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'''Earth''' is the third [[planet]] from the [[Sun]] and is the largest of the [[terrestrial planet]]s in the [[Solar System]], in both [[diameter]] and [[mass]].  It is also referred to as "the Earth", "Planet Earth", "[[Gaia (mythology)|Gaia]]", "[[Terra (mythology)|Terra]]",Note that by [[International Astronomical Union]] convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. [http://planetarynames.wr.usgs.gov/jsp/append5.jsp] Gazetteer of Planetary Nomenclature, USGS, and "the [[World]]". Home to millions of [[species|species]] [http://adsabs.harvard.edu/abs/1988Sci...241.1441M] including [[human|humans]].

'''Earth''' is the third [[planet]] from the [[Sun]] and is the largest of the [[terrestrial planet]]s in the [[Solar System]], in both [[diameter]] and [[mass]].  It is also referred to as "the Earth", "Planet Earth", "[[Gaia (mythology)|Gaia]]", "[[Terra (mythology)|Terra]]",Note that by [[International Astronomical Union]] convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. [https://planetarynames.wr.usgs.gov/jsp/append5.jsp] Gazetteer of Planetary Nomenclature, USGS, and "the [[World]]". Home to millions of [[species|species]] [https://adsabs.harvard.edu/abs/1988Sci...241.1441M] including [[human|humans]].

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About 71% of Earth's surface is covered with [[seawater|salt-water]] [[ocean]]s, the remainder consisting of [[continent]]s and [[island]]s; liquid water. As of 2007, water vapor has been detected in the atmosphere of extrasolar planets. See: Water vapour in the atmosphere of a transiting extrasolar planet, Nature, [http://www.nature.com/nature/journal/v448/n7150/abs/nature06002.html] The Earth formed about [[Age of the Earth|4.57 billion years]] ago, and life appeared on its surface within a billion years.  Since then, Earth's [[biosphere]] has significantly altered [[Earth's atmosphere|the atmosphere]] and other [[abiotic]] conditions on the planet. [[Oxygen_evolution#Oxygen_evolution_in_nature|Oxygenic photosynthesis]] evolved 2.7 billion years ago, [[Oxygen Catastrophe| forming]] the primarily [[nitrogen]]-[[oxygen]] [[atmosphere]] that exists today. This change led to the proliferation of [[aerobic organisms]] as well as to the formation of the [[ozone layer]] which, together with Earth's [[magnetic field]], blocks harmful radiation, permitting life on land.   

About 71% of Earth's surface is covered with [[seawater|salt-water]] [[ocean]]s, the remainder consisting of [[continent]]s and [[island]]s; liquid water. As of 2007, water vapor has been detected in the atmosphere of extrasolar planets. See: Water vapour in the atmosphere of a transiting extrasolar planet, Nature, [https://www.nature.com/nature/journal/v448/n7150/abs/nature06002.html] The Earth formed about [[Age of the Earth|4.57 billion years]] ago, and life appeared on its surface within a billion years.  Since then, Earth's [[biosphere]] has significantly altered [[Earth's atmosphere|the atmosphere]] and other [[abiotic]] conditions on the planet. [[Oxygen_evolution#Oxygen_evolution_in_nature|Oxygenic photosynthesis]] evolved 2.7 billion years ago, [[Oxygen Catastrophe| forming]] the primarily [[nitrogen]]-[[oxygen]] [[atmosphere]] that exists today. This change led to the proliferation of [[aerobic organisms]] as well as to the formation of the [[ozone layer]] which, together with Earth's [[magnetic field]], blocks harmful radiation, permitting life on land.   

Earth interacts with other objects in [[outer space]], including the [[Sun]] and the [[Moon]]. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a [[sidereal year]], which is equal to 36'''5'''.26 [[solar day]]s.<ref>The number of solar days is one less than the number of [[sidereal day]]s because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis. The Earth's axis of rotation is [[axial tilt|tilted]] 23.5° Ahrens, ''Global Earth Physics: A Handbook of Physical Constants'', p. 8.</ref> away from the [[perpendicular]] to its [[Orbital plane (astronomy)|orbital plane]], producing seasonal variations on the planet's surface with a period of one [[tropical year]]. Earth's only known [[natural satellite]], the Moon, which began orbiting it about 4.53 billion years ago, provides ocean [[tide]]s, stabilizes the axial tilt and gradually slows the planet's rotation. A [[comet]]ary bombardment during the early history of the planet played a role in the formation of the oceans. Later, [[asteroid]] impacts caused significant changes to the surface environment. Long term [[Milankovitch cycles|periodic changes]] in the orbit of the planet are believed to have caused the [[ice age]]s that have covered significant portions of the surface in glacial sheets.

Earth interacts with other objects in [[outer space]], including the [[Sun]] and the [[Moon]]. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a [[sidereal year]], which is equal to 36'''5'''.26 [[solar day]]s.<ref>The number of solar days is one less than the number of [[sidereal day]]s because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis. The Earth's axis of rotation is [[axial tilt|tilted]] 23.5° Ahrens, ''Global Earth Physics: A Handbook of Physical Constants'', p. 8.</ref> away from the [[perpendicular]] to its [[Orbital plane (astronomy)|orbital plane]], producing seasonal variations on the planet's surface with a period of one [[tropical year]]. Earth's only known [[natural satellite]], the Moon, which began orbiting it about 4.53 billion years ago, provides ocean [[tide]]s, stabilizes the axial tilt and gradually slows the planet's rotation. A [[comet]]ary bombardment during the early history of the planet played a role in the formation of the oceans. Later, [[asteroid]] impacts caused significant changes to the surface environment. Long term [[Milankovitch cycles|periodic changes]] in the orbit of the planet are believed to have caused the [[ice age]]s that have covered significant portions of the surface in glacial sheets.

==History==

==History==

Scientists have been able to reconstruct detailed information about the planet's past. Earth and the other planets in the [[Solar System]] formed 4.57&nbsp;billion&nbsp;years ago<ref name="age_earth" /> out of the [[solar nebula]], a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially [[molten]], the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object (sometimes called [[Giant impact hypothesis|Theia]] with about 10% of the Earth's mass, An impact origin of the Earth-Moon system, American Geophysical Union, [http://adsabs.harvard.edu/abs/2001AGUFM.U51A..02C] impacting the Earth in a glancing blow. Origin of the Moon in a giant impact near the end of the Earth's formation [http://www.nature.com/nature/journal/v412/n6848/abs/412708a0.html]. Some of this object's mass merged with the Earth and a portion was ejected into space, but enough material survived to form an orbiting [[natural satellite|moon]].

Scientists have been able to reconstruct detailed information about the planet's past. Earth and the other planets in the [[Solar System]] formed 4.57&nbsp;billion&nbsp;years ago<ref name="age_earth" /> out of the [[solar nebula]], a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially [[molten]], the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object (sometimes called [[Giant impact hypothesis|Theia]] with about 10% of the Earth's mass, An impact origin of the Earth-Moon system, American Geophysical Union, [https://adsabs.harvard.edu/abs/2001AGUFM.U51A..02C] impacting the Earth in a glancing blow. Origin of the Moon in a giant impact near the end of the Earth's formation [https://www.nature.com/nature/journal/v412/n6848/abs/412708a0.html]. Some of this object's mass merged with the Earth and a portion was ejected into space, but enough material survived to form an orbiting [[natural satellite|moon]].

Outgassing and [[Volcano|volcanic]] activity produced the primordial atmosphere. Condensing [[water vapor]], augmented by ice delivered by [[comet]]s, [[Origin of the world's oceans|produced the oceans]]. Source regions and time scales for the delivery of water to Earth, Meteoritics & Planetary Science, [http://adsabs.harvard.edu/abs/2000M&PS...35.1309M] The highly energetic chemistry is believed to have produced a self-replicating molecule around 4&nbsp;billion&nbsp;years ago, and half a billion years later, the [[last universal common ancestor|last common ancestor of all life]] existed.

Outgassing and [[Volcano|volcanic]] activity produced the primordial atmosphere. Condensing [[water vapor]], augmented by ice delivered by [[comet]]s, [[Origin of the world's oceans|produced the oceans]]. Source regions and time scales for the delivery of water to Earth, Meteoritics & Planetary Science, [https://adsabs.harvard.edu/abs/2000M&PS...35.1309M] The highly energetic chemistry is believed to have produced a self-replicating molecule around 4&nbsp;billion&nbsp;years ago, and half a billion years later, the [[last universal common ancestor|last common ancestor of all life]] existed.

Uprooting the tree of life, Scientific American.

Uprooting the tree of life, Scientific American.

