User:FreshYoMama/TWA/Earth

Source: Wikipedia, the free encyclopedia.
This page is about the planet. For other uses, see Earth (disambiguation) and Planet Earth (disambiguation).
"Third planet" redirects here. For other systems of numbering planets, see Planet § History. For the song, see 3rd Planet (song).


Earth
aj)
29.7827 km/s[5]
(107218 km/h; 66622 mph)
358.617°
Inclination
−11.26064°[5] to J2000 ecliptic
2023-Jan-04[7]
114.20783°[5]
Satellites
Physical characteristics
Mean radius
6371.0 km (3958.8 mi)[9]
Equatorial radius
6378.137 km (3963.191 mi)[10][11]
Polar radius
6356.752 km (3949.903 mi)[12]
Flattening1/298.257222101 (ETRS89)[13]
Circumference
Total: 510072000 km2 (196940000 sq mi)[15][n 5]

Land: 148940000 km2 (57510000 sq mi) – 29.2%

Ocean: 361132000 km2 (139434000 sq mi) – 70.8%
Sidereal rotation period
0.99726968 d[19]
(23h 56m 4.100s)
Equatorial rotation velocity
0.4651 km/s[20]
(1674.4 km/h; 1040.4 mph)
23.4392811°[4]
Albedo
Temperature287.91 K (14.76 °C) (blackbody temperature)[21]
Surface temp. min mean max
Celsius −89.2 °C[22] 14.76 °C (2022)[23] 56.7 °C[24]
Fahrenheit −128.5 °F 58.568 °F (2022) 134.0 °F
Surface equivalent dose rate0.274 μSv/h[25]
Atmosphere
Surface pressure
101.325 kPa (at MSL)
Composition by volume
  • 78.08% nitrogen (N2; dry air)[5]
  • 20.95% oxygen (O2)
  • ~ 1% water vapor (climate variable)
  • 0.9340% argon
  • 0.0413% carbon dioxide[26]
  • 0.00182% neon[5]
  • 0.00052% helium
  • 0.00019% methane
  • 0.00011% krypton
  • 0.00006% hydrogen

Earth is the third

originated and found habitability. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. Approximately 70.8% of Earth's surface is made up of the ocean, dwarfing Earth's polar ice, lakes, and rivers. The remaining 29.2% of Earth's surface is land, consisting of continents and islands. Earth's surface layer is formed of several slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Earth's liquid outer core generates the magnetic field that shapes the magnetosphere of Earth, deflecting destructive solar winds
.

The

Greenhouse gases in the atmosphere like carbon dioxide (CO2) trap a part of the energy from the Sun close to the surface. Water vapor is widely present in the atmosphere and forms clouds that cover most of the planet. More solar energy is received by tropical regions than polar regions and is redistributed by atmospheric and ocean circulation. A region's climate is governed not only by latitude but also by elevation and proximity to moderating oceans. In most areas, severe weather, such as tropical cyclones, thunderstorms, and heat waves
, occurs and greatly impacts life.

Earth is an

light-minutes away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. The Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes). The Earth's axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons. Earth is orbited by one permanent natural satellite, the Moon, which orbits Earth at 384,400 km (1.28 light seconds) and is roughly a quarter as wide as Earth. Through tidal locking, the Moon always faces the Earth with the same side, which causes tides, stabilizes Earth's axis, and gradually slows its rotation
.

Earth, like most other bodies in the Solar System,

billion years of Earth's history, the ocean formed and then life developed within it. Life spread globally and began to affect Earth's atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago, and have reached a population of 8 billion today. Humans depend on Earth's biosphere and natural resources for their survival, but have increasingly 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 life, causing widespread extinctions.[27]

Etymology

The

Germanic paganism: late Norse mythology included Jörð ('Earth'), a giantess often given as the mother of Thor.[29]

Historically, earth has been written in lowercase. From

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 remain common.[28] House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name (for example, "Earth's atmosphere") but writes it in lowercase when preceded by the (for example, "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"[30]

Occasionally, the name Terra

Latin) like Italian and Portuguese, while in other Romance languages the word gave rise to names with slightly altered spellings (like the Spanish Tierra and the French Terre). The Latinate form Gæa or Gaea (English: /ˈdʒiː.ə/) of the Greek poetic name Gaia (Γαῖα; Greek pronunciation: [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/.[33]

There are a number of adjectives for the planet Earth. From Earth itself comes earthly. From the Latin Terra comes terran /ˈtɛrən/,[34] terrestrial /təˈrɛstriəl/,[35] and (via French) terrene /təˈriːn/,[36] and from the Latin Tellus comes tellurian /tɛˈlʊəriən/[37] and telluric.[38]

Chronology

Main articles: History of Earth and Timeline of natural history

Formation

Further information: Early Earth and Hadean
Artist's impression of the early Solar System's protoplanetary disk, out of which Earth and other Solar System bodies formed

The oldest material found in the Solar System is dated to 4.5682+0.0002
−0.0004

nebular theory, planetesimals formed by accretion, with the primordial Earth being estimated as likely taking anywhere from 70 to 100 million years to form.[41]

Estimates of the age of the Moon range from 4.5 Ga to significantly younger.

