View source for Earth

{{about|the planet}}<!--Discuss on the talk page before changing this tag.-->
{{pp-semi-indef}}
{{Infobox Planet
| bgcolour=#c0c0ff
| name = Earth
| symbol = [[File:Earth symbol.svg|25px|Astronomical symbol of Earth]]
| image = [[File:The Earth seen from Apollo 17.jpg|240px|A color image of Earth, as seen from Apollo 17]]
| flag = {{flagicon|world}}
| caption = Famous "[[The Blue Marble|Blue Marble]]" photograph of Earth, taken from [[Apollo 17]]
| epoch = [[J2000.0]]<ref group=note>All astronomical quantities vary, both [[Secular phenomena|secularly]] and [[Frequency|periodically]]. The quantities given are the values at the instant [[J2000.0]] of the secular variation, ignoring all periodic variations.</ref>
| aphelion = 152,097,701&nbsp;km <br />1.0167103335&nbsp;[[astronomical unit|AU]]
| perihelion = 147,098,074&nbsp;km <br /> 0.9832898912&nbsp;AU
| semimajor = 149,597,887.5&nbsp;km <br /> 1.0000001124&nbsp;AU
| eccentricity = 0.016710219
| inclination = 1°34'43.3"<ref name=Allen294>{{cite book
 | title=Allen's Astrophysical Quantities
 | author=Allen, Clabon Walter; Cox, Arthur N.
 | publisher=Springer | year=2000 | isbn=0387987460
 | url=http://books.google.com/books?id=w8PK2XFLLH8C&pg=PA294
 | pages=294}}</ref> <br /> to [[Invariable plane]]
| asc_node = 348.73936°
| arg_peri = 114.20783°
| period = 365.256366&nbsp;days<br /> 1.0000175&nbsp;[[Julian year (astronomy)|yr]]
| avg_speed = 29.783&nbsp;km/s <br /> 107,218&nbsp;km/h
| satellites = 1&nbsp;(the&nbsp;[[Moon]])
| physical_characteristics = yes
| flattening = 0.0033528<ref name=iers/>
| equatorial_radius = 6,378.1&nbsp;km<ref name=iers>{{cite conference
 | author=IERS Working Groups
 | editor=McCarthy, Dennis D.; Petit, Gérard
 | title=General Definitions and Numerical Standards
 | year=2003 | booktitle=IERS Technical Note No. 32
 | publisher=U.S. Naval Observatory and Bureau International des Poids et Mesures
 | url=http://www.iers.org/MainDisp.csl?pid=46-25776
 | accessdate=2008-08-03 }}</ref>
| polar_radius = 6,356.8&nbsp;km<ref>{{cite book
 | first=Anny | last=Cazenave
 | editor=Ahrens, Thomas J. | year=1995
 | chaptertitle=Geoid, Topography and Distribution of Landforms
 | title=Global earth physics a handbook of physical constants
 | publisher=American Geophysical Union
 | location=Washington, DC | isbn=0-87590-851-9
 | url=http://www.agu.org/reference/gephys/5_cazenave.pdf
 | accessdate=2008-08-03 |format=PDF}}</ref>
| mean_radius = 6,371.0&nbsp;km<ref>{{cite book
 | author=Various | editor=David R. Lide | year=2000
 | title=Handbook of Chemistry and Physics
 | edition=81st | publisher=CRC
 | isbn=0849304814 }}</ref>
| 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)
| surface_area = 510,072,000&nbsp;km²<ref>{{cite journal
 | last = Pidwirny | first = Michael | date=2006-02-02
 | title=Surface area of our planet covered by oceans and continents.(Table 8o-1)
 | publisher=University of British Columbia, Okanagan
 | url=http://www.physicalgeography.net/fundamentals/8o.html
 | accessdate=2007-11-26
}}</ref><ref name=cia/><ref group=note name=surfacecover>Due to natural fluctuations, ambiguities surrounding [[ice shelves]], and mapping conventions for [[vertical datum]]s, exact values for land and ocean coverage are not meaningful. Based on data from the [[Vector Map]] and [http://www-gem.jrc.it/ Global Landcover] datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of the earth’s surface. Note that the ice shields of [[Antarctica]] and [[Greenland]] are counted as land, even though much of the rock which supports them lies below sea level.</ref>
148,940,000&nbsp;km²&nbsp;land&nbsp;&nbsp;(29.2&nbsp;%)<br />
361,132,000&nbsp;km²&nbsp;water&nbsp;(70.8&nbsp;%)
| volume = [[Volume of the Earth|1.0832073{{e|12}}]]&nbsp;km<sup>3</sup>
| mass = 5.9736{{e|24}}&nbsp;kg<ref name="earth_fact_sheet"/>
| density = 5.5153&nbsp;g/cm<sup>3</sup>
| surface_grav = [[Earth's gravity|9.780327]] [[metre per second squared|m/s²]]<ref name="yoder12">Yoder, C. F. (1995) p. 12.</ref><br />0.99732&nbsp;[[g-force|''g'']]
| escape_velocity = 11.186&nbsp;km/s&nbsp;
| sidereal_day = 0.99726968&nbsp;d<ref name=Allen296>{{cite book
 | title=Allen's Astrophysical Quantities
 | author=Allen, Clabon Walter; Cox, Arthur N.
 | publisher=Springer | year=2000 | isbn=0387987460
 | url=http://books.google.com/books?id=w8PK2XFLLH8C&pg=PA296
 | pages=296}}</ref><br />23{{smallsup|h}}&nbsp;56{{smallsup|m}}&nbsp;4.100{{smallsup|s}}
| rot_velocity = {{convert|1674.4|km/h|m/s|abbr=on}}
| axial_tilt = 23.439281°
| albedo = 0.367<ref name="earth_fact_sheet"/>
| adjectives = Terrestrial, Terran, [[Terra (mythology)|Telluric]], Tellurian, Earthly
| pronounce = {{IPA-en|ɝːθ||en-us-earth.ogg}}<ref>{{cite web
 | url=http://dictionary.cambridge.org/define.asp?dict=CALD&key=24538&ph=on
 | title=Earth (PLANET)
 | work=Cambridge Advanced Learner's Dictionary
 | publisher=Cambridge University Press
 | accessdate=2008-11-14 }}</ref>
| atmosphere = yes
| temperatures = yes
| temp_name1 = [[Kelvin]]
| min_temp_1 = 184&nbsp;K
| mean_temp_1 = 287&nbsp;K
| max_temp_1 = 331&nbsp;K
| temp_name2 = [[Celsius]]
| min_temp_2 = −89&nbsp;°C
| mean_temp_2 = 14&nbsp;°C
| max_temp_2 = 57.7&nbsp;°C
| surface_pressure = 101.3&nbsp;[[kPa]] ([[Sea Level|MSL]])
| 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"/>
|note=no
}}<!--

 Proper article text starts here

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'''Earth''' is the third [[planet]] from the [[Sun]], and the largest of the [[terrestrial planet]]s in the [[Solar System]] in terms of [[diameter]], [[mass]] and [[density]]. It is also referred to as ''the [[World]]'', the ''Blue Planet'',<ref group=note>''Blue Planet'' is used as the title of several films [[Blue Planet (film)|Blue Planet]] and [[The Blue Planet]], in the [[Life (magazine)|Life]] issue ''The Incredible Year '68'' featuring the [[Earthrise]] photo with lines from poet [[James Dickey]] ''Behold/The blue planet steeped in its dream/Of reality'' [http://yalepress.yale.edu/yupbooks/excerpts/poole_earthrise.pdf] page 7-8 [http://www.northjersey.com/entertainment/books/36520714.html], and in the title of the [[European Space Agency]] bulletin report ''Exploring the water cycle of the 'Blue Planet' [http://www.esa.int/esapub/bulletin/bulletin137/bul137b_drinkwater.pdf] </ref> and ''[[wikt:Terra|Terra]]''.<ref group=note>By [[International Astronomical Union]] convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. ''Cf.'' {{cite web
 | last=Blue | first=Jennifer | date=2007-07-05
 | url=http://planetarynames.wr.usgs.gov/jsp/append5.jsp
 | title=Descriptor Terms (Feature Types)
 | work=Gazetteer of Planetary Nomenclature
 | publisher=USGS | accessdate=2007-07-05 }}</ref>

