Earth hasa dynamic atmosphere, which sustains Earth's surface conditions and protects it from mostmeteoroids andUV-light at entry. It has a composition of primarilynitrogen andoxygen.Water vapor is widely present in the atmosphere,forming clouds that cover most of the planet. The water vapor acts as agreenhouse gas and, together with other greenhouse gases in the atmosphere, particularlycarbon dioxide (CO2), creates the conditions for both liquid surface water and water vapor to persist via the capturing ofenergy from the Sun's light. This process maintains the current average surface temperature of 14.76 °C (58.57 °F), at which water is liquid under normal atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with theequatorial region receiving more sunlight than the polar regions) driveatmospheric andocean currents, producing a globalclimate system with differentclimate regions, and a range of weather phenomena such asprecipitation, allowing components such asnitrogen tocycle.
Earth isrounded intoan ellipsoid witha circumference of about 40,000 kilometres (25,000 miles). It is thedensest planet in the Solar System. Of the fourrocky planets, it is the largest and most massive. Earth is about eightlight-minutes away from the Sun andorbits it, taking a year (about 365.25 days) to complete one revolution.Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes).Earth's axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons. Earth isorbited by one permanentnatural satellite, theMoon, which orbits Earth at 384,400 km (238,900 mi)—1.28 light seconds—and is roughly a quarter as wide as Earth. The Moon's gravity helps stabilize Earth's axis, causestides andgradually slows Earth's rotation.Tidal locking has made the Moon always face Earth with the same side.
Earth, like most other bodies in the Solar System,formed about 4.5 billion years ago from gas and dust in theearly Solar System. During the first billion years ofEarth's history, the ocean formed and thenlife developed within it. Life spread globally and has been altering Earth's atmosphere and surface, leading to theGreat Oxidation Event two billion years ago. Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth. Humans depend on Earth'sbiosphere and natural resources for their survival, but haveincreasingly impacted the planet's environment. Humanity's current impact on Earth's climate and biosphere is unsustainable, threatening the livelihood of humans and many other forms of life, andcausing widespread extinctions.[24]
Historically, "Earth" has been written in lowercase. Beginning with the use ofEarly Middle English, itsdefinite sense as "the globe" was expressed as "the earth". By the era ofEarly Modern English,capitalization of nouns began to prevail, andthe earth was also writtenthe Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given asEarth, by analogy with the names of theother planets, though "earth" and forms with "the earth" remain common.[25]House styles now vary:Oxford spelling recognizes the lowercase form as the more common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name, such as a description of the "Earth's atmosphere", but employs the lowercase when it is preceded by "the", such as "the atmosphere of the earth". It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"[27]
The nameTerra/ˈtɛrə/ occasionally is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others,[28] while in poetryTellus/ˈtɛləs/ has been used to denote personification of the Earth.[29]Terra is also the name of the planet in someRomance languages, languages that evolved fromLatin, like Italian andPortuguese, while in other Romance languages the word gave rise to names with slightly altered spellings, like theSpanishTierra and theFrenchTerre. The Latinate formGæa orGaea (English:/ˈdʒiː.ə/) of the Greek poetic nameGaia (Γαῖα;Ancient Greek:[ɡâi̯.a] or[ɡâj.ja]) is rare, though the alternative spellingGaia has become common due to theGaia hypothesis, in which case its pronunciation is/ˈɡaɪ.ə/ rather than the more classical English/ˈɡeɪ.ə/.[30]
There are a number of adjectives for the planet Earth. The word "earthly" is derived from "Earth". From theLatinTerra comesterran/ˈtɛrən/,[31]terrestrial/təˈrɛstriəl/,[32] and (via French)terrene/təˈriːn/,[33] and from the LatinTellus comestellurian/tɛˈlʊəriən/[34] andtelluric.[35]
A 2012 artistic impression of the earlySolar System'sprotoplanetary disk from which Earth and other Solar System bodies were formed
The oldest material found in theSolar System is dated to4.5682+0.0002 −0.0004Ga (billion years) ago.[36] By4.54±0.04 Ga the primordial Earth had formed.[37] The bodies inthe Solar System formed and evolved with the Sun. In theory, asolar nebula partitions a volume out of amolecular cloud by gravitational collapse, which begins to spin and flatten into acircumstellar disk, and then the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, anddust (includingprimordial nuclides). According tonebular theory,planetesimals formed byaccretion, with the primordial Earth being estimated as likely taking anywhere from 70 to 100 million years to form.[38]
Estimates of the age of the Moon range from 4.5 Ga to significantly younger.[39] Aleading hypothesis is that it was formed by accretion from material loosed from Earth after aMars-sized object with about 10% of Earth's mass, namedTheia, collided with Earth.[40] It hit Earth with a glancing blow and some of its mass merged with Earth.[41][42] Between approximately 4.0 and3.8 Ga, numerousasteroid impacts during theLate Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth.[43]
