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Quite Harmless.
{{<!-- DON'T PANIC!
And don't bother putting "MOSTLY HARMLESS" or "HARMLESS" or any Hitchhiker's Guide reference in here. It's been done to death. Find a gem with which to improve the article and you'll shine forever.

But it really is Mostly Harmless...
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otheruses}}
{{Planet Infobox/Earth}}
'''Earth''', also known as '''Terra''', and (mostly in the [[19th century]]) '''[[Terra (mythology)|Tellus]]''', is the third-closest [[planet]] to the [[Sun]]. It is the largest of the [[solar system]]'s [[terrestrial planet]]s, and the only planetary body that modern [[science]] confirms as harboring [[life]]. Scientific evidence indicates that the planet [[Age of the Earth|formed]] around 4.57 [[billion]] (4.57{{e|9}}) [[year]]s ago, and shortly thereafter (4.533 [[billion]] years ago) acquired its single [[natural satellite]], the [[Moon]].

Its [[astronomical symbol]] consists of a circled cross, representing a [[meridian]] and the [[equator]]; a variant puts the cross atop the circle ([[Unicode]]: &#x2295; or {{unicode|&#x2641;}}). Besides words derived from ''Terra'', such as ''terrestrial'', terms that refer to the Earth include ''tellur-'' ([[telluric]], [[tellurian]], from the [[Ancient Rome|Roman]] [[goddess]] ''Tellūs'') and ''geo-'' ([[geocentric]], [[geothermal]]; from the [[Ancient Greece|Greek]] [[goddess]] [[Gaia (mythology)|Gaia]]).

The word Earth has cognates in many modern as well as defunct - including ancient - languages. Examples in modern tongues include ''aarde'' in Dutch and ''Erde'' in German. The root also has cognates in extinct languages such as ''ertha'' in Old Saxon and ''ert'' (meaning 'ground') in Middle Irish. It is derived from [[Old English]] ''eor&#0240;e''. Taking into account [[metathesis (linguistics)|metathesis]], we can find cognates of the word Earth in the Latin ''terra'' and in the modern [[Romance Languages]] (i.e. ''tierra'' in Spanish).

Although a link to such Indo-European languages has not been proved, several Semitic languages have similar-sounding words for Earth: ''aard'' in Arabic, ''irtsitu'' in Assyrian, ''araa'' in Aramaic, ''erets'' in Phoenician (which appears in the [[Mesha Stele]]), and ארץ (''arets'', or ''erets'' when followed by a noun modifier) in [[Hebrew language|Hebrew]].

== Physical characteristics ==
{{main|Structure of the Earth}} , see also [[Geology]] (science)

[[Image:Earth-crust-cutaway-english.png|thumb|150px|left|Earth cutaway from core to exosphere. Partially to scale]]

<!-- This section has been moved to the article '''[[Structure of the Earth]]'''. A new 30-line summary section must be written from this main article to this "Physical characteristics" section. Help is welcome. -->
The Earth consists of several [[atmospheric]], [[hydrologic]], and mainly [[geologic]] layers. Its components are the [[Earth's atmosphere|atmosphere]], the [[hydrosphere]], the [[crust]], the [[mantle (geology)|mantle]], and its [[core]]. The [[biosphere]] is a tiny layer in this composition and is usually not considered part of the physical layers of the Earth.

The geologic component layers of the Earth are located at the following depths below surface:

* 0 to 60 km - [[Lithosphere]] (locally varies between 5 and 200 km)
** 0 to 35 km - Crust (locally varies between 5 and 70 km)
** 35 to 60 km - Uppermost part of mantle
* 35 to 2890 km - [[Mantle (geology)|Mantle]]
** 100 to 700 km - [[Asthenosphere]]
* 2890 to 5100 km - Outer Core
* 5100 to 6378 km - Inner Core

== Earth in the solar system ==

It takes the Earth 23 hours, 56 minutes and 4.091 seconds ([[sidereal day|1 sidereal day]]) to rotate around the [[axis]] connecting the [[north pole]] and the [[south pole]]. From Earth the main apparent motion of celestial bodies in the sky (except [[meteor]]s which are within the atmosphere and low-orbiting satellites) is the movement to the west at a rate of 15 &deg;/h = 15'/min, i.e., a Sun or Moon diameter every two minutes.

Earth orbits the Sun every 365.2564 mean solar days ([[sidereal year|1 sidereal year]]). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of ca. 1 &deg;/day, i.e., a Sun or Moon diameter every 12 hours eastward.