Home to millions of [[species]],<ref>{{cite journal
 | last = May | first = Robert M.
 | title=How many species are there on earth?
 | journal=Science | year=1988 | volume=241
 | issue=4872 | pages=1441–1449
 | url=http://adsabs.harvard.edu/abs/1988Sci...241.1441M
 | accessdate=2007-08-14
 | doi=10.1126/science.241.4872.1441
 | pmid=17790039 }}</ref> including [[human]]s, Earth is the only place in the [[universe]] where [[life]] is known to exist. The planet formed [[Age of the Earth|4.54 billion years]] ago,<ref name="age_earth1">{{cite book
 | first=G.B. | last=Dalrymple | year=1991
 | title=The Age of the Earth | publisher=Stanford University Press | location=California
 | isbn=0-8047-1569-6
}}</ref><ref name="age_earth2">{{cite web
 | last=Newman | first=William L. | date=2007-07-09
 | url=http://pubs.usgs.gov/gip/geotime/age.html
 | title=Age of the Earth
 | publisher=Publications Services, USGS
 | accessdate=2007-09-20
}}</ref><ref name="age_earth3">{{cite journal
 | last=Dalrymple | first=G. Brent | title=The age of the Earth in the twentieth century: a problem (mostly) solved
 | journal=Geological Society, London, Special Publications
 | year=2001 | volume=190 | pages=205–221 | url=http://sp.lyellcollection.org/cgi/content/abstract/190/1/205
 | accessdate=2007-09-20
 | doi = 10.1144/GSL.SP.2001.190.01.14
}}</ref><ref name="age_earth4">{{cite web
 | last=Stassen | first=Chris | date=2005-09-10 | url=http://www.talkorigins.org/faqs/faq-age-of-earth.html
 | title=The Age of the Earth | publisher=[[TalkOrigins Archive]] | accessdate=2008-12-30
}}</ref> and [[abiogenesis|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
 | first=Roy M. | last=Harrison
 | coauthors=Hester, Ronald E. | year=2002
 | title=Causes and Environmental Implications of Increased UV-B Radiation
 | publisher=Royal Society of Chemistry
 | isbn=0854042652 }}</ref> The physical properties of the Earth, as well as its geological history and orbit, allowed life to persist during this period. The world is expected to continue supporting life for another 1.5 billion years, after which the rising luminosity of the Sun will eliminate the biosphere.<ref name=carrington />

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 group=note>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
 | last=Malik
 | first=Tariq
 | title=Rover reveals Mars was once wet enough for life
 | publisher=[[Space.com]] (via [[MSNBC]])
 | date=2007-03-02
 | url=http://www.msnbc.msn.com/id/4202901/
 | accessdate=2007-08-28 }}</cite>
* <cite>{{cite news
<!-- no author given -->
 | title=Simulations Show Liquid Water Could Exist on Mars
 | work=Daily Headlines
 | publisher=[[University of Arkansas]]
 | date=2005-11-07
 | url=http://dailyheadlines.uark.edu/5717.htm
 | accessdate=2007-08-08 }}</cite></ref><ref group=note>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
 | author=Tinetti, G. ''et al.'' | title=Water vapour in the atmosphere of a transiting extrasolar planet
 | journal=Nature | month=July | year=2007 | volume=448
 | pages=169–171 | url=http://www.nature.com/nature/journal/v448/n7150/abs/nature06002.html | doi = 10.1038/nature06002 }}</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]].

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 group=note>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>Yoder, Charles F. (1995:8).</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. Between approximately 4.1 and 3.8 billion years ago, [[asteroid]] impacts during the [[Late Heavy Bombardment]] caused significant changes to the surface environment.

Both the [[mineral]] resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. The inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a [[flat Earth]] or in [[geocentric model|Earth being the center of the universe]], and a modern perspective of the world as an integrated environment that requires stewardship.

==Chronology==
{{main|History of the Earth}}
{{seealso|Geological history of Earth}}
Scientists have been able to reconstruct detailed information about the planet's past. The earliest dated solar system material is dated to 4.5672&nbsp;±&nbsp;0.0006&nbsp;billion&nbsp;years ago,<ref>{{cite journal
 | author=Bowring, S.
 | title=The Earth's early evolution | year=1995
 | doi=10.1126/science.7667634  | journal=Science
 | volume=269 | pages=1535 | pmid=7667634 }}</ref> and by 4.54&nbsp;billion&nbsp;years ago (within an uncertainty of 1%)<ref name="age_earth1" /><ref name="age_earth2" /><ref name="age_earth3" /><ref name="age_earth4" /> the
Earth and the other planets in the Solar System formed out of the [[solar nebula]]—a disk-shaped mass of dust and gas left over from the formation of the Sun. This assembly of the Earth through accretion was largely completed within 10–20&nbsp;million&nbsp;years.<ref>{{cite journal
 | last=Yin | first=Qingzhu | coauthors=Jacobsen, S. B.; Yamashita, K.; Blichert-Toft, J.; Télouk, P.; Albarède, F.
 | title=A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites
 | journal=Nature | year=2002 | volume=418 | issue=6901
 | pages=949–952 | doi=10.1038/nature00995 }}</ref> 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 shortly thereafter, 4.53&nbsp;billion&nbsp;years ago,<ref>{{cite journal
 | author=Kleine, Thorsten; Palme, Herbert; Mezger, Klaus; Halliday, Alex N. | title=Hf-W Chronometry of Lunar Metals and the Age and Early Differentiation of the Moon
 | journal=Science | volume=310 | issue=5754
 | date=2005-11-24 | pages=1671&ndash;1674
 | doi=10.1126/science.1118842
 | pmid=16308422 }}</ref> most likely 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 | year = 2001 | url = http://www.nature.com/nature/journal/v412/n6848/abs/412708a0.html 
 | doi = 10.1038/35089010 }}</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.

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="watersource">{{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>
The newly-formed Sun was only 70% of its present [[solar luminosity|luminosity]], yet evidence shows that the early oceans remained liquid&mdash;a contradiction dubbed the [[faint young Sun paradox]]. A combination of [[greenhouse gas]]es and higher levels of [[solar activity]] served to raise the Earth's surface temperature, preventing the oceans from freezing over.<ref>{{cite conference
 | author=Guinan, E. F.; Ribas, I. | editor=Benjamin Montesinos, Alvaro Gimenez and Edward F. Guinan
 | title=Our Changing Sun: The Role of Solar Nuclear Evolution and Magnetic Activity on Earth's Atmosphere and Climate
 | booktitle=ASP Conference Proceedings: The Evolving Sun and its Influence on Planetary Environments
 | location=San Francisco | isbn=1-58381-109-5
 | publisher=Astronomical Society of the Pacific
 | url=http://adsabs.harvard.edu/abs/2002ASPC..269...85G
 | accessdate=2009-07-27 }}</ref>

Two major models have been proposed for the rate of continental growth:<ref>{{cite book
 | first=John James William | last=Rogers
 | coauthors=Santosh, M. | year=2004
 | title=Continents and Supercontinents | pages=48
 | publisher=Oxford University Press US
 | isbn=0195165896 }}</ref> steady growth to the present-day<ref>{{cite journal | last=Hurley | first=P. M.
 | coauthors=Rand, J. R. | date=1969
 | title=Pre-drift continental nuclei
 | journal=Science | volume=164 |pages=1229–1242
 | doi=10.1126/science.164.3885.1229 | pmid=17772560 | month=Jun | author=Hurley, Pm; Rand, Jr | issue=3885 | issn=0036-8075 }}</ref> and rapid growth early in Earth history.<ref>{{cite journal|last=Armstrong|first=R.L.|date=1968|title=A model for the evolution of strontium and lead isotopes in a dynamic earth|journal=Rev. Geophys.|volume=6|pages=175–199|doi=10.1029/RG006i002p00175}}</ref>
Current research shows that the second option is most likely, with rapid initial growth of continental crust<ref>{{cite journal |doi=10.1016/S0040-1951(00)00055-X
 | title=Early formation and long-term stability of continents resulting from decompression melting in a convecting mantle
 | year=2000 | author=De Smet, J. | journal=Tectonophysics
 | volume=322 | pages=19 }}</ref> followed by a long-term steady continental area.<ref>{{cite journal
 | doi=10.1126/science.1117926 | year=2005 | month=December
 | author=Harrison, T.; Blichert-Toft, J.; Müller, W.; Albarede, F.; Holden, P.; Mojzsis, S.
 | title=Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 ga.
 | volume=310 | issue=5756 | pages=1947–50 | pmid=16293721
 | journal=Science }}</ref><ref>{{cite journal
 | doi=10.1016/S1367-9120(03)00134-2
 | title=Continental crustal growth and the supercontinental cycle: evidence from the Central Asian Orogenic Belt
 | year=2004 | author=Hong, D.
 | journal=Journal of Asian Earth Sciences | volume=23
 | pages=799 }}</ref><ref>{{cite journal
 | last=Armstrong | first=R. L. | date=1991
 | title=The persistent myth of crustal growth
 | journal=Australian Journal of Earth Sciences | volume=38
 | pages=613–630 | doi=10.1080/08120099108727995}}</ref> On [[Geologic time scale|time scales]] lasting hundreds of millions of years, the surface continually reshaped itself as continents formed and broke up. The continents migrated across the surface, occasionally combining to form a [[supercontinent]]. Roughly 750&nbsp;million&nbsp;years ago ([[annum|Ma]]), one of the earliest known supercontinents, [[Rodinia]], began to break apart. The continents later recombined to form [[Pannotia]], 600–540&nbsp;Ma, then finally [[Pangaea]], which broke apart 180&nbsp;Ma.<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 | doi=10.1511/2004.4.324 }}</ref>