The development of [[photosynthesis]] allowed the Sun's energy to be harvested directly by life forms; the resultant [[oxygen]] accumulated in the atmosphere and resulted in a layer of [[ozone]] (a form of [[molecular oxygen]] [O₃]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the [[endosymbiotic theory|development of complex cells]] called [[eukaryotes]].[http://adsabs.harvard.edu/abs/1965JAtS...22..225B] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful [[ultraviolet radiation]] by the [[ozone layer]], life colonized the surface of Earth.[http://www.nasa.gov/centers/ames/news/releases/2000/00_79AR.html] Astrobiologists Find Evidence of Early Life on Land, NASA

The development of [[photosynthesis]] allowed the Sun's energy to be harvested directly by life forms; the resultant [[oxygen]] accumulated in the atmosphere and resulted in a layer of [[ozone]] (a form of [[molecular oxygen]] [O₃]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the [[endosymbiotic theory|development of complex cells]] called [[eukaryotes]].[https://adsabs.harvard.edu/abs/1965JAtS...22..225B] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful [[ultraviolet radiation]] by the [[ozone layer]], life colonized the surface of Earth.[https://www.nasa.gov/centers/ames/news/releases/2000/00_79AR.html] Astrobiologists Find Evidence of Early Life on Land, NASA

As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a [[supercontinent]]. Roughly 750&nbsp;million&nbsp;years ago (mya), the earliest known supercontinent, [[Rodinia]], began to break apart. The continents later recombined to form [[Pannotia]], 600–540&nbsp;mya, then finally [[Pangaea]], which broke apart 180&nbsp;mya. How do supercontinents assemble?, American Scientist, [http://scienceweek.com/2004/sa040730-5.htm]

As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a [[supercontinent]]. Roughly 750&nbsp;million&nbsp;years ago (mya), the earliest known supercontinent, [[Rodinia]], began to break apart. The continents later recombined to form [[Pannotia]], 600–540&nbsp;mya, then finally [[Pangaea]], which broke apart 180&nbsp;mya. How do supercontinents assemble?, American Scientist, [https://scienceweek.com/2004/sa040730-5.htm]

Since the 1960s, it has been hypothesized that severe [[Glacier|glacial]] action between 750 and 580&nbsp;mya, during the [[Neoproterozoic]], covered much of the planet in a sheet of ice. This hypothesis has been termed "[[Snowball Earth]]", and is of particular interest because it preceded the [[Cambrian explosion]], when multicellular life forms began to proliferate. The Proterozoic Biosphere: A Multidisciplinary Study, Cambridge University Press, ISBN 0521366151

Since the 1960s, it has been hypothesized that severe [[Glacier|glacial]] action between 750 and 580&nbsp;mya, during the [[Neoproterozoic]], covered much of the planet in a sheet of ice. This hypothesis has been termed "[[Snowball Earth]]", and is of particular interest because it preceded the [[Cambrian explosion]], when multicellular life forms began to proliferate. The Proterozoic Biosphere: A Multidisciplinary Study, Cambridge University Press, ISBN 0521366151

Following the [[Cambrian explosion]], about 535&nbsp;mya, there have been five [[Extinction event|mass extinctions]]. Mass Extinctions in the Marine Fossil Record, Science [http://adsabs.harvard.edu/abs/1982Sci...215.1501R] The last extinction event occurred 65&nbsp;mya, when a meteorite collision probably triggered the extinction of the (non-avian) [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s, which then resembled shrews. Over the past 65&nbsp;million years, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright. The Evolution of Life on Earth, Scientific American[http://brembs.net/gould.html] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had, The impact of humans on continental erosion and sedimentation, Bulletin of the Geological Society of America

Following the [[Cambrian explosion]], about 535&nbsp;mya, there have been five [[Extinction event|mass extinctions]]. Mass Extinctions in the Marine Fossil Record, Science [https://adsabs.harvard.edu/abs/1982Sci...215.1501R] The last extinction event occurred 65&nbsp;mya, when a meteorite collision probably triggered the extinction of the (non-avian) [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s, which then resembled shrews. Over the past 65&nbsp;million years, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright. The Evolution of Life on Earth, Scientific American[https://brembs.net/gould.html] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had, The impact of humans on continental erosion and sedimentation, Bulletin of the Geological Society of America

[http://bulletin.geoscienceworld.org/cgi/content/abstract/119/1-2/140] affecting both the nature and quantity of other life forms.

[https://bulletin.geoscienceworld.org/cgi/content/abstract/119/1-2/140] affecting both the nature and quantity of other life forms.

The present pattern of [[ice age]]s began about 40&nbsp;mya, then intensified during the [[Pleistocene]] about 3&nbsp;mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40&ndash;100,000&nbsp;years. The last ice age ended 10,000&nbsp;years ago.[http://www.lakepowell.net/sciencecenter/paleoclimate.htm] Paleoclimatology - The Study of Ancient Climates, Paleontology Science Center

The present pattern of [[ice age]]s began about 40&nbsp;mya, then intensified during the [[Pleistocene]] about 3&nbsp;mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40&ndash;100,000&nbsp;years. The last ice age ended 10,000&nbsp;years ago.[https://www.lakepowell.net/sciencecenter/paleoclimate.htm] Paleoclimatology - The Study of Ancient Climates, Paleontology Science Center

==Composition and structure==

==Composition and structure==

Earth is a [[terrestrial planet]], meaning that it is a rocky body, rather than a [[gas giant]] such as [[Jupiter]]. It is the largest of the four solar terrestrial planets, both in terms of size and total mass. Of these four planets, Earth also has the highest density, the highest [[surface gravity]] and the strongest [[magnetic field]].[http://astrogeology.usgs.gov/HotTopics/index.php?/archives/147-Names-for-the-Columbia-astronauts-provisionally-approved.html]

Earth is a [[terrestrial planet]], meaning that it is a rocky body, rather than a [[gas giant]] such as [[Jupiter]]. It is the largest of the four solar terrestrial planets, both in terms of size and total mass. Of these four planets, Earth also has the highest density, the highest [[surface gravity]] and the strongest [[magnetic field]].[https://astrogeology.usgs.gov/HotTopics/index.php?/archives/147-Names-for-the-Columbia-astronauts-provisionally-approved.html]

Planetary Magnetism

Planetary Magnetism

===Shape===

===Shape===

The Earth's shape is very close to an [[oblate spheroid]]&mdash;a rounded shape with a bulge around the [[equator]]&mdash;although the precise shape (the [[geoid]]) varies from this by up to 100&nbsp;meters (327&nbsp;ft).[http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html] Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model, National Geodetic Survey, NOAA, The average diameter of the reference spheroid is about 12,742&nbsp;km (7,913&nbsp;mi). More approximately the distance is 40,000&nbsp;km/[[pi|π]] because the [[meter]] was originally defined as 1/10,000,000 of the distance from the equator to the [[north pole]] through [[Paris]], [[France]].[http://physics.nist.gov/cuu/Units/meter.html] NIST Reference on Constants, Units, and Uncertainty, NIST Physics Laboratory

The Earth's shape is very close to an [[oblate spheroid]]&mdash;a rounded shape with a bulge around the [[equator]]&mdash;although the precise shape (the [[geoid]]) varies from this by up to 100&nbsp;meters (327&nbsp;ft).[https://www.ngs.noaa.gov/PUBS_LIB/gislis96.html] Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model, National Geodetic Survey, NOAA, The average diameter of the reference spheroid is about 12,742&nbsp;km (7,913&nbsp;mi). More approximately the distance is 40,000&nbsp;km/[[pi|π]] because the [[meter]] was originally defined as 1/10,000,000 of the distance from the equator to the [[north pole]] through [[Paris]], [[France]].[https://physics.nist.gov/cuu/Units/meter.html] NIST Reference on Constants, Units, and Uncertainty, NIST Physics Laboratory

The [[rotation]] of the Earth creates the [[equatorial bulge]] so that the equatorial diameter is 43&nbsp;km (27&nbsp;mi) larger than the [[Geographical pole|pole]] to pole diameter. [http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML] Exploring the Ocean Basins with Satellite Altimeter Data

The [[rotation]] of the Earth creates the [[equatorial bulge]] so that the equatorial diameter is 43&nbsp;km (27&nbsp;mi) larger than the [[Geographical pole|pole]] to pole diameter. [https://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML] Exploring the Ocean Basins with Satellite Altimeter Data

The largest local deviations in the rocky surface of the Earth are [[Mount Everest]] (8,848&nbsp;m [29,028&nbsp;ft] above local [[sea level]]) and the [[Mariana Trench]] (10,911&nbsp;m [35,798&nbsp;ft] below local sea level). Hence compared to a perfect [[ellipsoid]], the Earth has a [[tolerance (engineering)|tolerance]] of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in [[billiard ball]]s. [http://www.wpa-pool.com/index.asp?content=rules_spec] WPA Tournament Table & Equipment Specifications, World Pool-Billiards Association,  Because of the bulge, the feature farthest from the center of the Earth is actually [[Chimborazo (volcano)|Mount Chimborazo]] in [[Ecuador]]. Did Edmund Hillary Climb the Wrong Mountain, Professional Surveyor, [http://www.profsurv.com/archive.php?issue=42&article=589]