The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is enough to cover one Earth diameter (~12,700 km) in 7 minutes, and one distance to the Moon (384,000 km) in 4 hours.

Earth has one [[natural satellite]], the [[Moon]], which orbits around Earth every [[month|27 1/3 days]]. From Earth this gives an apparent movement of the Moon with respect to the Sun and the stars at a rate of roughly 12 &deg;/day, i.e., a Moon diameter every hour eastward.

Viewed from Earth's north pole, the motion of Earth, its moon and their axial rotations are all [[counterclockwise]].

The orbital and axial planes are not precisely aligned: Earth's [[axial tilt|axis is tilted]] some 23.5 degrees against the Earth-Sun plane (which causes the [[season]]s); and the Earth-Moon plane is tilted about 5 degrees against the Earth-Sun plane (otherwise there would be an eclipse every month).

The [[Hill sphere]] (sphere of influence) of the Earth is about 1.5 Gm (930 thousand miles) in radius, within which one natural satellite (the [[Moon]]) comfortably orbits.

In an inertial reference frame, the Earth's axis undergoes a slow [[precession]]al motion with a period of some 25,800 years, as well as a [[nutation]] with a main period of 18.6 years. These motions are caused by the differential attraction of Sun and Moon on the equatorial bulge due to the Earth's oblateness. In a reference frame attached to the solid body of the Earth, its rotation is also slightly irregular due to [[polar motion]]. The polar motion is quasi-periodic, containing an annual component and a component with a 14-month period called the [[Chandler wobble]]. Also, the rotational velocity varies, a phenomenon known as [[Day|length of day]] variation.

In modern times, Earth's [[perihelion]] is always about [[January 3]], and [[aphelion]] is about [[July 4]]. For other eras, see [[precession]] and [[Milankovitch cycles]].

The Earth is sometimes referred to as the '''Third Rock from the Sun''' because, of the nine planets of our solar system, Earth is the third closest to the sun.

=== The Moon ===

[[Image:Earth-moon.jpg|thumb|''Earthrise'' as seen from the Moon on [[Apollo 8]], [[24 December]] [[1968]]]]
{{main|Moon}}
<center>
{| border="2" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; border: 1px #aaa solid; border-collapse: collapse; font-size: 95%;"
!style="background:#efefef;"|Name
!style="background:#efefef;"|Diameter (km)
!style="background:#efefef;"|Mass (kg)
!style="background:#efefef;"|[[Semi-major axis]] (km)
!style="background:#efefef;"|Orbital period
|-
|[[Moon]]
|align="center"|3,474.8
|7.349{{e|22}}
|align="center"|384,400
|27&nbsp;Days, 7&nbsp;hours, 43.7&nbsp;minutes
|}
</center>

The Moon, sometimes called 'Luna', is a relatively large terrestrial planet-like satellite, whose diameter is about one-quarter of the Earth's. With the exception of [[Pluto|Pluto's]] [[Charon]], it is the largest moon in the Solar system relative to the size of its planet. The [[natural satellite]]s orbiting other planets are called "moons", after Earth's Moon.

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

The Moon may dramatically affect the development of life by moderating the weather. Paleontological evidence and computer simulations show that Earth's [[axial tilt]] is stabilised by tidal interactions with the Moon. Some theorists believe that, without this stabilization against the [[torque]]s applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, as it appears to be with [[Mars (planet)|Mars]]. If Earth's axis of rotation were to approach the [[ecliptic|plane of the ecliptic]], extremely severe [[weather]] could result, as this would make seasonal differences extreme. 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. This remains a controversial subject, however, and further studies of Mars&mdash;which shares Earth's [[sidereal day|rotation period]] and [[axial tilt]], but not its large moon or liquid core&mdash;may provide additional insight.

The Moon is just far enough away to have, when seen from Earth, very nearly the same apparent angular size as the Sun (the Sun is 400 times larger, but the Moon is 400 times closer). This allows total [[eclipse]]s and annular eclipses to occur on Earth.

[[Image:Earth-Moon.jpg|thumb|center|700px|Earth and Moon to scale.]]

The most widely accepted theory of the Moon's origin, the [[Giant impact theory]], states that it was formed from the collision of a Mars-size [[protoplanet]] 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.

Earth also has at least one known co-orbital [[asteroid]], [[3753 Cruithne]].

