Earth has
a dynamic atmosphere, which sustains Earth's surface conditions and protects it from most
meteoroids and
UV-light at entry. It has a composition of primarily
nitrogen and
oxygen.
Water vapor is widely present in the atmosphere,
forming clouds that cover most of the planet. The water vapor acts as a
greenhouse gas and, together with other greenhouse gases in the atmosphere, particularly
carbon dioxide (CO2), creates the conditions for both liquid surface water and water vapor to persist via the capturing of
energy 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 the
equatorial region receiving more sunlight than the polar regions) drive
atmospheric and
ocean currents, producing a global
climate system with different
climate regions, and a range of weather phenomena such as
precipitation, allowing components such as
nitrogen to
cycle.
Historically, "Earth" has been written in lowercase. Beginning with the use of
Early Middle English, its
definite sense as "the globe" was expressed as "the earth". By the era of
Early Modern English,
capitalization of nouns began to prevail, and the earth was also written the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the
other planets, though "earth" and forms with "the earth" remain common.[24]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?"[26]
The name Terra/ˈtɛrə/ occasionally is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others,[27] while in poetry Tellus/ˈtɛləs/ has been used to denote personification of the Earth.[28]Terra is also the name of the planet in some
Romance languages, languages that evolved from
Latin, like Italian and
Portuguese, while in other Romance languages the word gave rise to names with slightly altered spellings, like the
SpanishTierra and the
FrenchTerre. The Latinate form Gæa or Gaea (English: /ˈdʒiː.ə/) of the Greek poetic name Gaia (Γαῖα; Ancient Greek:[ɡâi̯.a] or [ɡâj.ja]) is rare, though the alternative spelling Gaia has become common due to the
Gaia hypothesis, in which case its pronunciation is /ˈɡaɪ.ə/ rather than the more classical English /ˈɡeɪ.ə/.[29]
There are a number of adjectives for the planet Earth. The word "earthly" is derived from "Earth". From the
LatinTerra comes terran/ˈtɛrən/,[30]terrestrial/təˈrɛstriəl/,[31] and (via French) terrene/təˈriːn/,[32] and from the Latin Tellus comes tellurian/tɛˈlʊəriən/[33] and telluric.[34]
The oldest material found in the
Solar System is dated to 4.5682+0.0002 −0.0004Ga (billion years) ago.[35] By 4.54±0.04 Ga the primordial Earth had formed.[36] The bodies in
the Solar System formed and evolved with the Sun. In theory, a
solar nebula partitions a volume out of a
molecular cloud by gravitational collapse, which begins to spin and flatten into a
circumstellar disk, and then the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, and
dust (including
primordial nuclides). According to
nebular theory,
planetesimals formed by
accretion, with the primordial Earth being estimated as likely taking anywhere from 70 to 100 million years to form.[37]
Estimates of the age of the Moon range from 4.5 Ga to significantly younger.[38] A
leading hypothesis is that it was formed by accretion from material loosed from Earth after a
Mars-sized object with about 10% of Earth's mass, named
Theia, collided with Earth.[39] It hit Earth with a glancing blow and some of its mass merged with Earth.[40][41] Between approximately 4.1 and 3.8 Ga, numerous
asteroid impacts during the
Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth.[42]
As the molten outer layer of Earth cooled it
formed the first solid
crust, which is thought to have been
mafic in composition. The first
continental crust, which was more
felsic in composition, formed by the partial melting of this mafic crust.[49] The presence of grains of the
mineral zircon of Hadean age in
Eoarcheansedimentary rocks suggests that at least some felsic crust existed as early as 4.4 Ga, only 140
Ma after Earth's formation.[50] There are two main models of how this initial small volume of continental crust evolved to reach its current abundance:[51] (1) a relatively steady growth up to the present day,[52] which is supported by the radiometric dating of continental crust globally and (2) an initial rapid growth in the volume of continental crust during the
Archean, forming the bulk of the continental crust that now exists,[53][54] which is supported by isotopic evidence from
hafnium in
zircons and
neodymium in sedimentary rocks. The two models and the data that support them can be reconciled by large-scale
recycling of the continental crust, particularly during the early stages of Earth's history.[55]
New continental crust forms as a result of
plate tectonics, a process ultimately driven by the continuous loss of heat from Earth's interior. Over
the period of hundreds of millions of years, tectonic forces have caused areas of continental crust to group together to form
supercontinents that have subsequently broken apart. At approximately 750 Ma, one of the earliest known supercontinents,
Rodinia, began to break apart. The continents later recombined to form
Pannotia at 600–540 Ma, then finally
Pangaea, which also began to break apart at 180 Ma.[56]
The most recent pattern of
ice ages began about 40 Ma,[57] and then intensified during the
Pleistocene about 3 Ma.[58]High- and
middle-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating about every 21,000, 41,000 and 100,000 years.[59] The
Last 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.[60]
During the
Neoproterozoic, 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 the
Cambrian explosion, when multicellular life forms significantly increased in complexity.[72][73] Following the Cambrian explosion, 535 Ma, there have been at least five major