As the molten outer layer of Earth cooled itformed the first solidcrust, which is thought to have beenmafic in composition. The firstcontinental crust, which was morefelsic in composition, formed by the partial melting of this mafic crust.[50] The presence of grains of themineral zircon of Hadean age inEoarcheansedimentary rocks suggests that at least some felsic crust existed as early as4.4 Ga, only140 Ma after Earth's formation.[51] There are two main models of how this initial small volume of continental crust evolved to reach its current abundance:[52] (1) a relatively steady growth up to the present day,[53] which is supported by the radiometric dating of continental crust globally and (2) an initial rapid growth in the volume of continental crust during theArchean, forming the bulk of the continental crust that now exists,[54][55] which is supported by isotopic evidence fromhafnium inzircons andneodymium in sedimentary rocks. The two models and the data that support them can be reconciled by large-scalerecycling of the continental crust, particularly during the early stages of Earth's history.[56]
New continental crust forms as a result ofplate tectonics, a process ultimately driven by the continuous loss of heat from Earth's interior. Overthe period of hundreds of millions of years, tectonic forces have caused areas of continental crust to group together to formsupercontinents that have subsequently broken apart. At approximately750 Ma, one of the earliest known supercontinents,Rodinia, began to break apart. The continents later recombined to formPannotia at600–540 Ma, then finallyPangaea, which also began to break apart at180 Ma.[57]
The most recent pattern ofice ages began about40 Ma,[58] and then intensified during thePleistocene about3 Ma.[59]High- andmiddle-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating about every 21,000, 41,000 and 100,000 years.[60] TheLast Glacial Period, colloquially called the "last ice age", covered large parts of the continents, to the middle latitudes, in ice and ended about 11,700 years ago.[61]
An artist's impression of theArchean, theeon after Earth's formation, featuring roundstromatolites, which are early oxygen-producing forms of life from billions of years ago. After theLate Heavy Bombardment,Earth's crust had cooled, its water-rich barrensurface is marked bycontinents andvolcanoes, with the Moon still orbiting Earth half as far as it is today, appearing 2.8 times larger and producing strongtides.[72]
During theNeoproterozoic,1000 to 539 Ma, much of Earth might have been covered in ice. This hypothesis has been termed "Snowball Earth", and it is of particular interest because it preceded theCambrian explosion, when multicellular life forms significantly increased in complexity.[73][74] Following the Cambrian explosion,535 Ma, there have been at least five majormass extinctions and many minor ones.[75] Apart from the proposed currentHolocene extinction event, themost recent was66 Ma, whenan asteroid impact triggered the extinction of non-avian dinosaurs and other large reptiles, but largely spared small animals such as insects,mammals, lizards and birds. Mammalian life has diversified over the past66 Mys, and several million years ago, an Africanape species gained the ability to stand upright.[76][77] This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which led to theevolution of humans. Thedevelopment of agriculture, and thencivilization, led to humans having aninfluence on Earth and the nature and quantity of other life forms that continues to this day.[78]
Conjectured illustration of the scorched Earth after theSun has entered thered giant phase, about 5–7 billion years in the future
Earth's expected long-term future is tied to that of the Sun. Over the next1.1 billion years, solar luminosity will increase by 10%, and over the next3.5 billion years by 40%.[79] Earth's increasing surface temperature will accelerate theinorganic carbon cycle, possibly reducingCO2 concentration to levels lethally low for current plants (10 ppm forC4 photosynthesis) in approximately100–900 million years.[80][81] A lack of vegetation would result in the loss of oxygen in the atmosphere, making current animal life impossible.[82] Due to the increased luminosity, Earth's mean temperature may reach 100 °C (212 °F) in 1.5 billion years, and all ocean water will evaporate and be lost to space, which may trigger arunaway greenhouse effect, within an estimated 1.6 to 3 billion years.[83] Even if the Sun were stable and eternal, a significant fraction of the water in the modern oceans would descend into themantle, due to reduced steam venting from mid-ocean ridges as the core of the Earth slowly cools.[83][84]
The Sun willevolve to become ared giant in about5 billion years. Models predict that the Sun will expand to roughly 1 AU (150 million km; 93 million mi), about 250 times its present radius.[79][85] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, Earth will move to an orbit 1.7 AU (250 million km; 160 million mi) from the Sun when the star reaches its maximum radius, otherwise, with tidal effects, it may enter the Sun's atmosphere and be vaporized, with the heavier elements sinking to the core of the dying sun.[79]
To measure the local variation of Earth's topography,geodesy employs an idealized Earth producing ageoid shape. Such a shape is gained if the ocean is idealized, covering Earth completely and without any perturbations such as tides and winds. The result is a smooth but irregular geoid surface, providing a mean sea level as a reference level for topographic measurements.[96]
Earth's surface is the boundary between the atmosphere, and the solid Earth and oceans. Defined in this way, it has an area of about 510 million km2 (197 million sq mi).[12] Earth can be divided into twohemispheres: bylatitude into the polarNorthern andSouthern hemispheres; or bylongitude into the continentalEastern andWestern hemispheres.