== Geography ==
{{main|Geography}}

[[Image:Physical_world.jpg|thumb|333px|right|Physical map of the Earth ([[:Image:Physical_world.jpg|Medium]]) ([[:Image:World-map-2004-cia-factbook-large-2m.jpg|Large 2 MB)]]]]

'''Map references:'''

[[Time Zone]]s, [[Coordinate]]s.

'''Biggest geographic subdivision'''

[[Continent]]s, [[Ocean]]s

'''Area:'''

* ''Total:'' [[1 E14 m²|510.073 million]] [[square kilometre|km<sup>2</sup>]]
* ''[[Landform|land]]:'' 148.94 million km<sup>2</sup>
* ''[[Water]]:'' 361.132 million km<sup>2</sup>
* ''Note:'' 70.8 % of the world's surface is covered by water, 29.2 % is exposed land

'''Land boundaries:'''
the land boundaries in the world total 251,480 km (not counting shared boundaries twice)

'''Coastline:'''
356,000 km

'''Maritime claims:''' see [[United Nations Convention on the Law of the Sea]]

* ''Contiguous zone:'' 24 [[nautical mile|nautical miles]] (44.4 km) claimed by most, but can vary
* ''[[Continental shelf]]:'' 200 m depth claimed by most or to depth of exploitation; others claim 200 nautical miles (370.4 km) or to the edge of the continental margin
* ''Exclusive fishing zone:'' 200 nautical miles (370.4 km) claimed by most, but can vary
* ''Exclusive economic zone:'' 200 nautical miles (370.4 km) claimed by most, but can vary
* ''Territorial sea:'' 12 nautical miles (22.2 km) claimed by most, but can vary
* ''Note:'' boundary situations with neighboring states prevent many countries from extending their fishing or economic zones to a full 200 nautical miles (370.4 km)
* 42 nations and other areas are completely landlocked (see list of [[landlocked|landlocked countries]])

== Environment and Ecosystem ==
{{main|Biosphere}}, see also [[Life]].

Earth is the only place in the universe where life is known to exist. The planet's lifeforms are sometimes said to form a "[[biosphere]]". This biosphere is generally believed to have begun [[evolution|evolving]] about 3.5 billion (3.5{{e|9}}) years ago. The biosphere is divided into a number of [[biome]]s, inhabited by broadly similar [[flora (plants)|flora]] and [[fauna (animals)|fauna]]. On land, biomes are separated primarily by [[latitude]]. Terrestrial biomes lying within the [[Arctic Circle|Arctic]] and [[Antarctic Circle]]s are relatively barren of [[plant]] and [[animal]] life, while most of the more populous biomes lie near the [[Equator]].
[[Image:90 mile beach.jpg|thumb|right|220px|A familiar [[beach|scene]] on Earth which simultaneously shows the lithosphere, hydrosphere and atmosphere]]

=== Climate ===
{{main|Climate}}

Two large areas of polar [[climate]]s separated by two rather narrow [[temperate]] zones from a wide [[equator]]ial band of [[tropical]] to [[subtropical]] climates. [[precipitation (meteorology)|Precipitation]] patterns vary widely, ranging from several metres of water per year to less than a millimetre.

Ocean currents, particularly the spectacular [[thermohaline circulation]] which distributes heat energy from the equatorial oceans to the polar regions, are important determinators of climate.

=== Terrain ===
{{main|Extreme points of the world}}

'''Elevation extremes:''' (measured relative to [[sea level]])

* Lowest point on land: [[Dead Sea]] [[1 E2 m| &minus;417]] m
* Lowest point overall: [[Mariana Trench]] in the [[Pacific Ocean]] [[1 E4 m| &minus;10,924]] m <!-- Takuyo measurement; see Mariana Trench for details -->[http://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html]
* Highest point: [[Mount Everest]] [[1 E3 m|8,844]] m ([[2005]] est.)

=== Natural resources ===
{{main|Natural resource}}

* Earth's crust contains large deposits of [[fossil fuel]]s: ([[coal]], [[petroleum]], [[natural gas]], [[methane clathrate]]). These deposits are used by humans both for energy production and as feedstock for chemical production.
* Mineral [[ore]] bodies have been formed in Earth's crust by the action of [[erosion]] and [[plate tectonics]]. These ore bodies form concentrated sources for many [[metal]]s and other useful [[chemical element|element]]s.
* Earth's [[biosphere]] produces many useful biological products, 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.

Some of these resources, such as [[fossil fuel|mineral fuel]]s, are difficult to replenish on a short time scale, called [[non-renewable resources]]. The exploitation of non-renewable resources by human [[civilization]] has become a subject of significant controversy in modern [[environmentalism]] movements.

