|
*I am an organic farmer and horticulturist in Southeastern Pennsylvania.
{{otheruses}}
*I'm mostly interested in working on Wiki pages that are in my field, since that's what I know best. In particular, I am interested in entries on plants, plant pests, plant diseases, and organic horticulture and agriculture.
{{Planet Infobox/Earth}}
*If you are interested in bolstering the plant/horticulture/agriculture/agronomy/organic part of the wikipedia, I'd love to hear from you. This is all very new to me.
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== On plant names, etc. ==
'''Earth''', also known as '''the Earth''', '''[[Terra (mythology)|Terra]]''', and (mostly in the [[19th century]]) '''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]]. The planet [[Age of the Earth|formed]] around 4.57 [[billion]] (4.57×10<sup>9</sup>) [[year]]s ago, and shortly thereafter (4.533 billion years ago) acquired its single [[natural satellite]], the [[Moon]].
I definitely lean towards using binomials ("latin names") for plants, insects, etc. This is mostly because most plants have many common names, and common names often refer to more than one plant. Using binomials might seem contrary to the "English" nature of the 'pedia, but it's relatively easy to point pages on wiki. It's also just because folks "in the trade" (like me) frequently use binomials and generic names in spoken language, e.g. one is much more likely to hear "[[Epimedium]]" than "[[Barrenwort]]", or "[[Pachysandra terminalis]]" rather than "[[Japanese Spurge]]".
Its [[astronomical symbol]] consists of a circled cross, representing a [[meridian]] and the [[equator]]; a variant puts the cross atop the circle ([[Unicode]]: ⊕ or ♁). Besides words derived from ''Terra'', such as ''terrestrial'', terms which refer to the Earth include ''tellur-'' ([[telluric]], [[tellurian]], from the goddess ''Tellūs'') and ''geo-'' ([[geocentric]], [[geothermal]]).
I also generally use binomials accepted "in the trade", rather than the newer binomials used by the botanists. Again, cross-linking is the key.
==Physical characteristics==
{{main|Geology}}
===Shape===
The Earth's shape is that of an [[oblate]] [[spheroid]], with an average diameter of approximately 12,742 km. The rotation of the Earth causes the equator to bulge out slightly so that the equatorial diameter is 43 km larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are [[Mount Everest]] (8,850 m above local sea level) and the [[Mariana Trench]] (10,911 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a [[tolerance (engineering)|tolerance]] of about one part in about 584, or 0.17%. For perspective, this is less than the 0.22% tolerance allowed in [[billiards|pool balls]]. Due to the bulge, the feature farthest from the center of the Earth is actually [[Mount Chimborazo]] in [[Ecuador]]. The mass of the Earth is approximately 5,980 yottagrams (5.98 x 10<sup>24</sup> kg).
==Horticulture-stub==
===Structure===
Hi Johnny - it's made now: {{:Horticulture-stub}} - [[User:MPF|MPF]] 21:53, 13 November 2005 (UTC)
The interior of Earth, like that of the other [[terrestrial planets]], is chemically divided into an outer [[silicon|siliceous]] solid [[Crust (geology)|crust]], a highly viscous [[Mantle (geology)|mantle]], a liquid outer core that is much less viscous than the mantle, and a solid inner core. The liquid outer core gives rise to a weak [[magnetosphere|magnetic field]] due to the convection of its electrically conductive [[material]].
New material constantly finds its way to the surface through volcanoes and cracks in the ocean floors (see [[seafloor spreading]]). Many of the rocks now making up the Earth's crust formed less than 100 million (1×10<sup>8</sup>) years ago, however the oldest known mineral grains are 4.4 billion (4.4×10<sup>9</sup>) years old, indicating that the Earth has had a solid crust for at least that long [http://spaceflightnow.com/news/n0101/14earthwater/].
