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[[Image:lighterstill.jpg]][[Image:Earth.jpg|right|frame]]

[[Image:lighterstill.jpg]][[Image:Earth.jpg|right|frame]]

 

'''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]].

<center>For lessons on the [[topic]] of '''''Earth''''', follow [http://nordan.daynal.org/wiki/index.php?title=Category:Earth this link].</center>

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]].<ref>{{cite journal | author=Berkner, L. V.; Marshall, L. C. | title= On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere, Journal of Atmospheric Sciences, [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]].[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

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, [http://scienceweek.com/2004/sa040730-5.htm]

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. [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 [[mass]] of the Earth is approximately 5.98{{e|24}}&nbsp;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, [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=350422]

===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.<ref>{{cite web | author = Staff | date = February 27, 2004 | url = http://www.geolsoc.org.uk/template.cfm?name=lithosphere | title = Crust and Lithosphere | work = Plate Tectonics & Structural Geology | publisher = The Geological Survey

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]

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.[http://pubs.usgs.gov/gip/dynamic/understanding.html], Understanding plate motions, USGS

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.[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 [[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{{e|7}}&nbsp;km² (3.3{{e|9}}&nbsp;acres) of cropland and 3.4{{e|7}}&nbsp;km² (8.4{{e|9}}&nbsp;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,[http://www.wku.edu/~smithch/wallace/S453.htm]

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). [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 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{{e|9}}&nbsp;km³. The total surface area of the Earth is 5.1{{e|8}}&nbsp;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, [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.

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.[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

==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.<ref>{{cite web | last = Fisher | first = Rick | date = January, 30, 1996 | url = http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html | title = Astronomical Times National Radio Astronomy Observatory. 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.[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.)

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.[http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html] Earth Fact Sheet

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.<ref>{{cite web | last = Fisher | first = Rick | date = February 5, 1996 [http://www.cv.nrao.edu/~rfisher/Ephemerides/earth_rot.html] | title = Earth Rotation and Equatorial Coordinates, National Radio Astronomy Observatory

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]  

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.[http://www.usatoday.com/weather/tg/wseason/wseason.htm] Earth's tilt creates seasons  

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.<ref>{{cite journal [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.[http://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" />