=== Land use ===

* ''Arable land:'' 10%
* ''Permanent crops:'' 1%
* ''Permanent pastures:'' 26%
* ''Forests and woodland:'' 32%
* ''Urban areas:'' 1.5%
* ''Other:'' 30% (1993 est.)

'''Irrigated land:'''
2,481,250 km<sup>2</sup> ([[1993]] est.)

=== 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, [[flood]]s, [[drought]]s, and other calamities and [[disaster]]s.

Large areas are subject to [[overpopulation]], industrial disasters such as [[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]].

Long-term [[climate]] [[global warming|alteration]] due to enhancement of the [[greenhouse effect]] by human industrial [[carbon dioxide]] emissions is an increasing concern, the focus of intense study and debate.

=== Human geography ===
[[Image:Earthlights dmsp.jpg|333px|thumb|right|Earth at night, composite of pictures taken between October 1994 and March 1995]]
{{main|Human}}
On [[25 February]] [[2005]] the [[United Nations Population Division]] issued revised estimates and projected that the [[world population|world's population]] will reach 7 billion by [[2013]] and swell to 9.1 billion in [[2050]]. Most of the growth is expected to take place in [[developing nations]].

Nearly all [[human]]s currently reside on Earth: 6,411,000,000 inhabitants ([[January 5]] [[2005]] est.). It is estimated that only 1/8th of the surface of the earth is suitable for [[human]]s to live on - 3/4 is covered by [[ocean]]s, and half of the landmass is unsuitable, being [[desert]], high [[mountain]], etc. Coastal areas constitute the highest density.

Two humans are presently in orbit around Earth on board the [[International Space Station]]. The station crew is replaced with new personnel every six months. During the exchange there are more, and sometimes others are also [[space flight|traveling briefly above the atmosphere]].

In total, about 400 [[astronaut|people]] have been outside Earth (in space) as of [[2004]]. Out of those only twelve humans have ever walked on a world other than Earth, the men of [[Apollo 11]], [[Apollo 12|12]], [[Apollo 14|14]], [[Apollo 15|15]], [[Apollo 16|16]] and [[Apollo 17|17]] who walked on the [[Moon]] between [[1969]] and [[1972]].

See also [[space colonization]].

The northernmost settlement in the world is [[Alert, Nunavut|Alert]], [[Ellesmere Island]], [[Canada]]. The southernmost is the [[Amundsen-Scott South Pole Station]], in [[Antarctica]], almost exactly at the [[South Pole]].

There are 267 administrative divisions, including nations, dependent areas, other, and miscellaneous entries. Earth does not have a [[sovereignty | sovereign]] [[government]] with planet-wide authority. Independent sovereign [[nation|nations]] claim all of the land surface except [[Antarctica]]. There is a worldwide general [[international organization]], the [[United Nations]]. The United Nations is primarily an international discussion forum with only limited ability to pass and enforce [[international law|law]]s.

== Descriptions of Earth ==

Earth has often been personified as a [[deity]], in particular a [[goddess]] (''see [[Gaia (mythology)|Gaia]] and [[Mother Earth]]''). The [[China|Chinese]] earth goddess [[Hu-Tu]], is similar to Gaia, the deification of the earth. The patroness of fertility, her element is earth. In [[Norse mythology]], the earth goddess [[Jord]] was the mother of [[Thor]] and the daughter of [[Annar]].

Since Earth is rather large, it is not immediately obvious to the naked eye viewing from the surface that it is an oblate spheroid, bulging slightly at the equator and slightly flattened at the poles. In the past there were varying levels of belief in a [[flat Earth]] because of this, but ancient Greek philosophers and, in the [[Middle Ages]], thinkers as [[Thomas Aquinas]] knew that the Earth was a sphere.

Prior to the introduction of [[space flight]], this belief was countered with deductions based on observations of the secondary effects of the earth's shape and parallels drawn with the shape of other planets. [[Cartography]], the study and practice of mapmaking, and vicariously [[geography]], have historically been the disciplines devoted to depicting the earth. [[Surveying]], the determination of locations and distances, and to a somewhat lesser extent [[navigation]], the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

The technological developments of the latter half of the [[20th century]] are widely considered to have altered the public's perception of the Earth. A photo taken of Earth by ''[[Voyager 1]]'' inspired [[Carl Sagan]] to describe the planet as a "[[Pale Blue Dot]]". Earth has also been described as a massive [[spaceship]], with a [[life support system]] that requires maintenance, or as having a [[biosphere]] that forms one large [[organism]]. ''See [[Spaceship Earth]] and [[Gaia theory]].''