Taken as a whole, the Earth's composition by [[mass]] [http://earthref.org/cgi-bin/er.cgi?s=erda.cgi?n=547] is:
<dl><dd>
{|cellspacing=0 cellpadding=0
|[[iron]]:||align="right"|34||.1||%
|-
|[[oxygen]]:||align="right"|28||.2||%
|-
|[[silicon]]:||align="right"|17||.2||%
|-
|[[magnesium]]:||align="right"|15||.9||%
|-
|[[nickel]]:||align="right"|1||.6||%
|-
|[[calcium]]:||align="right"|1||.6||%
|-
|[[aluminium]]:||align="right"|1||.5||%
|-
|[[sulfur]]:||align="right"|0||.70||%
|-
|[[sodium]]:||align="right"|0||.25||%
|-
|[[titanium]]:||align="right"|0||.071||%
|-
|[[potassium]]:||align="right"|0||.019 ||%
|-
|other elements: ||align="right"|0||.53||%
|}
</dd></dl>
===Interior===
====Interior heat====
{{main|geothermal (geology)}}
The interior of Earth reaches temperatures of 5650 +/- 600 [[kelvin]]s [http://www.es.ucl.ac.uk/people/d-price/papers/153.pdf] [http://www.carnegieinstitution.org/news_010905.html]. The planet's internal heat was originally generated during its accretion (see [[gravitational binding energy]]), and since then additional heat has continued to be generated by the decay of [[radioactive]] elements such as [[uranium]], [[thorium]], and [[potassium]]. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun.
====Structure====
Earth's composition (by depth below surface):
*0 to 60 km - [[Lithosphere]] (locally varies 5-200 km)
**0 to 35 km - Crust (locally varies 5-70 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
===The core===
The average density of Earth is 5515 [[kilogram|kg]]/[[metre|m<sup>3</sup>]], making it the densest planet in the Solar system. Since the average density of surface material is only around 3000 kg/m<sup>3</sup>, we must conclude that denser materials exist within the core of the Earth. In its earliest stages, about 4.5 billion (4.5×10<sup>9</sup>) years ago, melting would have caused denser substances to sink towards the center in a process called [[planetary differentiation]], while less dense materials would have migrated to the crust. As a result, the core is largely composed of iron (80%), along with nickel and one or more light elements; while other dense elements, such as lead and uranium, are either too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see: [[felsic|felsic materials]]).
The core is divided into two parts, a solid inner core with a [[radius]] of ~1250 km and a liquid outer core extending beyond it to a radius of ~3500 km. The inner core is generally believed to be solid and composed primarily of iron and some nickel. Some have argued that the inner core may be in the form of a single iron [[crystal]]. The outer core surrounds the inner core and is believed to be composed of liquid iron mixed with liquid nickel and trace amounts of lighter elements. It is generally believed that convection in the outer core, combined with stirring caused by the Earth's rotation (see: [[Coriolis effect]]), gives rise to the [[Earth's magnetic field]] through a process described by the [[dynamo theory]]. The solid inner core is too hot to hold a permanent magnetic field (see: [[Curie temperature]]) but probably acts to stabilise the magnetic field generated by the liquid outer core.
Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet. In August [[2005]] a team of [[geophysicist]]s announced in the journal ''[[Nature (journal)|Nature]]'' that according to their estimates the earth's core rotated approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface [http://www.nytimes.com/2005/08/25/science/25cnd-core.html].
<!--~0-2°per [[year]] (''[[Neil F. Comins|Comins]] DEU-p.82'')-->
[[Image:Earth-crust-cutaway-english.png|thumb|350px|left|Earth cutaway from core to exosphere. Partially to scale]]
===Mantle===
{{main|Mantle (geology)}}
[[Earth's mantle]] extends to a depth of 2890 km. The [[pressure]], at the bottom of the mantle, is ~140 G[[pascal|Pa]] (1.4 M[[atmospheric pressure|atm]]). It is largely composed of substances rich in iron and magnesium. The melting point of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is [[plasticity (physics)|plastic]] (semi-molten). The [[viscosity]] of the upper mantle ranges between 10<sup>21</sup> and 10<sup>24</sup> [[pascal second|Pa·s]], depending on depth [http://www2.uni-jena.de/chemie/geowiss/geodyn/poster2.html]. Thus, the upper mantle can only flow very slowly.
Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting points of iron-rich substances are higher than pure iron. The core is composed almost entirely of pure iron, while iron rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-molten (as it is hot and they are under relatively little pressure), lower mantle iron-substances are solid (as they are under tremendous pressure), outer core pure iron is liquid as it has a very low melting point (despite enormous pressure), and the inner core is solid due to the overwhelming pressure found at the center of the planet.