===Evolution of life===
{{main|Evolutionary history of life}}

At present, Earth provides the only example of an environment that has given rise to the [[evolution]] of life.<ref>{{cite book | author=Purves, William Kirkwood; Sadava, David; Orians, Gordon H.; Heller, Craig
 | title=Life, the Science of Biology: The Science of Biology
 | publisher=Macmillan | page=455 | year=2001
 | isbn=0716738732 }}</ref> 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 | month=February
 | year=2000 | volume=282 | issue=6 | pages=90–95
 | doi=10.1038/nature03582 }}</ref> The development of [[photosynthesis]] allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed 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 | doi= 10.1175/1520-0469(1965)022<0225:OTOARO>2.0.CO;2 }}</ref> True multicellular organisms formed as cells within [[Colony (biology)|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 = 2002-11-29 | 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>

Since the 1960s, it has been hypothesized that severe [[Glacier|glacial]] action between 750 and 580&nbsp;Ma, 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. & Des Maris, D.
 | year=1992 | title= Late Proterozoic low-latitude global glaciation: the Snowball Earth
 | series= The Proterozoic Biosphere: A Multidisciplinary Study
 | pages=51–52 | publisher=Cambridge University Press
 | isbn=0521366151 }}</ref>

Following the Cambrian explosion, about 535&nbsp;Ma, 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 | pmid=17788674 }}</ref> The [[Cretaceous–Tertiary extinction event|last extinction event]] was 65&nbsp;Ma, 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 million years ago, 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 | month=October
 | year=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
 | author=Wilkinson, B. H.; McElroy, B. J.
 | title=The impact of humans on continental erosion and sedimentation | journal=Bulletin of the Geological Society of America | year=2007
 | volume=119 | issue=1–2 | pages=140–156 | url=http://bulletin.geoscienceworld.org/cgi/content/abstract/119/1-2/140
 | accessdate=2007-04-22 | doi = 10.1130/B25899.1 }}</ref> affecting both the nature and quantity of other life forms.

The present pattern of [[ice age]]s began about 40&nbsp;Ma and then intensified during the [[Pleistocene]] about 3&nbsp;Ma. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000&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>

===Future===
{{seealso|Risks to civilization, humans and planet Earth}}
[[File:Sun Life.png|700px|center|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 at the Sun's core, the [[Solar luminosity|star's total luminosity]] will slowly increase. The luminosity of the Sun will grow by 10 percent over the next 1.1&nbsp;[[Gigayear|Gyr]] (1.1&nbsp;billion years) 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
 | doi = 10.1086/173407 | bibcode= 1993ApJ...418..457S 
 | url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1993ApJ...418..457S&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf | format=PDF
 | accessdate=2008-07-08}}</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. | year=1988
 | title=Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus
 | journal=Icarus | volume=74 | pages=472–494
 | doi = 10.1016/0019-1035(88)90116-9 | url=http://adsabs.harvard.edu/abs/1988Icar...74..472K
 | accessdate=2007-03-31 }}</ref>

The Earth's increasing surface temperature will accelerate the [[inorganic]] [[Carbon cycle|CO<sub>2</sub> cycle]], reducing its concentration to 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>Ward and Brownlee (2002).</ref> After another billion years all surface water will have disappeared<ref name=carrington>{{cite news
 | first=Damian | last=Carrington
 | title=Date set for desert Earth
 | publisher=BBC News | date=2000-02-21 | 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 about another 500 million years,<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> although this may be extended up to {{nowrap|2.3 billion years}} if the nitrogen is removed from the atmosphere.<ref>{{cite journal
 | author=Li, King-Fai; Pahlevan, Kaveh; Kirschvink, Joseph L.; Yung, Yuk L. | year=2009
 | title=Atmospheric Pressure as a Natural Climate Regulator for a Terrestrial Planet with a Biosphere
 | journal=Proceedings of the National Academy of Sciences | volume=1=6 | issue=24 | pages=9576–9579 | url=http://www.gps.caltech.edu/~kfl/paper/Li_PNAS2009.pdf
 | accessdate=2009-07-19 | doi=10.1073/pnas.0809436106
 | pmid=19487662
}}</ref> Even if the Sun were eternal and stable, the continued internal cooling of the Earth would result in a loss of much of its CO<sub>2</sub> due to reduced [[volcanism]],<ref>{{cite journal
 | author=Guillemot, H.; Greffoz, V.
 | title=Ce que sera la fin du monde
 | journal=Science et Vie
 | month=March | year=2002 | volume=N° 1014
 | language=French }}</ref> and 35% of the water in the oceans would descend to the [[Mantle (geology)|mantle]] due to reduced steam venting from mid-ocean ridges.<ref>{{cite journal
 | last=Bounama | first=Christine | year=2001
 | coauthors=Siegfried Franck and Werner von Bloh
 | title=The fate of Earth’s ocean
 | journal=Hydrology and Earth System Sciences
 | volume=5 | issue=4 | pages=569–575
 | publisher=Potsdam Institute for Climate Impact Research | location=Germany | url=http://www.hydrol-earth-syst-sci.net/5/569/2001/hess-5-569-2001.pdf
 | accessdate=2009-07-03}}</ref>

The Sun, as part of its [[stellar evolution|evolution]], will become a [[red giant]] in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present radius, roughly {{convert|1|AU|km| lk=off | 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}} | volume=386
 | pages=155}}<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=2008-02-22 | 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 move to an orbit {{convert|1.7|AU|km| abbr=on}} from the Sun when the star reaches it maximum radius. Therefore, the planet is expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life will be destroyed because of the Sun's increased luminosity.<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" />

==Composition and structure==
{{main|Earth science}}
{{See|Earth physical characteristics tables}}
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]], the strongest magnetic field, and fastest rotation.<ref>{{cite web
 | last=Stern | first=David P. | date=2001-11-25
 | url= http://astrogeology.usgs.gov/HotTopics/index.php?/archives/147-Names-for-the-Columbia-astronauts-provisionally-approved.html
 | title=Planetary Magnetism | publisher=NASA
 | accessdate=2007-04-01 }}</ref> It also is the only terrestrial planet with active [[plate tectonics]].<ref>{{cite journal
 | last=Tackley | first=Paul J.
 | title=Mantle Convection and Plate Tectonics: Toward an Integrated Physical and Chemical Theory
 | journal=Science | date=2000-06-16
 | volume=288 | issue=5473 | pages=2002–2007
 | doi=10.1126/science.288.5473.2002
 | pmid=10856206 }}</ref>