For descriptions of the Earth in (science) fiction, see [[Earth in fiction]].

== See also ==

* '''Ecology''' [[Millennium Ecosystem Assessment]]
* '''Economy:''' [[World economy]]
* '''History'''
** [[Solar system#Origin and evolution of the solar system|Origin and evolution of the solar system]]
** [[Geologic time scale]]
** [[Timeline of evolution]]
** [[History|Human History]]
* '''Legal system:''' [[anarchy (word)|Anarchy]], [[International law]]
* '''Political:''' [[List of countries]]
* [[Geology]]
** [[Earthquake]]
** [[Plate tectonics]]
* [[Degree Confluence Project]]
* [[Equatorial bulge]]
* [[Earth in fiction]]
** [[Journey to the Center of the Earth]]
** [[Hollow Earth]]
* [[Extraterrestrial life]]
* [[Globus Cassus]]
* [[Extremes on Earth]]

== References ==

* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html NASA's Earth fact sheet]
* ''Discovering the Essential Universe'' (Second Edition) by Neil F. Comins (2001)
* [http://space.about.com/od/earth/a/earthinfo_2.htm space.about.com - Earth - Pictures and Astronomy Facts]

== External links ==
{{commons|Earth}}

* [http://geomag.usgs.gov USGS Geomagnetism Program]
* [http://www.geo.cornell.edu/geology/classes/isacks/velstruct.pdf] (pdf) - [[density]], [[pressure]], [[gravity]], P-wave and S-wave [[seismic wave]] velocities, and [[Poisson's ratio]] as a function of depth
* [http://worldwind.arc.nasa.gov/index.html Earth 3D Map from NASA] free downloadable (184.3 [[Megabyte|MB]]).
* [http://earth.google.com/ Google Earth] - [[google]] earth search system.
* [http://earthobservatory.nasa.gov/Newsroom/BlueMarble] Nasa Earth Observatory - the Blue Marble (very high resolution images)
{{Footer_SolarSystem}}

[[Category:Earth| ]]


<!-- The below are interlanguage links. -->
[[mt: Art]]
[[af:Aarde]]
[[ang:Eorðe]]
[[ar:أرض]]
[[ast:Tierra]]
[[bg:Земя (планета)]]
[[zh-min-nan:Tē-kiû]]
[[bn:পৃথিবী]]
[[ca:Terra]]
[[cs:Země]]
[[cy:Daear]]
[[da:Jorden]]
[[de:Erde]]
[[et:Maa (planeet)]]
[[el:Γη]]{{Link FA|el}}
[[es:Tierra]]
[[eo:Tero]]
[[eu:Lurra]]
[[fa:زمین]]
[[fr:Terre]]
[[fy:Ierde]]
[[gl:Terra (planeta)]]
[[gu:પૃથ્વી]]
[[ko:지구]]
[[hi:पृथ्वी]]
[[hr:Zemlja (planet)]]
[[io:Tero]]
[[id:Bumi]]
[[ilo:Daga (planeta)]]
[[ia:Terra]]
[[is:Jörðin]]
[[it:Terra]]
[[he:כדור הארץ]]
[[csb:Zemia]]
[[kw:Dor]]
[[sw:Ardhi]]
[[la:Terra]]
[[lv:Zeme]]
[[lb:Äerd]]
[[li:Eerd]]
[[ln:Mabelé]]
[[jbo:terdi]]
[[hu:Föld]]
[[ms:Bumi]]
[[mk:Земја]]
[[nl:Aarde (planeet)]]
[[ne:पृथ्वी]]
[[nds:Eer]]
[[ja:地球]]
[[no:Jorden]]
[[nn:Jorda]]
[[pl:Ziemia]]
[[pt:Terra]]{{Link FA|pt}}
[[ro:Pământ]]
[[ru:Земля (планета)]]
[[simple:Earth]]
[[sk:Zem]]
[[sl:Zemlja]]
[[sr:Земља (планета)]]{{Link FA|sr}}
[[su:Marcapada]]
[[fi:Maa]]
[[sv:Jorden]]
[[th:โลก]]
[[tl:Lupa (planeta)]]
[[tr:Yer (gezegen)]]
[[vi:Trái Đất]]
[[uk:Земля (планета)]]
[[zh:地球]]

Revision as of 11:49, 14 December 2005

Quite Harmless.

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