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.[43] It hit Earth with a glancing blow and some of its mass merged with Earth.[44][45] 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.[46]

After formation

Main article: Geological history of Earth
Pale orange dot artist's impression of the early Earth tinted orange by its methane-rich early atmosphere[47]

Earth's atmosphere and oceans were formed by volcanic activity and outgassing.[48] Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets.[49] Sufficient water to fill the oceans may have been on Earth since it formed.[50] In this model, atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity.[51] By 3.5 Ga, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[52]

As the molten outer layer of Earth cooled it

Ma after Earth's formation.[53] There are two main models of how this initial small volume of continental crust evolved to reach its current abundance:[54] (1) a relatively steady growth up to the present day,[55] 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,[56][57] 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.[58]

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.[59]

The most recent pattern of

middle-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating about every 21,000, 41,000 and 100,000 years.[62] 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.[63]

Origin of life and evolution

Main articles: Origin of life, Earliest known life forms, and History of life
Artist's impression of the Archean, the eon after Earth's formation, featuring round stromatolites which are early oxygen producing forms of life from billions of years ago. After the Late Heavy Bombardment, Earth's crust had cooled, its water-rich barren surface is marked by continents and volcanoes, with the Moon still orbiting Earth much closer than today, producing strong tides.[64]

Western Greenland,[70] and remains of biotic material found in 4.1 billion-year-old rocks in Western Australia.[71][72] The earliest direct evidence of life on Earth is contained in 3.45 billion-year-old Australian rocks showing fossils of microorganisms.[73][74]

During the

civilization, led to humans having an influence on Earth and the nature and quantity of other life forms that continues to this day.[79]

Future

Main article: Future of Earth
See also:
ppm for C4 photosynthesis) in approximately 100–900 million years.[81][82] The lack of vegetation will result in the loss of oxygen in the atmosphere, making animal life impossible.[83] 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.[84] 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.[84][85]

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.[80][86] 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.[80]

Geophysical characteristics

Further information: Geophysics

Size and shape

Main article: Figure of the Earth
Further information: Earth radius, Earth's circumference, Earth curvature, and Geomorphology
See also: List of highest mountains on Earth
mean sea level
, as in common topographic maps)

The shape of Earth is nearly spherical, with an average diameter of 12,742 kilometers (7,918 mi), making it the

oblate spheroid
.

Earth's shape furthermore has local

Chimborazo volcano in Ecuador (6,384.4 km or 3,967.1 mi) the farthest point.[92][93]
Parallel to the rigid land topography the Ocean exhibits a more dynamic topography.[94]

To measure the local variation of Earth's topography, geodesy employs an idealized Earth producing a shape called a geoid. Such a geoid 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 gravitational irregular geoid surface, providing a mean sea level (MSL) as a reference level for topographic measurements.[95]

Surface

Further information:
Peplosphere
Approxomately 70.8% of Earth's surface is ocean water, the remaining 29.2% is land. At the polar regions, Earth's ocean and land surface is mostly covered by ice, as depicted in this composite image of the ice cover over the Southern Ocean (in grey) and Antarctica (in white).

Earth's surface is the top layer of Earth's rigid or liquid structure, at the interface with its atmosphere. Earth as an idealized spheroid has a surface area of about 510 million km2 (197 million sq mi).[15] Earth can be divided into two hemispheres. Generally, Earth is divided by latitude into the polar Northern and Southern hemispheres, or by longitude into the continental Eastern and Western hemispheres. Regarding the surface distribution of land and water, Earth can be divided into an oceans-focused water hemisphere and a landmasses-focused land hemisphere.

Most of Earth's surface is made of water, in liquid form or in smaller amounts as ice. 70.8% or 361.13 million km2 (139.43 million sq mi) of Earth's surface consists of the interconnected ocean,[96] making it Earth's global ocean or world ocean.[97][98] This makes Earth, along with its vibrant hydrosphere, a water world[99][100] or ocean world,[101][102] particularly in Earth's early history when the ocean is thought to have possibly covered Earth completely.[103] The world ocean is commonly divided into the Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern Ocean, and Arctic Ocean, from largest to smallest. The ocean fills the oceanic basins. The ocean floor comprises abyssal plains, continental shelves, seamounts, submarine volcanoes,[88] oceanic trenches, submarine canyons, oceanic plateaus, and a globe-spanning mid-ocean ridge system.