The largest local deviations in the rocky surface of the Earth are [[Mount Everest]] (8,848&nbsp;m [29,028&nbsp;ft] above local [[sea level]]) and the [[Mariana Trench]] (10,911&nbsp;m [35,798&nbsp;ft] below local sea level). Hence compared to a perfect [[ellipsoid]], the Earth has a [[tolerance (engineering)|tolerance]] of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in [[billiard ball]]s. [https://www.wpa-pool.com/index.asp?content=rules_spec] WPA Tournament Table & Equipment Specifications, World Pool-Billiards Association,  Because of the bulge, the feature farthest from the center of the Earth is actually [[Chimborazo (volcano)|Mount Chimborazo]] in [[Ecuador]]. Did Edmund Hillary Climb the Wrong Mountain, Professional Surveyor, [https://www.profsurv.com/archive.php?issue=42&article=589]

The [[mass]] of the Earth is approximately 5.98 kg. It is composed mostly of [[iron]] (32.1%), [[oxygen]] (30.1%), [[silicon]] (15.1%), [[magnesium]] (13.9%), [[sulfur]] (2.9%), [[nickel]] (1.8%), [[calcium]] (1.5%), and [[aluminum]] (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to [[mass segregation]], the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%),

The [[mass]] of the Earth is approximately 5.98 kg. It is composed mostly of [[iron]] (32.1%), [[oxygen]] (30.1%), [[silicon]] (15.1%), [[magnesium]] (13.9%), [[sulfur]] (2.9%), [[nickel]] (1.8%), [[calcium]] (1.5%), and [[aluminum]] (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to [[mass segregation]], the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%),

and less than 1% trace elements. Chemical composition of Earth, Venus, and Mercury, Proceedings of the National Academy of Science, [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=350422]

and less than 1% trace elements. Chemical composition of Earth, Venus, and Mercury, Proceedings of the National Academy of Science, [https://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=350422]

The geochemist [[Frank Wigglesworth Clarke|F. W. Clarke]] calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the [[Earth's crust]] are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.

The geochemist [[Frank Wigglesworth Clarke|F. W. Clarke]] calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the [[Earth's crust]] are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.

The interior of the Earth, like that of the other [[terrestrial planets]], is [[chemical]]ly divided into layers. The Earth has an outer [[Silicate minerals|silicate]] solid [[Crust (geology)|crust]], a highly viscous [[Mantle (geology)|mantle]], a liquid [[outer core]] that is much less viscous than the mantle, and a solid [[inner core]]. The crust is separated from the mantle by the [[Mohorovičić discontinuity]], and the thickness of the crust varies: averaging 6&nbsp;km under the oceans and 30&ndash;50&nbsp;km on the continents.

The interior of the Earth, like that of the other [[terrestrial planets]], is [[chemical]]ly divided into layers. The Earth has an outer [[Silicate minerals|silicate]] solid [[Crust (geology)|crust]], a highly viscous [[Mantle (geology)|mantle]], a liquid [[outer core]] that is much less viscous than the mantle, and a solid [[inner core]]. The crust is separated from the mantle by the [[Mohorovičić discontinuity]], and the thickness of the crust varies: averaging 6&nbsp;km under the oceans and 30&ndash;50&nbsp;km on the continents.

The internal heat of the planet is most likely produced by the radioactive decay of [[Potassium|potassium-40]], [[Uranium|uranium-238]] and [[Thorium|thorium-232]] [[isotope]]s. All three have [[half-life]] decay periods of more than a billion years. Radioactive potassium may be major heat source in Earth's core, UC Berkeley News, [http://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml] At the center of the planet, the temperature may be up to 7,000 degrees and the pressure could reach 360&nbsp;[[GPa]]. The ''ab initio'' simulation of the Earth's core, Philosophical Transaction of the Royal Society of London, [http://chianti.geol.ucl.ac.uk/~dario/pubblicazioni/PTRSA2002.pdf] A portion of the core's thermal energy is transported toward the crust by [[Mantle plume]]s; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce [[Hotspot (geology)|hotspots]] and [[flood basalt]]s. Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails, Science, [http://www.sciencemag.org/cgi/content/abstract/246/4926/103]

The internal heat of the planet is most likely produced by the radioactive decay of [[Potassium|potassium-40]], [[Uranium|uranium-238]] and [[Thorium|thorium-232]] [[isotope]]s. All three have [[half-life]] decay periods of more than a billion years. Radioactive potassium may be major heat source in Earth's core, UC Berkeley News, [https://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml] At the center of the planet, the temperature may be up to 7,000 degrees and the pressure could reach 360&nbsp;[[GPa]]. The ''ab initio'' simulation of the Earth's core, Philosophical Transaction of the Royal Society of London, [https://chianti.geol.ucl.ac.uk/~dario/pubblicazioni/PTRSA2002.pdf] A portion of the core's thermal energy is transported toward the crust by [[Mantle plume]]s; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce [[Hotspot (geology)|hotspots]] and [[flood basalt]]s. Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails, Science, [https://www.sciencemag.org/cgi/content/abstract/246/4926/103]

===Tectonic plates===

===Tectonic plates===

According to '''plate tectonics theory''', which is currently accepted by nearly all of the scientists working in this area, the outermost part of the Earth's interior is made up of two layers: the [[lithosphere]], comprising the [[Crust (geology)|crust]], and the solidified uppermost part of the [[Earth's mantle|mantle]]. Below the lithosphere lies the [[asthenosphere]], which forms the inner part of the mantle. The asthenosphere behaves like a superheated and extremely [[viscous]] liquid.[http://www.geolsoc.org.uk/template.cfm?name=lithosphere]  

According to '''plate tectonics theory''', which is currently accepted by nearly all of the scientists working in this area, the outermost part of the Earth's interior is made up of two layers: the [[lithosphere]], comprising the [[Crust (geology)|crust]], and the solidified uppermost part of the [[Earth's mantle|mantle]]. Below the lithosphere lies the [[asthenosphere]], which forms the inner part of the mantle. The asthenosphere behaves like a superheated and extremely [[viscous]] liquid.[https://www.geolsoc.org.uk/template.cfm?name=lithosphere]  

The lithosphere essentially ''floats'' on the [[asthenosphere]] and is broken up into what are called [[tectonic plate]]s. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: [[Convergent boundary|convergent]], [[Divergent boundary|divergent]] and [[transform fault|transform]]. The last occurs where two plates move laterally relative to each other, creating a [[strike-slip fault]]. [[Earthquake]]s, [[volcano|volcanic activity]], [[mountain]]-building, and [[oceanic trench]] formation can occur along these plate boundaries.[http://pubs.usgs.gov/gip/dynamic/understanding.html], Understanding plate motions, USGS

The lithosphere essentially ''floats'' on the [[asthenosphere]] and is broken up into what are called [[tectonic plate]]s. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: [[Convergent boundary|convergent]], [[Divergent boundary|divergent]] and [[transform fault|transform]]. The last occurs where two plates move laterally relative to each other, creating a [[strike-slip fault]]. [[Earthquake]]s, [[volcano|volcanic activity]], [[mountain]]-building, and [[oceanic trench]] formation can occur along these plate boundaries.[https://pubs.usgs.gov/gip/dynamic/understanding.html], Understanding plate motions, USGS

The main plates are:[http://www.ees1.lanl.gov/Wohletz/SFT-Tectonics.htm] SFT and the Earth's Tectonic Plates, Los Alamos National Laboratory

The main plates are:[https://www.ees1.lanl.gov/Wohletz/SFT-Tectonics.htm] SFT and the Earth's Tectonic Plates, Los Alamos National Laboratory

Notable minor plates include the [[Indian Plate]], 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 actually fused with [[Indian Plate]] between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the [[Cocos Plate]] advancing at a rate of 75&nbsp;mm/yr[http://www-odp.tamu.edu/publications/170_SR/chap_07/chap_07.htm] Plate Tectonic Evolution of the Cocos-Nazca Spreading Center, Proceedings of the Ocean Drilling Program, Texas A&M University

Notable minor plates include the [[Indian Plate]], 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 actually fused with [[Indian Plate]] between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the [[Cocos Plate]] advancing at a rate of 75&nbsp;mm/yr[https://www-odp.tamu.edu/publications/170_SR/chap_07/chap_07.htm] Plate Tectonic Evolution of the Cocos-Nazca Spreading Center, Proceedings of the Ocean Drilling Program, Texas A&M University

(3.0&nbsp;in/yr) and the [[Pacific Plate]] moving 52–69&nbsp;mm/yr (2.1&ndash;2.7 in/yr). At the other extreme, the slowest-moving plate is the [[Eurasian Plate]], progressing at a typical rate of about 21&nbsp;mm/yr (0.8&nbsp;in/yr).[http://sideshow.jpl.nasa.gov/mbh/series.html]

(3.0&nbsp;in/yr) and the [[Pacific Plate]] moving 52–69&nbsp;mm/yr (2.1&ndash;2.7 in/yr). At the other extreme, the slowest-moving plate is the [[Eurasian Plate]], progressing at a typical rate of about 21&nbsp;mm/yr (0.8&nbsp;in/yr).[https://sideshow.jpl.nasa.gov/mbh/series.html]

===Surface===

===Surface===

Present day Earth [[terrain|altimetry]] and [[bathymetry]]. Data from the [[National Geophysical Data Center]]'s [http://www.ngdc.noaa.gov/seg/fliers/se-1104.shtml TerrainBase Digital Terrain Model].]]