===Shape===
{{main|Figure of the Earth}}
[[File:Terrestrial planet size comparisons.jpg|thumb|right|300px|Size comparison of inner planets (left to right): [[Mercury (planet)|Mercury]], [[Venus]], Earth and [[Mars]]]]
The shape of the Earth is very close to that of an [[oblate spheroid]], a sphere squished along the orientation from pole to pole such that there is a [[equatorial bulge|bulge]] around the [[equator]].<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>
This bulge results from the [[rotation]] of the Earth, and causes the diameter at the equator to be 43&nbsp;km larger than the [[Geographical pole|pole]] to pole diameter.<ref name="ngdc2006">{{cite web
 | author=Sandwell, D. T.; Smith, W. H. F.
 | date = 2006-07-07
 | url =http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML
 | title =Exploring the Ocean Basins with Satellite Altimeter Data
 | publisher = NOAA/NGDC | accessdate = 2007-04-21 }}</ref> The average diameter of the reference spheroid is about 12,742&nbsp;km, which is approximately 40,000&nbsp;km/[[pi|π]], as 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
 | author=Mohr, P.J.; Taylor, B.N.
 | month=October | year=2000
 | url=http://physics.nist.gov/cuu/Units/meter.html
 | title=Unit of length (meter)
 | work=NIST Reference on Constants, Units, and Uncertainty
 | publisher=NIST Physics Laboratory
 | accessdate=2007-04-23 }}</ref>

Local [[topography]] deviates from this idealized spheroid, though on a global scale, these deviations are very small: Earth has a [[tolerance (engineering)|tolerance]] of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in [[billiard ball]]s.<ref>{{cite web | author=Staff | month = November | year = 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> 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). Because of the equatorial 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 | pages=16–21
}}</ref><ref>{{cite journal
 | last=Sharp | first=David
 | title=Chimborazo and the old kilogram
 | journal=The Lancet | date=2005-03-05
 | volume=365 | issue=9462 | pages=831–832
 | doi=10.1016/S0140-6736(05)71021-7 }}</ref>

{| class="wikitable" style="float: right; clear: right; margin-left: 2em;"
|+ Chemical Composition of the Crust<ref>{{cite book
 | author=Brown, Geoff C.; Mussett, Alan E.
 | title=The Inaccessible Earth | edition=2nd | year=1981
 | page=166 | publisher=Taylor & Francis
 | isbn=0045500282 }}</ref><!-- After Ronov and Yaroshevsky (1969). -->
!rowspan="2"|Compound
!rowspan="2"|Formula
!colspan="2"|Composition
|-
!style="font-size: smaller;"|Continental
!style="font-size: smaller;"|Oceanic
|-
|[[silica]]
|style="text-align: center;"|SiO<sub>2</sub>
|style="text-align: right;"|60.2%
|style="text-align: right;"|48.6%
|-
|[[alumina]]
|style="text-align: center;"|Al<sub>2</sub>O<sub>3</sub>
|style="text-align: right;"|15.2%
|style="text-align: right;"|16.5%
|-
|[[Calcium oxide|lime]]
|style="text-align: center;"|CaO
|style="text-align: right;"|5.5%
|style="text-align: right;"|12.3%
|-
|[[Magnesia (mineral)|magnesia]]
|style="text-align: center;"|MgO
|style="text-align: right;"|3.1%
|style="text-align: right;"|6.8%
|-
|[[iron(II) oxide]]
|style="text-align: center;"|FeO
|style="text-align: right;"|3.8%
|style="text-align: right;"|6.2%
|-
|[[sodium oxide]]
|style="text-align: center;"|Na<sub>2</sub>O
|style="text-align: right;"|3.0%
|style="text-align: right;"|2.6%
|-
|[[potassium oxide]]
|style="text-align: center;"|K<sub>2</sub>O
|style="text-align: right;"|2.8%
|style="text-align: right;"|0.4%
|-
|[[iron(III) oxide]]
|style="text-align: center;"|Fe<sub>2</sub>O<sub>3</sub>
|style="text-align: right;"|2.5%
|style="text-align: right;"|2.3%
|-
|[[water (molecule)|water]]
|style="text-align: center;"|H<sub>2</sub>O
|style="text-align: right;"|1.4%
|style="text-align: right;"|1.1%
|-
|[[carbon dioxide]]
|style="text-align: center;"|CO<sub>2</sub>
|style="text-align: right;"|1.2%
|style="text-align: right;"|1.4%
|-
|[[titanium dioxide]]
|style="text-align: center;"|TiO<sub>2</sub>
|style="text-align: right;"|0.7%
|style="text-align: right;"|1.4%
|-
|[[phosphorus pentoxide]]
|style="text-align: center;"|P<sub>2</sub>O<sub>5</sub>
|style="text-align: right;"|0.2%
|style="text-align: right;"|0.3%
|-
!colspan="2"|Total
!style="text-align: right;"|99.6%
!style="text-align: right;"|99.9%
|}

===Chemical composition===
{{seealso|Abundance of elements on Earth}}
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 | doi=10.1073/pnas.77.12.6973
 | pmid=16592930 }}</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 group=note name=EB1911>{{1911|article=Petrology}}</ref>

===Internal structure===
{{main|Structure of the Earth}}
The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their [[chemical]] or physical ([[Rheology|rheological]]) properties. The outer layer of the Earth is a chemically distinct [[Silicate minerals|silicate]] solid [[crust]], which is underlain by a highly viscous solid mantle. 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 30–50&nbsp;km on the continents. The crust and the cold, rigid, top of the [[upper mantle]] are collectively known as the [[lithosphere]], and it is of the lithosphere that the [[tectonic plate]]s are comprised. Beneath the lithosphere is the [[asthenosphere]], a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660&nbsp;kilometers below the surface, spanning a [[transition zone]] that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid [[outer core]] lies above a solid [[inner core]].<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 | isbn=0-87590-851-9
 | url=http://www.agu.org/reference/gephys/15_tanimoto.pdf
 | format=PDF | accessdate=2007-02-03 }}</ref> The inner core may rotate at a slightly higher [[angular velocity]] than the remainder of the planet, advancing by 0.1–0.5° per year.<ref>{{cite journal
 | last=Kerr | first=Richard A. | title=Earth's Inner Core Is Running a Tad Faster Than the Rest of the Planet
 | journal=Science | date=2005-09-26 | volume=309
 | issue=5739 | pages=1313
 | doi=10.1126/science.309.5739.1313a
 | pmid=16123276 }}</ref>

{| class="wikitable" style="margin: 4px; margin-right: 0px; width: 100%;"
|+ Geologic 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 | doi=10.1073/pnas.76.9.4192
 | pmid=16592703 }}</ref>
|-
!rowspan="8" style="font-size: smaller; text-align: center; padding: 0px;"|[[File:Earth-crust-cutaway-english.svg|250px|center]]<br />Earth cutaway from core to exosphere. Not to scale.
!Depth<ref>{{cite web
 | last = Robertson | first = Eugene C.
 | date = 2001-07-26
 | url = http://pubs.usgs.gov/gip/interior/
 | title = The Interior of the Earth
 | publisher = USGS | accessdate = 2007-03-24
}}</ref><br /><span style="font-size: smaller;">km</span>
!style="vertical-align: bottom;"|Component Layer
!Density<br /><span style="font-size: smaller;">g/cm<sup>3</sup></span>
|-
|style="text-align: center;"|0–60
|Lithosphere<ref group="note">Locally varies between 5 and 200&nbsp;km.</ref>
|style="text-align: center;"| —
|- style="background: #FEFEFE;"
|style="text-align: center;"|0–35
|... Crust<ref group="note">Locally varies between 5 and 70 km.</ref>
|style="text-align: center;"| 2.2–2.9
|- style="background: #FEFEFE;"
|style="text-align: center;"|35–60
|... Upper mantle
|style="text-align: center;"| 3.4–4.4
|-
|style="text-align: center;"|35–2890
|Mantle
|style="text-align: center;"| 3.4–5.6
|- style="background: #FEFEFE;"
|style="text-align: center;"|100–700
|... Asthenosphere
|style="text-align: center;"| —
|-
|style="text-align: center;"|2890–5100
|Outer core
|style="text-align: center;"| 9.9–12.2
|-
|style="text-align: center;"|5100–6378
|Inner core
|style="text-align: center;"| 12.8–13.1
|}