Earth's land covers 29.2%, or 149 million km2 (58 million sq mi) of Earth's surface. The land surface includes many islands around the globe, but most of the land surface is taken by the four continentallandmasses, which are (in descending order):Africa-Eurasia,America (landmass),Antarctica, andAustralia (landmass).[106][107][108] These landmasses are further broken down and grouped into thecontinents. Theterrain of the land surface varies greatly and consists of mountains,deserts,plains,plateaus, and otherlandforms. The elevation of the land surface varies from a low point of −418 m (−1,371 ft) at theDead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top ofMount Everest. The mean height of land above sea level is about 797 m (2,615 ft).[109]
Land can becovered bysurface water, snow, ice, artificial structures or vegetation. Most of Earth's land hosts vegetation,[110] but considerable amounts of land areice sheets (10%,[111] not including the equally large area of land underpermafrost)[112] ordeserts (33%).[113]
Thepedosphere is the outermost layer of Earth's land surface and is composed of soil and subject tosoil formation processes. Soil is crucial for land to be arable. Earth's totalarable land is 10.7% of the land surface, with 1.3% being permanent cropland.[114][115] Earth has an estimated 16.7 million km2 (6.4 million sq mi) of cropland and 33.5 million km2 (12.9 million sq mi) of pastureland.[116]
The land surface and theocean floor form the top ofEarth's crust, which together with parts of theupper mantle formEarth's lithosphere. Earth's crust may be divided intooceanic andcontinental crust. Beneath the ocean-floor sediments, the oceanic crust is predominantlybasaltic, while the continental crust may include lower density materials such asgranite, sediments and metamorphic rocks.[117] Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the mass of the crust.[118]
As the tectonic plates migrate, oceanic crust issubducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes recycles the oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than100 Ma old. The oldest oceanic crust is located in the Western Pacific and is estimated to be200 Ma old.[124][125] By comparison, the oldest dated continental crust is4,030 Ma,[126] although zircons have been found preserved as clasts within Eoarchean sedimentary rocks that give ages up to4,400 Ma, indicating that at least some continental crust existed at that time.[51]
The seven major plates are thePacific,North American,Eurasian,African,Antarctic,Indo-Australian, andSouth American. Other notable plates include theArabian Plate, theCaribbean Plate, theNazca Plate off the west coast of South America and theScotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between50 and 55 Ma. The fastest-moving plates are the oceanic plates, with theCocos Plate advancing at a rate of 75 mm/a (3.0 in/year)[127] and the Pacific Plate moving 52–69 mm/a (2.0–2.7 in/year). At the other extreme, the slowest-moving plate is the South American Plate, progressing at a typical rate of 10.6 mm/a (0.42 in/year).[128]
Earth's interior, like that of the other terrestrial planets, is divided into layers by theirchemical or physical (rheological) properties. The outer layer is a chemically distinctsilicate solid crust, which is underlain by a highlyviscous solid mantle. The crust is separated from the mantle by theMohorovičić discontinuity.[131] The thickness of the crust varies from about 6 kilometres (3.7 mi) under the oceans to 30–50 km (19–31 mi) for the continents. The crust and the cold, rigid, top of theupper mantle are collectively known as the lithosphere, which is divided into independently moving tectonic plates.[132]
Beneath the lithosphere is theasthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km (250 and 410 mi) below the surface, spanning atransition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquidouter core lies above a solidinner core.[133] Earth's inner core may be rotating at a slightly higherangular velocity than the remainder of the planet, advancing by 0.1–0.5° per year, although both somewhat higher and much lower rates have also been proposed.[134] The radius of the inner core is about one-fifth of that of Earth.The density increases with depth. Among the Solar System's planetary-sized objects, Earth is theobject with the highest density.
