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'''Earth''' is a [[planet]] of the [[Solar system]] and the home-world of humanity. Earth is moving around the [[star]] with the official name [[Sol]] but commonly referred to as the [[Sun]]. Earth has one physical [[satellite]] with the name [[Luna]] (usually referred to as "the [[Moon]]"), however, it is argued that Earth and Moon are actually a single system, since they both revolve around the common center of mass, rather than one around the other.
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{{three other uses|scientific information on the Earth|the Earth's geography|World|religious beliefs|creation myth}}
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[[Image:Earthlights (NASA & NOAA).jpeg|thumb|400px|right|Image of Earth from space, showing the human population's effect on the planet through the [[light pollution]].]]
  
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Please note the formatting and layout of this infobox has been matched with the other bodies of the solar system. Please do not arbitrarily change it without discussion.
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3/4 of Earth surface is [[water]]. Humans live in the rest 1/4 and have polluted it so much that it is doubtful whether [[future]] [[generation]]s will be able to live as people have in the past.
  
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There are many [[society|societies]] on Earth, most notable are these of [[human]]s, but also [[ant]]s, [[bee]]s etc.
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{{Infobox Planet
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| bgcolour=#c0c0ff
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| name = Earth
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| symbol = [[Image:Earth symbol.svg|25px|Astronomical symbol of Earth]]
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| image = [[Image:The Earth seen from Apollo 17.jpg|240px|A color image of Earth as seen from Apollo 17.]]
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| flag = {{flagicon|world}}
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| caption = Famous "[[The Blue Marble|Blue Marble]]" photograph of Earth, taken from [[Apollo 17]].
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| epoch = [[J2000]]
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| aphelion = 152,097,701&nbsp;km <br />1.0167103335&nbsp;[[astronomical unit|AU]]
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| perihelion = 147,098,074&nbsp;km <br /> 0.9832898912&nbsp;AU
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| semimajor = 149,597,887.5&nbsp;km <br /> 1.0000001124&nbsp;AU
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| eccentricity = 0.016710219
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| inclination = Reference (0) <br /> 7.25° to [[Sun]]'s equator
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| asc_node = 348.73936°
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| arg_peri = 114.20783°
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| period = 365.256366&nbsp;days<br /> 1.0000175&nbsp;[[julian year (astronomy)|yr]]
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| avg_speed = 29.783&nbsp;km/s <br /> 107,218&nbsp;km/h
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| satellites = 1&nbsp;(the&nbsp;[[Moon]])
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| physical_characteristics = yes
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| flattening = 0.0033528<ref name=wgs84/>
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| equatorial_radius = 6,378.1&nbsp;km<ref name=wgs84>The [[WGS 84]] [[reference ellipsoid]].</ref>
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| polar_radius = 6,356.8&nbsp;km<ref name=wgs84/>
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| mean_radius = 6,371.0&nbsp;km<ref name=wgs84sphere> This is the radius that gives a sphere with the same volume as the [[WGS 84]] [[reference ellipsoid]].</ref>
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| circumference = 40,075.02&nbsp;km&nbsp;([[equator]]ial)<br>40,007.86&nbsp;km&nbsp;([[meridional]])<br>40,041.47&nbsp;km&nbsp;(mean)
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| surface_area = 510,072,000&nbsp;km²<ref>{{cite journal
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| last = Pidwirny | first = Michael
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| date=[[February 2]], [[2006]]
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| title=Surface area of our planet covered by oceans and continents.(Table 8o-1)
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| publisher=University of British Columbia, Okanagan
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| url=http://www.physicalgeography.net/fundamentals/8o.html
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| accessdate=2007-11-26 }}</ref>
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148,940,000&nbsp;km²&nbsp;land&nbsp;&nbsp;(29.2&nbsp;%)<br>
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361,132,000&nbsp;km²&nbsp;water&nbsp;(70.8&nbsp;%)
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| volume = 1.0832073{{e|12}}&nbsp;km³
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| mass = 5.9736{{e|24}}&nbsp;kg
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| density = 5.5153&nbsp;g/cm³
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| surface_grav = [[Earth's gravity|9.780327]] [[metre per second squared|m/s²]]<ref name="yoder12" /><br />0.99732&nbsp;[[g-force|''g'']]
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| escape_velocity = 11.186&nbsp;km/s&nbsp;<br />40,270&nbsp;km/h<br />
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| sidereal_day = 0.997258&nbsp;d<br />23<sup>h</sup>&nbsp;56<sup>m</sup>&nbsp;04.09054<sup>s</sup><ref name="yoder12">Yoder, C. F. (1995) p. 12.</ref>
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| rot_velocity = 465.11&nbsp;m/s
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| axial_tilt = 23.439281°
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| albedo = 0.367
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| adjectives = Terrestrial, Terran, [[Terra (mythology)|Telluric]], Tellurian, Earthly
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| atmosphere = yes
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| temperatures = yes
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| temp_name1 = [[Kelvin]]
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| min_temp_1 = 184&nbsp;K
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| mean_temp_1 = 287&nbsp;K
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| max_temp_1 = 331&nbsp;K
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| temp_name2 = [[Celsius]]
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| min_temp_2 = −89&nbsp;°C
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| mean_temp_2 = 14&nbsp;°C
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| max_temp_2 = 57.7&nbsp;°C
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| surface_pressure = 101.3&nbsp;[[kPa]] ([[Sea Level|MSL]])
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| atmosphere_composition = 78.08%&nbsp;[[Nitrogen]] (N<sub>2</sub>)<br /> 20.95%&nbsp;[[Oxygen]] (O<sub>2</sub>)<br /> 0.93%&nbsp;[[Argon]]<br /> 0.038%&nbsp;[[Carbon dioxide]]<br />About 1% [[water vapor]] (varies with [[climate]])<ref name="earth_fact_sheet"/>
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}}
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'''Earth''' ({{pronEng|ˈɝːθ}})<ref>[http://dictionary.cambridge.org/define.asp?dict=CALD&key=24538&ph=on Earth (PLANET)], entry in the ''Cambridge Advanced Learner's Dictionary'', Cambridge University Press, online.  Accessed 14-II-2008.</ref> is the third [[planet]] from the [[Sun]] and is the largest of the [[terrestrial planet]]s in the [[Solar System]] in [[diameter]], [[mass]] and [[density]]. It is also referred to as ''the Earth'', ''Planet Earth'', ''the [[World]]'', and ''[[Wiktionary:Terra|Terra]]''.<ref>Note that by [[International Astronomical Union]] convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. C.f.:<br />{{cite web
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| last=Blue | first=Jennifer | date=[[July 5]], [[2007]]
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| url=http://planetarynames.wr.usgs.gov/jsp/append5.jsp
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| title=Descriptor Terms (Feature Types)
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| work=Gazetteer of Planetary Nomenclature
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| publisher=USGS | accessdate=2007-07-05 }}</ref>
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Home to millions of [[species]],<ref>{{cite journal
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Earth is 1 [[astronomic unit|AU]] away from the Sun.
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| last = May | first = Robert M.
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| title=How many species are there on earth?
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| journal=Science | year=1999 | volume=241
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| issue=4872 | pages=1441-1449
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| url=http://adsabs.harvard.edu/abs/1988Sci...241.1441M
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| accessdate=2007-08-14 }}</ref> including [[human]]s, Earth is the only place in the [[universe]] where humans have found life.
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Scientific evidence indicates that the planet formed [[Age of the Earth|4.54 billion years]] ago,<ref name="age_earth">{{cite book
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| first=G.B. | last=Dalrymple | year=1991
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| title=The Age of the Earth
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| publisher=Stanford University Press | location=California
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| id=ISBN 0-8047-1569-6
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}}</ref><ref>{{cite web
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| last=Newman
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| first=William L.
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| date=[[July 9]], [[2007]]
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| url=http://pubs.usgs.gov/gip/geotime/age.html
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| title=Age of the Earth
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| publisher=Publications Services, USGS
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| accessdate=2007-09-20
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}}</ref><ref>{{cite journal
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| last=Dalrymple
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| first=G. Brent
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| title=The age of the Earth in the twentieth century: a problem (mostly) solved
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| journal=Geological Society, London, Special Publications
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| year=2001
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| volume=190
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| pages=205-221
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| url=http://sp.lyellcollection.org/cgi/content/abstract/190/1/205
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| accessdate=2007-09-20
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| doi = 10.1144/GSL.SP.2001.190.01.14 <!--Retrieved from URL by DOI bot-->
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}}</ref><ref>{{cite web
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| last=Stassen
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| first=Chris
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| date=[[September 10]], [[2005]]
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| url=http://www.talkorigins.org/faqs/faq-age-of-earth.html
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| title=The Age of the Earth
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| publisher=The TalkOrigins Archive
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| accessdate=2007-09-20
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}}</ref> 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, enabling the proliferation of [[aerobic organisms]] as well as the formation of the [[ozone layer]] which, together with Earth's [[magnetic field]], blocks harmful radiation, permitting life on land.<ref>{{cite book
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| first=Roy M. | last=Harrison
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| coauthors=Hester, Ronald E. | year=2002
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| title=Causes and Environmental Implications of Increased UV-B Radiation
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| publisher=Royal Society of Chemistry
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| id=ISBN 0854042652 }}</ref>
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Earth's [[Crust (geology)|outer surface]] is divided into several rigid segments, or [[tectonic plate]]s, that gradually migrate across the surface over periods of [[Geologic time scale|many millions of years]]. About 71% of the surface is covered with [[seawater|salt-water]] [[ocean]]s, the remainder consisting of [[continent]]s and [[island]]s; liquid [[water]], necessary for all known life, is not known to exist on any other planet's surface.<ref>Other planets in the solar system are either too hot or too cold to support liquid water. However, it is confirmed to have existed on the surface of Mars in the past, and may still appear today. See: <cite>{{cite news
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[[Category:Planets]]
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| author=Msnbc
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| title=Rover reveals Mars was once wet enough for life
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| publisher=NASA
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| date=[[March 2]], [[2007]]
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| url=http://www.msnbc.msn.com/id/4202901/
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| accessdate=2007-08-28 }}</cite><cite>{{cite news
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| author=Staff
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| title=Simulations Show Liquid Water Could Exist on Mars
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| publisher=University of Arkansas
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| date=[[November 7]], [[2005]]
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| url=http://dailyheadlines.uark.edu/5717.htm
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| accessdate=2007-08-08 }}</cite></ref><ref>As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: {{cite journal
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| last=G. Tinetti et al.
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| title=Water vapour in the atmosphere of a transiting extrasolar planet
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| journal=Nature | date=July, 2007 | volume=448
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| pages=169-171 | url=http://www.nature.com/nature/journal/v448/n7150/abs/nature06002.html | doi = 10.1038/nature06002 <!--Retrieved from URL by DOI bot-->
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}}</ref> Earth's interior remains active, with a thick layer of relatively solid [[Mantle (geology)|mantle]], a liquid outer core that generates a magnetic field, and a solid iron [[inner core]].
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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 365.26 [[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.</ref> The Earth's axis of rotation is [[axial tilt|tilted]] 23.4° away from the [[perpendicular]] to its [[Orbital plane (astronomy)|orbital plane]],<ref>Ahrens, ''Global Earth Physics: A Handbook of Physical Constants'', p. 8.</ref> producing seasonal variations on the planet's surface with a period of one [[tropical year]] (365.24 solar days). 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.<ref name="comet">{{cite journal | author=Morbidelli, A.; Chambers, J.; Lunine, J. I.; Petit, J. M.; Robert, F.; Valsecchi, G. B.; Cyr, K. E. | title=Source regions and time scales for the delivery of water to Earth | journal=Meteoritics & Planetary Science | year=2000 | volume=35 | issue=6 | pages=1309–1320 | url=http://adsabs.harvard.edu/abs/2000M&PS...35.1309M | accessdate=2007-03-06 }}</ref>  Later, [[asteroid]] impacts caused significant changes to the surface environment.
 