Present day Earth [[terrain|altimetry]] and [[bathymetry]]. Data from the [[National Geophysical Data Center]]'s [https://www.ngdc.noaa.gov/seg/fliers/se-1104.shtml TerrainBase Digital Terrain Model].]]

The Earth's [[terrain]] varies greatly from place to place. About 70.8%[http://www.physicalgeography.net/fundamentals/7h.html] Fundamentals of Physical Geography of the surface is covered by water, with much of the [[continental shelf]] below sea level. The submerged surface has mountainous features, including a globe-spanning [[mid-ocean ridge]] system, as well as undersea [[volcano]]es,<ref name="ngdc2006" /> [[oceanic trench]]es, [[submarine canyon]]s, [[oceanic plateau]]s and [[abyssal plain]]s. The remaining 29.2% not covered by water consists of [[mountains]], [[deserts]], [[plain]]s, [[plateau]]s, and other [[Geomorphology|geomorphologies]].

The Earth's [[terrain]] varies greatly from place to place. About 70.8%[https://www.physicalgeography.net/fundamentals/7h.html] Fundamentals of Physical Geography of the surface is covered by water, with much of the [[continental shelf]] below sea level. The submerged surface has mountainous features, including a globe-spanning [[mid-ocean ridge]] system, as well as undersea [[volcano]]es,<ref name="ngdc2006" /> [[oceanic trench]]es, [[submarine canyon]]s, [[oceanic plateau]]s and [[abyssal plain]]s. The remaining 29.2% not covered by water consists of [[mountains]], [[deserts]], [[plain]]s, [[plateau]]s, and other [[Geomorphology|geomorphologies]].

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and [[erosion]]. The surface features built up or deformed through plate tectonics are subject to steady [[weathering]] from [[Precipitation (meteorology)|precipitation]], thermal cycles, and chemical effects. [[Glaciation]], [[coastal erosion]], the build-up of [[coral reef]]s, and large meteorite impacts. [http://www.lpl.arizona.edu/SIC/impact_cratering/intro/]Terrestrial Impact Cratering and Its Environmental Effects, Lunar and Planetary Laboratory, also act to reshape the landscape.

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and [[erosion]]. The surface features built up or deformed through plate tectonics are subject to steady [[weathering]] from [[Precipitation (meteorology)|precipitation]], thermal cycles, and chemical effects. [[Glaciation]], [[coastal erosion]], the build-up of [[coral reef]]s, and large meteorite impacts. [https://www.lpl.arizona.edu/SIC/impact_cratering/intro/]Terrestrial Impact Cratering and Its Environmental Effects, Lunar and Planetary Laboratory, also act to reshape the landscape.

As the continental plates migrate across the planet, the ocean floor is [[Subduction|subducted]] under the leading edges. At the same time, upwellings of mantle material create a [[divergent boundary]] along [[mid-ocean ridge]]s. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.[http://www.soest.hawaii.edu/GG/ASK/plate-tectonics2.html] Pacific Plate Motion, University of Hawaii, [http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html] Age of the Ocean Floor Poster, NOAA

As the continental plates migrate across the planet, the ocean floor is [[Subduction|subducted]] under the leading edges. At the same time, upwellings of mantle material create a [[divergent boundary]] along [[mid-ocean ridge]]s. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.[https://www.soest.hawaii.edu/GG/ASK/plate-tectonics2.html] Pacific Plate Motion, University of Hawaii, [https://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html] Age of the Ocean Floor Poster, NOAA

The continental plates consist of lower density material such as the [[igneous rock]]s [[granite]] and [[andesite]]. Less common is [[basalt]], a denser volcanic rock that is the primary constituent of the ocean floors.[http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/part1.html] Layers of the Earth, Volcano World, [[Sedimentary rock]] is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust. [http://geology.csupomona.edu/drjessey/class/Gsc101/Weathering.html] Weathering and Sedimentary Rocks,  Cal Poly Pomona, 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 the Earth's surface include [[quartz]], the [[feldspar]]s, [[amphibole]], [[mica]], [[pyroxene]] and [[olivine]]. [http://natural-history.uoregon.edu/Pages/web/mineral.htm] Minerals, Museum of Natural History, Oregon, Common carbonate minerals include [[calcite]] (found in [[limestone]]), [[aragonite]] and [[dolomite]].[http://madmonster.williams.edu/geos.302/L.08.html] Carbonate sediments

The continental plates consist of lower density material such as the [[igneous rock]]s [[granite]] and [[andesite]]. Less common is [[basalt]], a denser volcanic rock that is the primary constituent of the ocean floors.[https://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/part1.html] Layers of the Earth, Volcano World, [[Sedimentary rock]] is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust. [https://geology.csupomona.edu/drjessey/class/Gsc101/Weathering.html] Weathering and Sedimentary Rocks,  Cal Poly Pomona, 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 the Earth's surface include [[quartz]], the [[feldspar]]s, [[amphibole]], [[mica]], [[pyroxene]] and [[olivine]]. [https://natural-history.uoregon.edu/Pages/web/mineral.htm] Minerals, Museum of Natural History, Oregon, Common carbonate minerals include [[calcite]] (found in [[limestone]]), [[aragonite]] and [[dolomite]].[https://madmonster.williams.edu/geos.302/L.08.html] Carbonate sediments

The [[pedosphere]] is the outermost layer of the Earth that is composed of [[soil]] and subject to [[pedogenesis|soil formation processes]]. It exists at the interface of the [[lithosphere]], [[Earth's atmosphere|atmosphere]], [[hydrosphere]] and [[biosphere]]. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.  [https://www.cia.gov/library/publications/the-world-factbook/geos/xx.html], The World Factbook, U.S. C.I.A. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3 km² (3.3 acres) of cropland and 3.4 km² (8.4 acres) of pastureland. Production Yearbook 1994, Food and Agriculture Organization of the United Nations,Rome, Italy ISBN 9250038445

The [[pedosphere]] is the outermost layer of the Earth that is composed of [[soil]] and subject to [[pedogenesis|soil formation processes]]. It exists at the interface of the [[lithosphere]], [[Earth's atmosphere|atmosphere]], [[hydrosphere]] and [[biosphere]]. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.  [https://www.cia.gov/library/publications/the-world-factbook/geos/xx.html], The World Factbook, U.S. C.I.A. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3 km² (3.3 acres) of cropland and 3.4 km² (8.4 acres) of pastureland. Production Yearbook 1994, Food and Agriculture Organization of the United Nations,Rome, Italy ISBN 9250038445

The elevation of the land surface of the Earth varies from the low point of −418&nbsp;m (−1,371&nbsp;ft) at the [[Dead Sea]], to a 2005-estimated maximum altitude of 8,848&nbsp;m (29,028&nbsp;ft) at the top of [[Mount Everest]]. The mean height of land above sea level is 686(2,250&nbsp;ft). The Permanence of Ocean Basins, The Geographical Journal,[http://www.wku.edu/~smithch/wallace/S453.htm]

The elevation of the land surface of the Earth varies from the low point of −418&nbsp;m (−1,371&nbsp;ft) at the [[Dead Sea]], to a 2005-estimated maximum altitude of 8,848&nbsp;m (29,028&nbsp;ft) at the top of [[Mount Everest]]. The mean height of land above sea level is 686(2,250&nbsp;ft). The Permanence of Ocean Basins, The Geographical Journal,[https://www.wku.edu/~smithch/wallace/S453.htm]

===Hydrosphere===

===Hydrosphere===

The abundance of water on Earth's surface is a unique feature that distinguishes the "[[Blue Planet]]" from others in the solar system. The Earth's hydrosphere consists chiefly of the [[oceans]], but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000&nbsp;m. The deepest underwater location is Challenger Deep of the [[Mariana Trench]] in the [[Pacific Ocean]] with a depth of −10,911&nbsp;m (35,798&nbsp;ft or 6.78&nbsp;mi). [http://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html] "Deep Ocean Studies", Ocean Studies, RAIN National Public Internet and Community Technology Center, Takuyo measurement; see [[Mariana Trench]] for details. The average depth of the oceans is 3,794&nbsp;m (12,447&nbsp;ft), more than five times the average height of the continents.