===Heat===
Earth's [[internal heat]] comes from a combination of [[Gravitational binding energy|residual heat from planetary accretion]] (about 20%) and heat produced through [[radioactive decay]] (80%).<ref name="turcotte">{{cite book
 | last=Turcotte | first=D. L. | coauthors=Schubert, G.
 | title=Geodynamics | publisher=Cambridge University Press
 | location=Cambridge, England, UK| date=2002 | edition=2
 | pages=136–137 | chapter=4 | isbn=978-0-521-66624-4 }}</ref> The major heat-producing isotopes in the Earth are [[Potassium|potassium-40]], [[Uranium|uranium-238]], uranium-235, and [[Thorium|thorium-232]].<ref>{{cite news
 | first=Robert | last=Sanders
 | title=Radioactive potassium may be major heat source in Earth's core | publisher=UC Berkeley News | date=2003-12-10 | 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> Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately 3&nbsp;billion&nbsp;years ago,<ref name="turcotte" /> would have increased temperature gradients within the Earth, increasing the rates of [[mantle convection]] and [[plate tectonics]], and allowing the production of igneous rocks such as [[komatiites]] that are not formed today.<ref>{{cite journal
 | last=Vlaar | first=N | title=Cooling of the earth in the Archaean: Consequences of pressure-release melting in a hotter mantle | year=1994 |journal=Earth and Planetary Science Letters
 | volume=121 | page=1 | doi=10.1016/0012-821X(94)90028-0
 | pages=1 }}</ref>

{| class="wikitable" border="1" style="text-align: center;"
|+ Present-day major heat-producing isotopes<ref name="T&S 137">{{cite book|last=Turcotte|first=D. L.|coauthors=Schubert, G.|title=Geodynamics|publisher=Cambridge University Press|location=Cambridge, England, UK|date=2002|edition=2|pages=137|chapter=4|isbn=978-0-521-66624-4}}</ref>
|-
! Isotope
! Heat release [W/kg isotope]
! Half-life [years]
! Mean mantle concentration [kg isotope/kg mantle]
! Heat release [W/kg mantle]
|-
| <sup>238</sup>U
| {{nowrap|9.46 × 10<sup>-5</sup>}}
| {{nowrap|4.47 × 10<sup>9</sup>}}
| {{nowrap|30.8 × 10<sup>-9</sup>}}
| {{nowrap|2.91 × 10<sup>-12</sup>}}
|-
| <sup>235</sup>U
| {{nowrap|5.69 × 10<sup>-4</sup>}}
| {{nowrap|7.04 × 10<sup>8</sup>}}
| {{nowrap|0.22 × 10<sup>-9</sup>}}
| {{nowrap|1.25 × 10<sup>-13</sup>}}
|-
| <sup>232</sup>Th
| {{nowrap|2.64 × 10<sup>-5</sup>}}
| {{nowrap|1.40 × 10<sup>10</sup>}}
| {{nowrap|124 × 10<sup>-9</sup>}}
| {{nowrap|3.27 × 10<sup>-12</sup>}}
|-
| <sup>40</sup>K
| {{nowrap|2.92 × 10<sup>-5</sup>}}
| {{nowrap|1.25 × 10<sup>9</sup>}}
| {{nowrap|36.9 × 10<sup>-9</sup>}}
| {{nowrap|1.08 × 10<sup>-12</sup>}}
|}

Total heat loss from the earth is {{nowrap|4.2 × 10<sup>13</sup> Watts}}.<ref name="heat loss">{{cite journal
 | doi=10.1029/JB086iB12p11535 | title=Oceans and Continents: Similarities and Differences in the Mechanisms of Heat Loss
 | year=1981 | last=Sclater | first=John G
 | journal=Journal of Geophysical Research | volume=86 |pages=11535
}}</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
 | pmid=17837768 }}</ref>
More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, majority of which occurs in the oceans due to the crust there being much thinner than that of the continents.<ref name="heat loss" />

===Tectonic plates===
{| class="wikitable" align="right" style="margin-left: 1em"
|+ [[List of tectonic plates|Earth's main plates]]<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>
|-
|colspan="2" style="font-size: smaller; text-align: center;"|[[File:Tectonic plates (empty).svg|250px]]
|-
!Plate name
!Area<br /><span style="font-size: smaller;">10<sup>6</sup>&nbsp;km²</span>
|-
| [[African Plate]]<ref group=note>Including the [[Somali Plate]], which is currently in the process of formation out of the African Plate. See: {{cite journal
 | first=Jean | last=Chorowicz | month=October | year=2005
 | title=The East African rift system
 | journal=Journal of African Earth Sciences
 | volume=43 | issue=1–3 | pages=379–410
 | doi=10.1016/j.jafrearsci.2005.07.019 }}</ref> ||style="text-align: center;"| 78.0
|-
| [[Antarctic Plate]] ||style="text-align: center;"| 60.9
|-
| [[Australian Plate]] ||style="text-align: center;"| 47.2
|-
| [[Eurasian Plate]] ||style="text-align: center;"| 67.8
|-
| [[North American Plate]] ||style="text-align: center;"| 75.9
|-
| [[South American Plate]] ||style="text-align: center;"| 43.6
|-
| [[Pacific Plate]] ||style="text-align: center;"| 103.3
|}
{{main|Plate tectonics}}
The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces 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 boundaries]], at which two plates come together, [[Divergent boundary|Divergent boundaries]], at which two plates are pulled apart, and [[Transform boundary|Transform boundaries]], in which two plates slide past one another laterally. [[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 = 1999-05-05 | url = http://pubs.usgs.gov/gip/dynamic/understanding.html | title = Understanding plate motions | publisher = USGS | accessdate = 2007-03-02 }}</ref>
The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates,<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>
and their motion is strongly coupled with patterns convection inside the [[Earth's mantle]].

As the tectonic plates migrate across the planet, the ocean floor is [[Subduction|subducted]] under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates [[mid-ocean ridge]]s. The combination of these processes continually recycles the [[oceanic crust]] back into the mantle. Because of this recycling, most of the ocean floor is less than 100 million years in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 million years.<ref>{{cite web | last = Duennebier | first = Fred
 | date = 1999-08-12 | 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 = 2007-03-07 | url = http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html
 | title = Age of the Ocean Floor Poster
 | publisher = NOAA | accessdate = 2007-03-14 }}</ref> By comparison, the oldest dated continental crust is 4030 million years old.<ref>{{cite journal|doi=10.1007/s004100050465|title=Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada|year=1999|author=Bowring, Samuel A.|journal=Contributions to Mineralogy and Petrology|volume=134|pages=3}}</ref>

Other notable 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=2000-11-20
 | 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 Earth}}
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 [[erosion and tectonics|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.lpi.usra.edu/science/kring/epo_web/impact_cratering/intro/index.html
 | title = Terrestrial Impact Cratering and Its Environmental Effects | publisher = Lunar and Planetary Laboratory | accessdate = 2007-03-22 }}</ref> also act to reshape the landscape.
[[File:AYool topography 15min.png|250px|left|thumb|Present day Earth [[terrain|altimetry]] and [[bathymetry]]. Data from the [[National Geophysical Data Center]]'s [http://www.ngdc.noaa.gov/mgg/topo/ TerrainBase Digital Terrain Model].]]

The [[continental crust]] consists 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.oregonstate.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=2008-07-24
 | url=https://www.cia.gov/library/publications/the-world-factbook/geos/XX.html
 | title=World | work=The World Factbook
 | publisher=Central Intelligence Agency
 | accessdate=2008-08-05
}}</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 | isbn=9250038445 }}</ref>

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 840&nbsp;m.<ref name=sverdrup>{{cite book
 | first=H. U. | last=Sverdrup
 | coauthors=Fleming, Richard H. | date=1942-01-01
 | title=The oceans, their physics, chemistry, and general biology
 | publisher=Scripps Institution of Oceanography Archives
 | url=http://repositories.cdlib.org/sio/arch/oceans/
 | accessdate=2008-06-13 }}</ref>

===Hydrosphere===
{{main|Hydrosphere}}
[[File:Earth elevation histogram 2.svg|thumb|300px|Elevation [[histogram]] of the surface of the Earth. Approximately 71% of the Earth's surface is covered with water.]]
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 −10,911.4&nbsp;m.<ref group="note">This is the measurement taken by the vessel ''[[Kaikō]]'' in March 1995 and is believed to be the most accurate measurement to date. See the [[Challenger Deep]] article for more details.</ref><ref>{{cite web | title=7,000 m Class Remotely Operated Vehicle ''KAIKO 7000'' | url=http://www.jamstec.go.jp/e/about/equipment/ships/kaiko7000.html
 | publisher=Japan Agency for Marine-Earth Science and Technology (JAMSTEC) | accessdate=2008-06-07}}</ref> The average depth of the oceans is 3,800&nbsp;m, more than four times the average height of the continents.<ref name=sverdrup/>