===The Crust===
The crust ranges from 5 to 70 km in depth. The thin parts are [[oceanic crust]] composed of dense ([[mafic]]) iron magnesium [[silicate]] [[Rock (geology)|rock]]s and underlie the ocean basins. The thicker crust is [[continental crust]] which is less dense and composed of ([[felsic]]) [[sodium]] potassium [[aluminium]] silicate rocks. The crust-mantle boundary occurs as two physically different events. Firstly, there is a discontinuity in the [[seismic wave|seismic]] velocity which is known as the [[Mohorovicic discontinuity|Mohorovičić discontinuity]] or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing [[feldspar|plagioclase feldspar]] (above) to rocks that contain no feldspars (below). The second event is a [[chemistry|chemical]] discontinuity between [[ultramafic]] cumulates and tectonized [[Peridotite|harzburgite]]s which has been observed from deep parts of the oceanic crust that have been [[obduction|obducted]] into the continental crust and preseved as [[ophiolites|ophiolite sequences]].
===Atmosphere===
{{main|Earth's atmosphere}}
Earth has a relatively thick [[Earth atmosphere|atmosphere]] composed of 78% [[nitrogen]], 21% [[oxygen]], and 1% [[argon]], plus traces of other gases including [[carbon dioxide]] and [[water (molecule)|water]] vapor. The atmosphere acts as a buffer between Earth and the Sun. The Earth's atmospheric composition is unstable and is maintained by the biosphere. Namely, the large amount of free diatomic oxygen is maintained through solar energy by the Earth's [[plants]], and without the plants supplying it, the oxygen in the atmosphere will over geological timescales combine with material from the surface of the Earth. Free oxygen in the atmosphere is a signature of life.
The layers, [[troposphere]], [[stratosphere]], [[mesosphere]], [[thermosphere]], and the [[exosphere]], vary around the globe and in response to seasonal changes.
The total mass of the atmosphere is about 5.1 × 10<sup>18</sup> kg, ca. 0.9 [[Parts per million|ppm]] of the Earth's total mass.
===Hydrosphere===
{{main|Ocean}}
Earth is the only planet in our [[solar system]] whose surface has liquid [[water (molecule)|water]]. Water covers 71% of Earth's surface (97% of it being sea water and 3% fresh water [http://earthobservatory.nasa.gov/Library/Water/]) and is divided into five oceans and seven [[continent]]s. Earth's [[orbit|solar orbit]], [[vulcanism]], [[gravity]], [[greenhouse effect]], [[magnetic field]] and oxygen-rich atmosphere seem to combine to make Earth a water planet.
Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a [[greenhouse effect]], Earth's water would freeze. [[Paleontology|Paleontological]] evidence indicates that at one point after blue-green bacteria ([[Cyanobacteria]]) had colonized the oceans, the greenhouse effect failed, and Earth's oceans may have completely frozen over for 10 to 100 million years in what is called a [[snowball Earth]] event.
On other planets, such as [[Venus]], gaseous water is destroyed (cracked) by solar [[ultraviolet]] radiation, and the [[hydrogen]] is [[ion]]ized and blown away by the [[solar wind]]. This effect is slow, but inexorable. This is one hypothesis explaining why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid [[mineral]]s.
In the Earth's atmosphere, a tenuous layer of [[ozone]] within the stratosphere absorbs most of this energetic ultraviolet radiation high in the atmosphere, reducing the [[cracking effect]]. The ozone, too, can only be produced in an atmosphere with a large amount of free diatomic oxygen, and so also is dependent on the biosphere ([[plant]]s). The [[magnetosphere]] also shields the [[ionosphere]] from direct scouring by the solar wind.
Finally, [[volcano|vulcanism]] continuously emits water [[vapor]] from the interior. Earth's [[plate tectonics]] recycle [[carbon]] and water as [[limestone]] rocks are [[subduction|subducted]] into the mantle and volcanically released as gaseous carbon dioxide and [[steam]]. It is estimated that the minerals in the mantle may contain as much as 10 times the water as in all of the current oceans, though most of this trapped water will never be released.