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==History==
 
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{{main|History of Earth}}
 
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Scientists have been able to reconstruct detailed information about the planet's past. Earth and the other planets in the Solar System formed 4.54&nbsp;billion&nbsp;years ago<ref name="age_earth" /> out of the [[solar nebula]], a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially [[molten]], the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object (sometimes called [[Giant impact hypothesis|Theia]]) with about 10% of the Earth's mass<ref>{{cite conference | author = Canup, R. M.; Asphaug, E. | title = An impact origin of the Earth-Moon system | booktitle = Abstract #U51A-02 | publisher = American Geophysical Union | date = Fall Meeting 2001 | url = http://adsabs.harvard.edu/abs/2001AGUFM.U51A..02C | accessdate = 2007-03-10 }}</ref> impacting the Earth in a glancing blow.<ref>{{cite journal | last = R. Canup and E. Asphaug | title = Origin of the Moon in a giant impact near the end of the Earth's formation | journal = Nature | volume = 412 | pages = 708–712 | date = 2001 | url = http://www.nature.com/nature/journal/v412/n6848/abs/412708a0.html }}</ref> Some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.
 
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Outgassing and [[Volcano|volcanic]] activity produced the primordial atmosphere. Condensing [[water vapor]], augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects [[Origin of the world's oceans|produced the oceans]].<ref name="comet"/> The highly energetic chemistry is believed to have produced a self-replicating molecule around 4&nbsp;billion&nbsp;years ago, and half a billion years later, the [[last universal common ancestor|last common ancestor of all life]] existed.<ref>{{cite journal | last=Doolittle | first=W. Ford | title=Uprooting the tree of life | journal=Scientific American | date=February, 2000 | volume=282 | issue=6 | pages=90–95 }}</ref>
 
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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]] [O<sub>3</sub>]) 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=Journal of Atmospheric Sciences | year=1965 | volume=22 | issue=3 | pages=225–261 | url=http://adsabs.harvard.edu/abs/1965JAtS...22..225B | accessdate=2007-03-05 }}</ref> 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.<ref>{{cite web | last = Burton | first = Kathleen | date = [[November 29]], [[2000]] | url = http://www.nasa.gov/centers/ames/news/releases/2000/00_79AR.html | title = Astrobiologists Find Evidence of Early Life on Land | publisher = NASA | accessdate = 2007-03-05 }}</ref>
 
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Beginning with almost no dry land, the total amount of surface lying above the oceans has steadily increased. During the past two billion years, for example, the total size of the continents
 
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has doubled.<ref name=ward_brownlee>Ward and Brownlee (2002)</ref> 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 (unit)|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.<ref>{{cite journal | author=Murphy, J. B.; Nance, R. D. | title=How do supercontinents assemble? | journal=American Scientist | year=1965 | volume=92 | pages=324–33 | url=http://scienceweek.com/2004/sa040730-5.htm | accessdate=2007-03-05 }}</ref>
 
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Since the 1960s, it has been hypothesized that severe [[Glacier|glacial]] action between 750 and 580&nbsp;mya, during the [[Neoproterozoic]], covered much of the planet in a sheet of ice. This hypothesis has been termed "[[Snowball Earth]]", and is of particular interest because it preceded the [[Cambrian explosion]], when multicellular life forms began to proliferate.<ref>{{cite book | last=Kirschvink | first=J. L. | editors=Schopf, J.W.; Klein, C. | year=1992 | title=The Proterozoic Biosphere: A Multidisciplinary Study | pages=51–52 | publisher=Cambridge University Press | id=ISBN 0521366151 }}</ref>
 
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Following the [[Cambrian explosion]], about 535&nbsp;mya, there have been five [[Extinction event|mass extinctions]].<ref>{{cite journal | author=Raup, D. M.; Sepkoski, J. J. | title=Mass Extinctions in the Marine Fossil Record | journal=Science | year=1982 | volume=215 | issue=4539 | pages=1501–1503 | url=http://adsabs.harvard.edu/abs/1982Sci...215.1501R | accessdate=2007-03-05  | doi = 10.1126/science.215.4539.1501 <!--Retrieved from Yahoo! by DOI bot-->}}</ref> The last extinction event occurred 65&nbsp;mya, when a meteorite collision probably triggered the extinction of the (non-avian) [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s, which then resembled shrews. Over the past 65&nbsp;million years, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright.<ref>{{cite journal | last = Gould | first = Stephan J. | title=The Evolution of Life on Earth | journal=Scientific American | date=October , 1994 | url=http://brembs.net/gould.html | accessdate=2007-03-05 }}</ref> This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,<ref>{{cite journal
 
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| author=Wilkinson, B. H.; McElroy, B. J.
 
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| title=The impact of humans on continental erosion and sedimentation
 
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| journal=Bulletin of the Geological Society of America
 
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| year=2007
 
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| volume=119
 
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| issue=1–2
 
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| pages=140–156
 
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| url=http://bulletin.geoscienceworld.org/cgi/content/abstract/119/1-2/140
 
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| accessdate=2007-04-22  | doi = 10.1130/B25899.1 <!--Retrieved from URL by DOI bot-->
 
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}}</ref> affecting both the nature and quantity of other life forms.
 
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The present pattern of [[ice age]]s began about 40&nbsp;mya, then intensified during the [[Pleistocene]] about 3&nbsp;mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40&ndash;100,000&nbsp;years. The last ice age ended 10,000&nbsp;years ago.<ref>{{cite web | author=Staff | url = http://www.lakepowell.net/sciencecenter/paleoclimate.htm | title = Paleoclimatology - The Study of Ancient Climates | publisher = Page Paleontology Science Center | accessdate = 2007-03-02 }}</ref>
 
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==Composition and structure==
 
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{{See|Earth physical characteristics tables}}
 
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Earth is a terrestrial planet, meaning that it is a rocky body, rather than a [[gas giant]] like [[Jupiter]]. It is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest [[surface gravity]] and the strongest magnetic field.<ref>{{cite web
 
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| last=Stern
 
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| first=David P.
 
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| date=[[November 25]], [[2001]]
 
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| url= http://astrogeology.usgs.gov/HotTopics/index.php?/archives/147-Names-for-the-Columbia-astronauts-provisionally-approved.html
 
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| title=Planetary Magnetism
 
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| publisher=NASA
 
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| accessdate=2007-04-01
 
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}}</ref>
 
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===Shape===
 
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{{main|Figure of the Earth}}
 
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[[Image:Terrestrial planet size comparisons.jpg|thumb|left|300px|Size comparison of inner planets (left to right): [[Mercury (planet)|Mercury]], [[Venus]], Earth, and [[Mars]]]]
 
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The Earth's shape is very close to an [[oblate spheroid]]&mdash;a rounded shape with a bulge around the [[equator]]&mdash;although the precise shape (the [[geoid]]) varies from this by up to 100&nbsp;meters.<ref>{{cite web | author=Milbert, D. G.; Smith, D. A. | url = http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html | title = Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model | publisher = National Geodetic Survey, NOAA | accessdate = 2007-03-07 }}</ref> The average diameter of the reference spheroid is about 12,742&nbsp;km. More approximately the distance is 40,000&nbsp;km/[[pi|Ï€]] because the [[meter]] was originally defined as 1/10,000,000 of the distance from the equator to the [[north pole]] through [[Paris]], [[France]].<ref>{{cite web
 
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| author=Mohr, P.J.; Taylor, B.N.
 
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| date=October, 2000
 
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| url=http://physics.nist.gov/cuu/Units/meter.html
 
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| title=Unit of length (meter)
 
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| work=NIST Reference on Constants, Units, and Uncertainty
 
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| publisher=NIST Physics Laboratory
 
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| accessdate=2007-04-23
 
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}}</ref>
 
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The [[rotation]] of the Earth creates the [[equatorial bulge]] so that the equatorial diameter is 43&nbsp;km larger than the [[Geographical pole|pole]] to pole diameter.<ref name="ngdc2006">{{cite web
 
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| author=Sandwell, D. T.; Smith, W. H. F.
 
