Changes

'''Earth''' ({{IPAEng|ɜ(ɹ)θ}}) 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]]".

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

Home to millions of [[species|species]] How many species are there on earth?

'''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]].

[http://adsabs.harvard.edu/abs/1988Sci...241.1441M] including [[human|humans]],

Earth is the only place in the [[universe]] known to harbor [[life]]. 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, necessary for life as we know it, is not known to exist on any other planet's surface. Other planets in the solar system are either too hot or too cold to support liquid water. However, it is believed to have existed on the surface of Mars in the past, and may still appear today. See: Simulations Show Liquid Water Could Exist on Mars, University of Arkansas, [http://dailyheadlines.uark.edu/5717.htm] As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. 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]] The Age of the Earth, Stanford University Press, California, ISBN 0-8047-1569-6 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.   

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

 

 

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.   

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.

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]

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]]

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]

[[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&nbsp;K 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]

===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 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:[http://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[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

(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).[http://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 [http://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

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]].

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. [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.

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.[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

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]]

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

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 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]

===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). [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

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]]. [http://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.[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.

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.

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°. [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]

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.[http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml]

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]]).[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.

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 [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]

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.[http://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]].[http://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.<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

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"[http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html] Moon 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  

==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.[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.

==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]]. [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]

===Natural resources and land use===

===Natural resources and land use===

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are [[non-renewable resources]], such as [[fossil fuel|mineral fuels]], that are difficult to replenish on a short time scale.

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are [[non-renewable resources]], such as [[fossil fuel|mineral fuels]], that are difficult to replenish on a short time scale.

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" />

===Human geography===

===Human geography===

The Earth at night, a composite of [[Defense Meteorological Satellite Program|DMSP]]/OLS ground illumination data on a simulated night-time image of the world. This image is not [[Photography|photographic]] and many features are brighter than they would appear to a direct observer.]]

The Earth at night, a composite of [[Defense Meteorological Satellite Program|DMSP]]/OLS ground illumination data on a simulated night-time image of the world. This image is not [[Photography|photographic]] and many features are brighter than they would appear to a direct observer.]] Earth has approximately 6,600,000,000 human inhabitants."LiveScience"

Earth has approximately 6,600,000,000 human inhabitants."LiveScience"

It is estimated that only one eighth of the surface of the Earth is suitable for [[human]]s to live on&mdash;three-quarters is covered by [[ocean]]s, and half of the land area is [[desert]] (14%) (82°28′N) The southernmost is the [[Amundsen-Scott South Pole Station]], in [[Antarctica]], almost exactly at the [[South Pole]]. (90°S)

It is estimated that only one eighth of the surface of the Earth is suitable for [[human]]s to live on&mdash;three-quarters is covered by [[ocean]]s, and half of the land area is [[desert]] (14%) (82°28′N) The southernmost is the [[Amundsen-Scott South Pole Station]], in [[Antarctica]], almost exactly at the [[South Pole]]. (90°S)

==Future==

==Future==

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium ash at the Sun's core, the [[Solar luminosity|star's total luminosity]] will slowly increase. The luminosity of the Sun will increase by 10 percent over the next 1.1 billion years (1.1&nbsp;[[Gigayear|Gyr]]), and by 40% over the next 3.5&nbsp;Gyr.

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium ash at the Sun's core, the [[Solar luminosity|star's total luminosity]] will slowly increase. The luminosity of the Sun will increase by 10 percent over the next 1.1 billion years (1.1&nbsp;[[Gigayear|Gyr]]), and by 40% over the next 3.5&nbsp;Gyr.

==External links==

==External links==

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

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

----

----

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

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

[[Category: General Reference]]

[[Category: General Reference]]