A composite image of Earth producedbyNASA.Orbital characteristicsEpochJ2000.0[n 1]Aphelion152098232km(1.01671388AU)[n 2]Perihelion147098290km(0.98329134 AU)[n 2]Semi-major axis149598261km(1.00000261 AU)[1]Eccentricity0.01671123[1]Orbital period365.256363004 d[2](1.000017421yr)Averageorbital speed29.78km/s[3](107200km/h)Mean anomaly357.51716deg[3]Inclination*.7.155deg toSunsequator;*.1.57869 deg[4]toinvariable plane.Longitude of ascending node348.73936deg[3][n 3]Argument of perihelion114.20783deg[3][n 4]Satellites*.One natural satellite;*.1070 operationalartificial satellites;*.21000pieces of debris over 10cmin size(as of 24 October 2013).[5]Physical characteristicsMean radius6371.0 km[6]Equatorialradius6378.1 km[7][8]Polarradius6356.8 km[9]Flattening0.0033528[10]Circumference*.40075.017 km(equatorial)[8]*.40007.86 km(meridional)[11][12]Surface area*.510072000km2[13][14][n 5]*.(148940000km2(29.2%)land*.361132000km2(70.8%)water)Volume1.08321×1012km3[3]Mass5.97219×1024kg[15](3.0×10-6Suns)Meandensity5.515 g/cm3[3]Surface gravity9.798m/s2[16](0.99732g)Moment of inertia factor0.3307[17]Escape velocity11.186 km/s[3]Siderealrotation period0.99726968 d[18](23h 56m 4.100s)Equatorial rotation velocity1,674.4 km/h (465.1 m/s)[19]Axial tilt23 deg 26min21.4119s[2]Albedo*.0.367geometric[3]*.0.306Bond[3]Surfacetemp.minmeanmaxKelvin184 K[20]288 K[21]330 K[22]Celsius−89.2°C15°C56.7°CAtmosphereSurfacepressure101.325kPa(atMSL)Composition*.78.08%nitrogen(N2)[3](dry air)*.20.95%oxygen(O2)*.0.930%argon*.0.039%carbon dioxide[23]*.~ 1%water vapor(climate-variable)Earth, also known astheworld,[25]Terra,[27]orGaia,[29]is the thirdplanetfrom theSun, thedensestplanet in theSolar System, the largest of the Solar Systems fourterrestrial planets, and the onlycelestial bodyknown to accommodatelife. It is home to millions ofspecies,[30]includingbillionsofhumans[31]who depend upon itsbiosphereandminerals. The Earths human population is divided amongabout two hundredindependent statesthat interact through diplomacy, conflict, travel, trade, andmedia.According to evidence from sources such asradiometric dating, Earth was formedaround four and a half billion yearsago. Within its first billion years,[32]life appearedin its oceans and began to affectits atmosphereand surface, promoting the proliferation ofaerobicas well asanaerobic organismsand causing the formation of the atmospheresozone layer. This layer andEarths magnetic fieldblockthe most life-threatening parts of theSuns radiation, so life was able toflourish on land as well as in water.[33]Since then, Earthsposition in the Solar System,its physical propertiesandits geological historyhave allowed life to persist.Earthslithosphereis divided into several rigid segments, ortectonic plates, that migrate across the surface over periods ofmany millions of years. Over 70% percent of Earths surface is covered with water,[34]with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to thehydrosphere. Earthspolesare mostly covered with ice that is the solid ice of theAntarctic ice sheetand thesea icethat is thepolar ice packs.The planets interiorremains active, with asolid ironinner core, a liquidouter corethat generates the magnetic field, and a thick layer of relatively solidmantle.Earthgravitationally interactswith other objects in space, especially the Sun and theMoon. During one orbit around the Sun, the Earth rotates about its own axis 366.26 times, creating 365.26solar days, or onesidereal year.[n 6]The Earths axis of rotation istilted23.4° away from theperpendicularof itsorbital plane, producing seasonal variations on the planets surface with a period of onetropical year(365.24 solar days).[35]The Moon is Earths onlynatural satellite. It began orbiting the Earth about4.53 billion years ago (bya). The Moons gravitational interaction with Earth stimulates oceantides, stabilizes the axial tilt, and gradually slows the planets rotation.Name and etymologyNASAs 2014Earth DayGlobalSelfiemosaic, composed of more than 50,000photographsfrom around the world.The modern EnglishEarthdeveloped from a wide variety ofMiddle Englishforms,[37]which derived from anOld Englishnoun most often spelledeorðe.