Earth's mass is approximately5.97×1024kg (5.970 Yg). It is composed mostly of iron (32.1%by mass),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 togravitational separation, the core is primarily composed of the denser elements: iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[135][50] The most common rock constituents of the crust areoxides. Over 99% of thecrust is composed of various oxides of eleven elements, principally oxides containing silicon (thesilicate minerals), aluminium, iron, calcium, magnesium, potassium, or sodium.[136][135]
A map ofheat flow from Earth's interior to the surface of Earth's crust, mostly along theoceanic ridges
The major contributors to Earth's internal heat are primordial heat (heat left over from Earth's formation) and radiogenic heat (heat produced by radioactive decay).[137] The major heat-producingisotopes within Earth arepotassium-40,uranium-238, andthorium-232.[138] At the center, the temperature may be up to 6,000 °C (10,830 °F),[139] and the pressure could reach 360 GPa (52 million psi).[140] Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's history, before isotopes with short half-lives were depleted, Earth's heat production was much higher. At approximately3 Gyr, twice the present-day heat would have been produced, increasing the rates ofmantle convection and plate tectonics, and allowing the production of uncommonigneous rocks such askomatiites that are rarely formed today.[141][142]
The mean heat loss from Earth is87 mW/m2, for a global heat loss of4.42×1013 W.[143] A portion of the core's thermal energy is transported toward the crust bymantle plumes, a form of convection consisting of upwellings of higher-temperature rock. These plumes can producehotspots andflood basalts.[144] More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated withmid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs under the oceans.[145]
The gravity of Earth is theacceleration that is imparted to objects due to the distribution of mass within Earth. Near Earth's surface,gravitational acceleration is approximately 9.8 m/s2 (32 ft/s2). Local differences in topography, geology, and deeper tectonic structure cause local and broad regional differences in Earth's gravitational field, known asgravity anomalies.[146]
A schematic view of Earth's magnetosphere withsolar wind flowing from left to right
The main part of Earth's magnetic field is generated in the core, the site of adynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth's surface, where it is, approximately, adipole. The poles of the dipole are located close to Earth's geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is3.05×10−5T, with amagnetic dipole moment of7.79×1022 Am2 at epoch 2000, decreasing nearly 6% per century (although it still remains stronger than its long time average).[147] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causessecular variation of the main field andfield reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[148][149]
The extent of Earth's magnetic field in space defines themagnetosphere. Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the day-side of the magnetosphere, to about 10 Earth radii, and extends the night-side magnetosphere into a long tail.[150] Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonicbow shock precedes the day-side magnetosphere within the solar wind.[151]Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates.[152][153] The ring current is defined by medium-energyparticles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field,[154] and theVan Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.[155][156] Duringmagnetic storms andsubstorms, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth'sionosphere, where atmospheric atoms can be excited and ionized, causing anaurora.[157]
Earth's rotation period relative to the Sun—its mean solar day—is86,400 seconds of mean solar time (86,400.0025SI seconds).[158] Because Earth's solar day is now slightly longer than it was during the 19th century due totidal deceleration, each day varies between0 and 2ms longer than the mean solar day.[159][160]
Earth's rotation period relative to thefixed stars, called itsstellar day by theInternational Earth Rotation and Reference Systems Service (IERS), is86,164.0989 seconds of mean solar time (UT1), or23h 56m 4.0989s.[2][n 10] Earth's rotation period relative to theprecessing or moving meanMarch equinox (when the Sun is at 90° on the equator), is86,164.0905 seconds of mean solar time (UT1)(23h 56m 4.0905s).[2] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[161]
Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near thecelestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.[162][163]
Exaggerated illustration of Earth's elliptical orbit around the Sun, marking that the orbital extreme points (apoapsis andperiapsis) are not the same as the four seasonal extreme points, theequinox andsolstice
Earth orbits the Sun, making Earth the third-closest planet to the Sun and part of theinner Solar System. Earth's average orbital distance is about 150 million km (93 million mi), which is the basis for theastronomical unit (AU) and is equal to roughly 8.3light minutes or 380 timesEarth's distance to the Moon. Earth orbits the Sun every 365.2564 meansolar days, or onesidereal year. With an apparent movement of the Sun in Earth's sky at a rate of about 1°/day eastward, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to themeridian.
