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| − | '''Earth''' is the third [[planet]] from the [[Sun]] and is the largest of the [[terrestrial planet]]s. Earth is also referred to as "the Earth", "Planet Earth", "[[Terra (mythology)|Terra]]", or "the [[World]]". | + | [[image:Earthandmoon2.jpg|right|300px]] |
| | + | '''Earth''' is the name for the flat plane on which humankind lives. It is the center of the universe around which other bodies, like the [[Sun]], revolve. |
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| − | This is the only planet known to have liquid [[water]] on the surface and the only place in the [[universe]] known to harbor [[life]]. Earth has a [[magnetic field]] that, together with a primarily [[nitrogen]]-[[oxygen]] atmosphere, protects the surface from [[radiation]] that is harmful to life. The atmosphere also serves as a shield that causes smaller [[meteor]]s to burn up before they strike the surface.
| + | Hellbound abortion-loving leftie scallions claim that the Earth is "round", and orbits at an average distance of about 93 million miles (150 million km) from the Sun in an almost circular orbit. It takes light (and other forms of electromagnetic radiation) approximately 500 seconds to travel from the Sun to the Earth, i.e. the distance can also be stated as "500 light-seconds." The distance to nearby stars is made by measuring the stellar [[parallax]] between observations when the earth is at opposite ends of its orbit, so the earth's orbit itself is a measuring stick for astronomical distances, and is known as the ''astronomical unit'' (A. U.) It is worth noting that if the Earth were slightly closer to the Sun, it would be too hot for life (just as Venus is) while if Earth were slightly further away from the Sun, water would freeze and life would be impossible. Many [[Christian|Christians]] believe that the Earth is the perfect distance away from the Sun and take this to be evidence of [[God]]'s existence. <ref>http://www.everystudent.com/features/isthere.html</ref> |
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| − | The Earth formed around [[Age of the Earth|4.57 billion years]] ago<ref name="age_earth">{{cite book | first=G.B. | last=Dalrymple | year=1991 | title=The Age of the Earth | publisher=Stanford University Press | location=California | id=ISBN 0-8047-1569-6 }}</ref> and its only known [[natural satellite]], the [[Moon]], began orbiting it around 4.53 billion years ago. At present the Earth orbits the Sun once for every roughly 365.25 times it rotates about its axis. The axial tilt of 23.4° produces seasonal variations on the surface. | + | The Earth is approximately 8000 miles (13000 km) in diameter, so its circumference is approximately 8000·π = about 25,000 miles (40,000 km). Its surface area is approximately 4·π (4000 mi)<sup>2</sup> = 200 million square miles (510 million km<sup>2</sup>). It has been said that Earth should really be called "water:" most of the Earth's surface, about 70%, is covered by water. In fact, the Pacific and Indian oceans alone cover about half of the Earth. |
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| − | [[Earth's atmosphere|Atmospheric]] conditions on Earth have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered in salt-water [[ocean]]s, and the remainder consists of continents and islands. The [[Crust (geology)|outer surface]] is divided into several [[tectonic plate]]s that gradually migrate across the surface over [[geologic time]] spans. The interior of the planet remains active, with a thick layer of convecting yet solid [[Mantle (geology)|mantle]], a liquid outer core that generates a [[magnetic field]], and a solid-iron inner core.
| + | Estimates by geologists of the age of the Earth give about 4.55 billion years to an accuracy of about 1%. This estimate is primarily based on radioactive dating of meteorites. In addition, most scientists also believe that the Earth formed by natural processes instead of being supernaturally created. Indeed, astronomers have detected dust lanes around other stars that indicate planet formation is a common and ongoing process. |
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| − | The space environment interacts with the Earth to a significant degree. The relatively large moon provides ocean [[tide]]s, stabilizes the [[Axial_tilt|axial tilt]] and has gradually modified the length of the planet's rotation period. A [[comet]]ary bombardment during the early history of the planet played a role in the formation of the oceans. Later, [[asteroid]] impacts caused significant changes to the surface environment. Long term [[Milankovitch cycles|periodic changes]] in the orbit of the planet are believed to have caused the [[ice age]]s that have covered significant portions of the surface in glacial sheets.
| + | Bible scholars have also estimated the age of the earth based on the Creation account in Genesis and the genealogical accounts in Numbers and other books of the Pentateuch. One famous estimate was published in 1650 by R. B. Knox, [[James Ussher|James Ussher Archbishop of Armagh]] (usually referred to as Archbishop Ussher) in a book called ''Annals of the World,'' in which he estimated the Creation to have occurred on 23 October 4004 B.C. Other Biblical scholars maintain that there are possible gaps in the genealogies, often using the ideas of the 19th century Calvinist theologian [[Benjamin Warfield]] on the issue. <ref>http://www.reasons.org/resources/apologetics/primeval_chronology.shtml</ref> However, [[James Barr]], regius professor of Hebrew at [[Oxford University]], wrote in 1984 the following: "… probably, so far as I know, there is no professor of Hebrew or Old Testament at any world-class university who does not believe that the writer(s) of Genesis 1–11 intended to convey to their readers the ideas that: … the figures contained in the Genesis genealogies provided by simple addition a chronology from the beginning of the world up to later stages in the biblical story.’" <ref>http://www.creationontheweb.com/content/view/1606/</ref> However, it should be noted that Barr himself rejects supernatural Christianity. <ref>http://www.reasons.org/resources/apologetics/pca_creation_study_committee_report.shtml</ref> |
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| − | ==History==
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| − | {{main|History of Earth}}
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| − | Based on the available evidence, current scientists have been able to reconstruct detailed information about the planet's past. Earth formed 4.567 billion years ago<ref name="age_earth" /> out of the [[solar nebula]], along with the Sun and the other planets. Initially [[molten]], the outer layer of the planet cooled to form a solid crust when water began accumulating in the atmosphere. The moon formed soon afterwards, possibly as the result of a Mars-sized object 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> known as [[Giant impact hypothesis|Theia]], impacting the Earth in a glancing blow.<ref>{{cite journal | last = R. Canup and E. Asphaug | title = Origin of the Moon in a giant impact near the end of the Earth's formation | journal = Nature | volume = 412 | pages = 708-712 | date = 2001 }}</ref> Most of this object's mass merged with the Earth, slightly increasing the
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| − | the planet's radius.