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| date = Jul7 26, 2006
 
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| url =http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML
 
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| title =Exploring the Ocean Basins with Satellite Altimeter Data
 
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| publisher = NOAA/NGDC
 
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| accessdate = 2007-04-21
 
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}}</ref> The largest local deviations in the rocky surface of the Earth are [[Mount Everest]] (8,848&nbsp;m above local [[sea level]]) and the [[Mariana Trench]] (10,911&nbsp;m 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.<ref>{{cite web | author=Staff | date = November, 2001 | url = http://www.wpa-pool.com/index.asp?content=rules_spec | title = WPA Tournament Table & Equipment Specifications | publisher = World Pool-Billiards Association | accessdate = 2007-03-10 }}</ref> Because of the bulge, the feature farthest from the center of the Earth is actually [[Chimborazo (volcano)|Mount Chimborazo]] in [[Ecuador]].<ref>{{cite journal | last = Senne | first = Joseph H. | title=Did Edmund Hillary Climb the Wrong Mountain | journal=Professional Surveyor | year=2000 | volume=20 | issue=5 | url=http://www.profsurv.com/archive.php?issue=42&article=589 | accessdate=2007-02-04 }}</ref>
 
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===Chemical composition===
 
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{{seealso|Abundance of elements on Earth}}
 
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{| class="wikitable" style="float: right; clear: right; margin-left: 2em;"
 
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|+ F. W. Clarke's Table of Crust Oxides
 
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!Compound
 
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!Formula
 
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!Composition
 
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|-
 
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|[[silica]]
 
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|style="text-align: center;"|SiO<sub>2</sub>
 
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|style="text-align: right;"|59.71%
 
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|-
 
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|[[alumina]]
 
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|style="text-align: center;"|Al<sub>2</sub>O<sub>3</sub>
 
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|style="text-align: right;"|15.41%
 
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|-
 
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|[[Calcium oxide|lime]]
 
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|style="text-align: center;"|CaO
 
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|style="text-align: right;"|4.90%
 
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|-
 
−
|[[Magnesia (mineral)|Magnesia]]
 
−
|style="text-align: center;"|MgO
 
−
|style="text-align: right;"|4.36%
 
−
|-
 
−
|[[sodium oxide]]
 
−
|style="text-align: center;"|Na<sub>2</sub>O
 
−
|style="text-align: right;"|3.55%
 
−
|-
 
−
|[[iron(II) oxide]]
 
−
|style="text-align: center;"|FeO
 
−
|style="text-align: right;"|3.52%
 
−
|-
 
−
|[[potassium oxide]]
 
−
|style="text-align: center;"|K<sub>2</sub>O
 
−
|style="text-align: right;"|2.80%
 
−
|-
 
−
|[[iron(III) oxide]]
 
−
|style="text-align: center;"|Fe<sub>2</sub>O<sub>3</sub>
 
−
|style="text-align: right;"|2.63%
 
−
|-
 
−
|[[water (molecule)|water]]
 
−
|style="text-align: center;"|H<sub>2</sub>O
 
−
|style="text-align: right;"|1.52%
 
−
|-
 
−
|[[titanium dioxide]]
 
−
|style="text-align: center;"|TiO<sub>2</sub>
 
−
|style="text-align: right;"|0.60%
 
−
|-
 
−
|[[phosphorus pentoxide]]
 
−
|style="text-align: center;"|P<sub>2</sub>O<sub>5</sub>
 
−
|style="text-align: right;"|0.22%
 
−
|-
 
−
!colspan="2"|Total
 
−
!style="text-align: right;"|99.22%
 
−
|}
 
−
 
−
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 [[aluminium]] (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.<ref>{{cite journal | author=Morgan, J. W.; Anders, E. | title=Chemical composition of Earth, Venus, and Mercury | journal=Proceedings of the National Academy of Science | year=1980 | volume=71 | issue=12 | pages=6973–6977 | url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=350422 | accessdate=2007-02-04 }}</ref>
 
−
 
−
The geochemist [[Frank Wigglesworth Clarke|F. W. Clarke]] calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the [[Earth's crust]] are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.<ref name=EB1911>{{1911|article=Petrology}}</ref>
 
−
 
−
===Internal structure===
 
−
{{main|Structure of the Earth}}
 
−
[[Image:Earth-crust-cutaway-english.svg|thumb|300px|right|Earth cutaway from core to exosphere. Not to scale.]]
 
−
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, 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.<ref>{{cite book
 
−
| first=Toshiro
 
−
| last=Tanimoto
 
−
| editor=Thomas J. Ahrens
 
−
| year=1995
 
−
| title=Crustal Structure of the Earth
 
−
| booktitle=Global Earth Physics: A Handbook of Physical Constants
 
−
| publisher=American Geophysical Union
 
−
| location=Washington, DC
 
−
| id=ISBN 0-87590-851-9
 
−
| url=http://www.agu.org/reference/gephys/15_tanimoto.pdf
 
−
| format=PDF
 
−
| accessdate=2007-02-03 }}</ref>
 
−
 
−
The geologic component layers of the Earth<ref>{{cite journal
 
−
| last = Jordan
 
−
| first = T. H.
 
−
| title=Structural Geology of the Earth's Interior
 
−
| journal=Proceedings National Academy of Science
 
−
| year=1979
 
−
| volume=76
 
−
| issue=9
 
−
| pages=4192–4200
 
−
| url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=411539
 
−
| accessdate=2007-03-24 }}</ref> are at the following depths below the surface:<ref>{{cite web
 
−
| last = Robertson
 
−
| first = Eugene C.
 
−
| date = [[July 26]], [[2001]]
 
−
| url = http://pubs.usgs.gov/gip/interior/
 
−
| title = The Interior of the Earth
 
−
| publisher = USGS
 
−
| accessdate = 2007-03-24
 
−
}}</ref>
 
−
 
−
{| class="wikitable"
 
−
!Depth<br><span style="font-size: smaller;">km</span>
 
−
!style="vertical-align: bottom;"|Layer
 
−
!Density<br><span style="font-size: smaller;">g/cm³</span>
 
−
|-
 
−
|style="text-align: center;"|0&ndash;60
 
−
|[[Lithosphere]] (locally varies between 5 and 200&nbsp;km)
 
−
|style="text-align: center;"| &mdash;
 
−
|- style="background: #FEFEFE;"
 
−
|style="text-align: center;"|0&ndash;35
 
−
|... [[Crust (geology)|Crust]] (locally varies between 5 and 70&nbsp;km)
 
−
|style="text-align: center;"| 2.2&ndash;2.9
 
−
|- style="background: #FEFEFE;"
 
−
|style="text-align: center;"|35&ndash;60
 
−
|... Uppermost part of mantle
 
−
|style="text-align: center;"| 3.4&ndash;4.4
 
−
|-
 
−
|style="text-align: center;"|35&ndash;2890
 
−
|[[Mantle (geology)|Mantle]]
 
−
|style="text-align: center;"| 3.4&ndash;5.6
 
−
|- style="background: #FEFEFE;"
 
−
|style="text-align: center;"|100&ndash;700
 
−
|... [[Asthenosphere]]
 
−
|style="text-align: center;"| &mdash;
 
−
|-
 
−
|style="text-align: center;"|2890&ndash;5100
 
−
|[[Outer core]]
 
−
|style="text-align: center;"| 9.9&ndash;12.2
 
−
|-
 
−
|style="text-align: center;"|5100&ndash;6378
 
−
|[[Inner core]]
 
−
|style="text-align: center;"| 12.8&ndash;13.1
 
−
|}
 
−
 
−
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.<ref>{{cite news | first=Robert | last=Sanders | title=Radioactive potassium may be major heat source in Earth's core | publisher=UC Berkeley News | date=[[December 10]], [[2003]] | url=http://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml | accessdate=2007-02-28 }}</ref> At the center of the planet, the temperature may be up to
 
−
7,000&nbsp;K and the pressure could reach 360&nbsp;[[GPa]].<ref>{{cite journal | author=Alfè, D.; Gillan, M. J.; Vocadlo, L.; Brodholt, J; Price, G. D. | title=The ''ab initio'' simulation of the Earth's core | journal= Philosophical Transaction of the Royal Society of London | year=2002 | volume=360 | issue=1795 | pages=1227–1244 | url=http://chianti.geol.ucl.ac.uk/~dario/pubblicazioni/PTRSA2002.pdf | format=PDF | accessdate=2007-02-28 }}</ref> 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.<ref>{{cite journal
 
−
| author=Richards, M. A.; Duncan, R. A.; Courtillot, V. E.
 
−
| title=Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails
 
−
| journal=Science | year=1989 | volume=246
 
−
| issue=4926 | pages=103–107
 
−
| url=http://adsabs.harvard.edu/abs/1989Sci...246..103R
 
−
| doi=10.1126/science.246.4926.103 | accessdate=2007-04-21 }}</ref>
 
−
 
−
===Tectonic plates===
 
−
{{main|Plate tectonics}}
 
−
[[Image:Plates tect2 en.svg|thumb|right|300px|A map illustrating the Earth's major plates.]]
 
−
According to plate tectonics theory, 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 upper mantle. The asthenosphere behaves like a superheated material that is in a semi-fluidic, plastic-like state.<ref>{{cite web
 
−
| first=Courtney | last=Seligman | year=2008
 
−
| url = http://cseligman.com/text/planets/innerstructure.htm
 
−
| title = The Structure of the Terrestrial Planets
 
−
| work = Online Astronomy eText Table of Contents
 
−
| publisher = cseligman.com | accessdate = 2008-02-28 }}</ref>
 
−
 
−
The lithosphere essentially ''floats'' on the asthenosphere and is broken up into what are called [[List of tectonic plates|tectonic plates]]. 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 boundary|transform]]. The last occurs where two plates move laterally relative to each other, creating a [[strike-slip fault]]. [[Earthquake]]s, volcanic activity, [[Orogeny|mountain-building]], and [[oceanic trench]] formation can occur along these plate boundaries.<ref>{{cite web | author=Kious, W. J.; Tilling, R. I. | date = May 5, 1999 | url = http://pubs.usgs.gov/gip/dynamic/understanding.html | title = Understanding plate motions | publisher = USGS | accessdate = 2007-03-02 }}</ref>
 
−
 
−
The main plates are:<ref>{{cite web | author=Brown, W. K.; Wohletz, K. H. | year = 2005 | url = http://www.ees1.lanl.gov/Wohletz/SFT-Tectonics.htm | title = SFT and the Earth's Tectonic Plates | publisher = Los Alamos National Laboratory | accessdate = 2007-03-02 }}</ref>
 
−
 
−
{| class="wikitable"
 
−
!Plate name
 
−
!Area<br><span style="font-size: smaller;">10<sup>6</sup>&nbsp;km²</span>
 
−
!Covering
 
−
|-
 
−
| [[African Plate]] ||style="text-align: center;"| 61.3 ||style="text-align: center;"| [[Africa]]
 
−
|-
 
−
| [[Antarctic Plate]] ||style="text-align: center;"| 60.9 ||style="text-align: center;"| [[Antarctica]]
 
−
|-
 
−
| [[Australian Plate]] ||style="text-align: center;"| 47.2 ||style="text-align: center;"| [[Australia]]
 
−
|-
 
−
| [[Eurasian Plate]] ||style="text-align: center;"| 67.8 ||style="text-align: center;"| [[Asia]] and [[Europe]]
 
−
|-
 
−
| [[North American Plate]] ||style="text-align: center;"| 75.9 ||style="text-align: center;"| [[North America]] and north-east [[Siberia]]
 
−
|-
 
−
| [[South American Plate]] ||style="text-align: center;"| 43.6 ||style="text-align: center;"| [[South America]]
 
−
|-
 
−
| [[Pacific Plate]] ||style="text-align: center;"| 103.3 ||style="text-align: center;"| [[Pacific Ocean]]
 
−
|}
 
−
 
−
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<ref>{{cite web
 
−
| author=Meschede, M.; Udo Barckhausen, U.
 