The abundance of water on Earth's surface is a unique feature that distinguishes the "[[Blue Planet]]" from others in the solar system. The Earth's hydrosphere consists chiefly of the [[oceans]], but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000&nbsp;m. The deepest underwater location is Challenger Deep of the [[Mariana Trench]] in the [[Pacific Ocean]] with a depth of −10,911&nbsp;m (35,798&nbsp;ft or 6.78&nbsp;mi). [https://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html] "Deep Ocean Studies", Ocean Studies, RAIN National Public Internet and Community Technology Center, Takuyo measurement; see [[Mariana Trench]] for details. The average depth of the oceans is 3,794&nbsp;m (12,447&nbsp;ft), more than five times the average height of the continents.

The mass of the oceans is approximately 1.35 e|18 ;metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386 e|9&nbsp;km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7&nbsp;km (approximately 1.7&nbsp;mi). The total volume of the Earth's oceans is: 1.4 km³. The total surface area of the Earth is 5.1 km². So, to [[Orders of approximation|first approximation]], the average depth would be the ratio of the two, or 2.7km. About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.Igor A. Shiklomanov, [http://espejo.unesco.org.uy] World Water Resources and their use Beginning of the 21st century" Prepared in the Framework of IHP UNESCO, State Hydrological Institute, St. Petersburg.

The mass of the oceans is approximately 1.35 e|18 ;metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386 e|9&nbsp;km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7&nbsp;km (approximately 1.7&nbsp;mi). The total volume of the Earth's oceans is: 1.4 km³. The total surface area of the Earth is 5.1 km². So, to [[Orders of approximation|first approximation]], the average depth would be the ratio of the two, or 2.7km. About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.Igor A. Shiklomanov, [https://espejo.unesco.org.uy] World Water Resources and their use Beginning of the 21st century" Prepared in the Framework of IHP UNESCO, State Hydrological Institute, St. Petersburg.

About 3.5% of the total mass of the oceans consists of [[salt]]. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[http://www.astrobio.net/news/article223.html],Salt of the Early Earth,NASA Astrobiology Magazine. The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms. [http://seis.natsci.csulb.edu/rmorris/oxy/oxy4.html] NASA Astrobiology Magazine, Sea water has an important influence on the world's

About 3.5% of the total mass of the oceans consists of [[salt]]. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[https://www.astrobio.net/news/article223.html],Salt of the Early Earth,NASA Astrobiology Magazine. The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms. [https://seis.natsci.csulb.edu/rmorris/oxy/oxy4.html] NASA Astrobiology Magazine, Sea water has an important influence on the world's

climate, with the oceans acting as a large [[heat reservoir]].[http://earthobservatory.nasa.gov/Study/HeatBucket/]Earth's Big heat Bucket, NASA Earth Observatory. Shifts in the oceanic temperature distribution

climate, with the oceans acting as a large [[heat reservoir]].[https://earthobservatory.nasa.gov/Study/HeatBucket/]Earth's Big heat Bucket, NASA Earth Observatory. Shifts in the oceanic temperature distribution

can cause significant weather shifts, such as the

can cause significant weather shifts, such as the

[[El Niño-Southern Oscillation]]. [http://science.hq.nasa.gov/oceans/physical/SST.html]

[[El Niño-Southern Oscillation]]. [https://science.hq.nasa.gov/oceans/physical/SST.html]

===Atmosphere===

===Atmosphere===

The [[atmospheric pressure]] on the surface of the Earth averages 101.325&nbsp;[[kPa]], with a [[scale height]] of about 8.5&nbsp;km.<ref name="earth_fact_sheet"/> It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The atmosphere protects the Earth's life forms by absorbing [[ultraviolet]] [[solar radiation]], moderating temperature, transporting water vapor, and providing useful gases.[http://www.nasa.gov/audience/forstudents/9-12/features/912_liftoff_atm.html] Earth's Atmosphere,NASA. It also serves as a shield that causes small [[meteor]]s to burn up before they strike the surface.

The [[atmospheric pressure]] on the surface of the Earth averages 101.325&nbsp;[[kPa]], with a [[scale height]] of about 8.5&nbsp;km.<ref name="earth_fact_sheet"/> It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The atmosphere protects the Earth's life forms by absorbing [[ultraviolet]] [[solar radiation]], moderating temperature, transporting water vapor, and providing useful gases.[https://www.nasa.gov/audience/forstudents/9-12/features/912_liftoff_atm.html] Earth's Atmosphere,NASA. It also serves as a shield that causes small [[meteor]]s to burn up before they strike the surface.

In a phenomenon known as the [[greenhouse effect]], trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the net temperature. Carbon dioxide, water vapor, methane and ozone are the primary [[greenhouse gas]]es in the Earth's atmosphere. Without this heat-retention effect, the average surface temperature would be −18&nbsp;°C and life would likely not exist.

In a phenomenon known as the [[greenhouse effect]], trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the net temperature. Carbon dioxide, water vapor, methane and ozone are the primary [[greenhouse gas]]es in the Earth's atmosphere. Without this heat-retention effect, the average surface temperature would be −18&nbsp;°C and life would likely not exist.

====Weather and climate====

====Weather and climate====

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11&nbsp;km (about 4&nbsp;mi) of the planet's surface. This lowest layer is called the [[troposphere]]. 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 heat energy.[http://www.nasa.gov/worldbook/weather_worldbook.html] World Book Online Reference Center,NASA/World Book, Inc.  

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11&nbsp;km (about 4&nbsp;mi) of the planet's surface. This lowest layer is called the [[troposphere]]. 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 heat energy.[https://www.nasa.gov/worldbook/weather_worldbook.html] World Book Online Reference Center,NASA/World Book, Inc.  

The primary atmospheric circulation bands consist of the [[trade winds]] in the [[equator]]ial region below 30° latitude and the [[westerlies]] in the mid-latitudes between

The primary atmospheric circulation bands consist of the [[trade winds]] in the [[equator]]ial region below 30° latitude and the [[westerlies]] in the mid-latitudes between

30° and 60°. [http://earthguide.ucsd.edu/virtualmuseum/climatechange1/cc1syllabus.shtml] Ocean currents are also important factors in determining climate, particularly the [[thermohaline circulation]] that distributes heat energy from the equatorial oceans to the polar regions.[http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html]

30° and 60°. [https://earthguide.ucsd.edu/virtualmuseum/climatechange1/cc1syllabus.shtml] Ocean currents are also important factors in determining climate, particularly the [[thermohaline circulation]] that distributes heat energy from the equatorial oceans to the polar regions.[https://www.pik-potsdam.de/~stefan/thc_fact_sheet.html]

When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as [[Precipitation (meteorology)|precipitation]].<ref name="moran2005" /> Most of the water is then transported back to lower elevations by [[river]] systems, usually returning to the oceans or being deposited into [[lake]]s. 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]], topological features and temperature differences determine the average precipitation that falls in each region.[http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml]

When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as [[Precipitation (meteorology)|precipitation]].<ref name="moran2005" /> Most of the water is then transported back to lower elevations by [[river]] systems, usually returning to the oceans or being deposited into [[lake]]s. 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]], topological features and temperature differences determine the average precipitation that falls in each region.[https://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml]

The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the [[equator]] to the polar regions, these are the [[tropics|tropical]] (or equatorial), [[Subtropics|subtropical]], [[temperate]] and [[Polar region|polar]] climates.Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform [[air mass]]es. The commonly-used [[Köppen climate classification]] system (as modified by [[Wladimir Köppen]]'s student Rudolph Geiger) has five broad groups (humid tropics, [[Desert|arid]], humid middle latitudes, [[Continental climate|continental]] and cold polar), which are further divided into more specific subtypes<ref name="berger2002" />

The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the [[equator]] to the polar regions, these are the [[tropics|tropical]] (or equatorial), [[Subtropics|subtropical]], [[temperate]] and [[Polar region|polar]] climates.Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform [[air mass]]es. The commonly-used [[Köppen climate classification]] system (as modified by [[Wladimir Köppen]]'s student Rudolph Geiger) has five broad groups (humid tropics, [[Desert|arid]], humid middle latitudes, [[Continental climate|continental]] and cold polar), which are further divided into more specific subtypes<ref name="berger2002" />

====Upper atmosphere====

====Upper atmosphere====

Above the troposphere, the atmosphere is usually divided into the [[stratosphere]], [[mesosphere]], and [[thermosphere]].<ref name="atmosphere" /> Each of these layers 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 Earth's magnetic fields interact with the [[solar wind]]).[http://scienceweek.com/2004/rmps-23.htm] Stratosphere and Weather; Discovery of the Stratosphere, Science WeekAn important part of the atmosphere for [[life on Earth]] is the [[ozone layer]], a component of the stratosphere that partially shields the surface from ultraviolet light. The [[Kármán line]], defined as 100&nbsp;km (62&nbsp;mi) above the Earth's surface, is a working definition for the boundary between atmosphere and space.