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| − | Outgassing and [[Volcano|volcanic]] activity produced the primordial atmosphere. Condensing [[water vapor]], augmented by ice delivered by [[comet]]s, [[Origin of the world's oceans|produced the oceans]].<ref>{{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 highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the [[last universal common ancestor|last common ancestor of all life]] existed.<ref>{{cite journal | last=Doolittle | first=W. Ford | title=Uprooting the tree of life | journal=Scientific American | date=February , 2000 | volume=282 | issue=6 | pages=90-95 }}</ref>
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| − | The development of [[photosynthesis]] allowed the sun's energy to be harvested directly by life forms; the resultant [[oxygen]] accumulated in the atmosphere and gave rise to the [[ozone layer]]. The incorporation of smaller cells within larger ones resulted in the [[endosymbiotic theory|development of complex cells]] called [[eukaryotes]].<ref>{{cite journal | author=Berkner, L. V.; Marshall, L. C. | title= On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere | journal=Journal of Atmospheric Sciences | year=1965 | volume=22 | issue=3 | pages=225-261 | url=http://adsabs.harvard.edu/abs/1965JAtS...22..225B | accessdate=2007-03-05 }}</ref> True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful [[ultraviolet radiation]] by the ozone layer, life colonized the surface of Earth.<ref>{{cite web | last = Burton | first = Kathleen | date = [[November 29]], [[2000]] | url = http://www.nasa.gov/centers/ames/news/releases/2000/00_79AR.html | title = Astrobiologists Find Evidence of Early Life on Land | publisher = NASA | accessdate = 2007-03-05 }}</ref>
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| − | Over hundreds of millions of years, continents formed and broke up as the surface of Earth continually reshaped itself. The continents have migrated across the surface of the Earth, occasionally combining to form a [[supercontinent]]. Roughly 750 million years ago (mya), the earliest known supercontinent [[Rodinia]], began to break apart. The continents later recombined to form [[Pannotia]], 600–540 mya, then finally [[Pangaea]], which broke apart 180 mya.<ref>{{cite journal | author=Murphy, J. B.; Nance, R. D. | title=How do supercontinents assemble? | journal=American Scientist | year=1965 | volume=92 | pages=324–33 | url=http://scienceweek.com/2004/sa040730-5.htm | accessdate=2007-03-05 }}</ref>
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| − | Since the 1960s, it has been hypothesized that severe [[Glacier|glacial]] action between [[1 E16 s|750 and 580 mya]], during the [[Neoproterozoic]], covered much of the planet in a sheet of ice. This hypothesis has been termed "[[Snowball Earth]]", and is of particular interest because it preceded the [[Cambrian explosion]], when multicellular life forms began to proliferate.<ref>{{cite book | last=Kirschvink | first=J. L. | editors=Schopf, J.W.; Klein, C. | year=1992 | title=The Proterozoic Biosphere: A Multidisciplinary Study | pages=51-52 | publisher=Cambridge University Press | id=ISBN 0521366151 }}</ref>
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| − | Following the [[Cambrian explosion]], about 535 mya, there have been five [[Extinction event|mass extinctions]].<ref>{{cite journal | author=Raup, D. M.; Sepkoski, J. J. | title=Mass Extinctions in the Marine Fossil Record | journal=Science | year=1982 | volume=215 | issue=4539 | pages=1501-1503 | url=http://adsabs.harvard.edu/abs/1982Sci...215.1501R | accessdate=2007-03-05 }}</ref> The last extinction event occurred 65 mya, when a meteorite collision probably triggered the extinction of the (non-avian) [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s, which then resembled shrews. Over the past 65 mya, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright.<ref>{{cite journal | last = Gould | first = Stephan J. | title=The Evolution of Life on Earth | journal=Scientific American | date=October , 1994 | url=http://brembs.net/gould.html | accessdate=2007-03-05 }}</ref> This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had, affecting both the nature and quantity of other life forms.
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| − | The present pattern of [[ice age]]s began about 40 mya, then intensified during the [[Pleistocene]] about 3 mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.<ref>{{cite web | author=Anonymous | url = http://www.lakepowell.net/sciencecenter/paleoclimate.htm | title = Paleoclimatology - The Study of Ancient Climates | publisher = Page Paleontology Science Center | accessdate = 2007-03-02 }}</ref>
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| − | {{-}}
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| − | ==Composition and structure==
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| − | [[Image:Terrestrial planet size comparisons.jpg|thumb|right|300px|Size comparison of terrestrial planets (left to right): [[Mercury (planet)|Mercury]], Venus, Earth, and [[Mars]]]]
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| − | ===Shape===
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| − | {{main|Figure of the Earth}}
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| − | The Earth's shape is very close to an [[oblate]] [[spheroid]]—a rounded shape with a bulge around the [[equator]]—although the precise shape (the [[geoid]]) varies from this by up to 100 metres (327 ft).<ref>{{cite web | author=Milbert, D. G.; Smith, D. A. | url = http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html | title = Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model | publisher = National Geodetic Survey, NOAA | accessdate = 2007-03-07 }}</ref> The average diameter of the reference spheroid is about 12,742 km (7,913 mi). More approximately the distance is 40,000 km/[[pi|π]] because the [[metre]] was originally defined as 1/10,000,000 of the distance from the equator to the [[north pole]] through [[Paris]], [[France]].
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| − | The [[rotation]] of the Earth creates the [[equator]]ial bulge so that the equatorial diameter is 43 km (27 mi) larger than the [[Geographical pole|pole]] to pole diameter. The largest local deviations in the rocky surface of the Earth are [[Mount Everest]] (8,848 m [29,028 ft] above local [[sea level]]) and the [[Mariana Trench]] (10,911 m [35,798 ft] below local sea level). Hence compared to a perfect [[ellipsoid]], the Earth has a [[tolerance (engineering)|tolerance]] of about one part in about 584, or 0.17%. For comparison, this is less than the 0.22% tolerance allowed in [[billiard ball]]s.<ref>{{cite web | date = November, 2001 | url = http://www.wpa-pool.com/index.asp?content=rules_spec | title = WPA Tournament Table & Equipment Specifications | publisher = World Pool-Billiards Association | accessdate = 2007-03-10 }}</ref> Because of the bulge, the feature farthest from the center of the Earth is actually [[Chimborazo (volcano)|Mount Chimborazo]] in [[Ecuador]].<ref>{{cite journal | last = Senne | first = Joseph H. | title=Did Edmund Hillary Climb the Wrong Mountain | journal=Professional Surveyor | year=2000 | volume=20 | issue=5 | url=http://www.profsurv.com/archive.php?issue=42&article=589 | accessdate=2007-02-04 }}</ref>
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| − |
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| − | ===Chemical composition===
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| − | {{seealso|Abundance of the chemical elements#Abundance of elements on Earth}}
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| − |
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| − | {| {{prettytable}} style="float: right;"
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| − | |+ F. W. Clarke's Table of Crust Oxides
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| − | !Compound
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| − | !Formula
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| − | !Composition
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| − | |-
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| − | |[[silica]]
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| − | |style="text-align: center;"|SiO<sub>2</sub>
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| − | |style="text-align: right;"|59.71%
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| − | |-
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| − | |[[alumina]]
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| − | |style="text-align: center;"|Al<sub>2</sub>O<sub>3</sub>
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| − | |style="text-align: right;"|15.41%
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| − | |-
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| − | |[[Calcium oxide|lime]]
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| − | |style="text-align: center;"|CaO
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| − | |style="text-align: right;"|4.90%
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| − | |-
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| − | |[[Magnesia (mineral)|Magnesia]]
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| − | |style="text-align: center;"|MgO
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| − | |style="text-align: right;"|4.36%
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| − | |-
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| − | |[[sodium oxide]]