At Earth's

ocean surface is covered by a seasonally variable amount of sea ice that often connects with polar land and ice sheets, forming polar ice caps
.

Earth's land is 29.2%, or 148.94 million km2 (57.51 million sq mi) of Earth's surface area. Earth's land consists of many islands around the globe, but mainly of four continental landmasses, which are, from largest to smallest: Africa-Eurasia, America (landmass), Antarctica, and Australia (landmass).[104][105][106] These landmasses are further broken down and grouped into the continents. The terrain varies greatly and consists of mountains, deserts, plains, plateaus, and other landforms. The elevation of the land surface varies from the 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).[107]

Land can be covered by surface water, snow, ice, artificial structures or vegetation. Most of Earth's land hosts vegetation,[108] but ice sheets (10 %)[109] and deserts (33 %)[110] occupy a considerable amount of it.

The pedosphere is the outermost layer of Earth's continental 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.[111][112] 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.[113]

The pedosphere and the ocean, with its ocean floor, rest on

upper mantle form Earth's lithosphere
. Earth's crust is divided into oceanic and continental crusts, while the latter consists of lower density material such as the igneous rocks granite and andesite. Basalt, a denser volcanic rock primarily constitutes the lithosphere of the ocean floors.[114]

Relief of Earth's crust.

Earth's surface

ocean water
.

Erosion and tectonics, volcanic eruptions, flooding, weathering, glaciation, the growth and decomposition of biomass into soil or other remains such as coral reefs, and meteorite impacts are among the processes that constantly reshape Earth's crust over geological time.[115][116]

Sedimentary rock is formed from the accumulation of sediment that becomes buried and compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the crust.[117] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on Earth's surface include quartz, feldspars, amphibole, mica, pyroxene and olivine.[118] Common carbonate minerals include calcite (found in limestone) and dolomite.[119]

Tectonic plates

Main article: Plate tectonics
Shows the extent and boundaries of tectonic plates, with superimposed outlines of the continents they support
Earth's major plates, which are:[120]

Earth's mechanically rigid outer layer of

transform boundaries, two plates slide past one another laterally. Along these plate boundaries, earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur.[121] The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates.[122]

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.[53]

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]

Internal structure

Main article: Internal structure of Earth
Geologic layers of Earth[128]
Illustration of Earth's cutaway, not to scale
Depth[129]
(km) 		Component
layer name 		Density
(g/cm3)
0–60
Lithosphere[n 8]
 —
0–35 Crust[n 9] 2.2–2.9
35–660
Upper mantle
 3.4–4.4
660–2890
Lower mantle
 3.4–5.6
100–700 Asthenosphere —
2890–5100 Outer core 9.9–12.2
5100–6378 Inner core 12.8–13.1

Earth's interior, like that of the other terrestrial planets, is divided into layers by their

viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity.[130] The thickness of the crust varies from about 6 kilometers (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

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. Density increases with depth, as described in the table on the right.

Among the Solar System's planetary-sized objects Earth is the

object with the highest density
.

Chemical composition

See also:
Abundance of elements on Earth

mass segregation, the core region is estimated to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[134]

The most common rock constituents of the crust are nearly all oxides: chlorine, sulfur, and fluorine are the important exceptions to this and their total amount in any rock is usually much less than 1%. Over 99% of the crust is composed of 11 oxides, principally silica, alumina, iron oxides, lime, magnesia, potash, and soda.[135][134]

Internal heat

Main article: Earth's internal heat budget
oceanic ridges

The major heat-producing

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 because the crust there is much thinner than that of the continents.[143]

Gravitational field

Main article: Gravity of Earth

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]

Magnetic field

Main article: Earth's magnetic field
Diagram showing the magnetic field lines of Earth's magnetosphere. The lines are swept back in the anti-solar direction under the influence of the solar wind.
Schematic of Earth's magnetosphere, with the solar wind flows from left to right

The main part of Earth's magnetic field is generated in the core, the site of 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 dayside of the magnetosphere, to about 10 Earth radii, and extends the nightside 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 dayside 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

aurora.[155]

Orbit and rotation

Rotation

Main article: Earth's rotation
Earth's rotation imaged by Deep Space Climate Observatory, showing axis tilt

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

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).[4] 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]

Orbit

Main articles:
Earth's location
periapsis) are not the same as the four seasonal extreme points (equinox and solstice
)