Above the troposphere, the atmosphere is usually divided into the [[stratosphere]], [[mesosphere]], and [[thermosphere]].<ref name="atmosphere" /> Each of these layers 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 Earth's magnetic fields interact with the [[solar wind]]).[https://scienceweek.com/2004/rmps-23.htm] Stratosphere and Weather; Discovery of the Stratosphere, Science WeekAn important part of the atmosphere for [[life on Earth]] is the [[ozone layer]], a component of the stratosphere that partially shields the surface from ultraviolet light. The [[Kármán line]], defined as 100&nbsp;km (62&nbsp;mi) above the Earth's surface, is a working definition for the boundary between atmosphere and space.

Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can [[escape velocity|escape]] from the planet's gravity. This results in a slow but steady [[Atmospheric escape|leakage of the atmosphere into space]]. Because unfixed [[hydrogen]] has a low molecular weight, it can achieve [[escape velocity]] more readily and it leaks into outer space at a greater rate.Liu, S. C.; Donahue, T. M., The Aeronomy of Hydrogen in the Atmosphere of the Earth | journal=Journal of Atmospheric Sciences [http://adsabs.harvard.edu/abs/1974JAtS...31.1118L]For this reason, the Earth's current environment is [[Redox|oxidizing]], rather than [[Redox|reducing]], with consequences for the [[chemical]] nature of [[life]] which developed on the planet. The oxygen-rich atmosphere also preserves much of the surviving hydrogen by locking it up in water molecules. [http://www.mansfield.ohio-state.edu/~sabedon/biol1010.htm]

Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can [[escape velocity|escape]] from the planet's gravity. This results in a slow but steady [[Atmospheric escape|leakage of the atmosphere into space]]. Because unfixed [[hydrogen]] has a low molecular weight, it can achieve [[escape velocity]] more readily and it leaks into outer space at a greater rate.Liu, S. C.; Donahue, T. M., The Aeronomy of Hydrogen in the Atmosphere of the Earth | journal=Journal of Atmospheric Sciences [https://adsabs.harvard.edu/abs/1974JAtS...31.1118L]For this reason, the Earth's current environment is [[Redox|oxidizing]], rather than [[Redox|reducing]], with consequences for the [[chemical]] nature of [[life]] which developed on the planet. The oxygen-rich atmosphere also preserves much of the surviving hydrogen by locking it up in water molecules. [https://www.mansfield.ohio-state.edu/~sabedon/biol1010.htm]

===Magnetic field===

===Magnetic field===

The [[Earth's magnetic field]] is shaped roughly as a [[magnetic dipole]], with the poles currently located proximate to the planet's geographic poles. According to [[dynamo theory]], the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in [[geomagnetic reversal|field reversals]] at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[http://farside.ph.utexas.edu/teaching/plasma/lectures/node69.html] = MHD dynamo theory

The [[Earth's magnetic field]] is shaped roughly as a [[magnetic dipole]], with the poles currently located proximate to the planet's geographic poles. According to [[dynamo theory]], the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in [[geomagnetic reversal|field reversals]] at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[https://farside.ph.utexas.edu/teaching/plasma/lectures/node69.html] = MHD dynamo theory

The field forms the [[magnetosphere]], which deflects particles in the [[solar wind]]. The sunward edge of the [[bow shock]] is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the [[Van Allen radiation belt]]s, a pair of concentric, [[torus]]-shaped regions of energetic [[charged particle]]s. When the [[plasma (physics)|plasma]] enters the Earth's atmosphere at the magnetic poles, it forms the [[Aurora (astronomy)|aurora]].[http://www-spof.gsfc.nasa.gov/Education/wmap.html] Exploration of the Earth's Magnetosphere.  

The field forms the [[magnetosphere]], which deflects particles in the [[solar wind]]. The sunward edge of the [[bow shock]] is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the [[Van Allen radiation belt]]s, a pair of concentric, [[torus]]-shaped regions of energetic [[charged particle]]s. When the [[plasma (physics)|plasma]] enters the Earth's atmosphere at the magnetic poles, it forms the [[Aurora (astronomy)|aurora]].[https://www-spof.gsfc.nasa.gov/Education/wmap.html] Exploration of the Earth's Magnetosphere.  

==Orbit and rotation==

==Orbit and rotation==

Relative to the background stars, it takes the Earth, on average, 23&nbsp;hours, 56&nbsp;minutes and 4.091&nbsp;seconds ([[sidereal day|one sidereal day]]) to rotate around the [[Axis of rotation|axis]] that connects the [[north pole|north]] and the [[south pole]]s.[http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html]. From Earth, the main apparent motion of celestial bodies in the sky (except that of [[meteor]]s within the atmosphere and low-orbiting satellites) is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes. (The apparent sizes of the Sun and the Moon are approximately the same.)

Relative to the background stars, it takes the Earth, on average, 23&nbsp;hours, 56&nbsp;minutes and 4.091&nbsp;seconds ([[sidereal day|one sidereal day]]) to rotate around the [[Axis of rotation|axis]] that connects the [[north pole|north]] and the [[south pole]]s.[https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html]. From Earth, the main apparent motion of celestial bodies in the sky (except that of [[meteor]]s within the atmosphere and low-orbiting satellites) is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes. (The apparent sizes of the Sun and the Moon are approximately the same.)

Earth orbits the Sun at an average distance of about 150&nbsp;million kilometers (93.2&nbsp;million miles) every 365.2564&nbsp;mean&nbsp;solar&nbsp;days ([[sidereal year|1 sidereal&nbsp;year]]). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1°/day (or a Sun or Moon diameter every 12&nbsp;hours) eastward. Because of this motion, on average it takes 24 hours&mdash;a [[Solar time|solar day]]&mdash;for Earth to complete a full rotation about its axis so that the Sun returns to the [[Meridian (astronomy)|meridian]]. The orbital speed of the Earth averages about 30&nbsp;km/s (108,000&nbsp;km/h or 67,000&nbsp;mi/h), which is fast enough to cover the planet's diameter (about 12,600&nbsp;km [7,800&nbsp;mi]) in seven minutes, and the distance to the Moon (384,000&nbsp;km or 238,000&nbsp;mi) in four hours.[http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html] Earth Fact Sheet

Earth orbits the Sun at an average distance of about 150&nbsp;million kilometers (93.2&nbsp;million miles) every 365.2564&nbsp;mean&nbsp;solar&nbsp;days ([[sidereal year|1 sidereal&nbsp;year]]). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1°/day (or a Sun or Moon diameter every 12&nbsp;hours) eastward. Because of this motion, on average it takes 24 hours&mdash;a [[Solar time|solar day]]&mdash;for Earth to complete a full rotation about its axis so that the Sun returns to the [[Meridian (astronomy)|meridian]]. The orbital speed of the Earth averages about 30&nbsp;km/s (108,000&nbsp;km/h or 67,000&nbsp;mi/h), which is fast enough to cover the planet's diameter (about 12,600&nbsp;km [7,800&nbsp;mi]) in seven minutes, and the distance to the Moon (384,000&nbsp;km or 238,000&nbsp;mi) in four hours.[https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html] Earth Fact Sheet

The [[Moon]] revolves with the Earth around a common [[barycenter]] every 27.32&nbsp;days relative to the background stars. When combined with the Earth–Moon system's common revolution around the Sun, the period of the [[synodic month]], from new moon to new moon, is 29.53&nbsp;days. Viewed from the [[celestial pole|celestial north pole]], the motion of Earth, the Moon and their axial rotations are all [[counter-clockwise]]. The orbital and axial planes are not precisely aligned: Earth's [[axial tilt|axis is tilted]] some 23.5&nbsp;degrees from the perpendicular to the Earth–Sun plane (which causes the [[season]]s); and the Earth–Moon plane is tilted about 5&nbsp;degrees against the Earth-Sun plane (without this tilt, there would be an eclipse every two weeks, alternating between [[lunar eclipse]]s and [[solar eclipse]]s).<ref name="moon_fact_sheet"[http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html] Moon Fact Sheet

The [[Moon]] revolves with the Earth around a common [[barycenter]] every 27.32&nbsp;days relative to the background stars. When combined with the Earth–Moon system's common revolution around the Sun, the period of the [[synodic month]], from new moon to new moon, is 29.53&nbsp;days. Viewed from the [[celestial pole|celestial north pole]], the motion of Earth, the Moon and their axial rotations are all [[counter-clockwise]]. The orbital and axial planes are not precisely aligned: Earth's [[axial tilt|axis is tilted]] some 23.5&nbsp;degrees from the perpendicular to the Earth–Sun plane (which causes the [[season]]s); and the Earth–Moon plane is tilted about 5&nbsp;degrees against the Earth-Sun plane (without this tilt, there would be an eclipse every two weeks, alternating between [[lunar eclipse]]s and [[solar eclipse]]s).<ref name="moon_fact_sheet"[https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html] Moon Fact Sheet

Because of the axial tilt of the Earth, the position of the Sun in the sky (as seen by an observer on the surface) varies over the course of the year. For an observer at a northern latitude, when the northern pole is tilted toward the Sun the day lasts longer and the Sun climbs higher in the sky. This results in warmer average temperatures from the increase in solar radiation reaching the surface. When the northern pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the [[arctic circle]], an extreme case is reached where there is no daylight at all for part of the year. (This is called a [[polar night]].)