[36]It has cognates in everyGermanic languageand theirproto-Germanicroot has been reconstructedas*erþō. In its earliest appearances,eorðewas already being used to translate the many senses ofLatinterraandGreekγῆ(gē): the ground,[39]its soil,[41]dry land,[44]the humanworld,[46]the surface of the world (including the sea),[49]and the globe itself.[51]As withTerraandGaia, Earth was apersonified goddessinGermanic paganism: theAngleswere listed byTacitusamong thedevoteesofNerthus[52]and laterNorse mythologyincludedJörð, a giantess often given as the mother ofThor.[53]Originally,earthwas written in lowercase and, fromearly Middle English, itsdefinitesense as the globe was expressed astheearth. Byearly Modern English, many nouns were capitalized andthe earthbecame(and often remained)the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is simply given asEarth, by analogy with the names of theother planets.[36]House stylesnow vary:Oxford spellingrecognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes Earth when appearing as a name (e.g., Earths atmosphere) but writes it in lowercase when preceded bythe(e.g.,the atmosphere of the earth). It almost always appears in lowercase in colloquial expressions such as what onearth are you doing?[54]Composition and structureMain article:Earth scienceFurther information:Earth physical characteristics tablesEarth is a terrestrial planet, meaning that it is a rocky body, rather than agas giantlikeJupiter. It is the largestof the four terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, thehighestsurface gravity, the strongest magnetic field, and fastest rotation,[55]and is probably the only one with activeplate tectonics.[56]ShapeMain article:Figure of the EarthStratocumulus clouds over the Pacific,viewed from orbitThe shape of the Earth approximates anoblate spheroid, a sphere flattenedalong the axis from pole to pole such that there is abulgearound theequator.[57]This bulge results from therotationof the Earth, and causes the diameter at the equator to be43km(kilometer) larger than thepole-to-pole diameter.[58]For this reason the furthest point on the surface from the Earths center of mass is theChimborazovolcano inEcuador.[59]The average diameter of the reference spheroid is about12742km, which is approximately 40,000 km/π, as themeterwas originally defined as 1/10,000,000 of the distance from the equator to theNorth PolethroughParis,France.[60]Localtopographydeviates from this idealized spheroid, although on a globalscale, these deviations are small: Earth has atoleranceof about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed inbilliard balls.[61]The largest local deviations in the rocky surface of the Earth areMount Everest(8,848 m above local sea level) and theMariana Trench(10911mbelow local sea level). Due to the equatorial bulge, the surface locations farthest from the center of the Earth are the summits ofMount ChimborazoinEcuadorandHuascaráninPeru.[62][63][64]Chemical composition of the crust[65]CompoundFormulaCompositionContinentalOceanicsilicaSiO260.2%48.6%aluminaAl2O315.2%16.5%limeCaO5.5%12.3%magnesiaMgO3.1%6.8%iron(II) oxideFeO3.8%6.2%sodium oxideNa2O3.0%2.6%potassium oxideK2O2.8%0.4%iron(III) oxideFe2O32.5%2.3%waterH2O1.4%1.1%carbon dioxideCO21.2%1.4%titanium dioxideTiO20.7%1.4%phosphorus pentoxideP2O50.2%0.3%Total99.6%99.9%Chemical compositionSee also:Abundance of elements on EarthThe mass of the Earth is approximately5.98×1024kg. It is composed mostly ofiron(32.1%),oxygen(30.1%),silicon(15.1%),magnesium(13.9%),sulfur(2.9%),nickel(1.8%),calcium(1.5%), andaluminium(1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due tomass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[66]The geochemistF. W. Clarkecalculatedthat a little more than 47% of the Earthscrustconsists of oxygen. The more common rock constituents of theEarths crust are nearly all oxides; chlorine, sulfur and fluorine
Posted on: Tue, 19 Aug 2014 10:26:34 +0000
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