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| − | |style="text-align: center;"|Na<sub>2</sub>O
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| − | |style="text-align: right;"|3.55%
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| − | |-
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| − | |[[iron(II) oxide]]
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| − | |style="text-align: center;"|FeO
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| − | |style="text-align: right;"|3.52%
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| − | |-
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| − | |[[potassium oxide]]
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| − | |style="text-align: center;"|K<sub>2</sub>O
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| − | |style="text-align: right;"|2.80%
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| − | |-
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| − | |[[iron(III) oxide]]
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| − | |style="text-align: center;"|Fe<sub>2</sub>O<sub>3</sub>
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| − | |style="text-align: right;"|2.63%
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| − | |-
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| − | |[[water (molecule)|water]]
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| − | |style="text-align: center;"|H<sub>2</sub>O
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| − | |style="text-align: right;"|1.52%
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| − | |-
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| − | |[[titanium dioxide]]
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| − | |style="text-align: center;"|TiO<sub>2</sub>
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| − | |style="text-align: right;"|0.60%
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| − | |-
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| − | |[[phosphorus pentoxide]]
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| − | |style="text-align: center;"|P<sub>2</sub>O<sub>5</sub>
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| − | |style="text-align: right;"|0.22%
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| − | |-
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| − | !colspan="2"|Total
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| − | !style="text-align: right;"|99.22%
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| − | |}
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| − |
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| − | The [[mass]] of the Earth is approximately 5.98 {{e|24}} kg. It is composed mostly of [[iron]] (32.1%), [[oxygen]] (30.1%), [[magnesium]] (13.9%), [[aluminum]] (1.4%), [[silicon]] (15.1%), [[sulfur]] (2.9%), [[calcium]] (1.5%), and [[nickel]] (1.8%), 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%),
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| − | and less than 1% trace elements.<ref>{{cite journal | author=Morgan, J. W.; Anders, E. | title=Chemical composition of Earth, Venus, and Mercury | journal=Procedings of the National Academy of Science | year=1980 | volume=71 | issue=12 | pages=6973–6977 | url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=350422 | accessdate=2007-02-04 }}</ref>
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| − | The geochemist [[Frank Wigglesworth Clarke|F. W. Clarke]] calculated that a little more than 47% of the earth's crust consists of oxygen. The more common rock constituents of the [[Earth's crust]] are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.<ref name=EB1911>{{1911|article=Petrology}}</ref>
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| − | {{-}}
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| − |
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| − | ===Internal structure===
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| − | {{main|Structure of the Earth}}
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| − | [[Image:Earth layers model.png|thumb|right|Earth cutaway from core to exosphere. Partially to scale]]
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| − | [[Image:Slice_earth.svg|thumb|right|Schematic view of the interior of Earth. 1. continental crust - 2. oceanic crust - 3. upper mantle - 4. lower mantle - 5. outer core - 6. inner core - A: [[Mohorovičić discontinuity]] - B: [[Gutenberg Discontinuity]] - C: [[Lehmann discontinuity]]]]
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| − | The interior of the Earth, like that of the other [[terrestrial planets]], is [[chemical]]ly divided into layers. The Earth has an outer [[Silicate minerals|silicate]] solid [[Crust (geology)|crust]], a highly viscous [[Mantle (geology)|mantle]], a liquid [[outer core]] that is much less viscous than the mantle, and a solid [[inner core]].
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| − | The geologic component layers of the Earth<ref>T. H. Jordan, "[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=411539 Structural Geology of the Earth's Interior]", ''Proceedings National Academy of Science'', 1979, Sept., 76(9): 4192–4200.</ref> are at the following depths below the surface:
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| − |
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| − | {| class="wikitable"
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| − | !colspan=2|Depth
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| − | !rowspan=2 valign="bottom"|Layer
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| − | |-
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| − | !Kilometres
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| − | !Miles
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| − | |-
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| − | |style="text-align: center;"|0–60
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| − | |style="text-align: center;"|0–37
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| − | |[[Lithosphere]] (locally varies between 5 and 200 km)
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| − | |- style="background: #FEFEFE;"
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| − | |style="text-align: center;"|0–35
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| − | |style="text-align: center;"|0–22
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| − | |... [[Crust (geology)|Crust]] (locally varies between 5 and 70 km)
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| − | |- style="background: #FEFEFE;"
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| − | |style="text-align: center;"|35–60
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| − | |style="text-align: center;"|22–37
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| − | |... Uppermost part of mantle
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| − | |-
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| − | |style="text-align: center;"|35–2890
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| − | |style="text-align: center;"|22–1790
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| − | |[[Mantle (geology)|Mantle]]
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| − | |- style="background: #FEFEFE;"
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| − | |style="text-align: center;"|100–700
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| − | |style="text-align: center;"|62–435
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| − | |... [[Asthenosphere]]
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| − | |-
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| − | |style="text-align: center;"|2890–5100
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| − | |style="text-align: center;"|1790–3160
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| − | |Outer core
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| − | |-
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| − | |style="text-align: center;"|5100–6378
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| − | |style="text-align: center;"|3160–3954
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| − | |[[Inner core]]
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| − | |}
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| − |
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| − | The internal heat of the planet is most likely produced by the radioactive decay of [[Potassium|potassium-40]], [[Uranium|uranium-238]] and [[Thorium|thorium-232]]
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| − | [[isotope]]s. All three have [[half-life]] decay periods of more than a billion years.<ref>{{cite news | first=Robert | last=Sanders | title=Radioactive potassium may be major heat source in Earth's core | publisher=UC Berkeley News | date=December 10, 2003 | url=http://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml | accessdate=2007-02-28 }}</ref> At the center of the planet, the temperature may be up to
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| − | 7,000 K and the pressure could reach 360 [[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>
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| − |
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| − | ===Tectonic plates===
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| − | {{main|Plate tectonics}}[[Image:Tectonic plates.png|thumb|right|380px|A map pointing out the Earth's major plates.]]