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;km<sup>3</sup>. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7&nbsp;km.<ref group=note>The total volume of the Earth's oceans is: 1.4{{e|9}}&nbsp;km<sup>3</sup>. The total surface area of the Earth is 5.1{{e|8}}&nbsp;km². So, to 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://webworld.unesco.org/water/ihp/db/shiklomanov/
 | 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 = 2002-06-11 | 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 = 2006-04-24 | 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 =2005-06-21
 | 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 8&nbsp;km at the poles to 17&nbsp;km at the equator, with some variation due to weather and seasonal factors.<ref>{{cite web
 | last=Geerts | first=B. | coauthors=Linacre, E.
 | url=http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/tropo.html
 | title=The height of the tropopause | month=November | year=1997
 | work=Resources in Atmospheric Sciences
 | publisher=University of Wyoming
 | accessdate=2006-08-10 }}</ref>

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 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 = 2003-10-08 | 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>

The primary atmospheric circulation bands consist of the [[trade winds]] in the equatorial region below 30° latitude and the [[westerlies]] in the mid-latitudes between
30° and 60°.<ref name="berger2002">{{cite web
 | last = Berger | first = Wolfgang H. | year=2002
 | url = http://earthguide.ucsd.edu/virtualmuseum/climatechange1/cc1syllabus.shtml
 | title = The Earth's Climate System
 | publisher = University of California, San Diego
 | accessdate = 2007-03-24 }}</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
 | first=Stefan | last=Rahmstorf | year=2003
 | url =http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html
 | title =The Thermohaline Ocean Circulation
 | publisher =Potsdam Institute for Climate Impact Research
 | accessdate = 2007-04-21 }}</ref>

[[File:Air masses 2.jpg|left|thumb|300px|Source regions of global [[air mass]]es]]
Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere.
When atmospheric conditions permit an uplift of warm, humid air, this water condenses and 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
 | author=Various | date = 1997-07-21
 | url = http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml
 | title = The Hydrologic Cycle
 | publisher = University of Illinois
 | accessdate = 2007-03-24 }}</ref>

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
 | author=Staff | url = http://www.ace.mmu.ac.uk/eae/Climate/Older/Climate_Zones.html
 | title = Climate Zones | publisher = UK Department for Environment, Food and Rural Affairs
 | accessdate = 2007-03-24 }}</ref> Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used [[Köppen climate classification]] system (as modified by [[Wladimir Köppen]]'s student Rudolph Geiger) has five broad groups (humid tropics, [[Desert|arid]], humid middle latitudes, [[Continental climate|continental]] and cold polar), which are further divided into more specific subtypes.<ref name="berger2002" />

====Upper atmosphere====
[[File: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.]]
{{seealso|Outer space}}
Above the troposphere, the atmosphere is usually divided into the [[stratosphere]], [[mesosphere]], and [[thermosphere]].<ref name="atmosphere" /> Each of these layers has a different [[lapse rate]], defining the rate of change in temperature with height. Beyond these, the [[exosphere]] thins out into the [[magnetosphere]]. This is 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
 | first=S. Sanz Fernández | last=de Córdoba
 | date =2004-06-21
 | url =http://www.un.org/members/list.shtml
 | title =100&nbsp;km. Altitude Boundary for Astronautics
 | publisher =Fédération Aéronautique Internationale
 | accessdate = 2007-04-21 }}</ref>

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 than other gasses.<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 | doi=10.1175/1520-0469(1974)031<1118:TAOHIT>2.0.CO;2 }}</ref> The leakage of hydrogen into space is a contributing factor in pushing the Earth from an initially [[redox|reducing]] state to its current [[Redox|oxidizing]] one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.<ref>{{cite journal
|title=Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth
|author=David C. Catling, Kevin J. Zahnle, Christopher P. McKay
|journal=Science
|volume=293
|issue=5531
|pages=839–843
|url=http://www.sciencemag.org/cgi/content/full/293/5531/839
|doi=10.1126/science.1061976
|year=2001
|pmid=11486082}}</ref>
Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet.<ref>{{cite web
 | last = Abedon | first = Stephen T.
 | date = 1997-03-31 | url = http://www.mansfield.ohio-state.edu/~sabedon/biol1010.htm
 | title = History of Earth
 | publisher = Ohio State University
 | accessdate = 2007-03-19 }}</ref> In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of [[methane]] in the upper atmosphere.<ref>{{cite journal
 | last=Hunten | first=D. M. | coauthors=Donahue, T. M.
 | title=Hydrogen loss from the terrestrial planets
 | journal=Annual review of earth and planetary sciences
 | year=1976 | volume=4 | pages=265–292
 | url=http://adsabs.harvard.edu/abs/1976AREPS...4..265H
 | accessdate=2008-11-07
 | doi=10.1146/annurev.ea.04.050176.001405 }}</ref>

===Magnetic field===
[[File:Dipole field.svg|thumb|right|300px|The [[Earth's magnetic field]], which approximates a dipole.]]
{{main|Earth's magnetic field}}
The [[Earth's magnetic field]] is shaped roughly as a [[magnetic dipole]], with the poles currently located proximate to the planet's geographic poles. According to [[dynamo theory]], the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in [[geomagnetic reversal|field reversals]] at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.<ref>{{cite web | last = Fitzpatrick | first = Richard | date = 2006-02-16 | 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
 | last =Campbell | first =Wallace Hall
 | title =Introduction to Geomagnetic Fields
 | publisher =Cambridge University Press | year =2003
 | location =New York | pages =57
 | isbn = 0521822068}}</ref>

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 = 2005-07-08 | url = http://www-spof.gsfc.nasa.gov/Education/wmap.html | title = Exploration of the Earth's Magnetosphere | publisher = NASA | accessdate = 2007-03-21 }}</ref>
{{-}}

==Orbit and rotation==
===Rotation===
{{main|Earth's rotation}}
[[File:AxialTiltObliquity.png|thumb|right|280px|Earth's axial tilt (or [[obliquity]]) and its relation to the [[Rotation|rotation axis]] and [[Orbital plane (astronomy)|plane of orbit]].]]
Earth's rotation period relative to the Sun—its mean solar day—is 86,400&nbsp;seconds of mean solar time. Each of these seconds is slightly longer than an [[SI]] second because Earth's solar day is now slightly longer than it was during the 19th century due to [[tidal acceleration]].<ref>{{cite web
 | title=Leap seconds
 | publisher=Time Service Department, USNO
 | url=http://tycho.usno.navy.mil/leapsec.html
 | accessdate=2008-09-23 }}</ref>

Earth's rotation period relative to the [[fixed star]]s, called its ''stellar day'' by the [[International Earth Rotation and Reference Systems Service]] (IERS), is {{nowrap|86164.098903691 seconds}} of mean solar time (UT1), or {{nowrap |23{{smallsup|h}} 56{{smallsup|m}} 4.098903691{{smallsup|s}}. }}<ref name=IERS>{{cite web
 | author=Staff | date=2007-08-07
 | url=http://hpiers.obspm.fr/eop-pc/models/constants.html
 | title=Useful Constants | publisher=International Earth Rotation and Reference Systems Service (IERS)
 | accessdate=2008-09-23 }}</ref><ref group=note name=Aoki>Aoki, the ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".—{{cite journal
 | last=Aoki | first=S. | coauthors=Kinoshita, H.; Guinot, B.; Kaplan, G. H.; McCarthy, D. D.; Seidelmann, P. K.
 | title=The new definition of universal time
 | journal=Astronomy and Astrophysics | year=1982
 | volume=105 | issue=2 | pages=359–361
 | url=http://adsabs.harvard.edu/abs/1982A&A...105..359A
 | accessdate=2008-09-23 }}</ref> Earth's rotation period relative to the [[precession (astronomy)|precessing]] or moving mean vernal [[equinox]], misnamed its ''[[sidereal day]]'', is {{nowrap|86164.09053083288 seconds}} of mean solar time (UT1) {{nowrap|(23{{smallsup|h}} 56{{smallsup|m}} 4.09053083288{{smallsup|s}})}}.<ref name=IERS/> Thus the sidereal day is shorter than the stellar day by about 8.4 ms.<ref>{{cite book
 | last=Seidelmann | first=P. Kenneth | year=1992
 | title=Explanatory Supplement to the Astronomical Almanac
 | pages=48 | publisher=University Science Books
 | location=Mill Valley, CA | isbn=0-935702-68-7 }}</ref> The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005<ref>{{cite web
 | author=Staff | url=http://hpiers.obspm.fr/eop-pc/earthor/ut1lod/lod-1623.html
 | title=IERS Excess of the duration of the day to 86400s ... since 1623 | publisher=International Earth Rotation and Reference Systems Service (IERS)
 | accessdate=2008-09-23 }}—Graph at end.</ref> and 1962–2005.<ref>{{cite web
 | author=Staff | url=http://web.archive.org/web/20070813203913/http://hpiers.obspm.fr/eop-pc/earthor/ut1lod/figure3.html
 | title=IERS Variations in the duration of the day 1962–2005 | publisher=International Earth Rotation and Reference Systems Service (IERS)
 | accessdate=2008-09-23 }}</ref>