The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth's diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance from Earth to the Moon, 384,400 km (238,900 mi), in about 3.5 hours.[3]
The Moon and Earth orbit a commonbarycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common orbit around the Sun, the period of thesynodic month, from new moon to new moon, is 29.53 days. Viewed from thecelestial north pole, the motion of Earth, the Moon, and their axial rotations are allcounterclockwise. Viewed from a vantage point above the Sun and Earth's north poles, Earth orbits in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth'saxis is tilted some 23.44 degrees from the perpendicular to the Earth–Sun plane (theecliptic), and the Earth-Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating betweenlunar eclipses andsolar eclipses.[3][164]
TheHill sphere, or thesphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius.[165][n 11] This is the maximum distance at which Earth's gravitational influence is stronger than that of the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.[165] Earth, along with the Solar System, is situated in theMilky Way and orbits about 28,000 light-years from its center. It is about 20 light-years above thegalactic plane in theOrion Arm.[166]
Earth's axial tilt causing different angles of seasonal illumination at different orbital positions around the Sun
The axial tilt of Earth is approximately 23.439281°[2] with the axis of the plane of theEarth's orbit by definition pointing always towards theCelestial Poles. Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes the seasonal change in climate, with summer in theNorthern Hemisphere occurring when theTropic of Cancer is facing the Sun, and in theSouthern Hemisphere when theTropic of Capricorn faces the Sun. In each instance, winter occurs simultaneously in the opposite hemisphere.
During the summer, the day lasts longer, and the Sun climbs higher in the sky. In winter, the climate becomes cooler and the days shorter.[167] Above theArctic Circle and below theAntarctic Circle there is no daylight at all for part of the year, causing apolar night, and this night extends for several months at the poles themselves. These same latitudes also experience amidnight sun, where the sun remains visible all day.[168][169]
By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and theequinoxes, when Earth's rotational axis is aligned with its orbital axis. In the Northern Hemisphere,winter solstice currently occurs around 21 December;summer solstice is near 21 June, spring equinox is around 20 March andautumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.[170]
The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergonutation; a slight, irregular motion with a main period of 18.6 years.[171] The orientation (rather than the angle) of Earth's axis also changes over time,precessing around in a complete circle over each 25,800-year cycle; this precession is the reason for the difference between a sidereal year and atropical year. Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth's equatorial bulge. The poles also migrate a few meters across Earth's surface. Thispolar motion has multiple, cyclical components, which collectively are termedquasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called theChandler wobble. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.[172]
Earth's annual orbit is elliptical rather than circular, and its closest approach to the Sun is calledperihelion. In modern times, Earth's perihelion occurs around 3 January, and itsaphelion around 4 July. These dates shift over time due to precession and changes to the orbit, the latter of which follows cyclical patterns known asMilankovitch cycles. The annual change in the Earth–Sun distance causes an increase of about 6.8% in solar energy reaching Earth at perihelion relative to aphelion.[173][n 12] Because the Southern Hemisphere is tilted toward the Sun at about the same time that Earth reaches the closest approach to the Sun, the Southern Hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the Southern Hemisphere.[174]
The Moon is a relatively large,terrestrial,planet-like natural satellite, with a diameter about one-quarter of Earth's. It is the largest moon in the Solar System relative to the size of its planet, althoughCharon is larger relative to thedwarf planetPluto.[175][176] The natural satellites of other planets are also referred to as "moons", after Earth's.[177] The most widely accepted theory of the Moon's origin, thegiant-impact hypothesis, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains the Moon's relative lack of iron and volatile elements and the fact that its composition is nearly identical to that of Earth's crust.[41] Computer simulations suggest that two blob-like remnants of this protoplanet could be inside the Earth.[178][179]
The gravitational attraction between Earth and the Moon causeslunar tides on Earth.[180] The same effect on the Moon has led to itstidal locking: its rotation period is the same as the time it takes to orbit Earth. As a result, it always presents the same face to the planet.[181] As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to thelunar phases.[182] Due to theirtidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm/a (1.5 in/year). Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 μs/yr—add up to significant changes.[183] During theEdiacaran period, for example, (approximately620 Ma) there were 400±7 days in a year, with each day lasting 21.9±0.4 hours.[184]
The Moon may have dramatically affected the development of life by moderating the planet's climate.Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[185] Some theorists think that without this stabilization against thetorques applied by the Sun and planets to Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting large changes over millions of years, as is the case for Mars, though this is disputed.[186][187]
Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. Theangular size (orsolid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[163] This allows total and annular solar eclipses to occur on Earth.[188]
As of September 2021[update], there are 4,550 operational, human-madesatellites orbiting Earth.[193] There are also inoperative satellites, includingVanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of trackedspace debris.[n 13] Earth's largest artificial satellite is theInternational Space Station (ISS).[194]
A view of Earth with itsglobal ocean andcloud cover, which dominate Earth's surface andhydrosphere; at Earth'spolar regions, its hydrosphere forms larger areas of ice cover.