Earth orbits the Sun, making Earth the third-closest planet to the Sun and part of the

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 to the Moon, 384,000 km (239,000 mi), in about 3.5 hours.[5]

The Moon and Earth orbit a common

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.[5][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 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]

Axial tilt and seasons

Main article: Axial tilt § Earth
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°

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]

In modern times, Earth's

aphelion around 4 July. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing 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]

Earth–Moon system

Further information: Satellite system (astronomy)

Moon

Main articles: Moon, Lunar theory, and Orbit of the Moon
Earth–Moon system seen from Mars

The Moon is a relatively large, terrestrial, planet-like natural satellite, with a diameter about one-quarter of the 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 planet Pluto.[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 (among other things) 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.[44]

The gravitational attraction between Earth and the Moon causes tides on Earth.[176] 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.[177] As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases.[178] 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.[179] 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.[180]

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.[181] 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.[182][183]

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.[184]

Asteroids and artificial satellites

Main articles: Near-Earth object and Claimed moons of Earth
artificial satellites and space debris around Earth in geosynchronous and low Earth orbit

Earth's

near-Earth asteroid 2006 RH120 makes close approaches to the Earth–Moon system roughly every twenty years. During these approaches, it can orbit Earth for brief periods of time.[188]

As of September 2021[update], there are 4,550 operational, human-made satellites orbiting Earth.[8] There are also inoperative satellites, including Vanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of tracked space debris.[n 3] Earth's largest artificial satellite is the International Space Station.[189]

Hydrosphere

Main article: Hydrosphere
global ocean and cloud cover, which dominate Earth's surface and hydrosphere. At Earth's polar
regions Earth's 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

metric 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).[190] 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).[191] About 97.5% of the water is saline; the remaining 2.5% is fresh water.[192][193] Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.[194]

In Earth's coldest regions, snow survives over the summer and

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.[195] 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.[196]

The average

heat reservoir.[200] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño–Southern Oscillation.[201]

The abundance of 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.[202] 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.[203]

Atmosphere

Main article: Atmosphere of Earth
stratospheric blue sky at the horizon, and above this a line of green airglow of the lower thermosphere around an altitude of 100 km, at the edge of space
.

The atmospheric pressure at Earth's sea level averages 101.325 kPa (14.696 psi),[204] with a scale height of about 8.5 km (5.3 mi).[5] 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.[204] Water vapor content varies between 0.01% and 4%[204] but averages about 1%.[5] Clouds cover around two thirds of Earth's surface, more so over oceans than land.[205] 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.[206]

Earth's

solar radiation, permitting life on land.[207] 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.[208] 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),[209] and life on Earth probably would not exist in its current form.[210]

Weather and climate

Main articles: Weather and Climate
Eastern Pacific and the Americas as seen from space
Worldwide Köppen climate classifications

Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space.[211] 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.[212] 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.[213]

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°.[214] 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.[215]

Earth receives 1361 W/m2 of 

polar climates.[219]

Further factors that affect a location's climates are its

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.[222] Finally, temperatures decrease with height causing mountainous areas to be colder than low-lying areas.[223]

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.[213] 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.[224]

The commonly used

hot deserts, such as Death Valley, and can fall as low as −89 °C (−128 °F) in Antarctica.[226][227]

Upper atmosphere

Earth's atmosphere as it appears from space, as bands of different colours at the horizon. From the bottom, afterglow illuminates the troposphere in orange with silhouettes of clouds, and the stratosphere in white and blue. Next the mesosphere (pink area) extends to just below the edge of space at one hundred kilometers and the pink line of airglow of the lower thermosphere (invisible), which hosts green and red aurorae over several hundred kilometers.

The upper atmosphere, the atmosphere above the troposphere,[228] is usually divided into the stratosphere, mesosphere, and thermosphere.[208] 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.[229] 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.[230]

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.[231] 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.[232] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[233] 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.[234]

Life on Earth

Main articles:
History of life on Earth
ocean surface
(low in purple; high in yellow)

Earth is the only known place that was and has 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

Great oxidation event
.