mass extinctions and many minor ones.[74] Apart from the proposed current
Holocene extinction event, the
most recent was 66 Ma, when
an 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 past 66 Mys, and several million years ago, an African
ape species gained the ability to stand upright.[75][76] This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which led to the
evolution of humans. The
development of agriculture, and then
civilization, led to humans having an
influence on Earth and the nature and quantity of other life forms that continues to this day.[77]
Earth's expected long-term future is tied to that of the Sun. Over the next 1.1 billion years, solar luminosity will increase by 10%, and over the next 3.5 billion years by 40%.[78] Earth's increasing surface temperature will accelerate the
inorganic carbon cycle, possibly reducing CO2 concentration to levels lethally low for current plants (10
ppm for
C4 photosynthesis) in approximately 100–900 million years.[79][80] A lack of vegetation would result in the loss of oxygen in the atmosphere, making current animal life impossible.[81] 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 a
runaway greenhouse effect, within an estimated 1.6 to 3 billion years.[82] Even if the Sun were stable, a fraction of the water in the modern oceans will descend to the
mantle, due to reduced steam venting from mid-ocean ridges.[82][83]
The Sun will
evolve to become a
red giant in about 5 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.[78][84] 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.[78]
To measure the local variation of Earth's topography,
geodesy employs an idealized Earth producing a
geoid 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 (MSL) as a reference level for topographic measurements.[95]
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 two
hemispheres: by
latitude into the polar
Northern and
Southern hemispheres; or by
longitude into the continental
Eastern and
Western 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 continental
landmasses, which are (in descending order):
Africa-Eurasia,
America (landmass),
Antarctica, and
Australia (landmass).[105][106][107] These landmasses are further broken down and grouped into the
continents. The
terrain of the land surface varies greatly and consists of mountains,
deserts,
plains,
plateaus, and other
landforms. The elevation of the land surface varies from a low point of −418 m (−1,371 ft) at the
Dead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top of
Mount Everest. The mean height of land above sea level is about 797 m (2,615 ft).[108]
Land can be
covered by
surface water, snow, ice, artificial structures or vegetation. Most of Earth's land hosts vegetation,[109] but considerable amounts of land are
ice sheets (10%,[110] not including the equally large area of land under
permafrost)[111] or
deserts (33%).[112]
The
pedosphere is the outermost layer of Earth's land surface and is composed of soil and subject to
soil formation processes. Soil is crucial for land to be arable. Earth's total
arable land is 10.7% of the land surface, with 1.3% being permanent cropland.[113][114] 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.[115]
The land surface and the
ocean floor form the top of
Earth's crust, which together with parts of the
upper mantle form
Earth's lithosphere. Earth's crust may be divided into
oceanic and
continental crust. Beneath the ocean-floor sediments, the oceanic crust is predominantly
basaltic, while the continental crust may include lower density materials such as
granite, sediments and metamorphic rocks.[116] Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the mass of the crust.[117]
As the tectonic plates migrate, oceanic crust is
subducted 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 than 100 Ma old. The oldest oceanic crust is located in the Western Pacific and is estimated to be 200 Ma old.[123][124] By comparison, the oldest dated continental crust is 4,030 Ma,[125] although zircons have been found preserved as clasts within Eoarchean sedimentary rocks that give ages up to 4,400 Ma, indicating that at least some continental crust existed at that time.[50]
The seven major plates are the
Pacific,
North American,
Eurasian,
African,
Antarctic,
Indo-Australian, and
South American. Other notable plates include the
Arabian Plate, the
Caribbean Plate, the
Nazca Plate off the west coast of South America and the
Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and 55 Ma. The fastest-moving plates are the oceanic plates, with the
Cocos Plate advancing at a rate of 75 mm/a (3.0 in/year)[126] 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).[127]
Earth's interior, like that of the other terrestrial planets, is divided into layers by their
chemical or physical (
rheological) properties. The outer layer is a chemically distinct
silicate solid crust, which is underlain by a highly
viscous solid mantle. The crust is separated from the mantle by the
Mohorovičić discontinuity.[130] 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 the
upper mantle are collectively known as the lithosphere, which is divided into independently moving tectonic plates.[131]
Beneath the lithosphere is the
asthenosphere, 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 a
transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid
outer core lies above a solid
inner core.[132] Earth's inner core may be rotating at a slightly higher
angular 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.[133] 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 the
object with the highest density.