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| − | According to '''plate tectonics theory''' currently accepted by the vast majority of scientists working in this area, the outermost part of the Earth's interior is made up of two layers: the [[lithosphere]] comprising the [[Crust (geology)|crust]], and the solidified uppermost part of the [[Earth's mantle|mantle]]. Below the lithosphere lies the [[asthenosphere]], which comprises the inner part of the mantle. The asthenosphere behaves like a superheated and extremely [[viscous]] liquid.<ref>{{cite web | date = February 27, 2004 | url = http://www.geolsoc.org.uk/template.cfm?name=lithosphere | title = Crust and Lithosphere | work = Plate Tectonics & Structural Geology | publisher = The Geological Survey | accessdate = 2007-03-11 }}</ref>
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| − | The lithosphere essentially ''floats'' on the [[asthenosphere]] and is broken up into what are called [[tectonic plate]]s. These plates move in relation to one another at one of three types of plate boundaries: [[Convergent boundary|convergent]], [[Divergent boundary|divergent]], and [[Transform fault|transform]]. [[Earthquake]]s, [[volcano|volcanic activity]], [[mountain]]-building, and [[oceanic trench]] formation occur along plate boundaries.<ref>{{cite web | author=Kious, W. J.; Tilling, R. I. | date = May 5, 1999 | url = http://pubs.usgs.gov/gip/dynamic/understanding.html | title = Understanding plate motions | publisher = USGS | accessdate = 2007-03-02 }}</ref>
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| − | The main plates are:<ref>{{cite web | author=Brown, W. K.; Wohletz,K. H. | year = 2005 | url = http://www.ees1.lanl.gov/Wohletz/SFT-Tectonics.htm | title = SFT and the Earth's Tectonic Plates | publisher = Los Alamos National Laboratory | accessdate = 2007-03-02 }}</ref>
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| − |
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| − | {| {{prettytable}}
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| − | !rowspan="2"|Plate name
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| − | !colspan="2"|Area
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| − | !rowspan="2"|Covering
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| − | |-
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| − | !10<sup>6</sup> km²
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| − | !10<sup>6</sup> mi²
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| − | |-
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| − | | [[African Plate]] ||style="text-align: center;"| 61.3 ||style="text-align: center;"| 23.7 || [[Africa]]
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| − | |-
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| − | | [[Antarctic Plate]] ||style="text-align: center;"| 60.9 ||style="text-align: center;"| 23.5 || [[Antarctica]]
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| − | |-
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| − | | [[Australian Plate]] ||style="text-align: center;"| 47.2 ||style="text-align: center;"| 18.2 || [[Australia]]
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| − | |-
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| − | | [[Eurasian Plate]] ||style="text-align: center;"| 67.8 ||style="text-align: center;"| 26.2 || [[Asia]] and [[Europe]]
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| − | |-
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| − | | [[North American Plate]] ||style="text-align: center;"| 75.9 ||style="text-align: center;"| 29.3 || [[North America]] and north-east [[Siberia]]
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| − | |-
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| − | | [[South American Plate]] ||style="text-align: center;"| 43.6 ||style="text-align: center;"| 16.8 || [[South America]]
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| − | |-
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| − | | [[Pacific Plate]] ||style="text-align: center;"| 103.3 ||style="text-align: center;"| 39.9 || [[Pacific Ocean]]
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| − | |}
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| − |
| |
| − | Notable minor plates include the [[Indian Plate]], the [[Arabian Plate]], the [[Caribbean Plate]], the [[Nazca Plate]] and the [[Scotia Plate]]. 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 88 mm/yr (3.5 in/yr) and the Pacific Plate moving 80 mm/yr (3.1 in/yr). At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a rate of 7 mm/yr (0.3 in/yr).<ref>{{cite web | first=Hilma | last=Miles | date = October 27, 2003 | url = http://www.bbm.me.uk/portsdown/PH_063_PlateTec.htm | title = The Theory of Plate Tectonics | accessdate = 2007-03-02 }}</ref>
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| − |
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| − | [[Image:AYool_topography_15min.png|320px|thumb|Present day [[Earth]] [[terrain|altimetry]] and [[bathymetry]]. Data from the [[National Geophysical Data Center]]'s [http://www.ngdc.noaa.gov/seg/fliers/se-1104.shtml TerrainBase Digital Terrain Model].]]
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| − | <!-- [[Image:Elevation flat polar quartic.jpg|320px|thumb|[[Map projection|Equal-area]] map of the earth colored to show elevation]] -->
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| − |
| |
| − | ===Surface===
| |
| − | {{main|Landforms|Extreme points of the world}}
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| − | The Earth's [[terrain]] varies greatly from place to place. About 70.8% 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 [[oceanic trench]]es, [[submarine canyon]]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]].
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| − |
| |
| − | The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and [[erosion]]. The surface features built up or deformed through plate tectonics are subject to steady [[weathering]] from [[precipitation]], thermal cycles, and chemical effects. [[Glaciation]], [[coastal erosion]], the build-up of [[coral reef]]s, and large meteorite impacts also act to reshape the landscape.
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| − |
| |
| − | 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]], [[Earth's atmosphere|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 | date = February 8, 2007 | url = https://www.cia.gov/cia/publications/factbook/geos/xx.html | title = The World Factbook | publisher = U.S. C.I.A. | accessdate = 2007-02-25 }}</ref> Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 × 10<sup>9</sup> [[acre]]s of cropland and 8.4 × 10<sup>9</sup> acres of pastureland.<ref>FAO, 1995, "United Nations Food and Agricultural Organization Production Yearbook", 49.</ref>
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| − |
| |
| − | [[Image:Earth elevation histogram.svg|thumb|350px|Elevation [[histogram]] of the surface of the Earth—approximately 71% of the Earth's surface is covered with water.]]
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| − |
| |
| − | The elevation of the land surface of the Earth varies from the low point of [[1 E2 m|−418]] m (−1,371 ft) at the [[Dead Sea]], to a 2005-estimated maximum altitude of [[1 E3 m|8,848]] m (29,028 ft) at the top of [[Mount Everest]]. The mean height of land above sea level is 686 m (426 ft).<ref name="hr_mill">{{cite journal | last = Mill | first = Hugh Robert | title=The Permanence of Ocean Basins | journal=The Geographical Journal | year=1893 | volume=1 | issue=3 | pages=230-234 | url=http://www.wku.edu/~smithch/wallace/S453.htm | accessdate=2007-02-25 }}</ref>
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| − |
| |
| − | ===Hydrosphere===
| |
| − | {{main|Hydrosphere}}
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| − |
| |
| − | The abundance of water on Earth surface is a unique feature that distinguishes the "[[Blue Planet]]" from others in the solar system. Approximately 70.8 percent of the Earth is covered by water and only 29.2 percent is terra firma—solid earth.
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| − |
| |
| − | 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 m. The deepest underwater location is Challenger Deep of the [[Mariana Trench]] in the [[Pacific Ocean]] with a depth of [[1 E4 m|−10,911]] m (35,798 ft or 6.78 mi).<ref name="rain.org">{{cite web | url = http://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html | title = "Deep Ocean Studies" | work = Ocean Studies | publisher = RAIN National Public Internet and Community Technology Center | accessdate = 2006-04-02
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| − | }}</ref><!-- Takuyo measurement; see Mariana Trench for details --> The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents.<ref name="hr_mill" />
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| − |
| |
| − | The mass of the oceans is approximately 1.35 × 10<sup>18</sup> tonnes, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386 × 10<sup>9</sup> km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km (approximately 1.7 mi).<ref>The total volume of the Earth's oceans is: 1.4 × 10<sup>9</sup> km³. The total surface area of the Earth is 5.1 × 10<sup>8<sup> km². So the average depth would be the ratio of the two, or 2.7 km.</ref> About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.<ref>{{cite web | author = Igor A. Shiklomanov ''et al'' | year = 1999 | url = http://espejo.unesco.org.uy/ | title = World Water Resources and their use Beginning of the 21st Century" Prepared in the Framework of IHP UNESCO | publisher = State Hydrological Institute, St. Petersburg | accessdate = 2006-08-10 }}</ref>
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| − |
| |
| − | ===Atmosphere===
| |
| − | {{main|Earth's atmosphere|Climate}}
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| − | {{seealso|Outer space}}
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| − |
| |
| − | 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 km (about 4 mi) of the planet's surface. This lowest layer is called the [[troposphere]]. Further up, the atmosphere is usually divided into the [[stratosphere]], [[mesosphere]], and [[thermosphere]]. Beyond these, the [[exosphere]] thins out into the [[magnetosphere]] (where the Earth's magnetic fields interact with the [[solar wind]]).<!--This implies that it doesn't interact with the solar wind at lower altitudes; is that right? Ans.: Yes, but there are indirect effects.--> 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 a 100 km (62 mi) above the Earth's surface, is a working definition for the boundary between atmosphere and space.