−
| date=[[November 20]], [[2000]]
 
−
| url = http://www-odp.tamu.edu/publications/170_SR/chap_07/chap_07.htm
 
−
| title = Plate Tectonic Evolution of the Cocos-Nazca Spreading Center
 
−
| work=Proceedings of the Ocean Drilling Program
 
−
| publisher = Texas A&M University
 
−
| accessdate = 2007-04-02
 
−
}}</ref>  and the [[Pacific Plate]] moving 52–69&nbsp;mm/yr. At the other extreme, the slowest-moving plate is the [[Eurasian Plate]], progressing at a typical rate of about 21&nbsp;mm/yr.<ref>{{cite web
 
−
| author=Staff
 
−
| url = http://sideshow.jpl.nasa.gov/mbh/series.html
 
−
| title = GPS Time Series
 
−
| publisher = NASA JPL
 
−
| accessdate = 2007-04-02 }}</ref>
 
−
 
−
===Surface===
 
−
{{main|Landform|Extreme points of the World}}
 
−
[[Image:AYool topography 15min.png|300px|left|thumb|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%<ref name="Pidwirny2006">{{cite web
 
−
| last = Pidwirny
 
−
| first = Michael
 
−
| year = 2006
 
−
| url = http://www.physicalgeography.net/fundamentals/7h.html
 
−
| title = Fundamentals of Physical Geography
 
−
| edition = 2nd Edition
 
−
| publisher = PhysicalGeography.net
 
−
| accessdate = 2007-03-19 }}</ref> 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<ref>{{cite web | last = Kring | first = David A. | url = http://www.lpl.arizona.edu/SIC/impact_cratering/intro/ | title = Terrestrial Impact Cratering and Its Environmental Effects | publisher = Lunar and Planetary Laboratory | accessdate = 2007-03-22 }}</ref> 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.<ref>{{cite web | last = Duennebier | first = Fred | date = August 12, 1999 | url = http://www.soest.hawaii.edu/GG/ASK/plate-tectonics2.html | title = Pacific Plate Motion | publisher = University of Hawaii | accessdate = 2007-03-14 }}</ref><ref>{{cite web | author=Mueller, R.D.; Roest, W.R.; Royer, J.-Y.; Gahagan, L.M.; Sclater, J.G. | date = March 7, 2007 | url = http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html | title = Age of the Ocean Floor Poster | publisher = NOAA | accessdate = 2007-03-14 }}</ref>
 
−
 
−
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.<ref>{{cite web | author=Staff | url = http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/part1.html | title = Layers of the Earth | publisher = Volcano World | accessdate = 2007-03-11 }}</ref> [[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.<ref>{{cite web | last=Jessey | first=David | url = http://geology.csupomona.edu/drjessey/class/Gsc101/Weathering.html | title = Weathering and Sedimentary Rocks | publisher = Cal Poly Pomona | accessdate = 2007-03-20 }}</ref> 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]].<ref>{{cite web | author=Staff | url = http://natural-history.uoregon.edu/Pages/web/mineral.htm | title = Minerals | publisher = Museum of Natural History, Oregon | accessdate = 2007-03-20 }}</ref> Common carbonate minerals include [[calcite]] (found in [[limestone]]), [[aragonite]] and [[dolomite]].<ref>{{cite web
 
−
| last=Cox
 
−
| first=Ronadh
 
−
| year=2003
 
−
| url=http://madmonster.williams.edu/geos.302/L.08.html
 
−
| title=Carbonate sediments
 
−
| publisher=Williams College
 
−
| accessdate=2007-04-21
 
−
}}</ref>
 
−
 
−
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]], atmosphere, [[hydrosphere]] and [[biosphere]]. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.<ref name="cia">{{cite web | author=Staff | date = February 8, 2007 | url = https://www.cia.gov/library/publications/the-world-factbook/geos/xx.html | title = The World Factbook | publisher = U.S. C.I.A. | accessdate = 2007-02-25 }}</ref> 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 of cropland and 3.4{{e|7}}&nbsp;km² of pastureland.<ref>{{cite book
 
−
| author=FAO Staff
 
−
| year=1995
 
−
| title=FAO Production Yearbook 1994
 
−
| edition=Volume 48
 
−
| publisher=Food and Agriculture Organization of the United Nations
 
−
| location=Rome, Italy
 
−
| id=ISBN 9250038445 }}</ref>
 
−
 
−
[[Image:Earth elevation histogram 2.svg|thumb|350px|Elevation [[histogram]] of the surface of the Earth&mdash;approximately 71% of the Earth's surface is covered with water.]]
 
−
 
−
The elevation of the land surface of the Earth varies from the low point of −418&nbsp;m at the [[Dead Sea]], to a 2005-estimated maximum altitude of 8,848&nbsp;m at the top of [[Mount Everest]]. The mean height of land above sea level is 686&nbsp;m.<ref name="hr_mill">{{cite journal | last = Mill | first = Hugh Robert | title=The Permanence of Ocean Basins | journal=The Geographical Journal | year=1893 | volume=1 | issue=3 | pages=230–234 | url=http://www.wku.edu/~smithch/wallace/S453.htm | accessdate=2007-02-25 }}</ref>
 
−
 
−
===Hydrosphere===
 
−
{{main|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.<ref name="rain.org">{{cite web | author=Staff | url = http://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html | title = "Deep Ocean Studies" | work = Ocean Studies | publisher = RAIN National Public Internet and Community Technology Center | accessdate = 2006-04-02
 
−
}}</ref><ref>Takuyo measurement; see [[Mariana Trench]] for details.</ref> The average depth of the oceans is 3,794&nbsp;m, more than five times the average height of the continents.<ref name="hr_mill" />
 
−
 
−
The mass of the oceans is approximately 1.35{{e|18}}&nbsp;[[metric ton]]s, 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.<ref>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 2.7&nbsp;km.</ref> 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.<ref>{{cite web | author = Igor A. Shiklomanov ''et al'' | year = 1999 | url = http://espejo.unesco.org.uy/ | title = World Water Resources and their use Beginning of the 21st century" Prepared in the Framework of IHP UNESCO | publisher = State Hydrological Institute, St. Petersburg | accessdate = 2006-08-10 }}</ref>
 
−
 
−
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.<ref>{{cite web | last = Mullen | first = Leslie | date = June 11, 2002 | url = http://www.astrobio.net/news/article223.html | title = Salt of the Early Earth | publisher = NASA Astrobiology Magazine | accessdate = 2007-03-14 }}</ref> The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.<ref>{{cite web | last = Morris | first = Ron M. | url = http://seis.natsci.csulb.edu/rmorris/oxy/oxy4.html | title = Oceanic Processes | publisher = NASA Astrobiology Magazine | accessdate = 2007-03-14 }}</ref> Sea water has an important influence on the world's
 
−
climate, with the oceans acting as a large [[heat reservoir]].<ref>{{cite web | last = Scott | first = Michon | date = [[April 24]], [[2006]] | url = http://earthobservatory.nasa.gov/Study/HeatBucket/ | title = Earth's Big heat Bucket | publisher = NASA Earth Observatory | accessdate = 2007-03-14
 
−
}}</ref> Shifts in the oceanic temperature distribution
 
−
can cause significant weather shifts, such as the
 
−
[[El Niño-Southern Oscillation]].<ref>{{cite web
 
−
| first=Sharron
 
−
| last=Sample
 
−
| date =June 21, 2005
 
−
| url =http://science.hq.nasa.gov/oceans/physical/SST.html
 
−
| title =Sea Surface Temperature
 
−
| publisher =NASA
 
−
| accessdate = 2007-04-21
 
−
}}</ref>
 
−
 
−
===Atmosphere===
 
−
{{main|Earth's 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 height of the [[troposphere]] varies with [[latitude]], ranging between 7&nbsp;km  at the poles to 17&nbsp;km  at the equator, with some variation due to weather factors.{{Fact|date=January 2008}}
 
−
 
−
Earth's [[biosphere]] has significantly altered its [[atmosphere]]. [[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 enabled the proliferation of [[aerobic organisms]] as well as the formation of the [[ozone layer]] which, together with Earth's magnetic field, blocks [[ultraviolet]] [[solar radiation]], permitting life on land. Other atmospheric functions important to life on Earth's include transporting water vapor, providing useful gases, causing small [[meteor]]s to burn up before they strike the surface, and moderating temperature.<ref name="atmosphere">{{cite web | author=Staff | date = October 8, 2003 | url = http://www.nasa.gov/audience/forstudents/9-12/features/912_liftoff_atm.html | title = Earth's Atmosphere | publisher = NASA | accessdate = 2007-03-21 }}</ref> This last phenomenon is known as the [[greenhouse effect]]: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average 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.<ref name="Pidwirny2006" />
 
−
 
−
====Weather and climate====
 
−
{{main|Weather|Climate}}
 
−
The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11&nbsp;km of the planet's surface. This lowest layer is called the [[troposphere]]. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is [[atmospheric circulation]] that drives the [[weather]] and [[climate]] through redistribution of heat energy.<ref name="moran2005">{{cite web | last=Moran | first=Joseph M. | year=2005 | url=http://www.nasa.gov/worldbook/weather_worldbook.html | title=Weather | work=World Book Online Reference Center | publisher=NASA/World Book, Inc. | accessdate=2007-03-17 }}</ref>
 
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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
 
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30° and 60°.<ref name="berger2002">{{cite web
 
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| last = Berger
 
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| first = Wolfgang H.
 
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| year=2002
 
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| url = http://earthguide.ucsd.edu/virtualmuseum/climatechange1/cc1syllabus.shtml
 
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| title = The Earth's Climate System
 
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| publisher = University of California, San Diego
 
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| accessdate = 2007-03-24
 
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}}</ref> 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.<ref>{{cite web
 
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| first=Stefan
 
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| last=Rahmstorf
 
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| year=2003
 
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| url =http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html
 
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| title =The Thermohaline Ocean Circulation
 
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| publisher =Potsdam Institute for Climate Impact Research
 
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| accessdate = 2007-04-21
 
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}}</ref>
 
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[[Image:Air masses 2.jpg|left|thumb|300px|Source regions of global [[air mass]]es.]]
 
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Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere.
 
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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.<ref>{{cite web
 
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| author=Various
 
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| date = [[July 21]], [[1997]]
 
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| url = http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml
 
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| title = The Hydrologic Cycle
 
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| publisher = University of Illinois
 
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| accessdate = 2007-03-24
 
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}}</ref>
 
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The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the [[equator]] to the polar regions, these are the [[tropics|tropical]] (or equatorial), [[Subtropics|subtropical]], [[temperate]] and [[Polar region|polar]] climates.<ref>{{cite web
 
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| author=Staff
 
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| url = http://www.ace.mmu.ac.uk/eae/Climate/Older/Climate_Zones.html
 
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| title = Climate Zones
 
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| publisher = UK Department for Environment, Food and Rural Affairs
 
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| accessdate = 2007-03-24
 
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}}</ref> Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform [[air mass]]es. The commonly-used [[Köppen climate classification]] system (as modified by [[Wladimir Köppen]]'s student Rudolph Geiger) has five broad groups (humid tropics, [[Desert|arid]], humid middle latitudes, [[Continental climate|continental]] and cold polar), which are further divided into more specific subtypes.<ref name="berger2002" />
 
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====Upper atmosphere====
 
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[[Image:Full moon partially obscured by atmosphere.jpg|thumb|right|300px|This view from orbit shows the full Moon partially obscured by the Earth's atmosphere. ''NASA image.]]
 