Earth's mass is approximately 5.97×1024kg (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%), and
aluminium (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to
gravitational 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.[134][49] The most common rock constituents of the crust are
oxides. Over 99% of the
crust is composed of various oxides of eleven elements, principally oxides containing silicon (the
silicate minerals), aluminium, iron, calcium, magnesium, potassium, or sodium.[135][134]
The major heat-producing
isotopes within Earth are
potassium-40,
uranium-238, and
thorium-232.[136] At the center, the temperature may be up to 6,000 °C (10,830 °F),[137] and the pressure could reach 360
GPa (52 million
psi).[138] 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 approximately 3
Gyr, twice the present-day heat would have been produced, increasing the rates of
mantle convection and plate tectonics, and allowing the production of uncommon
igneous rocks such as
komatiites that are rarely formed today.[139][140]
The mean heat loss from Earth is 87 mW m−2, for a global heat loss of 4.42×1013 W.[141] A portion of the core's thermal energy is transported toward the crust by
mantle plumes, a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce
hotspots and
flood basalts.[142] More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated with
mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs under the oceans.[143]
The gravity of Earth is the
acceleration 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 as
gravity anomalies.[144]
The main part of Earth's magnetic field is generated in the core, the site of a
dynamo 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, a
dipole. 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 is 3.05×10−5T, with a
magnetic dipole moment of 7.79×1022 Am2 at epoch 2000, decreasing nearly 6% per century (although it still remains stronger than its long time average).[145] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes
secular variation of the main field and
field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[146][147]
The extent of Earth's magnetic field in space defines the
magnetosphere. 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.[148] Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonic
bow shock precedes the day-side magnetosphere within the solar wind.[149]Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates.[150][151] The ring current is defined by medium-energy
particles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field,[152] and the
Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.[153][154] During
magnetic storms and
substorms, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's
ionosphere, where atmospheric atoms can be excited and ionized, causing an
aurora.[155]
Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025
SI seconds).[156] Because Earth's solar day is now slightly longer than it was during the 19th century due to
tidal deceleration, each day varies between 0 and 2
ms longer than the mean solar day.[157][158]
Earth's rotation period relative to the
fixed stars, called its stellar day by the
International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (
UT1), or 23h 56m 4.0989s.[2][n 10] Earth's rotation period relative to the
precessing or moving mean
March equinox (when the Sun is at 90° on the equator), is 86,164.0905 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.[159]
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 the
celestial 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.[160][161]
Earth orbits the Sun, making Earth the third-closest planet to the Sun and part of the
inner Solar System. Earth's average orbital distance is about 150 million km (93 million mi), which is the basis for the
astronomical unit (AU) and is equal to roughly 8.3
light minutes or 380 times
Earth's distance to the Moon. Earth orbits the Sun every 365.2564 mean
solar days, or one
sidereal 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 the
meridian.
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 common
barycenter 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 the
synodic month, from new moon to new moon, is 29.53 days. Viewed from the
celestial north pole, the motion of Earth, the Moon, and their axial rotations are all
counterclockwise. 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's
axis is tilted some 23.44 degrees from the perpendicular to the Earth–Sun plane (the
ecliptic), 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 between
lunar eclipses and
solar eclipses.[3][162]
The
Hill sphere, or the
sphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius.[163][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.[163] Earth, along with the Solar System, is situated in the
Milky Way and orbits about 28,000
light-years from its center. It is about 20 light-years above the
galactic plane in the
Orion Arm.[164]
The axial tilt of Earth is approximately 23.439281°[2] with the axis of its orbit plane, always pointing towards the
Celestial 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 the
Northern Hemisphere occurring when the
Tropic of Cancer is facing the Sun, and in the
Southern Hemisphere when the
Tropic 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.[165] Above the
Arctic Circle and below the
Antarctic Circle there is no daylight at all for part of the year, causing a