The total mass of the hydrosphere is about 1.4 × 10<sup>21</sup> kg, ca. 0.023 % of the Earth's total mass.
[[Image:Earth_satellite_plane.jpg|thumb|300px|right|A [[plate carrée projection]] of a composite satellite image of Earth]]
==Earth in the Solar System==
It takes 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 °/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 °/day, i.e. a Sun or Moon diameter every 12 hours eastward.
The orbital speed of the Earth averages about 30 km/s, 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 °/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 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 × 10<sup>22</sup>
|align="center"|384,400
|27 Days, 7 hours, 43.7 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. Without this stabilization against the [[torque]]s applied by the Sun and planets to the Earth's equatorial bulge, some theorists believe that 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—which shares Earth's [[sidereal day|rotation period]] and [[axial tilt]], but not its large moon or liquid core—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. Here is a diagram showing the relative sizes of the Earth and the Moon and the distance between the two (click to enlarge):
[[Image:Earth-Moon.jpg|thumb|center|499px|Earth and Moon to scale (click to enlarge)]]
The most widely accepted theory of the Moon's origin states that it was formed from the collision of a Mars-sized [[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. See [[giant impact theory]].
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>]]
*''[[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|Life}}
Earth is the only place 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 (3.5×10<sup>9</sup>) 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| −417]] m
*Lowest point overall: [[Mariana Trench]] in the [[Pacific Ocean]] [[1 E4 m| −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|8,850]] m ([[1999]] 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 population===
[[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'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 [[inhabitant]]s ([[January 5]] [[2005]] est.)
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]].
See also [[space colonization]].
<br style="clear:both;" />
{{Social Infobox/Human}}
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]].
'''Age structure:'''
*''0 to 14 years:'' 1,819,000,000 (29.9 %)
**''male:'' 932,800,000 (15.4 %)
**''female:'' 886,000,000 (14.6 %)
*''15 to 64 years:'' 3,841,000,000 (63.2 %)
**''male:'' 1,942,000,000 (32.0 %)
**''female:'' 1,898,000,000 (31.2 %)
*''65 years and over:'' 419,100,000 (6.9 %)
**''male:'' 184,100,000 (3.0 %)
**''female:'' 235,000,000 (3.9 %) ([[2000]] est.)
''' [[Population growth rate]]:'''
1.14% (2004 est.); 73 mln/year (200,000/day); 1 in 32,000 per day
'''[[Birth rate]]:'''
22 births/1,000 population (2000 est.); 140 mln/year; 1 in 17,000 per day
'''[[Death rate]]:'''
9 deaths/1,000 population (2000 est.); 60 mln/year; 1 in 41,000 per day
'''[[Sex ratio]]:'''
*''at birth:'' 1.05 male(s)/female
*''under 15 years:'' 1.05 male(s)/female
*''15 to 64 years:'' 1.02 male(s)/female
*''65 years and over:'' 0.78 male(s)/female
*''total population:'' 1.01 male(s)/female (2000 est.)
'''[[Infant mortality rate]]:'''
54 deaths/1,000 live births (2000 est.)
'''[[Life expectancy]] at birth:'''
*''total population:'' 64 years
*''male:'' 62 years
*''female:'' 65 years (2000 est.)
'''Total [[fertility rate]]:'''
2.8 children born/woman (2000 est.)
====Government====
Earth does not have a [[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.
'''Administrative divisions:'''
267 nations, dependent areas, other, and miscellaneous entries
<br style="clear:both;" />
==Descriptions of Earth==
Earth has often been personified as a [[deity]], in particular a [[goddess]]. See [[Gaia (mythology)|Gaia]] and [[Mother Earth]]. The chinese earth goddess [[Hu-Tu]], is similar to Gaia, the deification of the earth. The patroness of fertility, 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. 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 to somewhat lesser extent geography, the study of what is where on earth and why, have historically been the diciplines 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 requsite 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. See [[Spaceship Earth]].
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 of life]]
**[[History|Human History]]
* '''Legal system:''' [[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]]
==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/ earth.google.com] - [[google]] earth search system.
{{Footer_SolarSystem}}
[[Category:Earth| ]]
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