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{{seealso|Outer space}}
 
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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]]).<ref>{{cite web | author=Staff | year = 2004 | url = http://scienceweek.com/2004/rmps-23.htm | title = Stratosphere and Weather; Discovery of the Stratosphere | publisher = Science Week | accessdate = 2007-03-14 }}</ref> An 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 above the Earth's surface, is a working definition for the boundary between atmosphere and space.<ref>{{cite web
 
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| first=S. Sanz Fernández
 
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| last=de Córdoba
 
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| date =June 21, 2004
 
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| url =http://www.un.org/members/list.shtml
 
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| title =100&nbsp;km. Altitude Boundary for Astronautics
 
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| publisher =Fédération Aéronautique Internationale
 
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| accessdate = 2007-04-21
 
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}}</ref>
 
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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.<ref>{{cite journal | author=Liu, S. C.; Donahue, T. M. | title=The Aeronomy of Hydrogen in the Atmosphere of the Earth | journal=Journal of Atmospheric Sciences | year=1974 | volume=31 | issue=4 | pages=1118–1136 | url=http://adsabs.harvard.edu/abs/1974JAtS...31.1118L | accessdate=2007-03-02 }}</ref> 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.<ref>{{cite web | last = Abedon | first = Stephen T. | date = March 31, 1997 | url = http://www.mansfield.ohio-state.edu/~sabedon/biol1010.htm | title = History of Earth | publisher = Ohio State University | accessdate = 2007-03-19 }}</ref>
 
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[[Image:Dipole field.jpg|thumb|right|300px|The [[Earth's magnetic field]], which approximates a dipole.]]
 
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===Magnetic field===
 
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{{main|Earth's magnetic field}}
 
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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.<ref>{{cite web | last = Fitzpatrick | first = Richard | date = [[February 16]], [[2006]] | url = http://farside.ph.utexas.edu/teaching/plasma/lectures/node69.html | title = MHD dynamo theory | publisher = NASA WMAP | accessdate = 2007-02-27 }}</ref><ref name=campbelwh>{{cite book
 
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| last =Campbell
 
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| first =Wallace Hall
 
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| authorlink =
 
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| coauthors =
 
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| title =Introduction to Geomagnetic Fields
 
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| publisher =Cambridge University Press
 
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| date =2003
 
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| location =New York
 
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| pages =p57
 
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| url =
 
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| doi =
 
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| id =
 
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| isbn = 0521822068}}</ref>
 
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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]].<ref>{{cite web | last = Stern | first = David P. | date = July 8, 2005 | url = http://www-spof.gsfc.nasa.gov/Education/wmap.html | title = Exploration of the Earth's Magnetosphere | publisher = NASA | accessdate = 2007-03-21 }}</ref>
 
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{{-}}
 
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==Orbit and rotation==
 
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{{main|Earth's rotation}}
 
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[[Image:Rotating earth (large).gif|thumb|right|175px|An animation showing the rotation of the Earth as seen from the northern hemisphere of the solar system.]]
 
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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 from west to east.<ref>{{cite web | last = Fisher | first = Rick | date = January, 30, 1996 | url = http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html | title = Astronomical Times | publisher = National Radio Astronomy Observatory | accessdate = 2007-03-21 }}</ref> 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.)
 
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Earth orbits the Sun at an average distance of about 150&nbsp;million kilometers 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), which is fast enough to cover the planet's diameter (about 12,600&nbsp;km) in seven minutes, and the distance to the Moon (384,000&nbsp;km) in four hours.<ref name="earth_fact_sheet">{{cite web | last = Williams | first = David R. | date = September 1, 2004 | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html | title = Earth Fact Sheet | publisher = NASA | accessdate = 2007-03-17 }}</ref>
 
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[[Image:PaleBlueDot.jpg|thumb|left|195px|[[Pale Blue Dot|Earth seen as a tiny dot]] by the [[Voyager 1]] spacecraft, more than 6 billion kilometers from Earth.]]
 
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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">{{cite web | last = Williams | first = David R. | date = September 1, 2004 | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html | title = Moon Fact Sheet | publisher = NASA | accessdate = 2007-03-21 }}</ref><ref name="earth_fact_sheet" />
 
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Because of the axial tilt of the Earth, the position of the Sun in the sky (as seen by an observer on the surface) varies over the course of the year. For an observer at a northern latitude, when the northern pole is tilted toward the Sun the day lasts longer and the Sun climbs higher in the sky. This results in warmer average temperatures from the increase in solar radiation reaching the surface. When the northern pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the [[arctic circle]], an extreme case is reached where there is no daylight at all for part of the year. (This is called a [[polar night]].)
 
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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.
 
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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 | url = http://www.cv.nrao.edu/~rfisher/Ephemerides/earth_rot.html | title = Earth Rotation and Equatorial Coordinates | publisher = National Radio Astronomy Observatory | accessdate = 2007-03-21 }}</ref>
 
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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.<ref>{{cite web | last = Williams | first = Jack | date = December 20, 2005 | url = http://www.usatoday.com/weather/tg/wseason/wseason.htm | title = Earth's tilt creates seasons | publisher = USAToday | accessdate = 2007-03-17 }}</ref>
 
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The [[Hill sphere]] ([[gravity|gravitational]] sphere of influence) of the Earth is about 1.5&nbsp;Gm (or 1,500,000 [[kilometer]]s)  in radius.<ref>{{cite web | author=Vázquez, M.; Montañés Rodríguez, P.; Palle, E. | year=2006 | url =http://www.iac.es/folleto/research/preprints/files/PP06024.pdf | title = The Earth as an Object of Astrophysical Interest in the Search for Extrasolar Planets | publisher = Instituto de Astrofísica de Canarias | accessdate = 2007-03-21 }}</ref><ref>For the Earth, the Hill radius is
 
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:<math>\begin{smallmatrix} R_H = a\left ( \frac{m}{3M} \right )^{\frac{1}{3}} \end{smallmatrix}</math>,
 
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where ''m'' is the mass of the Earth, ''a'' is an Astronomical Unit, and ''M'' is the mass of the Sun. So the radius in A.U. is about:
 
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<math>\begin{smallmatrix} \left ( \frac{1}{3 \cdot 332,946} \right )^{\frac{1}{3}} = 0.01 \end{smallmatrix}</math>.</ref> This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.
 
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==Moon==
 
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{{main|Moon}}
 
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{| class="wikitable"
 
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!Name || Diameter || Mass || [[Semi-major axis]] || Orbital period
 
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|-
 
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|rowspan="2"|[[Moon]]
 
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| style="text-align: center" | 3,474.8&nbsp;km
 
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| style="text-align: center" | 7.349{{e|22}}&nbsp;kg
 
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| style="text-align: center" | 384,400&nbsp;km
 
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|rowspan="2"|27&nbsp;days, 7&nbsp;hours, 43.7&nbsp;minutes
 
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|-
 
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| style="text-align: center" | 2,159.2&nbsp;mi
 
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| style="text-align: center" | 8.1{{e|19}}&nbsp;(short) tons
 
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| style="text-align: center" | 238,700&nbsp;mi
 
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|}
 
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The Moon is a relatively large, [[Terrestrial planet|terrestrial]], planet-like satellite, with a diameter about one-quarter of the Earth's. It is the largest moon in the solar system relative to the size of its planet. ([[Charon (moon)|Charon]] is larger relative to the [[dwarf planet]] [[Pluto]].) The natural satellites orbiting other planets are called "moons" after Earth's Moon.
 
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The gravitational attraction between the Earth and Moon cause [[tides]] on Earth. The same effect on the Moon has led to its [[tidal locking]]: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the [[lunar phase]]s: The dark part of the face is separated from the light part by the [[terminator (solar)|solar terminator]].
 
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Because of their [[Tidal acceleration|tidal interaction]], the Moon recedes from Earth at the rate of approximately 38&nbsp;[[Millimetre|mm]] a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 [[Microsecond|µs]] a year—add up to significant changes.<ref>{{cite web
 
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| author=Espenak, F.; Meeus, J.
 
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| date = February 7, 2007
 
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| url = http://sunearth.gsfc.nasa.gov/eclipse/SEcat5/secular.html
 
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| title = Secular acceleration of the Moon
 
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| publisher = NASA
 
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| accessdate = 2007-04-20
 
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}}</ref> During the [[Devonian]] period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.<ref>{{cite web
 
−
| first=Hannu K. J.
 
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| last=Poropudas
 
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| date =December 16, 1991
 
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| url = http://www.skepticfiles.org/origins/coralclo.htm
 
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| title = Using Coral as a Clock
 
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| publisher = Skeptic Tank
 
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| accessdate = 2007-04-20
 
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}}</ref>
 
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The Moon may have dramatically affected the development of life by moderating the planet's climate. [[Paleontology|Paleontological]] evidence and computer simulations show that Earth's [[axial tilt]] is stabilized by tidal interactions with the Moon.<ref>{{cite journal
 
−
| author=Laskar, J.; Robutel, P.; Joutel, F.; Gastineau, M.; Correia, A.C.M.;
 
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Levrard, B.
 
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| title=A long-term numerical solution for the insolation quantities of the Earth
 
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| journal=Astronomy and Astrophysics
 
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| year=2004
 
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| volume=428
 
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| pages=261–285
 
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| url=http://adsabs.harvard.edu/abs/2004A&A...428..261L
 
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| accessdate=2007-03-31  | doi = 10.1051/0004-6361:20041335 <!--Retrieved from URL by DOI bot-->
 
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}}</ref> Some theorists believe that without this stabilization against the [[torque]]s applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, as it appears to be for [[Mars (planet)|Mars]]. If Earth's axis of rotation were to approach the [[ecliptic|plane of the ecliptic]], extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during ''summer'' and directly away during ''winter''. [[Planetary science|Planetary scientists]] who have studied the effect claim that this might kill all large animal and higher plant life.<ref>{{cite journal
 
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| author=Williams, D.M.; J.F. Kasting
 
−
| title=Habitable planets with high obliquities
 
−
| journal=Lunar and Planetary Science
 
−
| year=1996
 
−
| volume=27
 
−
| pages=1437–1438
 
−
| url=http://adsabs.harvard.edu/abs/1996LPI....27.1437W
 
−
| accessdate=2007-03-31 }}</ref> However, this is a controversial subject, and further studies of Mars—which has a similar [[sidereal day|rotation period]] and [[axial tilt]] as Earth, but not its large Moon or liquid core—may settle the matter.
 