Earth's hydrosphere is the sum of Earth's water and its distribution. Most of Earth's hydrosphere consists of Earth's global ocean. Earth's hydrosphere also consists of water in the atmosphere and on land, including clouds, inland seas, lakes, rivers, and underground waters. The mass of the oceans is approximately 1.35×1018metric tons or about 1/4400 of Earth's total mass. The oceans cover an area of 361.8 million km2 (139.7 million sq mi) with a mean depth of 3,682 m (12,080 ft), resulting in an estimated volume of 1.332 billion km3 (320 million cu mi).[195]
If all of Earth's crustal surface were at the same elevation as a smooth sphere, the depth of the resulting world ocean would be 2.7 to 2.8 km (1.68 to 1.74 mi).[196] About 97.5% of the water issaline; the remaining 2.5% isfresh water.[197][198] Most fresh water, about 68.7%, is present as ice inice caps andglaciers.[199] The remaining 30% isground water, 1%surface water (covering only 2.8% of Earth's land)[200] and other small forms of fresh water deposits such aspermafrost,water vapor in the atmosphere, biological binding, etc.[201][202]
In Earth's coldest regions, snow survives over the summer andchanges into ice. This accumulated snow and ice eventually forms intoglaciers, bodies of ice that flow under the influence of their own gravity.Alpine glaciers form in mountainous areas, whereas vastice sheets form over land in polar regions. The flow of glaciers erodes the surface, changing it dramatically, with the formation ofU-shaped valleys and other landforms.[203]Sea ice in the Arctic covers an area about as big as the United States, although it is quickly retreating as a consequence of climate change.[204]
The averagesalinity of Earth's oceans is about 35 grams of salt per kilogram of seawater (3.5% salt).[205] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[206] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[207] Sea water has an important influence on the world's climate, with the oceans acting as a largeheat reservoir.[208] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as theEl Niño–Southern Oscillation.[209]
The abundance of water, particularly liquid water, on Earth's surface is a unique feature that distinguishes it from other planets in theSolar System. Solar System planets with considerable atmospheres do partly host atmospheric water vapor, but they lack surface conditions for stable surface water.[210] Despite somemoons showing signs of large reservoirs ofextraterrestrial liquid water, with possibly even more volume than Earth's ocean, all of them arelarge bodies of water under a kilometers thick frozen surface layer.[211]
Theatmospheric pressure at Earth's sea level averages 101.325 kPa (14.696 psi),[212] with ascale height of about 8.5 km (5.3 mi).[3] A dry atmosphere is composed of 78.084%nitrogen, 20.946% oxygen, 0.934%argon, and trace amounts of carbon dioxide and other gaseous molecules.[212]Water vapor content varies between 0.01% and 4%[212] but averages about 1%.[3]Clouds cover around two-thirds of Earth's surface, more so over oceans than land.[213] The height of thetroposphere varies with latitude, ranging between 8 km (5 mi) at the poles to 17 km (11 mi) at the equator, with some variation resulting from weather and seasonal factors.[214]
Earth'sbiosphere has significantly altered itsatmosphere.Oxygenic photosynthesis evolved2.7 Gya,forming the primarily nitrogen–oxygen atmosphere of today.[63] This change enabled the proliferation ofaerobic organisms and, indirectly, the formation of the ozone layer due to the subsequentconversion of atmosphericO2 intoO3. The ozone layer blocksultravioletsolar radiation, permitting life on land.[215] Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[216] This last phenomenon is thegreenhouse effect: trace molecules within the atmosphere serve to capturethermal energy emitted from the surface, thereby raising the average temperature. Water vapor, carbon dioxide,methane,nitrous oxide, andozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C (0 °F), in contrast to the current +15 °C (59 °F),[217] and life on Earth probably would not exist in its current form.[218]
Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space.[219] Three-quarters of the atmosphere's mass is contained within the first 11 km (6.8 mi) of the surface; this lowest layer is called the troposphere.[220] Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower-density air then rises and is replaced by cooler, higher-density air. The result isatmospheric circulation that drives the weather and climate through redistribution of thermal energy.[221]
The primary atmospheric circulation bands consist of thetrade winds in the equatorial region below 30° latitude and thewesterlies in the mid-latitudes between 30° and 60°.[222]Ocean heat content andcurrents are also important factors in determining climate, particularly thethermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.[223]
Earth receives 1361 W/m2 of solar irradiance.[224][225] The amount of solar energy that reaches Earth's surface decreases with increasing latitude. At higher latitudes, the sunlight reaches the surface at lower angles, and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C (0.7 °F) per degree of latitude from the equator.[226] Earth's surface can be subdivided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial),subtropical,temperate andpolar climates.[227]
Further factors that affect a location's climates are itsproximity to oceans, the oceanic and atmospheric circulation, and topology.[228] Places close to oceans typically have colder summers and warmer winters, due to the fact that oceans can store large amounts of heat. The wind transports the cold or the heat of the ocean to the land.[229] Atmospheric circulation also plays an important role: San Francisco and Washington DC are both coastal cities at about the same latitude. San Francisco's climate is significantly more moderate as the prevailing wind direction is from sea to land.[230] Finally, temperaturesdecrease with height causing mountainous areas to be colder than low-lying areas.[231]
Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and falls to the surface asprecipitation.[221] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. Thiswater cycle is a vital mechanism for supporting life on land and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topographic features, and temperature differences determine the average precipitation that falls in each region.[232]
Earth's night-side upper atmosphere appearing from the bottom as bands ofafterglow illuminating thetroposphere in orange with silhouettes of clouds, and thestratosphere in white and blue. Next themesosphere (pink area) extends to the orange and faintly green line of the lowestairglow, at about one hundred kilometers at theedge of space and the lower edge of thethermosphere (invisible). Continuing with green and red bands ofaurorae stretching over several hundred kilometers.