Because of the axial tilt of the Earth, the position of the Sun in the sky (as seen by an observer on the surface) varies over the course of the year. For an observer at a northern latitude, when the northern pole is tilted toward the Sun the day lasts longer and the Sun climbs higher in the sky. This results in warmer average temperatures from the increase in solar radiation reaching the surface. When the northern pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the [[arctic circle]], an extreme case is reached where there is no daylight at all for part of the year. (This is called a [[polar night]].)

This variation in the climate (because of the direction of the Earth's axial tilt) results in the [[season]]s. By astronomical convention, the four seasons are determined by the [[solstice]]s&mdash;the point in the orbit of maximum axial tilt toward or away from the Sun&mdash;and the [[equinox]]es, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about [[December 21]], summer solstice is near [[June 21]], spring equinox is around [[March 20]] and autumnal equinox is about [[September 23]]. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

This variation in the climate (because of the direction of the Earth's axial tilt) results in the [[season]]s. By astronomical convention, the four seasons are determined by the [[solstice]]s&mdash;the point in the orbit of maximum axial tilt toward or away from the Sun&mdash;and the [[equinox]]es, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about [[December 21]], summer solstice is near [[June 21]], spring equinox is around [[March 20]] and autumnal equinox is about [[September 23]]. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo a slight, irregular motion (known as [[nutation]]) with a main period of 18.6&nbsp;years. The orientation (rather than the angle) of the Earth's axis also changes over time, [[precession|precessing]] around in a complete circle over each 25,800&nbsp;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 Moon on the Earth's [[equatorial bulge]]. From the perspective of the Earth, the poles also migrate a few meters across the 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]]. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.[http://www.cv.nrao.edu/~rfisher/Ephemerides/earth_rot.html]  

The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo a slight, irregular motion (known as [[nutation]]) with a main period of 18.6&nbsp;years. The orientation (rather than the angle) of the Earth's axis also changes over time, [[precession|precessing]] around in a complete circle over each 25,800&nbsp;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 Moon on the Earth's [[equatorial bulge]]. From the perspective of the Earth, the poles also migrate a few meters across the 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]]. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.[https://www.cv.nrao.edu/~rfisher/Ephemerides/earth_rot.html]  

In modern times, Earth's [[perihelion]] occurs around [[January 3]], and the [[aphelion]] around [[July 4]] (for other eras, see [[precession (astronomy)|precession]] and [[Milankovitch cycles]]). The changing Earth-Sun distance results in an increase of about 6.9%<ref>Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% the energy at aphelion.</ref> in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the 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. However, 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.[http://www.usatoday.com/weather/tg/wseason/wseason.htm] Earth's tilt creates seasons  

In modern times, Earth's [[perihelion]] occurs around [[January 3]], and the [[aphelion]] around [[July 4]] (for other eras, see [[precession (astronomy)|precession]] and [[Milankovitch cycles]]). The changing Earth-Sun distance results in an increase of about 6.9%<ref>Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% the energy at aphelion.</ref> in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the 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. However, 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.[https://www.usatoday.com/weather/tg/wseason/wseason.htm] Earth's tilt creates seasons  

==Observation==

==Observation==

Earth was first photographed from space by [[Explorer 6]] in 1959.[http://www.nasa.gov/centers/goddard/pdf/106420main_explorers.pdf] Explorers: Searching the Universe Forty Years Later | publisher = NASA/Goddard | accessdate = 2007-03-05 }}</ref> [[Yuri Gagarin]] became the first human to view Earth from space in 1961. The crew of the [[Apollo 8]] was the first to view an Earth-rise from lunar orbit in 1968. In 1972 the crew of the [[Apollo 17]] produced the famous "[[The Blue Marble|Blue Marble]]" photograph of the planet Earth (see [[#top|top of page]]). NASA archivist Mike Gentry has speculated that "The Blue Marble" is the most widely distributed image in human history.

Earth was first photographed from space by [[Explorer 6]] in 1959.[https://www.nasa.gov/centers/goddard/pdf/106420main_explorers.pdf] Explorers: Searching the Universe Forty Years Later | publisher = NASA/Goddard | accessdate = 2007-03-05 }}</ref> [[Yuri Gagarin]] became the first human to view Earth from space in 1961. The crew of the [[Apollo 8]] was the first to view an Earth-rise from lunar orbit in 1968. In 1972 the crew of the [[Apollo 17]] produced the famous "[[The Blue Marble|Blue Marble]]" photograph of the planet Earth (see [[#top|top of page]]). NASA archivist Mike Gentry has speculated that "The Blue Marble" is the most widely distributed image in human history.

==Habitability==

==Habitability==

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble, and sufficient energy to sustain [[metabolism]]. [http://astrobiology.arc.nasa.gov/roadmap/g1.html] Astrobiology Roadmap, NASA, Lockheed Martin The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.Habitable Planets for Man, American Elsevier Publishing, Co.[http://www.rand.org/pubs/reports/R414/]ISBN 0-444-00092-5]

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble, and sufficient energy to sustain [[metabolism]]. [https://astrobiology.arc.nasa.gov/roadmap/g1.html] Astrobiology Roadmap, NASA, Lockheed Martin The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.Habitable Planets for Man, American Elsevier Publishing, Co.[https://www.rand.org/pubs/reports/R414/]ISBN 0-444-00092-5]

===Biosphere===

===Biosphere===

Large deposits of [[Fossil fuel]]s are obtained from the Earth's crust, consisting of [[coal]], [[petroleum]], [[natural gas]] and [[methane clathrate]]. These deposits are used by [[human]]s both for energy production and as feedstock for chemical production. Mineral [[ore]] bodies have also been formed in Earth's crust through a process of [[Ore genesis]], resulting from actions of [[erosion]] and [[plate tectonics]]. These bodies form concentrated sources for many [[metal]]s and other useful [[chemical element|elements]].

Large deposits of [[Fossil fuel]]s are obtained from the Earth's crust, consisting of [[coal]], [[petroleum]], [[natural gas]] and [[methane clathrate]]. These deposits are used by [[human]]s both for energy production and as feedstock for chemical production. Mineral [[ore]] bodies have also been formed in Earth's crust through a process of [[Ore genesis]], resulting from actions of [[erosion]] and [[plate tectonics]]. These bodies form concentrated sources for many [[metal]]s and other useful [[chemical element|elements]].

The Earth's [[biosphere]] produces many useful biological products for humans, including (but far from limited to) [[food]], [[wood]], [[pharmaceutical]]s, oxygen, and the recycling of many organic wastes. The land-based [[ecosystem]] depends upon [[topsoil]] and fresh water, and the oceanic [[ecosystem]] depends upon dissolved nutrients washed down from the land.[http://www.sciencemag.org/cgi/content/full/299/5607/673?ijkey]

The Earth's [[biosphere]] produces many useful biological products for humans, including (but far from limited to) [[food]], [[wood]], [[pharmaceutical]]s, oxygen, and the recycling of many organic wastes. The land-based [[ecosystem]] depends upon [[topsoil]] and fresh water, and the oceanic [[ecosystem]] depends upon dissolved nutrients washed down from the land.[https://www.sciencemag.org/cgi/content/full/299/5607/673?ijkey]

The estimated amount of irrigated land in 1993 was 2,481,250&nbsp;km².<ref name="cia" />

The estimated amount of irrigated land in 1993 was 2,481,250&nbsp;km².<ref name="cia" />

Independent sovereign [[nation]]s claim all of the planet's land surface, with the exception of some parts of [[Antarctica]]. As of 2007 there are [[List of sovereign states|201 sovereign states]], including the 192 [[United Nations member states]]. In addition, there are 59 [[Dependent territory|dependent territories]], and a number of [[List of autonomous areas by country|autonomous areas]], [[List of territorial disputes|territories under dispute]] and other entities. Historically, Earth has never had a [[sovereignty|sovereign]] [[government]] with authority over the entire globe, although a number of nation-states have striven for [[world domination]] and failed.