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Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. The [[angular size]] (or [[solid angle]]) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant. This allows total and annular [[eclipse]]s to occur on Earth.
 
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{{-}}
 
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[[Image:Earth-Moon2.jpg|thumb|center|800px| A scale representation of the relative sizes of, and distance between, Earth and Moon.]]
 
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The most widely accepted theory of the Moon's origin, the [[Giant impact hypothesis|giant impact theory]], states that it formed from the collision of a Mars-size [[protoplanet]] called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.<ref>{{cite journal
 
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| last = R. Canup and E. Asphaug
 
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| title = Origin of the Moon in a giant impact near the end of the Earth's formation
 
−
| journal = Nature
 
−
| volume = 412
 
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| pages = 708–712
 
−
| date = 2001 }}</ref>
 
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Earth has at least two [[Quasi-satellite|co-orbital asteroids]], [[3753 Cruithne]] and [[2002 AA29|2002 AA<sub>29</sub>]].<ref>{{cite news
 
−
| first=David
 
−
| last=Whitehouse
 
−
| title=Earth's little brother found
 
−
| publisher=BBC News
 
−
| date=[[October 21]], [[2002]]
 
−
| url=http://news.bbc.co.uk/1/hi/sci/tech/2347663.stm
 
−
| accessdate=2007-03-31 }}</ref>
 
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==Habitability==
 
−
{{seealso|Planetary habitability}}
 
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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]].<ref>{{cite web | author=Staff | date = September, 2003 | url = http://astrobiology.arc.nasa.gov/roadmap/g1.html | title = Astrobiology Roadmap | publisher = NASA, Lockheed Martin | accessdate = 2007-03-10 }}</ref> 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.<ref>{{cite book | first=Stephen H. | last=Dole | year=1970 | title=Habitable Planets for Man | edition=2nd edition | publisher=American Elsevier Publishing Co. | url=http://www.rand.org/pubs/reports/R414/ | accessdate=2007-03-11 | id=ISBN 0-444-00092-5 }}</ref>
 
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===Biosphere===
 
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{{main|Biosphere}}
 
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The planet's life forms are sometimes said to form a "[[biosphere]]". This biosphere is generally believed to have begun [[evolution|evolving]] about 3.5&nbsp;billion years ago. Earth is the only place in the universe where life is known to exist. Some scientists believe that Earth-like biospheres might be [[Rare Earth hypothesis|rare]].<ref>{{cite book
 
−
| author=Ward, P. D.; Brownlee, D.
 
−
| date=[[January 14]], [[2000]]
 
−
| title=Rare Earth: Why Complex Life is Uncommon in the Universe
 
−
| edition=1st edition
 
−
| publisher=Springer-Verlag
 
−
| location=New York
 
−
| id=ISBN 0387987010 }}</ref>
 
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The biosphere is divided into a number of [[biome]]s, inhabited by broadly similar [[plants]] and [[animals]]. On land primarily [[latitude]] and height above the sea level separates biomes. Terrestrial biomes lying within the [[Arctic Circle|Arctic]], [[Antarctic Circle]] or in high altitudes are relatively barren of [[plant]] and [[animal]] life, while the greatest [[Latitudinal gradients in species diversity|latitudinal diversity of species]] is found at the [[Equator]].<ref>{{cite journal
 
−
| last = Hillebrand
 
−
| first = Helmut
 
−
| title=On the Generality of the Latitudinal Gradient
 
−
| journal=American Naturalist
 
−
| year=2004
 
−
| volume=163
 
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| issue=2
 
−
| pages=192–211 }}</ref>
 
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===Natural resources and land use===
 
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{{main|Natural resource}}
 
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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.
 
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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]].<ref>{{cite web
 
−
| author=Staff
 
−
| date=[[November 24]], [[2006]]
 
−
| url=http://www.utexas.edu/tmm/npl/mineralogy/Mineral_Genesis/
 
−
| title=Mineral Genesis: How do minerals form?
 
−
| publisher=Non-vertebrate Paleontology Laboratory, Texas Memorial Museum
 
−
| accessdate=2007-04-01
 
−
}}</ref> These bodies form concentrated sources for many [[metal]]s and other useful [[chemical element|elements]].
 
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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
 
−
| last = Rona
 
−
| first = Peter A.
 
−
| title=Resources of the Sea Floor
 
−
| journal=Science
 
−
| year=2003
 
−
| volume=299
 
−
| issue=5607
 
−
| pages=673–674
 
−
| url=http://www.sciencemag.org/cgi/content/full/299/5607/673?ijkey=AHVbRrqUsmdHY&keytype=ref&siteid=sci
 
−
| accessdate=2007-02-04  | doi = 10.1126/science.1080679 <!--Retrieved from URL by DOI bot-->
 
−
}}</ref> Humans also live on the [[Ecological footprint|land]] by using [[building material]]s to construct [[home|shelters]]. In 1993, human use of land is approximately:
 
−
 
−
{| class="wikitable"
 
−
!Land use
 
−
!Percentage
 
−
|-
 
−
| ''Arable land:'' ||style="text-align: right;"| 13.13%<ref name="cia" />
 
−
|-
 
−
| ''Permanent crops:'' ||style="text-align: right;"| 4.71%<ref name="cia" />
 
−
|-
 
−
| ''Permanent pastures:'' ||style="text-align: right;"| 26%
 
−
|-
 
−
| ''Forests and woodland:'' ||style="text-align: right;"| 32%
 
−
|-
 
−
| ''Urban areas:'' ||style="text-align: right;"| 1.5%
 
−
|-
 
−
| ''Other:'' ||style="text-align: right;"| 30%
 
−
|}
 
−
 
−
The estimated amount of irrigated land in 1993 was 2,481,250&nbsp;km².<ref name="cia" />
 
−
 
−
===Natural and environmental hazards===
 
−
Large areas are subject to extreme [[weather]] such as tropical [[cyclone]]s, [[hurricane]]s, or [[typhoon]]s that dominate life in those areas. Many places are subject to [[earthquake]]s, [[landslide]]s, [[tsunami]]s, [[volcano|volcanic eruptions]], [[tornado]]es, [[sinkhole]]s, [[blizzard]]s, [[flood]]s, [[drought]]s, and other calamities and [[disaster]]s.
 
−
 
−
Many localized areas are subject to human-made [[pollution]] of the air and water, [[acid rain]] and toxic substances, loss of vegetation ([[overgrazing]], [[deforestation]], [[desertification]]), loss of [[wildlife]], [[species]] [[extinction]], [[soils retrogression and degradation|soil degradation]], soil depletion, [[erosion]], and introduction of [[invasive species]].
 
−
 
−
A [[scientific consensus]] exists linking human activities to [[global warming]] due to industrial [[carbon dioxide]] emissions. This is predicted to produce changes such as the melting of [[glacier]]s and [[ice sheet]]s, more extreme temperature ranges, significant changes in weather conditions and a [[Sea level rise|global rise in average sea levels]].<ref>{{cite web
 
−
| author=Staff
 
−
| date = [[February 2]], [[2007]]
 
−
| url = http://www.un.org/apps/news/story.asp?NewsID=21429&Cr=climate&Cr1=change
 
−
| title = Evidence is now ‘unequivocal’ that humans are causing global warming – UN report
 
−
| publisher = United Nations
 
−
| accessdate = 2007-03-07 }}</ref>
 
−
 
−
===Human geography===
 
−
{{main|Human geography}}
 
−
{{Earth Labelled Map|width=400|float=right}}
 
−
[[Image:Earthlights dmsp.jpg|400px|right|thumb|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 6,671,226,000 human inhabitants as of July 2007.<ref name="LiveScience">
 
−
{{cite news
 
−
| first= Leonard
 
−
| last= David
 
−
| url= http://www.livescience.com/othernews/060224_world_population.html
 
−
| title= Planet's Population Hit 6.5 Billion Saturday
 
−
| work= Live Science
 
−
| date= [[February 2]], [[2006]]
 
−
| accessdate= 2006-04-02
 
−
}}
 
−
</ref> Projections indicate that the [[world population|world's human population]] will reach seven billion in 2013 and 9.2&nbsp;billion<ref>{{cite web
 
−
| author=Staff
 
−
| url = http://www.un.org/esa/population/publications/wpp2006/wpp2006.htm
 
−
| title = World Population Prospects: The 2006 Revision
 
−
| publisher = United Nations
 
−
| accessdate = 2007-03-07 }}</ref> in 2050. Most of the growth is expected to take place in [[developing nations]]. Human [[population density]] varies widely around the world, but a majority live in [[Asia]]. By 2020, 60% of the world's population is expected to be living in [[Urban area|urban]], rather than [[rural]], areas.<ref>{{cite web
 
−
| author = Staff
 
−
| year = 2007
 
−
| url = http://www.prb.org/Educators/TeachersGuides/HumanPopulation/PopulationGrowth/QuestionAnswer.aspx
 
−
| title = Human Population: Fundamentals of Growth: Growth
 
−
| publisher = Population Reference Bureau
 
−
| accessdate = 2007-03-31
 
−
}}</ref>
 
−
 
−
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 either [[desert]] (14%),<ref>{{cite journal
 
−
| author=Peel, M. C.; Finlayson, B. L.; McMahon, T. A.
 
−
| title=Updated world map of the Köppen-Geiger climate classification
 
−
| journal=Hydrology and Earth System Sciences Discussions
 
−
| year=2007
 
−
| volume=4
 
−
| pages=439–473
 
−
| url=http://overview.sref.org/1812-2116/hessd/2007-4-439
 
−
| accessdate=2007-03-31 }}</ref> high mountains (27%),<ref>{{cite web
 
−
| author=Staff
 
−
| url = http://www.biodiv.org/programmes/default.shtml
 
−
| title = Themes & Issues
 
−
| publisher = Secretariat of the Convention on Biological Diversity
 
−
| accessdate = 2007-03-29
 
−
}}</ref> or other less suitable terrain. The northernmost permanent settlement in the world is [[Alert, Nunavut|Alert]], on [[Ellesmere Island]] in [[Nunavut]], [[Canada]].<ref>{{cite web
 
−
| author = Staff
 
−
| date = 2006-08-15
 
−
| url = http://www.img.forces.gc.ca/org/cfiog/alert_e.asp
 
−
| title = Canadian Forces Station (CFS) Alert
 
−
| publisher = Information Management Group
 
−
| accessdate = 2007-03-31
 
−
}}</ref> (82°28′N) The southernmost is the [[Amundsen-Scott South Pole Station]], in [[Antarctica]], almost exactly at the [[South Pole]]. (90°S)
 
−
 
−
Independent sovereign [[nation]]s claim all of the planet's land surface, with the exception of some parts of [[Antarctica]]. As of 2007 there are [[List of sovereign states|201 sovereign states]], including the 192 [[United Nations member states]]. In addition, there are 59 [[Dependent territory|dependent territories]], and a number of [[List of autonomous areas by country|autonomous areas]], [[List of territorial disputes|territories under dispute]] and other entities. Historically, Earth has never had a [[sovereignty|sovereign]] [[government]] with authority over the entire globe, although a number of nation-states have striven for [[world domination]] and failed.
 