Apart from [[meteor]]s within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth's sky 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.<ref>{{cite book
 | last=Zeilik | first=M. | coauthors=Gregory, S. A.
 | title=Introductory Astronomy & Astrophysics
 | edition=4th | pages=56
 | publisher=Saunders College Publishing
 | isbn=0030062284 | year=1998}}</ref><ref name=angular>{{cite web
 | last=Williams | first=David R. | date=2006-02-10
 | url=http://nssdc.gsfc.nasa.gov/planetary/planetfact.html
 | title=Planetary Fact Sheets
 | publisher=NASA | accessdate=2008-09-28
}}—See the apparent diameters on the Sun and Moon pages.</ref>

===Orbit===
{{Main|Earth's orbit}}
Earth orbits the Sun at an average distance of about 150&nbsp;million kilometers every 365.2564&nbsp;mean&nbsp;solar&nbsp;days, or one&nbsp;[[sidereal year]]. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter every 12&nbsp;hours. Because of this motion, on average it takes 24&nbsp;hours—a [[Solar time|solar day]]—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 = 2004-09-01 | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html | title = Earth Fact Sheet | publisher = NASA | accessdate = 2007-03-17 }}</ref>

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]]. Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about the Sun. 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, 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="earth_fact_sheet" /><ref name="moon_fact_sheet">{{cite web | last = Williams | first = David R. | date = 2004-09-01 | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html | title = Moon Fact Sheet | publisher = NASA | accessdate = 2007-03-21 }}</ref>

The [[Hill sphere]], or [[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 |format=PDF}}</ref><ref group=note>For the Earth, the [[Hill radius]] is

:<math>\begin{smallmatrix} R_H = a\left ( \frac{m}{3M} \right )^{\frac{1}{3}} \end{smallmatrix}</math>,

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:

<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.

[[File:236084main MilkyWay-full-annotated.jpg|thumb|Illustration of the [[Milky Way Galaxy]], showing the location of the [[Sun]].]]

Earth, along with the Solar System, is situated in the [[Milky Way]] [[galaxy]], orbiting about 28,000&nbsp;[[Light-year|light years]] from the center of the galaxy. It is currently about 20&nbsp;light years above the galaxy's [[equatorial plane]] in the [[Orion Arm|Orion spiral arm]].<ref>{{cite web
 | author=Astrophysicist team | date=2005-12-01
 | url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/030827a.html
 | title=Earth's location in the Milky Way | publisher=NASA
 | accessdate=2008-06-11 }}</ref>

===Axial tilt and seasons===
{{main|Axial tilt}}
Because of the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This results in [[season]]al change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the [[Arctic Circle]], an extreme case is reached where there is no daylight at all for part of the year—a [[polar night]]. In the southern hemisphere the situation is exactly reversed, with the [[South Pole]] oriented opposite the direction of the North Pole.

[[File:Earth and Moon from Mars PIA04531.jpg|200px|thumb|left|Earth and Moon from Mars, imaged by [[Mars Global Surveyor]]. From space, the Earth can be seen to go through phases similar to the [[lunar phases|phases of the Moon]].]]
By astronomical convention, the four seasons are determined by the [[solstice]]s—the point in the orbit of maximum axial tilt toward or away from the Sun—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.<ref>{{cite web
 | last=Bromberg | first=Irv | date=2008-05-01
 | url=http://www.sym454.org/seasons/
 | title=The Lengths of the Seasons (on Earth)
 | publisher=University of Toronto
 | accessdate=2008-11-08 }}</ref>

The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo [[nutation]]; a slight, irregular motion 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 = 1996-02-05 | 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>

In modern times, Earth's [[perihelion]] occurs around January 3, and the [[aphelion]] around July 4. However, these dates change over time due to [[precession (astronomy)|precession]] and other orbital factors, which follow cyclical patterns known as [[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 = 2005-12-20 | url = http://www.usatoday.com/weather/tg/wseason/wseason.htm | title = Earth's tilt creates seasons | publisher = USAToday | accessdate = 2007-03-17 }}</ref>

==Moon==
{| class="wikitable" style="float: right; margin-left: 0.5em;"
|+ '''Characteristics'''
|-
| Diameter || 3,474.8&nbsp;km<br />2,159.2&nbsp;mi
|-
| Mass || 7.349{{e|22}}&nbsp;kg<br />8.1{{e|19}}&nbsp;(short)&nbsp;tons
|-
| [[Semi-major axis]] || 384,400&nbsp;km<br />238,700&nbsp;mi
|-
| Orbital period || 27&nbsp;d 7&nbsp;h 43.7&nbsp;m
|}
{{main|Moon}}

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.

The gravitational attraction between the Earth and Moon causes [[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]].

Because of their [[Tidal acceleration|tidal interaction]], the Moon recedes from Earth at the rate of approximately 38&nbsp;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
 | author=Espenak, F.; Meeus, J.
 | date = 2007-02-07
 | url = http://sunearth.gsfc.nasa.gov/eclipse/SEcat5/secular.html
 | title = Secular acceleration of the Moon
 | publisher = NASA
 | accessdate = 2007-04-20
}}</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.
 | last=Poropudas
 | date =1991-12-16
 | url = http://www.skepticfiles.org/origins/coralclo.htm
 | title = Using Coral as a Clock
 | publisher = Skeptic Tank
 | accessdate = 2007-04-20
}}</ref>

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.;
Levrard, B.
 | title=A long-term numerical solution for the insolation quantities of the Earth
 | journal=Astronomy and Astrophysics
 | year=2004
 | volume=428
 | pages=261–285
 | url=http://adsabs.harvard.edu/abs/2004A&A...428..261L
 | accessdate=2007-03-31 | doi = 10.1051/0004-6361:20041335
}}</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, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.<ref>{{cite journal
 | last=Murray | first=N. | coauthors=Holman, M.
 | title=The role of chaotic resonances in the solar system
 | journal=Nature | year=2001
 | volume=410 | issue=6830 | pages=773–779
 | url=http://arxiv.org/abs/astro-ph/0111602v1
 | accessdate=2008-08-05
 | doi=10.1038/35071000 }}</ref> 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
 | 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.

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.<ref name=angular /> This allows total and annular [[eclipse]]s to occur on Earth.
{{-}}
[[File:Earth Moon Scale.jpg|thumb|center|800px|A scale representation of the relative sizes of, and distance between, Earth and Moon.]]

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
 | 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
 | year = 2001
 | doi = 10.1038/35089010 }}</ref>

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=2002-10-21
 | url=http://news.bbc.co.uk/1/hi/sci/tech/2347663.stm
 | accessdate=2007-03-31 }}</ref>

==Habitability==
{{seealso|Planetary habitability}}
[[File:Habitable zone-en.svg|thumb|320px|right|A range of theoretical habitable zones with stars of different mass (our [[solar system]] at center). Not to scale.]]

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 | month = September | year = 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 | publisher=American Elsevier Publishing Co. | url=http://www.rand.org/pubs/reports/R414/ | accessdate=2007-03-11 | isbn=0-444-00092-5 }}</ref>

===Biosphere===
{{main|Biosphere}}

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=2000-01-14
 | title=Rare Earth: Why Complex Life is Uncommon in the Universe
 | edition=1st
 | publisher=Springer-Verlag
 | location=New York
 | isbn=0387987010 }}</ref>

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
 | issue=2
 | pages=192–211
 | doi=10.1086/381004 }}</ref>

===Natural resources and land use===
{{main|Natural resource}}

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

Large deposits of [[fossil fuel]]s are obtained from the Earth's crust, consisting of [[coal]], [[petroleum]], [[natural gas]] and [[methane clathrate]]. These deposits are used by humans 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=2006-11-24 | 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]].