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| − |
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| − | [[Image:Full moon partially obscured by atmosphere.jpg|thumb|right|This view from orbit shows the full moon partially obscured by the Earth's atmosphere. ''NASA image.]]
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| − |
| |
| − | The [[atmospheric pressure]] on the surface of the Earth averages 101.325 [[kPa]], with a [[scale height]] of about 6 km. It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The atmosphere protects the Earth's life forms by absorbing [[ultraviolet]] [[solar radiation]], moderating temperature, transporting water vapor, and providing useful gases.
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| − |
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| − | Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can [[escape velocity|escape]] from the planet's gravity. This results in a slow but steady [[Atmospheric escape|leakage of the atmosphere into space]]. Because unfixed [[hydrogen]] has a low molecular weight, it can achieve [[escape velocity]] more readily and it leaks into outer space at a greater rate.<ref>{{cite journal | author=Liu, S. C.; Donahue, T. M. | title=The Aeronomy of Hydrogen in the Atmosphere of the Earth | journal=Journal of Atmospheric Sciences | year=1974 | volume=31 | issue=4 | pages=1118-1136 | url=http://adsabs.harvard.edu/abs/1974JAtS...31.1118L | accessdate=2007-03-02 }}</ref> For this reason, the Earth's environment is [[oxidizing]], with consequences for the [[chemical]] nature of [[life]] which developed on the planet.
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| − |
| |
| − | The atmosphere is one of the principal components in determining [[weather]] and [[climate]]. Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit, this water condenses and settles to the surface as [[precipitation]]. 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.
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| − |
| |
| − | The most prominent features of the Earth's climate are the climate zones, which occur in matching bands of latitude on each side of the equator.
| |
| − |
| |
| − | {| {{prettytable}} style="margin-left: auto; margin-right: auto;"
| |
| − | !Climate<br />zone
| |
| − | !Approximate<br />latitude
| |
| − | !Average<br />temperature
| |
| − | |-
| |
| − | | [[Polar region]] || Poles to [[polar circle]]s ||style="text-align: center;"| 0°C
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| − | |-
| |
| − | | [[Temperate]] || Polar circles to 40° ||style="text-align: center;"| 8°C
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| − | |-
| |
| − | | [[Subtropics]] || 40º to 23.5° ||style="text-align: center;"| 16°C
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| − | |-
| |
| − | | [[Tropics]] || 23.5º to [[equator]] ||style="text-align: center;"| 24°C
| |
| − | |}
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| − |
| |
| − | [[precipitation (meteorology)|Precipitation]] patterns vary widely, ranging from several metres of water per year to less than a millimetre. [[Atmospheric circulation]], topological features and temperature differences determine the average precipitation that falls in each region.
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| − |
| |
| − | Ocean currents are important factors in determining climate, particularly the [[thermohaline circulation]] which distributes heat energy from the equatorial oceans to the polar regions.
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| − |
| |
| − | ===Magnetic field===
| |
| − | {{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 a field reversal about once every 700,000 years.<ref>{{cite web | last = Fitzpatrick | first = Richard | date = [[February 16]], [[2006]] | url = http://farside.ph.utexas.edu/teaching/plasma/lectures/node69.html | title = MHD dynamo theory | publisher = NASA WMAP | accessdate = 2007-02-27 }}</ref>
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| − |
| |
| − | The field forms the [[magnetosphere]], which deflects particles in the [[solar wind]]. The [[bow shock]] is located about at 13.5 ''R''<sub>E</sub>, or Earth radii. 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<!--Check: energetic is different from charged?-->. When the [[plasma (physics)|plasma]] enters the Earth's atmosphere at the magnetic poles, it forms the [[Aurora (astronomy)|aurora]].
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| − | {{-}}
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| − |
| |
| − | ==Orbit and rotation==
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| − | [[Image:Rotating earth (small).gif|thumb|right|165px|An animation showing the rotation of the Earth.]]
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| − | It takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds ([[sidereal day|one sidereal day]]) to rotate around the [[Axis of rotation|axis]] that connects the [[north pole|north]] and the [[south pole]]s. From Earth, the main apparent motion of celestial bodies in the sky (except that of [[meteor]]s within the atmosphere and low-orbiting satellites) is to the west at a rate of 15°/h = 15'/min, i.e., an apparent Sun or Moon diameter every two minutes.
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| − |
| |
| − | Earth orbits the Sun at an average distance of about 150 million kilometres (93.2 million miles) 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 about 1°/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 or 67,000 mi/h), which is enough to cover the planet's diameter (~12,600 km or ~7,800 mi) in seven minutes, and the distance to the Moon (384,000 km or 238,000 mi) in four hours.
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| − |
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| − | [[image:PaleBlueDot.jpg|thumb|left|195px|[[Pale Blue Dot|Earth seen as a tiny dot]] by the [[Voyager 1]] spacecraft, four billion miles from Earth]]
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| − | The [[Moon]] revolves with the Earth around a common [[barycenter]], from fixed star to fixed star, every 27.32 days. 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 days. The [[Hill sphere]] ([[gravity|gravitational]] sphere of influence) of the Earth is about 1.5 Gm (930,000 miles) in radius.
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| − | 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 (without a tilt, there would be an eclipse every two weeks, alternating between [[lunar eclipse]]s and [[solar eclipse]]s).
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| − |
| |
| − | The axial tilt of the Earth causes the [[season]]s. 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|equinoxes]], when the tilt is minimized. 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]].
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| − |
| |
| − | In an inertial reference frame, the Earth's axis undergoes a slow [[precession]] 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 Earth's equatorial bulge because of its oblateness. In a reference frame attached to the solid body of the Earth, its rotation is also slightly irregular from [[polar motion]]. The polar motion is quasi-periodic, containing an annual component and a component with a 14-month period called the [[Chandler wobble]]. In addition, the rotational velocity varies, in a phenomenon known as [[Day|length of day]] variation.
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| − |
| |
| − | In modern times, Earth's [[perihelion]] occurs around [[January 3]], and the [[aphelion]] around [[July 4]]. For other eras, see [[precession]] and [[Milankovitch cycles]].
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| − |
| |
| − | ==Observation==
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| − | Earth was first photographed from space by [[Explorer 6]] in 1959.<ref>{{cite web | author=Anonymous | date = October, 1998 | url = http://www.nasa.gov/centers/goddard/pdf/106420main_explorers.pdf | format=PDF | title = Explorers: Searching the Universe Fourty Years Later | publisher = NASA/Goddard | accessdate = 2007-03-05 }}</ref> [[Yuri Gagarin]] became the first human to view Earth from space in 1961. The crew of the [[Apollo 8]] was the first to view an earth-rise from lunar orbit in 1968. In 1972 the crew of the [[Apollo 17]] produced the famous "[[The Blue Marble|Blue Marble]]" photograph of the planet Earth (see top of page). NASA archivist Mike Gentry has speculated that "The Blue Marble" is the most widely distributed image in human history.
