1335:is central body's equatorial radius ( 318:, widely used for map-making and the 493:Effect on gravitational acceleration 65:adding citations to reliable sources 36: 1595: 24: 1841:10.1111/j.1365-246X.1983.tb01868.x 1798:10.1111/j.1365-246X.1961.tb06801.x 1248: 1221: 1200: 1174: 236:, the imaginary surface used as a 25: 1975: 1401: 1126:A related quantity is the body's 1931:IERS – Geopotential model (2010) 1563:Astronomical object § Shape 1379:is the product of the universal 41: 1922: 609:longitude of the ascending node 516:Blue arrow: the resultant force 374: mi) between the radii or 52:needs additional citations for 1903: 1885: 1848: 1805: 1762: 1727:Astrophysics and Space Science 1717: 1672: 1617: 1589: 970: 944: 907: 886: 635: 471:gravitational potential energy 13: 1: 1583: 1383:and the central body's mass ( 622:undergo secular precessions. 340: mi) to the Equator and 218:the ellipsoid, including the 1568:Clairaut's theorem (gravity) 143:is a difference between the 7: 1858:Canadian Journal of Physics 1556: 1005:is the rotation period and 550:, on the northern coast of 248:and the centrifugal force. 182: 10: 1980: 1658:10.1103/PhysRevD.84.124001 1128:second dynamic form factor 599:with the exception of the 579:Effect on satellite orbits 532:gravitational acceleration 496: 407: 192: 186: 29: 18:Second dynamic form factor 1812:King-Hele, D. G. (1983). 1769:King-Hele, D. G. (1961). 1748:10.1007/s10509-014-1915-x 1703:10.1007/s12036-013-9186-4 1071:is the body density and 392:of 1/298.257223563. The 1893:"Rotational Flattening" 1740:2014Ap&SS.352..493R 1381:constant of gravitation 1122:is the total body mass. 246:gravitational potential 1320:is the central body's 1261: 1116: 1065: 1039: 999: 977: 914: 854: 836:gravitational constant 828: 803: 657: 613:argument of pericenter 527: 420: 251:The difference of the 240:from which to measure 179: 1262: 1117: 1066: 1064:{\displaystyle \rho } 1040: 1000: 978: 915: 855: 829: 804: 658: 506: 417: 408:Further information: 193:Further information: 177: 1578:Planetary flattening 1146: 1075: 1055: 1009: 989: 924: 866: 844: 818: 668: 647: 61:improve this article 1871:2012CaJPh..90..883R 1832:1983GeoJ...74....7K 1819:Geophysical Journal 1789:1961GeoJ....4....3K 1776:Geophysical Journal 1695:2013JApA...34..341R 1650:2011PhRvD..84l4001I 1603:Scientific American 1417: 860:is the mean radius, 603:. If the reference 499:Theoretical gravity 199:Figure of the Earth 1409: 1257: 1112: 1094: 1061: 1035: 1033: 995: 973: 968: 910: 905: 850: 824: 799: 653: 627:general relativity 528: 509:Red arrow: gravity 421: 180: 76:"Equatorial bulge" 1879:10.1139/p2012-081 1627:Physical Review D 1554: 1553: 1255: 1184: 1093: 1032: 998:{\displaystyle T} 967: 904: 853:{\displaystyle a} 834:is the universal 827:{\displaystyle G} 797: 772: 754: 720: 707: 656:{\displaystyle f} 524:centrifugal force 511:Green arrow: the 460:centripetal force 326:is assumed to be 232:also affects the 220:tallest mountains 157:centrifugal force 137: 136: 129: 111: 16:(Redirected from 1971: 1939: 1938: 1936: 1926: 1920: 1919: 1917: 1907: 1901: 1900: 1889: 1883: 1882: 1852: 1846: 1845: 1843: 1809: 1803: 1802: 1800: 1766: 1760: 1759: 1721: 1715: 1714: 1676: 1670: 1669: 1643: 1621: 1615: 1614: 1612: 1610: 1593: 1418: 1408: 1396: 1394: 1391: 1388: 1378: 1367: 1365: 1362: 1356: 1345: 1343: 1340: 1334: 1319: 1307: 1305: 1303: 1300: 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Index

Second dynamic form factor
equatorial ridge

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"Equatorial bulge"
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equatorial
diameters
planet
centrifugal force
rotation
oblate spheroid
sphere

Earth radii
Earth ellipsoid
Figure of the Earth
Earth
tallest mountains
oceanic trenches
Earth's rotation
sea level
reference frame

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