polar night, and this night extends for several months at the poles themselves. These same latitudes also experience a
midnight sun, where the sun remains visible all day.[166][167]
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 the
equinoxes, 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 and
autumnal 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.[168]
The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergo
nutation; a slight, irregular motion with a main period of 18.6 years.[169] 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 a
tropical 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. This
polar motion has multiple, cyclical components, which collectively are termed
quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the
Chandler wobble. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.[170]
Earth's annual orbit is elliptical rather than circular, and its closest approach to the Sun is called
perihelion. In modern times, Earth's perihelion occurs around 3 January, and its
aphelion around 4 July. These dates shift over time due to precession and changes to the orbit, the latter of which follows cyclical patterns known as
Milankovitch 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.[171][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.[172]
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, although
Charon is larger relative to the
dwarf planetPluto.[173][174] The natural satellites of other planets are also referred to as "moons", after Earth's.[175] The most widely accepted theory of the Moon's origin, the
giant-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.[40] Computer simulations suggest that two blob-like remnants of this protoplanet could be inside the Earth.[176][177]
The gravitational attraction between Earth and the Moon causes
lunar tides on Earth.[178] The same effect on the Moon has led to its
tidal 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.[179] As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the
lunar phases.[180] Due to their
tidal 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.[181] During the
Ediacaran period, for example, (approximately 620 Ma) there were 400±7 days in a year, with each day lasting 21.9±0.4 hours.[182]
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.[183] Some theorists think that without this stabilization against the
torques 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.[184][185]
Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The
angular size (or
solid 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.[161] This allows total and annular solar eclipses to occur on Earth.[186]
As of September 2021[update], there are 4,550 operational, human-made
satellites orbiting Earth.[191] There are also inoperative satellites, including
Vanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of tracked
space debris.[n 13] Earth's largest artificial satellite is the
International Space Station (ISS).[192]
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).[193]
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).[194] About 97.5% of the water is
saline; the remaining 2.5% is
fresh water.[195][196] Most fresh water, about 68.7%, is present as ice in
ice caps and
glaciers.[197] The remaining 30% is
ground water, 1%
surface water (covering only 2.8% of Earth's land)[198] and other small forms of fresh water deposits such as
permafrost,
water vapor in the atmosphere, biological binding, etc.[199][200]
In Earth's coldest regions, snow survives over the summer and
changes into ice. This accumulated snow and ice eventually forms into
glaciers, bodies of ice that flow under the influence of their own gravity.
Alpine glaciers form in mountainous areas, whereas vast
ice sheets form over land in polar regions. The flow of glaciers erodes the surface, changing it dramatically, with the formation of
U-shaped valleys and other landforms.[201]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.[202]
The average
salinity of Earth's oceans is about 35 grams of salt per kilogram of seawater (3.5% salt).[203] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[204] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[205] Sea water has an important influence on the world's climate, with the oceans acting as a large
heat reservoir.[206] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the
El Niño–Southern Oscillation.[207]
The abundance of water, particularly liquid water, on Earth's surface is a unique feature that distinguishes it from other planets in the
Solar System. Solar System planets with considerable atmospheres do partly host atmospheric water vapor, but they lack surface conditions for stable surface water.[208] Despite some
moons showing signs of large reservoirs of
extraterrestrial liquid water, with possibly even more volume than Earth's ocean, all of them are
large bodies of water under a kilometers thick frozen surface layer.[209]
The
atmospheric pressure at Earth's sea level averages 101.325 kPa (14.696 psi),[210] with a
scale 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.[210]Water vapor content varies between 0.01% and 4%[210] but averages about 1%.[3]Clouds cover around two-thirds of Earth's surface, more so over oceans than land.[211] The height of the
troposphere 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.[212]
Earth's
biosphere has significantly altered its
atmosphere.