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| − |
| |
| − | [[Image:Earth and Moon from Mars PIA04531.jpg|200px|thumb|left|Earth and Moon from Mars, imaged by [[Mars Global Surveyor]].]]
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| − |
| |
| − | From [[Outer space|space]], the Earth can be seen to go through phases similar to the phases of the [[lunar phases|Moon]] and Venus. This appearance is caused by light that reflects off the Earth as it moves around the [[Sun]]. The phases seen depend upon the observer's location in space, and the rate is determined by their orbital velocity. The phases of the Earth can be simulated by shining light on a globe of the Earth.
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| − |
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| − | A observer on Mars would be able to see the Earth go through phases similar to those that an Earth-bound observer sees the [[phases of Venus]] (as discovered be [[Galileo]]). It can be shown that an imaginary observer on the [[Sun]] would not see the Earth going through phases. The sun observer would only be able to see the lit side of the earth.
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| − |
| |
| − | ==Moon==
| |
| − | {{main|Moon}}
| |
| − | {| class="wikitable"
| |
| − | !Name || Diameter || Mass || [[Semi-major axis]] || Orbital period
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| − | |-
| |
| − | |rowspan="2"|[[Moon]]
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| − | | style="text-align: center" | 3,474.8 km
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| − | | style="text-align: center" | 7.349{{e|22}} km
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| − | | style="text-align: center" | 384,400 km
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| − | |rowspan="2"|27 days, 7 hours, 43.7 minutes
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| − | |-
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| − | | style="text-align: center" | 2,159.2 mi
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| − | | style="text-align: center" | 8.10{{e|19}} tons
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| − | | style="text-align: center" | 238,700 mi
| |
| − | |}
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| − | [[Image:Earth-moon.jpg|thumb|right|''Earthrise'' as seen from lunar orbit on ''[[Apollo 8]]'', [[24 December]] [[1968]].]]
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| − | The Moon, sometimes called 'Luna', is a relatively large, 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 [[dwarf planet]] [[Pluto]].) The [[natural satellite]]s orbiting other planets are called "moons", after Earth's Moon.
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| − |
| |
| − | The gravitational attraction between the Earth and Moon cause [[tides]] on Earth. The same effect on the Moon has led to its [[tidal locking]]: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the [[lunar phase]]s: The dark part of the face is separated from the light part by the [[terminator (solar)|solar terminator]].
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| − |
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| − | Because of their [[Tidal acceleration|tidal interaction]], the Moon recedes from Earth at the rate of approximately 38 [[Millimetre|mm]] (1.5 in) a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 17 [[Microsecond|µs]] a year—add up to significant changes. During the [[Devonian]] period, there were 400 days in a year, with each day lasting 21.8 hours.
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| − |
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| − | The Moon may dramatically affect the development of life by taming the weather. Paleontological evidence and computer simulations show that Earth's [[axial tilt]] is stabilized by tidal interactions with the Moon.<ref>Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A.C.M., Levrard, B., 2004, "[http://www.edpsciences.org/articles/aa/abs/2004/46/aa1335/aa1335.html A long-term numerical solution for the insolation quantities of the Earth]", ''Astronomy and Astrophysics'', 428, pp. 261–85.</ref> Some theorists believe that without this stabilization against the [[torque]]s applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, as it appears to be for [[Mars (planet)|Mars]]. If Earth's axis of rotation were to approach the [[ecliptic|plane of the ecliptic]], extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during ''summer'' and directly away during ''winter''. [[Planetary science|Planetary scientists]] who have studied the effect claim that this might kill all large animal and higher plant life.<ref>Williams, D.M., J.F. Kasting, 1997, "Habitable planets with high obliquities", ''Icarus'' 129, 254–68.</ref> However, this is a controversial subject, 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 settle the matter.<!--"Shares" may imply "has exactly the same", which is not the case for either variable.-->
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| − |
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| − | Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent angular size (same [[solid angle]]) as the Sun (the Sun is 400 times larger, and the Moon is 400 times closer). This allows total [[eclipse]]s and annular eclipses to occur on Earth.
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| − | {{-}}
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| − | [[Image:Earth-Moon2.jpg|thumb|center|800px|The relative sizes of and distance between Earth and Moon, to scale]]
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| − |
| |
| − | The most widely accepted theory of the Moon's origin, the [[Giant impact hypothesis|giant impact theory]], states that it formed from the collision of a Mars-size [[protoplanet]] 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.
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| − |
| |
| − | Earth has at least two [[co-orbital satellite]]s, the [[asteroid]]s [[3753 Cruithne]] and [[2002 AA29|2002 AA<sub>29</sub>]].
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| − |
| |
| − | ==Habitability==
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| − | {{seealso|Planetary habitability}}
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| − |
| |
| − | A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble, and sufficient energy to sustain [[metabolism]].<ref>{{cite web | date = September, 2003 | url = http://astrobiology.arc.nasa.gov/roadmap/g1.html | title = Astrobiology Roadmap | publisher = NASA, Lockheed Martin | accessdate = 2007-03-10 }}</ref> The distance of the Earth from the Sun, as well as it's orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.<ref>{{cite book | first=Stephen H. | last=Dole | year=1970 | title=Habitable Planets for Man | edition=2nd edition | publisher=American Elsevier Publishing Co. | url=http://www.rand.org/pubs/reports/R414/ | accessdate=2007-03-11 | id=ISBN 0-444-00092-5 }}</ref>
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| − |
| |
| − | ===Biosphere===
| |
| − | {{main|Biosphere}}
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| − |
| |
| − | The planet's life forms are sometimes said to form a "[[biosphere]]". This biosphere is generally believed to have begun [[evolution|evolving]] about 3.5 billion (3.5 {{e|9}}) years ago. Earth is the only place in the universe officially recognized by the communities of Earth where life is absolutely known to exist, and some scientists believe that [[Rare Earth hypothesis|biospheres might be rare]].
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| − |
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| − | The biosphere is divided into a number of [[biome]]s, inhabited by broadly similar [[flora (plants)|flora]] and [[fauna (animals)|fauna]]. 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 most of the more populous biomes lie near the [[Equator]].
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| − |
| |
| − | ===Natural resources and land use===
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| − | {{main|Natural resource}}
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| − |
| |
| − | The Earth provides resources that are exploitable by humans for useful purposes. Some of these 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 near the surface by human [[civilization]] has become a subject of significant controversy in modern [[environmentalism]] movements.
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| − |
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| − | Large deposits of [[Fossil fuel]]s are obtained from the Earth's crust: ([[coal]], [[petroleum]], [[natural gas]], [[methane clathrate]]). These deposits are used by [[human]]s both for energy production and as feedstock for chemical production. Mineral [[ore]] bodies have also been formed in Earth's crust by the action of [[erosion]] and [[plate tectonics]]. These bodies form concentrated sources for many [[metal]]s and other useful [[chemical element|element]]s.