−
 
−
The [[United Nations]] is a worldwide [[international organization|intergovernmental organization]] that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict. It is not, however, a world government. While the U.N. provides a mechanism for [[international law]] and, when the consensus of the membership permits, armed intervention,<ref>{{cite web | author=Staff | url = http://www.un.org/law/ | title = International Law | publisher = United Nations | accessdate = 2007-03-27 }}</ref> it serves primarily as a forum for international diplomacy.
 
−
 
−
In total, about 400 people have been outside the Earth's atmosphere as of 2004, and, of these, [[Apollo program|twelve]] have walked on the Moon. Normally the only humans in space are those on the [[International Space Station]]. The station's crew of three people is usually replaced every six months.{{-}}
 
−
 
−
==Cultural viewpoint==
 
−
[[Image:AS8-13-2329.jpg|thumb|left|The first photograph ever taken of an "Earthrise," on [[Apollo 8]].]]
 
−
 
−
===Etymology===
 
−
The name ''Earth'' originates from the 8th century [[Old English language|Anglo-Saxon]] word ''erda'', which means ground or soil. In [[Old English]] the word became ''eorthe'', then ''erthe'' in [[Middle English]].<ref>{{cite book
 
−
| date=July 2005 | title=Random House Unabridged Dictionary
 
−
| publisher=Random House | id=ISBN 0-375-42599-3 }}</ref> Earth was first used as the name of the sphere of the Earth around 1400.<ref>{{cite web
 
−
| last = Harper| first = Douglas | date = November 2001
 
−
| url = http://www.etymonline.com/index.php?term=earth
 
−
| title = Earth | publisher = Online Etymology Dictionary
 
−
| accessdate = 2007-08-07 }}</ref> It is the only planet whose name in English is not derived from [[Greek mythology|Greco]]-[[Roman mythology]].
 
−
 
−
The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle. This symbol is known as the wheel cross, sun cross, Odin's cross or Woden's cross. Although it has been used in various cultures for different purposes, it came to represent the compass points, earth and the land. Another version of the symbol is a cross on top of a circle; a stylized [[globus cruciger]] that was also used as an early astronomical symbol for the planet Earth.<ref>{{cite book
 
−
| first=Carl G. | last=Liungman | year=2004
 
−
| chapter=Group 29: Multi-axes symmetric, both soft and straight-lined, closed signs with crossing lines
 
−
| title=Symbols -- Encyclopedia of Western Signs and Ideograms
 
−
| pages=pp. 281&ndash;282 | publisher=Ionfox AB
 
−
| location=New York | id=ISBN 91-972705-0-4 }}</ref>
 
−
 
−
===Religious beliefs===
 
−
Earth has often been personified as a [[deity]], in particular a [[goddess]]. In many cultures the [[mother goddess]], also called the Mother Earth, is also portrayed as a [[Fertility god|fertility deity]].  See also ''[[Graha]]''. To the [[Aztec]], Earth was called [[Tonantzin]]&mdash;"our mother"; to the [[Incas]], Earth was called [[Pachamama]]&mdash;"mother earth". The [[China|Chinese]] Earth goddess [[Hou-T'u]]<ref>{{cite book | first=E. T. C. | last=Werner | year=1922 | title=Myths & Legends of China | publisher=George G. Harrap & Co. Ltd. | location=New York | url=http://www.gutenberg.org/etext/15250 | accessdate=2007-03-14 }}</ref> is similar to [[Gaia (mythology)|Gaia]], the Greek goddess personifying the Earth. To [[Hindu]]s it is called [[Bhuma Devi]], the Goddess of Earth. In [[Norse mythology]], the Earth goddess [[Jord]] was the mother of [[Thor]] and the daughter of [[Annar]]. [[Ancient Egyptian religion|Ancient Egyptian mythology]] is different from that of other cultures because Earth is male, [[Geb]], and sky is female, [[Nut (goddess)|Nut]].
 
−
 
−
In many [[religion]]s, [[creation myth]]s exist, recalling a story involving the creation of the Earth by a supernatural [[deity]] or deities. A variety of religious groups, often associated with [[fundamentalism|fundamentalist]] branches of [[Protestantism]],<ref name=Dutch2002>{{cite journal | author = Dutch, S.I. | year = 2002 | title = Religion as belief versus religion as fact | journal = Journal of Geoscience Education | volume = 50 | issue = 2 | pages = 137-144 | url=http://nagt.org/files/nagt/jge/abstracts/Dutch_v50n2p137.pdf | accessdate = 2008-04-28}}</ref> or [[Islam]],<ref>{{cite book
 
−
| author = Taner Edis  | year = 2003 | title = A World Designed by God: Science and Creationism in Contemporary Islam | publisher=Amherst: Prometheus | url = http://www2.truman.edu/~edis/writings/articles/CFI-2001.pdf | isbn = 1-59102-064-6 | accessdate = 2008-04-28}}</ref> assert that their [[Hermeneutics|interpretations]] of the accounts of creation in [[religious text|sacred texts]] are [[Creation science|literal truth]] and should be considered alongside or replace conventional scientific accounts of the formation of the Earth, and the origin and development of life.<ref name=Ross2005>{{cite journal | author = Ross, M.R. | year = 2005 | title = Who Believes What? Clearing up Confusion over Intelligent Design and Young-Earth Creationism | journal = Journal Of Geoscience Education | volume = 53 | issue = 3 | pages = 319 | url = http://www.nagt.org/files/nagt/jge/abstracts/Ross_v53n3p319.pdf  | accessdate = 2008-04-28}}</ref> Such assertions are opposed by both the [[scientific community]],<ref>{{cite journal |author=Pennock RT |title=Creationism and intelligent design |journal=Annu Rev Genomics Hum Genet |volume=4 |issue= |pages=143–63 |year=2003 |pmid=14527300 |doi=10.1146/annurev.genom.4.070802.110400}}</ref><ref>[http://books.nap.edu/openbook.php?record_id=11876&page=R1 Science, Evolution, and Creationism] National Academy Press, Washington, DC 2005</ref> and other religious groups.<ref name=Colburn2006>{{cite journal | author = Colburn, A. | coauthors = Henriques, L. | year = 2006 | title = Clergy views on evolution, creationism, science, and religion | journal = Journal of Research in Science Teaching | volume = 43 | issue = 4 | pages = 419-442 | doi = 10.1002/tea.20109}}</ref><ref name=Glass1984>{{cite book | author = Roland Mushat Frye | year = 1983 | title = Is God a Creationist? The Religious Case Against Creation-Science. | publisher = Scribner's | isbn = 0-68417-993-8}}</ref><ref name=Gould1997>{{cite journal | author = Gould, S.J. | year = 1997 | title = Nonoverlapping magisteria | journal = Natural History | volume = 106 | issue = 2 | pages = 16-22 | url = http://www.jbburnett.com/resources/gould_nonoverlapping.pdf  | accessdate = 2008-04-28}}</ref> For a prominent example, see the [[creation-evolution controversy]].
 
−
 
−
===Exploration and mapping===
 
−
In the ancient past there were varying levels of belief in a [[flat Earth]], with the [[Mesopotamian mythology|Mesopotamian]] culture portraying the world as a flat disk afloat in an ocean. The spherical form of the Earth was suggested by early [[Greek philosophers]]; a belief espoused by [[Pythagoras]]. By the [[Middle Ages]]&mdash;as evidenced by thinkers such as [[Thomas Aquinas]]&mdash;European belief in a [[spherical Earth]] was widespread.<ref>{{cite web | last = Russell | first = Jeffrey B. | url = http://www.asa3.org/ASA/topics/history/1997Russell.html | title = The Myth of the Flat Earth | publisher = American Scientific Affiliation | accessdate = 2007-03-14 }}; but see also [[Cosmas Indicopleustes]]</ref> Prior to circumnavigation of the planet and the introduction of [[space flight]], belief in a spherical Earth was based on observations of the secondary effects of the Earth's shape and parallels drawn with the shape of other planets.<ref>{{cite web
 
−
| last = Jacobs
 
−
| first = James Q.
 
−
| date =February 1, 1998
 
−
| url =http://www.jqjacobs.net/astro/aegeo.html
 
−
| title =Archaeogeodesy, a Key to Prehistory
 
−
| accessdate = 2007-04-21
 
−
}}</ref>
 
−
 
−
[[Cartography]], the study and practice of map making, and vicariously [[geography]], have historically been the disciplines devoted to depicting the Earth. [[Surveying]], the determination of locations and distances, to a lesser extent [[navigation]], the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.
 
−
 
−
===Modern perspective===
 
−
{{see also|Spaceship Earth|Gaia theory|Geocentric orbit}}
 
−
 
−
The technological developments of the latter half of the 20th century are widely considered to have altered the public's perception of the Earth. Before space flight, the popular image of Earth was of a green world. [[Science fiction]] artist [[Frank R. Paul]] provided perhaps the first image of a cloudless ''blue'' planet (with sharply defined land masses) on the back cover of the July 1940 issue of ''[[Amazing Stories]]'', a common depiction for several decades thereafter.<ref name="fja">
 
−
{{cite book
 
−
| last = Ackerman | first = Forrest J
 
−
| authorlink = Forrest J Ackerman | year = 1997
 
−
| title = Forrest J Ackerman's World of Science Fiction
 
−
| publisher = RR Donnelley & Sons Company
 
−
| location = Los Angeles | id = ISBN 1-57544-069-5
 
−
| pages = 116–117 }}</ref>
 
−
 
−
[[Image:Earth and Moon from Mars PIA04531.jpg|200px|thumb|right|Earth and Moon from Mars, imaged by [[Mars Global Surveyor]]. From [[Outer space|space]], the Earth can be seen to go through phases similar to the [[lunar phases|phases of the Moon]].]]
 