The upper atmosphere, the atmosphere above the troposphere,[236] is usually divided into thestratosphere,mesosphere, andthermosphere.[216] Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, theexosphere thins out into the magnetosphere, where the geomagnetic fields interact with the solar wind.[237] Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. TheKármán line, defined as 100 km (62 mi) above Earth's surface, is a working definition for the boundary between the atmosphere andouter space.[238]
Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth's gravity. This causes a slow but steadyloss of the atmosphere into space. Because unfixedhydrogen has a lowmolecular mass, it can achieveescape velocity more readily, and it leaks into outer space at a greater rate than other gases.[239] The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface from an initiallyreducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is thought to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[240] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[241] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[242]
An animation of the changing density ofproductive vegetation on land (low in brown; heavy in dark green) and phytoplankton at theocean surface (low in purple; high in yellow)
Earth is the only known place that has ever beenhabitable for life. Earth's life developed in Earth's early bodies of water some hundred million years after Earth formed. Earth's life has been shaping and inhabiting many particularecosystems on Earth and has eventually expanded globally forming an overarching biosphere.[243]
Earth provides liquid water—an environment where complexorganic molecules can assemble and interact, and sufficient energy to sustain ametabolism.[247] Plants and other organisms take upnutrients from water, soils and the atmosphere. These nutrients are constantly recycled between different species.[248]
Originating from earlierprimates in Eastern Africa 300,000years agohumans have since been migrating and with the advent of agriculture in the 10th millennium BC increasinglysettling Earth's land.[254] In the 20th centuryAntarctica had been the last continent to see a first and until today limited human presence.
Human population has since the 19th century grown exponentially to seven billion in the early 2010s,[255] and is projected to peak at around ten billion in the second half of the 21st century.[256] Most of the growth is expected to take place insub-Saharan Africa.[256]
Distribution anddensity of human population varies greatly around the world with the majority living in south to eastern Asia and 90% inhabiting only theNorthern Hemisphere of Earth,[257] partly due to thehemispherical predominance of the world's land mass, with 68% of the world's land mass being in the Northern Hemisphere.[258] Furthermore, since the 19th century humans have increasingly converged into urban areas with the majority living in urban areas by the 21st century.[259]
Beyond Earth's surface humans have lived on a temporary basis, with only a few special-purpose deepunderground andunderwater presences and a fewspace stations. The human population virtually completely remains on Earth's surface, fully depending on Earth and the environment it sustains. Since the second half of the 20th century, some hundreds of humans have temporarilystayed beyond Earth, a tiny fraction of whom have reached another celestial body, the Moon.[260][261]
Earth has been subject to extensive human settlement, and humans have developed diverse societies and cultures. Most of Earth's land has been territorially claimed since the 19th century bysovereign states (countries) separated bypolitical borders, and205 such states exist today,[262] with only parts of Antarctica and a few small regionsremaining unclaimed.[263] Most of these states together form the United Nations, the leading worldwideintergovernmental organization,[264] which extends human governanceover the ocean andAntarctica, and therefore all of Earth.