Independent sovereign [[nation]]s claim all of the planet's land surface, with the exception of some parts of [[Antarctica]]. As of 2007 there are [[List of sovereign states|201 sovereign states]], including the 192 [[United Nations member states]]. In addition, there are 59 [[Dependent territory|dependent territories]], and a number of [[List of autonomous areas by country|autonomous areas]], [[List of territorial disputes|territories under dispute]] and other entities. Historically, Earth has never had a [[sovereignty|sovereign]] [[government]] with authority over the entire globe, although a number of nation-states have striven for [[world domination]] and failed.

The [[United Nations]] is a worldwide [[international organization|intergovernmental organization]] that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict. It is not, however, a world government. While the U.N. provides a mechanism for [[international law]] and, when the consensus of the membership permits, armed intervention,[http://www.un.org/law/]it serves primarily as a forum for international diplomacy.

The [[United Nations]] is a worldwide [[international organization|intergovernmental organization]] that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict. It is not, however, a world government. While the U.N. provides a mechanism for [[international law]] and, when the consensus of the membership permits, armed intervention,[https://www.un.org/law/]it serves primarily as a forum for international diplomacy.

In total, about 400 people have been outside the Earth's atmosphere as of 2004, and, of these, [[Apollo program|twelve]] have walked on the [[Moon]]. Normally the only humans in space are those on the [[International Space Station]]. The station's crew of three people is usually replaced every six months.

In total, about 400 people have been outside the Earth's atmosphere as of 2004, and, of these, [[Apollo program|twelve]] have walked on the [[Moon]]. Normally the only humans in space are those on the [[International Space Station]]. The station's crew of three people is usually replaced every six months.

Earth has often been personified as a [[deity]], in particular a [[goddess]]. In many cultures the [[mother goddess]], also called the [[Mother Earth]], is also portrayed as a [[Fertility god|fertility deity]].

Earth has often been personified as a [[deity]], in particular a [[goddess]]. In many cultures the [[mother goddess]], also called the [[Mother Earth]], is also portrayed as a [[Fertility god|fertility deity]].

To the [[Aztec]], Earth was called [[Tonantzin]"our mother". The [[China|Chinese]] Earth goddess [[Hou-T'u]] Myths & Legends of China [http://www.gutenberg.org/etext/15250] is similar to [[Gaia (mythology)|Gaia]], the Greek goddess personifying the Earth. To [[Hindu]]s it is called [[Bhuma Devi]], the Goddess of Earth. In [[Norse mythology]], the Earth goddess [[Jord]] was the mother of [[Thor]] and the daughter of [[Annar]]. [[Ancient Egyptian religion|Ancient Egyptian mythology]] is different from that of other cultures because Earth is male, [[Geb]], and sky is female, [[Nut (goddess)|Nut]].

To the [[Aztec]], Earth was called [[Tonantzin]"our mother". The [[China|Chinese]] Earth goddess [[Hou-T'u]] Myths & Legends of China [https://www.gutenberg.org/etext/15250] is similar to [[Gaia (mythology)|Gaia]], the Greek goddess personifying the Earth. To [[Hindu]]s it is called [[Bhuma Devi]], the Goddess of Earth. In [[Norse mythology]], the Earth goddess [[Jord]] was the mother of [[Thor]] and the daughter of [[Annar]]. [[Ancient Egyptian religion|Ancient Egyptian mythology]] is different from that of other cultures because Earth is male, [[Geb]], and sky is female, [[Nut (goddess)|Nut]].

In many [[religion]]s, [[Origin belief|accounts of creation of the Earth]] exist, recalling a story involving the creation of the Earth by a supernatural [[deity]] or deities.

In many [[religion]]s, [[Origin belief|accounts of creation of the Earth]] exist, recalling a story involving the creation of the Earth by a supernatural [[deity]] or deities.

In the ancient past there were varying levels of belief in a [[flat Earth]], with the [[Mesopotamian mythology|Mesopotamian]] culture portraying the world as a flat disk afloat in an ocean. The spherical form of the Earth was suggested by early [[Greek philosophers]]; a belief espoused by [[Pythagoras]]. By the [[Middle Ages]]&mdash;as evidenced by thinkers such as [[Thomas Aquinas]]&mdash;European belief in a [[spherical Earth]] was widespread.[http://www.asa3.org/ASA/topics/history/1997Russell.html], The Myth of the Flat Earth , American Scientific Affiliation,  but see also [[Cosmas Indicopleustes]]. Prior to the introduction of [[space flight]], belief in a spherical Earth was based on observations of the secondary effects of the Earth's shape and parallels drawn with the shape of other planets.

In the ancient past there were varying levels of belief in a [[flat Earth]], with the [[Mesopotamian mythology|Mesopotamian]] culture portraying the world as a flat disk afloat in an ocean. The spherical form of the Earth was suggested by early [[Greek philosophers]]; a belief espoused by [[Pythagoras]]. By the [[Middle Ages]]&mdash;as evidenced by thinkers such as [[Thomas Aquinas]]&mdash;European belief in a [[spherical Earth]] was widespread.[https://www.asa3.org/ASA/topics/history/1997Russell.html], The Myth of the Flat Earth , American Scientific Affiliation,  but see also [[Cosmas Indicopleustes]]. Prior to the introduction of [[space flight]], belief in a spherical Earth was based on observations of the secondary effects of the Earth's shape and parallels drawn with the shape of other planets.

[[Cartography]], the study and practice of map making, and vicariously [[geography]], have historically been the disciplines devoted to depicting the Earth. [[Surveying]], the determination of locations and distances, to a lesser extent [[navigation]], the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

[[Cartography]], the study and practice of map making, and vicariously [[geography]], have historically been the disciplines devoted to depicting the Earth. [[Surveying]], the determination of locations and distances, to a lesser extent [[navigation]], the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

==External links==

==External links==

* [http://www.wikimapia.org/ WikiSatellite view of Earth at WikiMapia]

* [https://www.wikimapia.org/ WikiSatellite view of Earth at WikiMapia]

* [http://geomag.usgs.gov USGS Geomagnetism Program]

* [https://geomag.usgs.gov USGS Geomagnetism Program]

* [http://earthobservatory.nasa.gov/Newsroom/BlueMarble NASA Earth Observatory]

* [https://earthobservatory.nasa.gov/Newsroom/BlueMarble NASA Earth Observatory]

* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Earth Earth Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]

* [https://solarsystem.nasa.gov/planets/profile.cfm?Object=Earth Earth Profile] by [https://solarsystem.nasa.gov NASA's Solar System Exploration]

* [http://www.co-intelligence.org/newsletter/comparisons.html The size of Earth compared with other planets/stars]

* [https://www.co-intelligence.org/newsletter/comparisons.html The size of Earth compared with other planets/stars]

* [http://www.nasa.gov/centers/goddard/earthandsun/earthshape.html Climate changes causes the earth's shape to change - Nasa]

* [https://www.nasa.gov/centers/goddard/earthandsun/earthshape.html Climate changes causes the earth's shape to change - Nasa]

* [http://www.funonthenet.in/content/view/282/31/ Beautiful Views of Planet Earth] Pictures of Earth from space

* [https://www.funonthenet.in/content/view/282/31/ Beautiful Views of Planet Earth] Pictures of Earth from space

* [http://www.flashearth.com/ Flash Earth] A Flash-based viewer for satellite and [[aerial photography|aerial]] imagery of the Earth

* [https://www.flashearth.com/ Flash Earth] A Flash-based viewer for satellite and [[aerial photography|aerial]] imagery of the Earth

* [http://www.professores.uff.br/hjbortol/arquivo/2006.1/applets/earth_en.html Java 3D Earth's Globe]

* [https://www.professores.uff.br/hjbortol/arquivo/2006.1/applets/earth_en.html Java 3D Earth's Globe]

* [http://www.projectshum.org/Planets/earth.html Projectshum.org's Earth fact file] (for younger folk)

* [https://www.projectshum.org/Planets/earth.html Projectshum.org's Earth fact file] (for younger folk)

* [http://www.geody.com/?world=terra Geody Earth] World's search engine that supports [[Google Earth]], [[NASA World Wind]], [[Celestia]], GPS, and other applications.

* [https://www.geody.com/?world=terra Geody Earth] World's search engine that supports [[Google Earth]], [[NASA World Wind]], [[Celestia]], GPS, and other applications.

* [http://reference.aol.com/earth Planet Earth] From AOL Research & Learn: Photos, quizzes and info about Earth's climate, creatures and science.

* [https://reference.aol.com/earth Planet Earth] From AOL Research & Learn: Photos, quizzes and info about Earth's climate, creatures and science.

* [http://reference.aol.com/earth/earth-from-space Earth From Space] Some Photos From the Exhibit

* [https://reference.aol.com/earth/earth-from-space Earth From Space] Some Photos From the Exhibit

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[http://en.wikipedia.org/wiki/Earth See more]

[https://en.wikipedia.org/wiki/Earth See more]

[[Category: General Reference]]

[[Category: General Reference]]

[[Category: Earth Science]]

[[Category: Earth Science]]