Earth's life has over time greatly diversified, allowing the biosphere to have different

water depths, planetary temperature latitudes and on land also with different humidity. Earth's species diversity and biomass reaches a peak in shallow waters and with forests, particularly in equatorial, warm and humid conditions. While freezing polar regions and high altitudes, or extremely arid areas are relatively barren of plant and animal life.[237]

Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain a metabolism.[238] Plants and other organisms take up nutrients from water, soils and the atmosphere. These nutrients are constantly recycled between different species.[239]

Extreme weather, such as

global warming.[243] This is driving changes such as the melting of glaciers and ice sheets, a global rise in average sea levels, increased risk of drought and wildfires, and migration of species to colder areas.[244]

Human geography

Main article: Human geography
See also: World
A composite image of artificial light emissions at night on a map of Earth

Originating from earlier

eastern Africa 300,000 years ago humans have since been migrating and with the advent of agriculture in the 10th millennium BC increasingly settling Earth's land. 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,[245] and is projected to peak at around ten billion in the second half of the 21st century.[246] Most of the growth is expected to take place in sub-Saharan Africa.[246]

Distribution and

hemispherical predominance of the world's land mass, with 68% of the world's land mass being in the Northern Hemisphere.[248] Furthermore, since the 19th century humans have increasingly converged into urban areas with the majority living in urban areas by the 21st century.[249]

Beyond Earth's surface humans have lived on a temporary basis, with only special purpose deep

underwater presence, and a few space stations. Human population virtually completely remains on Earth's surface, fully depending on Earth and the environment it sustains. Humans have gone and temporarily stayed beyond Earth with some hundreds of people, since the latter half of the 20th century, and only a fraction of them reaching another celestial body, the Moon.[250][251]

Humans have developed diverse

over the ocean and Antarctica
, and therefore all of Earth.

Natural resources and land use

Main articles: Natural resource and Land use
Earth's land use for human agriculture

Earth has resources that have been exploited by humans.[255] Those termed non-renewable resources, such as fossil fuels, are only replenished over geological timescales.[256] Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.[257] These deposits are used by humans both for energy production and as feedstock for chemical production.[258] 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.[259] These metals and other elements are extracted by mining, a process which often brings environmental and health damage.[260]

Earth's biosphere produces many useful biological products for humans, including food,

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.[261] 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.[262] Of the 12–14% of ice-free land that is used for croplands, 2 percentage points were irrigated in 2015.[263] Humans use building materials to construct shelters.[264]

Humans and the environment

Main articles: Human impact on the environment and Climate change
The graph from 1880 to 2020 shows natural drivers exhibiting fluctuations of about 0.3 degrees Celsius. Human drivers steadily increase by 0.3 degrees over 100 years to 1980, then steeply by 0.8 degrees more over the past 40 years.
Change in average surface air temperature and drivers for that change. Human activity has caused increased temperatures, with natural forces adding some variability.[265]

Human activities have impacted Earth's environments. Through activities such as the burning of fossil fuels, humans have been increasing the amount of

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.[244]

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.[268][269] 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.[270]

Cultural and historical viewpoint

Main articles: Earth in culture and Earth symbol
Woman seeing the Earth from space through a window
Tracy Caldwell Dyson in the Cupola module of the International Space Station observing the Earth below

fertility deity.[273] Creation myths in many religions involve the creation of Earth by a supernatural deity or deities.[273] The Gaia hypothesis, developed in the mid-20th century, compared Earth's environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability.[274][275][276]

Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, called the overview effect, emphasizing its beauty, uniqueness and apparent fragility.[277][278] In particular, this caused a realization of the scope of effects from human activity on Earth's environment. Enabled by science, particularly Earth observation,[279] humans have started to take action on environmental issues globally,[280] acknowledging the impact of humans and the interconnectedness of Earth's environments.

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.[281][282] 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.[283]

It was only during the 19th century that geologists realized

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.[285][286]

See also

Notes

  1. J2000.0
    of the secular variation, ignoring all periodic variations.
  2. ISBN 978-0-643-09949-4
    .
  3. ^ a b 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. Retrieved 18 April 2018.
  4. ^ Earth's circumference 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.
  5. ^ Due to natural fluctuations, ambiguities surrounding ice shelves, and mapping conventions for vertical datums, exact values for land and ocean coverage are not meaningful. Based on data from the Vector Map and Global Landcover Archived 26 March 2015 at the Wayback Machine datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of Earth's surface. The ice sheets of Antarctica and Greenland are counted as land, even though much of the rock that supports them lies below sea level.
  6. ^ If Earth were shrunk to the size of a billiard ball, some areas of Earth such as large mountain ranges and oceanic trenches would feel like tiny imperfections, whereas much of the planet, including the Great Plains and the abyssal plains, would feel smoother.[90]
  7. doi:10.1016/j.jafrearsci.2005.07.019
    .
  8. ^ Locally varies between 5 and 200 km.
  9. ^ Locally varies between 5 and 70 km.
  10. Bibcode:1982A&A...105..359A
    .
  11. Hill radius
    is , where m is the mass of Earth, a is an astronomical unit, and M is the mass of the Sun. So the radius in AU is about .
  12. ^ 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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