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| − |
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| − | The Earth's [[biosphere]] produces many useful biological products for humans, including (but far from limited to) [[food]], [[wood]], [[pharmaceutical]]s, oxygen, and the recycling of many organic wastes. The land-based [[ecosystem]] depends upon [[topsoil]] and fresh water, and the oceanic [[ecosystem]] depends upon dissolved nutrients washed down from the land. Humans also live on the [[Ecological footprint|land]] by using [[building material]]s to construct [[homes|shelters]]. In 1993, human use of land is approximately:
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| − |
| |
| − | {| {{prettytable}}
| |
| − | !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%
| |
| − | |}
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| − |
| |
| − | The estimated amount of irrigated land in 1993 was 2,481,250 km².
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| − |
| |
| − | ===Natural and environmental hazards===
| |
| − |
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| − | 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.
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| − |
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| − | 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]]. Human activities are also producing long-term climate alteration due to industrial carbon dioxide emissions. This is expected to produce changes such as the melting of glaciers and [[Arctic]] ice, more extreme temperatures, significant changes in weather conditions and a global rise in average sea levels.<ref>{{cite web | date = February 2, 2007 | url = http://www.un.org/apps/news/story.asp?NewsID=21429&Cr=climate&Cr1=change | title = Evidence is now ‘unequivocal’ that humans are causing global warming – UN report | publisher = United Nations | accessdate = 2007-03-07 }}</ref>
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| − |
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| − | ===Human geography===
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| − | {{main|Human geography}}
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| − | {{Earth Labelled Map}}
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| − | {{-}}
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| − | Earth has approximately 6,600,000,000 human inhabitants.<ref>Currently it is closer to 6.6 billion than 6.5 billion. It will reach 6.6 billion in June 2007.</ref><ref name="LiveScience">
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| − | {{cite news
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| − | | first= Leonard
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| − | | last= David
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| − | | url= http://www.livescience.com/othernews/060224_world_population.html
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| − | | title= Planet's Population Hit 6.5 Billion Saturday
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| − | | work= Live Science
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| − | | date= [[2006-02-24]]
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| − | | accessdate= 2006-04-02
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| − | }}
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| − | </ref>
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| − |
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| − | Projections indicate that the [[world population|world's human population]] will reach seven billion in 2013 and 9.1 billion in 2050 (2005 [[United Nations|UN]] estimates). Most of the growth is expected to take place in [[developing nations]]. Human [[population density]] varies widely around the world.
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| − |
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| − | It is estimated that only one eighth of the surface of the Earth is suitable for [[human]]s to live on — three-quarters is covered by [[ocean]]s, and half of the land area is [[desert]], high [[mountain]]s or other unsuitable terrain.
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| − |
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| − | The northernmost permanent settlement in the world is [[Alert, Nunavut|Alert]], on [[Ellesmere Island]] in [[Nunavut]], [[Canada]]. (82°28′N) The southernmost is the [[Amundsen-Scott South Pole Station]], in [[Antarctica]], almost exactly at the [[South Pole]]. (90°S)
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| − |
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| − | [[image:Earthlights_dmsp.jpg|380px|center|thumb|The Earth at night, a composite of DMSP/OLS ground illumination data on a simulated night-time image of the world. This image is not photographic and many features are brighter than they would appear to a direct observer.]]
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| − | 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]]s claim all of the land surface except for some segments of [[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.
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| − |
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| − | In total, about 400 people have been outside the Earth's atmosphere as of 2004, and of these, twelve have walked on the [[Moon]]. Normally the only humans in space are those on the [[International Space Station]]. The station's crew of three people is usually replaced every 6 months. ''See [[human spaceflight]].''
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| − | {{-}}
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| − |
| |
| − | ==Human viewpoint==
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| − | [[Image:AS8-13-2329.jpg|thumb|left|The first time an "Earth-rise" was seen from the moon.]]
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| − | Earth has often been personified as a [[deity]], in particular a [[goddess]] (''see [[Gaia (mythology)|Gaia]] and [[Mother Earth]]''). The [[China|Chinese]] Earth goddess [[Hou-Tu]] is similar to Gaia, the deification of the Earth. As 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]]. Ancient Egyptian mythology is different from that of other cultures because Earth is male, [[Geb]], and sky is female, [[Nut (goddess)]].
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| − |
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| − | Although commonly thought to be a sphere, the Earth is actually an [[oblate spheroid]]. It bulges slightly at the equator and is slightly flattened at the poles. In the past there were varying levels of belief in a [[flat Earth]], but ancient [[Greek philosophers]] and, in the [[Middle Ages]], thinkers such as [[Thomas Aquinas]] believed that [[Spherical Earth|it was spherical]].
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| − | A 19th-century organization called the [[Flat Earth Society]] advocated the even-then discredited idea that the Earth was actually [[disc]]-shaped, with the [[North Pole]] at its center and a 150 [[foot (unit of length)|foot]] (50 [[metre|m]]) high wall of ice at the outer edge. It and similar organizations continued to promote this idea, based on religious beliefs and [[Conspiracy theory|conspiracy theories]], through the 1970s. Today, the subject is more frequently treated [[tongue-in-cheek]] or with mockery.
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| − |
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| − | Prior to the introduction of [[space flight]], these inaccurate beliefs were 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 map making, and vicariously [[geography]], have historically been the disciplines devoted to depicting the Earth. [[Surveying]], the determination of locations and distances, to a lesser extent [[navigation]], the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.
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| − |
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| − | The technological developments of the latter half of the 20th century are widely considered to have altered the public's perception of the Earth. Before space flight, the popular image of Earth was of a green world. [[Science fiction]] artist [[Frank R. Paul]] provided perhaps the first image of a cloudless ''blue'' planet (with sharply defined land masses) on the back cover of the July 1940 issue of ''[[Amazing Stories]]'', a common depiction for several decades thereafter.<ref name="fja">
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| − | {{cite book
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| − | | last = Ackerman
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| − | | first = Forrest J
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| − | | authorlink = Forrest J Ackerman
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| − | | year = 1997
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| − | | title = Forrest J Ackerman's World of Science Fiction
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| − | | publisher = RR Donnelley & Sons Company
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| − | | location = Los Angeles
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| − | | id = ISBN 1-57544-069-5
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| − | | pages = 116-117
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| − | }}
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| − | </ref>
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| − | [[Apollo 17]]'s 1972 "[[The Blue Marble|Blue Marble]]" photograph of Earth from [[cislunar space]] became the current iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. A photo taken of a distant Earth by ''[[Voyager 1]]'' in 1990 inspired [[Carl Sagan]] to describe the planet as a "[[Pale Blue Dot]]."<ref name="seti-pbd">
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| − | {{cite web
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| − | | url = http://gtrc911.quaker.org/pale_blue_dot.html
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| − | | title = Pale Blue Dot
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| − | | publisher = SETI@home
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| − | | accessdate = 2006-04-02
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| − | }}
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| − | </ref>
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| − | Earth has also been described as a massive [[Spacecraft|spaceship]], with a [[life support system]] that requires maintenance, or as having a [[biosphere]] that forms one large [[organism]].