Earth has resources that have been exploited by humans.[265] Those termednon-renewable resources, such asfossil fuels, are only replenished over geological timescales.[266] Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.[267] These deposits are used by humans both for energy production and as feedstock for chemical production.[268] Mineralore bodies have also been formed within the crust through a process ofore genesis, resulting from actions ofmagmatism, erosion, and plate tectonics.[269] These metals and other elements are extracted by mining, a process which often brings environmental and health damage.[270]
Earth's biosphere produces many useful biological products for humans, including food, wood,pharmaceuticals, oxygen, and the recycling of organic waste. The land-based ecosystem depends upontopsoil and fresh water, and the oceanic ecosystem depends on dissolved nutrients washed down from the land.[271] In 2019, 39 million km2 (15 million sq mi) of Earth's land surface consisted of forest and woodlands, 12 million km2 (4.6 million sq mi) was shrub and grassland, 40 million km2 (15 million sq mi) were used for animal feed production and grazing, and 11 million km2 (4.2 million sq mi) were cultivated as croplands.[272] Of the 12–14% of ice-free land that is used for croplands, 2percentage points were irrigated in 2015.[273] Humans usebuilding materials to construct shelters.[274]
Change in average surface air temperature and drivers for that change. Human activity has caused increased temperatures, with natural forces adding some variability.[275]
Human activities have impacted Earth's environments. Through activities such as the burning of fossil fuels, humans have been increasing the amount ofgreenhouse gases in the atmosphere, alteringEarth's energy budget and climate.[252][276] It is estimated that global temperatures in the year 2020 were 1.2 °C (2.2 °F) warmer than the preindustrial baseline.[277] This increase in temperature, known asglobal warming, has contributed to themelting of glaciers,rising sea levels, increased risk of drought and wildfires, and migration of species to colder areas.[253]
The concept ofplanetary boundaries was introduced to quantify humanity's impact on Earth. Of the nine identified boundaries, five have been crossed:Biosphere integrity, climate change, chemical pollution, destruction of wild habitats and thenitrogen cycle are thought to have passed the safe threshold.[278][279] As of 2018, no country meets the basic needs of its population without transgressing planetary boundaries. It is thought possible to provide all basic physical needs globally within sustainable levels of resource use.[280]
Scientific investigation has resulted in several culturally transformative shifts in people's view of the planet. Initial belief in aflat Earth was gradually displaced inAncient Greece by the idea of aspherical Earth, which was attributed to both the philosophersPythagoras andParmenides.[292][293] Earth was generally believed to bethe center of the universe until the 16th century, when scientists first concluded that it wasa moving object, one of the planets of the Solar System.[294]
It was only during the 19th century that geologists realizedEarth's age was at least many millions of years.[295]Lord Kelvin usedthermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864, sparking a vigorous debate on the subject; it was only when radioactivity andradioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth's age was established, proving the planet to be billions of years old.[296][297]
^All astronomical quantities vary, bothsecularly andperiodically. The quantities given are the values at the instantJ2000.0 of the secular variation, ignoring all periodic variations.
^aphelion =a × (1 +e); perihelion =a × (1 –e), wherea is the semi-major axis ande is the eccentricity. The difference between Earth's perihelion and aphelion is 5 million kilometers.—Wilkinson, John (2009).Probing the New Solar System. CSIRO Publishing. p. 144.ISBN978-0-643-09949-4.
^Earth'scircumference is almost exactly 40,000 km because the meter was calibrated on this measurement—more specifically, 1/10-millionth of the distance between the poles and the equator.
^Due to natural fluctuations, ambiguities surroundingice shelves, and mapping conventions forvertical datums, exact values for land and ocean coverage are not meaningful. Based on data from theVector Map andGlobal LandcoverArchived 26 March 2015 at theWayback Machine datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of Earth's surface. The ice sheets ofAntarctica andGreenland are counted as land, even though much of the rock that supports them lies below sea level.
^Source for minimum,[19] mean,[20] and maximum[21] surface temperature
^ If Earth were shrunk to the size of abilliard ball, some areas of Earth such as large mountain ranges and oceanic trenches would feel like tiny imperfections, whereas much of the planet, including theGreat Plains and theabyssal plains, would feel smoother.[91]
^The ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".—Aoki, S.; Kinoshita, H.; Guinot, B.; Kaplan, G. H.; McCarthy, D. D.; Seidelmann, P. K. (1982). "The new definition of universal time".Astronomy and Astrophysics.105 (2):359–361.Bibcode:1982A&A...105..359A.
^For Earth, theHill radius is, wherem is the mass of Earth,a is an astronomical unit, andM is the mass of the Sun. So the radius in AU is about.
^Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% of the energy at aphelion.
^As of 4 January 2018, the United States Strategic Command tracked a total of 18,835 artificial objects, mostly debris. See:Anz-Meador, Phillip; Shoots, Debi, eds. (February 2018)."Satellite Box Score"(PDF).Orbital Debris Quarterly News.22 (1): 12.Archived(PDF) from the original on 2 April 2019. Retrieved18 April 2018.
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