−
 
−
Earth was first photographed from space by [[Explorer 6]] in 1959.<ref>{{cite web | author=Staff | date = October, 1998 | url = http://www.nasa.gov/centers/goddard/pdf/106420main_explorers.pdf | format=PDF | title = 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 from [[cislunar space]] (see [[#top|top of page]]). This became an iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. NASA archivist Mike Gentry has speculated that "The Blue Marble" is the most widely distributed image in human history. A photo taken of a distant Earth by ''[[Voyager 1]]'' in 1990 inspired [[Carl Sagan]] to describe the planet as a "[[Pale Blue Dot]]."<ref name="seti-pbd">
 
−
{{cite web
 
−
| author=Staff
 
−
| url = http://gtrc911.quaker.org/pale_blue_dot.html
 
−
| title = Pale Blue Dot | publisher = SETI@home
 
−
| accessdate = 2006-04-02 }}</ref> Earth has also been described as a massive [[Spacecraft|spaceship]], with a [[life support system]] that requires maintenance,<ref>{{cite book
 
−
| first=R. Buckminster | last=Fuller
 
−
| authorlink=Buckminster Fuller | year=1963
 
−
| title=[[Operating Manual for Spaceship Earth]]
 
−
| edition=First edition | publisher=E.P. Dutton & Co.
 
−
| location=New York | id=ISBN 0-525-47433-1
 
−
| url=http://www.futurehi.net/docs/OperatingManual.html
 
−
| accessdate=2007-04-21 }}</ref> or as having a [[biosphere]] that forms one large [[organism]].<ref>{{cite book
 
−
| first=James E. | last=Lovelock
 
−
| authorlink=James Lovelock | year=1979
 
−
| title=Gaia: A New Look at Life on Earth
 
−
| edition=First edition | publisher=Oxford University Press
 
−
| location=Oxford | id=ISBN 0-19-286030-5 }}</ref>
 
−
 
−
Over the past two centuries a growing [[environmental movement]] has emerged that is concerned about humankind's effects on the Earth. The key issues of this socio-political movement are the [[conservation movement|conservation]] of [[natural resource]]s, elimination of [[pollution]], and the usage of land. Environmentalists advocate [[sustainability|sustainable]] management of resources and [[stewardship]] of the [[natural environment]] through changes in public policy and individual behavior. Of particular concern is the large-scale exploitation of [[non-renewable resources]]. Changes sought by the environmental movements are sometimes in conflict with commercial interests due to the additional costs associated with managing the environmental impact of those interests.<ref>{{cite web
 
−
| last = Meyer | first = Stephen M.
 
−
| date = [[August 18]], [[2002]]
 
−
| url = http://web.mit.edu/polisci/mpepp/
 
−
| title = MIT Project on Environmental Politics & Policy
 
−
| publisher = Massachusetts Institute of Technology
 
−
| accessdate = 2006-08-10 }}</ref>
 
−
 
−
==Future==
 
−
{{seealso|Risks to civilization, humans and planet Earth}}
 
−
[[Image:Sun Life.png|550px|The life cycle of the Sun.]]
 
−
 
−
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.<ref name="sun_future">{{cite journal
 
−
| author=Sackmann, I.-J.; Boothroyd, A. I.; Kraemer, K. E.
 
−
| title=Our Sun. III. Present and Future
 
−
| journal=Astrophysical Journal
 
−
| year=1993
 
−
| volume=418
 
−
| pages=457–468
 
−
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...418..457S
 
−
| accessdate=2007-03-31  | doi = 10.1086/173407 <!--Retrieved from URL by DOI bot-->
 
−
}}</ref> Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet's oceans.<ref>{{cite journal
 
−
| last = Kasting
 
−
| first = J.F.
 
−
| title=Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus
 
−
| journal=Icarus
 
−
| year=1988
 
−
| volume=74
 
−
| pages=472–494
 
−
| url=http://adsabs.harvard.edu/abs/1988Icar...74..472K
 
−
| accessdate=2007-03-31  | doi = 10.1016/0019-1035(88)90116-9 <!--Retrieved from URL by DOI bot-->
 
−
}}</ref>
 
−
 
−
The Earth's increasing surface temperature will accelerate the [[inorganic]] [[Carbon cycle|CO<sub>2</sub> cycle]], reducing its concentration to the lethal levels for plants (10 [[Parts-per notation|ppm]] for [[C4 carbon fixation|C4 photosynthesis]]) in 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.<ref name=ward_brownlee/> But even if the Sun were eternal and stable, the continued internal cooling of the Earth would have resulted in a loss of much of its atmosphere and oceans (due to lower [[volcanism]]).<ref>{{cite journal
 
−
| author=Guillemot, H.; Greffoz, V.
 
−
| title=Ce que sera la fin du monde
 
−
| journal=Science et Vie
 
−
| date=Mars 2002
 
−
| volume=N° 1014
 
−
| language=French }}</ref> After another billion years the surface water will have completely disappeared<ref>{{cite news
 
−
| first=Damian
 
−
| last=Carrington
 
−
| title=Date set for desert Earth
 
−
| publisher=BBC News
 
−
| date=[[February 21]], [[2000]]
 
−
| url=http://news.bbc.co.uk/1/hi/sci/tech/specials/washington_2000/649913.stm
 
−
| accessdate=2007-03-31 }}</ref> and the mean global temperature will reach 70°C.<ref name=ward_brownlee/> The Earth is expected to be effectively habitable for another 500 million years or so.<ref>{{cite web | first=Robert | last=Britt | url=http://www.space.com/scienceastronomy/solarsystem/death_of_earth_000224.html | title=Freeze, Fry or Dry: How Long Has the Earth Got? | date=2000-02-25}}</ref>
 
−
 
−
The Sun, as part of its [[stellar evolution|evolution]], will expand to a [[red giant]] in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present size, roughly {{convert|1|AU|km| lk=on | abbr=on}}.<ref name="sun_future" /><ref name="sun_future_schroder">{{cite journal | first=K.-P. | last=Schröder | coauthors=Smith, Robert Connon | year=2008 | title=Distant future of the Sun and Earth revisited |
 
−
doi=10.1111/j.1365-2966.2008.13022.x | journal=Monthly Notices of the Royal Astronomical Society | id={{arxiv|0801.4031}}}}<br />See also {{cite news | url=http://space.newscientist.com/article/dn13369-hope-dims-that-earth-will-survive-suns-death.html?feedId=online-news_rss20 | title=Hope dims that Earth will survive Sun's death | date=[[22 February]] [[2008]] | work=NewScientist.com news service | first=Jason | last=Palmer | accessdate=2008-03-24 }}</ref> Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will be in an orbit {{convert|1.7|AU|km| abbr=on}} from the Sun when the star reaches it maximum radius. Therefore, the planet is thought to escape envelopment by the expanded Sun's sparse outer atmosphere, though most (if not all) existing life would have been destroyed by the Sun's proximity to Earth.<ref name="sun_future" /> However, a more recent simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be destroyed.<ref name="sun_future_schroder" />
 
−
{{-}}
 
−
 
−
==Notes==
 
−
{{Reflist|2}}
 
−
 
−
==References==
 
−
* {{cite book | first=Neil F. | last=Comins | year=2001
 
−
| title=Discovering the Essential Universe
 
−
| edition=Second Edition | publisher=W. H. Freeman
 
−
| url=http://adsabs.harvard.edu/abs/2003deu..book.....C
 
−
| accessdate=2007-03-17 | id=ISBN 0-7167-5804-0 }}
 
−
* {{cite web | editor=Kirk Munsell | date = [[October 19]], [[2006]] | url = http://sse.jpl.nasa.gov/planets/profile.cfm?Object=Earth&Display=OverviewLong| title = Solar System Exploration: Earth | publisher = NASA | accessdate = 2007-03-17 }}
 
−
*{{cite book
 
−
| first=Peter D. | last=Ward
 
−
| coauthors=Donald Brownlee | year=2002
 
−
| title=The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of Our World
 
−
| publisher=Times Books, Henry Holt and Company
 
−
| isbn=0-8050-6781-7 }}
 
−
* {{cite web
 
−
| last = Williams | first = David R.
 
−
| date = [[September 1]], [[2004]]
 
−
| url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
 
−
| title = Earth Fact Sheet | publisher = NASA
 
−
| accessdate = 2007-03-17 }}
 
−
* {{cite book
 
−
| last=Yoder | first=Charles F.
 
−
| editor=T. J. Ahrens | year=1995
 
−
| title=Global Earth Physics: A Handbook of Physical Constants
 
−
| publisher=American Geophysical Union | location=Washington
 
−
| url=http://www.agu.org/reference/gephys.html
 
−
| accessdate=2007-03-17 | id=ISBN 0875908519 }}
 
−
 
−
==External links==
 
−
{{sisterlinks|Earth}}
 
−
{{portal|Solar System|Solar system.jpg}}
 
−
{{portal|Earth sciences|Terra.png}}
 
−
* [http://geomag.usgs.gov USGS Geomagnetism Program]
 
−
* [http://earthobservatory.nasa.gov/Newsroom/BlueMarble NASA Earth Observatory]
 
−
* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Earth Earth Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
 
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* [http://www.co-intelligence.org/newsletter/comparisons.html The size of Earth compared with other planets/stars]
 
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* [http://www.nasa.gov/centers/goddard/earthandsun/earthshape.html Climate changes cause Earth's shape to change - NASA]
 
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* [http://www.geody.com/?world=terra Geody Earth] World's search engine that supports [[Google Earth]], [[NASA World Wind]], [[Celestia]], GPS, and other applications.
 
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* [http://entertainment.timesonline.co.uk/tol/arts_and_entertainment/the_tls/article3159404.ece "The age of the earth"]: an article by [[Richard Fortey|Richard A. Fortey]] in the [[The Times Literary Supplement|TLS]], [[January 9]] [[2008]]
 
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{{Solar System}}
 
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{{Earth}}
 
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{{Earth's location}}
 
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{{Time Persons of the Year 1976-2000}}
 
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[[Category:Earth]]
 
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[[be:Планета Зямля]]
 
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[[bg:Земя]]
 
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[[bat-smg:Žemė]]
 
 
[[zh:地球]]
 
[[zh:地球]]

Latest revision as of 18:55, 2 January 2010

Earth is a planet of the Solar system and the home-world of humanity. Earth is moving around the star with the official name Sol but commonly referred to as the Sun. Earth has one physical satellite with the name Luna (usually referred to as "the Moon"), however, it is argued that Earth and Moon are actually a single system, since they both revolve around the common center of mass, rather than one around the other.

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Image of Earth from space, showing the human population's effect on the planet through the light pollution.

3/4 of Earth surface is water. Humans live in the rest 1/4 and have polluted it so much that it is doubtful whether future generations will be able to live as people have in the past.

There are many societies on Earth, most notable are these of humans, but also ants, bees etc.

Earth is 1 AU away from the Sun.

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