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
 | pmid = 12560541
}}</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 = 2007-02-02
 | 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}}
{{seealso|World}}
{{Continents navmap}}

[[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, and to a lesser extent [[navigation]], the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

Earth has approximately 6,740,000,000 human inhabitants as of November 2008.<ref name="World Population Clock">
{{cite web
 | first=
 | last= United States Census Bureau
 | url= http://www.census.gov/ipc/www/popclockworld.html
 | title= World POP Clock Projection
 | work= United States Census Bureau International Database
 | date= 2008-01-07
 | accessdate= 2008-01-07
}}
</ref> Projections indicate that the [[world population|world's human population]] will reach seven billion in 2013 and 9.2&nbsp;billion in 2050.<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> 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 humans to live on—three-quarters is covered by oceans, 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://www.hydrol-earth-syst-sci-discuss.net/4/439/2007/hessd-4-439-2007.html
 | 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.tscm.com/alert.html
 | 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)

[[File: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.]]
Independent sovereign [[nation]]s claim the planet's entire 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.<ref name=cia /> 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.<ref>{{cite book
 | first=Paul | last=Kennedy
 | authorlink=Paul Kennedy | year=1989
 | title=[[The Rise and Fall of the Great Powers]]
 | edition=1st | publisher=Vintage
 | isbn=0679720197 }}</ref>

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.<ref>{{cite web
 | url=http://www.un.org/aboutun/charter/
 | title=U.N. Charter Index
 | publisher=United Nations
 | accessdate=2008-12-23 }}</ref> 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.

The first human to orbit the Earth was [[Yuri Gagarin]] on April 12, 1961.<ref>{{cite book
 | first=Betsy | last=Kuhn | year=2006
 | title=The race for space: the United States and the Soviet Union compete for the new frontier | page=34
 | publisher=Twenty-First Century Books | isbn=0822559846
}}</ref> In total, about 400 people visited [[outer space]] and reached Earth orbit as of 2004, and, of these, [[Apollo program|twelve]] have walked on the Moon.<ref>{{cite book
 | first=Lee | last=Ellis | year=2004 | title=Who's who of NASA Astronauts | publisher=Americana Group Publishing | isbn=0966796144
}}</ref><ref>{{cite book
 | first=David | last=Shayler | first2=Bert
 | last2=Vis | year=2005 | title=Russia's Cosmonauts: Inside the Yuri Gagarin Training Center
 | publisher=Birkhäuser | isbn=0387218947
}}</ref><ref>{{cite web
 | last=Wade | first=Mark | date=2008-06-30 | url=http://www.astronautix.com/articles/aststics.htm
 | accessdate=2008-12-23 | title=Astronaut Statistics
 | publisher=Encyclopedia Astronautica }}</ref> Normally the only humans in space are those on the [[International Space Station]]. The station's crew, currently six people, is usually replaced every six months.<ref>{{cite web
 | date=2007-01-16 | url=http://www.nasa.gov/mission_pages/station/news/ISS_Reference_Guide.html | title=Reference Guide to the International Space Station
 | publisher=NASA | accessdate=2008-12-23 }}</ref> Humans traveled the farthest from the planet in 1970, when [[Apollo 13]] crew was 400,171&nbsp;km away from Earth.<ref>{{cite news
 | first=Auslan | last=Cramb | publisher=Telegraph
 | title=Nasa's Discovery extends space station
 | date=2007-10-28 | url=http://www.telegraph.co.uk/earth/earthnews/3311903/Nasas-Discovery-extends-space-station.html
 | acc3essdate=2009-03-23
}}</ref><ref>{{cite web
 | first=Vic | last=Stathopoulos | date=2009-01-08
 | title=Apollo Spacecraft | url=http://www.aerospaceguide.net/spaceexploration/apollo.html
 | accessdate=2009-03-23 }}</ref>
{{-}}

==Cultural viewpoint==
{{main|Earth in culture}}
[[File:AS8-13-2329.jpg|thumb|right|The first photograph ever taken by astronauts of an "Earthrise", from [[Apollo 8]]]]
The name "Earth" was derived from the [[Old English|Anglo-Saxon]] word ''erda'', which means ground or soil. It became ''eorthe'' in [[Old English]], then ''erthe'' in [[Middle English]].<ref>{{cite book
 | month=July | year=2005
 | title=Random House Unabridged Dictionary
 | publisher=Random House | isbn=0-375-42599-3
 | author= }}</ref> The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.<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=281–282 | publisher=Ionfox AB
 | location=New York | isbn=91-972705-0-4 }}</ref>

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 deity]]. [[Creation myth]]s in many religions recall 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 | format=PDF }}</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
 | format=PDF}}</ref> assert that their [[Hermeneutics|interpretations]] of these creation myths 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
 | format=PDF}}</ref> Such assertions are opposed by the [[scientific community]]<ref>{{cite journal
 | author=Pennock, R. T.
 | title=Creationism and intelligent design
 | journal=Annu Rev Genomics Hum Genet | volume=4
 | 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 =Frye, Roland Mushat
 | 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|format=PDF}}</ref> A prominent example is the [[creation-evolution controversy]].

In the past there were varying levels of belief in a [[flat Earth]],<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> but this was displaced by the concept of a [[spherical Earth]] due to observation and circumnavigation.<ref>{{cite web
 | last = Jacobs | first = James Q. | date =1998-02-01
 | url =http://www.jqjacobs.net/astro/aegeo.html
 | title =Archaeogeodesy, a Key to Prehistory
 | accessdate = 2007-04-21 }}</ref> The human perspective regarding the Earth has changed following the advent of spaceflight, and the biosphere is now widely viewed from a globally integrated perspective.<ref>{{cite book
 | first=R. Buckminster | last=Fuller
 | authorlink=Buckminster Fuller | year=1963
 | title=[[Operating Manual for Spaceship Earth]]
 | edition=First | publisher=E.P. Dutton & Co.
 | location=New York | isbn=0-525-47433-1
 | url=http://www.futurehi.net/docs/OperatingManual.html
 | accessdate=2007-04-21 }}</ref><ref>{{cite book
 | first=James E. | last=Lovelock
 | authorlink=James Lovelock | year=1979
 | title=Gaia: A New Look at Life on Earth
 | edition=First | publisher=Oxford University Press
 | location=Oxford | isbn=0-19-286030-5 }}</ref>
This is reflected in a growing [[environmental movement]] that is concerned about humankind's effects on the planet.<ref>For example: {{cite book
 | first=Anthony J. | last=McMichael | year=1993
 | title=Planetary Overload: Global Environmental Change and the Health of the Human Species
 | publisher=Cambridge University Press
 | isbn=0521457599 }}</ref>
{{-}}

==See also==
{{portal|Earth sciences|Terra.png}}
{{portal|Solar System|Solar system.jpg}}
* [[:Template:Earth|List of Earth-related topics]]
* [[Topic outline of Earth science]]
** [[List of Earth science topics]]
* [[Topic outline of geography]]
** [[List of geography topics]]
* [[Topic outline of geology]]
** [[List of geology topics]]
*[[Eratosthenes#Eratosthenes' measurement of the Earth's circumference]]

==Notes==
<div class="references-small">
<references group=note/>
</div>

==References==
{{Reflist|2}}

==Bibliography==
* {{cite book | first=Neil F. | last=Comins | year=2001
 | title=Discovering the Essential Universe
 | edition=Second | publisher=W. H. Freeman
 | url=http://adsabs.harvard.edu/abs/2003deu..book.....C
 | accessdate=2007-03-17 | isbn=0-7167-5804-0 }}
* {{cite web | editor=Kirk Munsell | date = 2006-10-19
 | 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 = 2004-09-01
 | 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 | isbn=0875908519 }}

==External links==
{{sisterlinks|Earth}}
* [http://geomag.usgs.gov USGS Geomagnetism Program]
* [http://earthobservatory.nasa.gov NASA Earth Observatory]
* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Earth Earth Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://www.nasa.gov/centers/goddard/earthandsun/earthshape.html Climate changes cause Earth's shape to change - NASA]
* [http://eol.jsc.nasa.gov/Coll/weekly.htm The Gateway to Astronaut Photography of Earth]

{{Solar System}}
{{Earth}}
{{Earth's location}}
{{Physical Earth}}
{{Nature nav}}
{{featured article}}


[[Category:Earth| ]]
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