Oxygenic photosynthesis evolved 2.7 Gya,
forming the primarily nitrogen–oxygen atmosphere of today.[62] This change enabled the proliferation of
aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent
conversion of atmospheric O2 into O3. The ozone layer blocks
ultravioletsolar radiation, permitting life on land.[213] 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.[214] This last phenomenon is the
greenhouse effect: trace molecules within the atmosphere serve to capture
thermal energy emitted from the surface, thereby raising the average temperature. Water vapor, carbon dioxide,
methane,
nitrous oxide, and
ozone 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),[215] and life on Earth probably would not exist in its current form.[216]
Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space.[217] 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.[218] 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 is
atmospheric circulation that drives the weather and climate through redistribution of thermal energy.[219]
The primary atmospheric circulation bands consist of the
trade winds in the equatorial region below 30° latitude and the
westerlies in the mid-latitudes between 30° and 60°.[220]Ocean heat content and
currents are also important factors in determining climate, particularly the
thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.[221]
Earth receives 1361 W/m2 of
solar irradiance.[222][223] 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.[224] 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 and
polar climates.[225]
Further factors that affect a location's climates are its
proximity to oceans, the oceanic and atmospheric circulation, and topology.[226] 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.[227] 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.[228] Finally, temperatures
decrease with height causing mountainous areas to be colder than low-lying areas.[229]
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 as
precipitation.[219] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This
water 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.[230]
The upper atmosphere, the atmosphere above the troposphere,[234] is usually divided into the
stratosphere,
mesosphere, and
thermosphere.[214] Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the
exosphere thins out into the magnetosphere, where the geomagnetic fields interact with the solar wind.[235] 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. The
Kármán line, defined as 100 km (62 mi) above Earth's surface, is a working definition for the boundary between the atmosphere and
outer space.[236]
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 steady
loss of the atmosphere into space. Because unfixed
hydrogen has a low
molecular mass, it can achieve
escape velocity more readily, and it leaks into outer space at a greater rate than other gases.[237] The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface from an initially
reducing 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.[238] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[239] 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.[240]
Earth is the only known place that has ever been
habitable 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 particular
ecosystems on Earth and has eventually expanded globally forming an overarching biosphere.[241]
Earth provides liquid water—an environment where complex
organic molecules can assemble and interact, and sufficient energy to sustain a
metabolism.[245] Plants and other organisms take up
nutrients from water, soils and the atmosphere. These nutrients are constantly recycled between different species.[246]
Originating from earlier
primates in Eastern Africa 300,000years ago
humans have since been migrating and with the advent of agriculture in the 10th millennium BC increasingly
settling Earth's land.[252] In the 20th century
Antarctica 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,[253] and is projected to peak at around ten billion in the second half of the 21st century.[254] Most of the growth is expected to take place in
sub-Saharan Africa.[254]
Distribution and
density of human population varies greatly around the world with the majority living in south to eastern Asia and 90% inhabiting only the
Northern Hemisphere of Earth,[255] partly due to the
hemispherical predominance of the world's land mass, with 68% of the world's land mass being in the Northern Hemisphere.[256] Furthermore, since the 19th century humans have increasingly converged into urban areas with the majority living in urban areas by the 21st century.[257]
Beyond Earth's surface humans have lived on a temporary basis, with only a few special-purpose deep
underground and
underwater presences and a few
space 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 temporarily
stayed beyond Earth, a tiny fraction of whom have reached another celestial body, the Moon.[258][259]
Earth has resources that have been exploited by humans.[263] Those termed
non-renewable resources, such as
fossil fuels, are only replenished over geological timescales.[264] Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.[265] These deposits are used by humans both for energy production and as feedstock for chemical production.[266] Mineral
ore bodies have also been formed within the crust through a process of
ore genesis, resulting from actions of
magmatism, erosion, and plate tectonics.[267] These metals and other elements are extracted by mining, a process which often brings environmental and health damage.[268]
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 upon
topsoil and fresh water, and the oceanic ecosystem depends on dissolved nutrients washed down from the land.[269] 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.[270] Of the 12–14% of ice-free land that is used for croplands, 2
percentage points were irrigated in 2015.[271] Humans use
building materials to construct shelters.[272]
Human activities have impacted Earth's environments. Through activities such as the burning of fossil fuels, humans have been increasing the amount of
greenhouse gases in the atmosphere, altering
Earth's energy budget and climate.[250][274] It is estimated that global temperatures in the year 2020 were 1.2 °C (2.2 °F) warmer than the preindustrial baseline.[275] This increase in temperature, known as
global warming, has contributed to the
melting of glaciers,
rising sea levels, increased risk of drought and wildfires, and migration of species to colder areas.[251]
The concept of
planetary 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 the
nitrogen cycle are thought to have passed the safe threshold.[276][277] 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.[278]
Scientific investigation has resulted in several culturally transformative shifts in people's view of the planet. Initial belief in a
flat Earth was gradually displaced in
Ancient Greece by the idea of a
spherical Earth, which was attributed to both the philosophers
Pythagoras and
Parmenides.[290][291] Earth was generally believed to be
the center of the universe until the 16th century, when scientists first concluded that it was
a moving object, one of the planets of the Solar System.[292]
It was only during the 19th century that geologists realized
Earth's age was at least many millions of years.[293]Lord Kelvin used
thermodynamics 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 and
radioactive 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.[294][295]
^All astronomical quantities vary, both
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J2000.0 of the secular variation, ignoring all periodic variations.
^aphelion = a × (1 + e); perihelion = a × (1 – e), where a is the semi-major axis and e 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.
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^Source for minimum,[19] mean,[20] and maximum[21] surface temperature
^ If Earth were shrunk to the size of a
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^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.
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