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| − |
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| − | {{see also|Spaceship Earth|Gaia theory}}
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| − |
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| − | ==Future==
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| − | <!-- Red giants can have different diameters depending on their mass and the stage of evolution. -->
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| − | [[Image:Sun_Red_Giant.jpg|thumb|right|200px|Artist's conception of the remains of artificial structures on the Earth after the Sun enters its red giant phase and swells to roughly 100 times its current size.]]
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| − | [[Image:redgiants.svg|thumb|200px|This shows a comparison between the red [[supergiant]] [[Antares]] and the Sun. The black circle is the size of the orbit of Mars. [[Arcturus]] is also included in the picture for comparison]]
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| − | The future of the planet is closely tied to that of the [[Sun]]. The [[Solar luminosity|luminosity of the Sun]] will continue to steadily increase, growing from the current luminosity by 10% in 1.1 billion years (1.1 [[Gigayear|Gyr]]) and up to 40% in 3.5 Gyr.<ref name="sun_future">I.J. Sackmann, A.I. Boothroyd, K.E. Kraemer, "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...418..457S Our Sun. III. Present and Future.]", ''Astrophysical Journal'', vol. 418, pp. 457.</ref> Climate models indicate that the increase in radiation reaching the Earth is likely to have dire consequences, including possible loss of the oceans.<ref>J.F. Kasting, 1988, "[http://adsabs.harvard.edu/abs/1988Icar...74..472K Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus]", ''Icarus'', 74, pp. 472-494.</ref>
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| − |
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| − | The Sun, as part of its solar lifespan, will expand to a [[red giant]] in 5 Gyr. Models predict that the Sun will expand out to about 99% of the distance to the Earth's present orbit (1 [[astronomical unit]], or AU). However, by that time, the orbit of the Earth may have expanded to about 1.7 [[astronomical unit|AUs]] because of the diminished mass of the Sun. The planet might thus escape envelopment.<ref name="sun_future" /> <!-- What would the atmosphere of the Sun be like at this distance? -->
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| − |
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| − | The increased heat will accelerate the inorganic CO<sub>2</sub> cycle, reducing its concentration to the lethal dose for plants (10 ppm for C4 photosynthesis) in 900 million years. But even if the Sun were eternal and stable, the continued internal cooling of the Earth would have resulted in a loss of much of its atmosphere and oceans (due to lower [[volcanism]]).<ref name="CO2 death">(French) Science&Vie n°1014 (March 2002)</ref> More specifically, for Earth's oceans, the lower temperatures in the crust will permit their water to leak more deeply than today (at certain depth the water is evaporating). After a billion years the oceans will have completely disappeared.
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| − | {{-}}
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| − |
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| − | ==See also==
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| − |
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| − | {| class="wikitable"
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| − | !align=left| Subtopic !!align=left| Links
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| − | |-
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| − | | Art || [[Landscape art]]
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| − | |-
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| − | | Astronomy || [[Darwin (ESA)]] · [[Terrestrial Planet Finder]]
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| − | |-
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| − | | Ecology || [[Earth Day]] · [[Millennium Ecosystem Assessment]]
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| − | |-
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| − | | Economy || [[World economy]]
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| − | |-
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| − | | Fiction || [[Hollow Earth]] · [[Journey to the Center of the Earth]] · [[Earth in fiction]]
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| − | |-
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| − | | Geography,<br/> [[Geology]] || [[Continents]] · [[Timezones]] · [[Degree Confluence Project]] · [[Earthquake]] · [[Extremes on Earth]] · [[Plate tectonics]] · [[Equatorial bulge]] · [[Structure of the Earth]]
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| − | |-
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| − | | History || [[Geologic time scale]] · [[History of Earth]] · [[History of the World|Human history]] · [[Solar system#Origin and evolution of the solar system|Origin and evolution of the solar system]] · [[Timeline of evolution]]
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| − | |-
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| − | | Imaging || [[Google Earth]] · [[World Wind]]
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| − | |-
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| − | | Law || [[International law]]
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| − | |-
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| − | | Language || [[Lexicography of Earth]]
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| − | |-
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| − | | Politics || [[List of countries]]
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| − | |}
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| | | | |
| | ==References== | | ==References== |
| − | * [http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html NASA's Earth fact sheet]
| + | <references/> |
| − | * ''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==
| + | |
| − | {{404}}
| + | |
| − | {{sisterlinks|Earth}}
| + | |
| − | * [http://www.wikimapia.org/ WikiSatellite view of Earth at WikiMapia]
| + | |
| − | * [http://geomag.usgs.gov USGS Geomagnetism Program]
| + | |
| − | * {{PDFlink|[http://www.geo.cornell.edu/geology/classes/isacks/velstruct.pdf Overview of the Seismic Structure of Earth]}}
| + | |
| − | * [http://earthobservatory.nasa.gov/Newsroom/BlueMarble NASA Earth Observatory]
| + | |
| − | * [http://www.co-intelligence.org/newsletter/comparisons.html The size of Earth compared with other planets/stars]
| + | |
| − | * [http://www.funonthenet.in/content/view/282/31/ Beautiful Views of Planet Earth] Pictures of Earth from space
| + | |
| − | * [http://www.flashearth.com/ Flash Earth] A Flash-based viewer for satellite and [[aerial photography|aerial]] imagery of the Earth
| + | |
| − | * [http://www.professores.uff.br/hjbortol/arquivo/2006.1/applets/earth_en.html Java 3D Earth's Globe]
| + | |
| − | * [http://www.projectshum.org/Planets/earth.html Projectshum.org's Earth fact file] (for younger folk)
| + | |
| − | * [http://www.geody.com/?world=terra Geody Earth] World's search engine that supports [[Google Earth]], [[NASA World Wind]], [[Celestia]], GPS, and other applications.
| + | |
| − | * [http://reference.aol.com/earth Planet Earth] From AOL Research & Learn: Photos, quizzes and info about Earth's climate, creatures and science.
| + | |
| − | * [http://reference.aol.com/earth/earth-from-space Earth From Space] Some Photos From the Exhibit
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| − | <!--Other languages-->
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| − | [[Category:Earth| ]]
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| − | [[Category:Geography| ]]
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| − | [[Category:Geology| ]]
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| − | [[Category:Terrestrial planets]]
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| − | [[Category:Planets of the Solar System]]
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| − | {{Link FA|de}}
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| − | {{Link FA|sr}}
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Hellbound abortion-loving leftie scallions claim that the Earth is "round", and orbits at an average distance of about 93 million miles (150 million km) from the Sun in an almost circular orbit. It takes light (and other forms of electromagnetic radiation) approximately 500 seconds to travel from the Sun to the Earth, i.e. the distance can also be stated as "500 light-seconds." The distance to nearby stars is made by measuring the stellar parallax between observations when the earth is at opposite ends of its orbit, so the earth's orbit itself is a measuring stick for astronomical distances, and is known as the astronomical unit (A. U.) It is worth noting that if the Earth were slightly closer to the Sun, it would be too hot for life (just as Venus is) while if Earth were slightly further away from the Sun, water would freeze and life would be impossible. Many Christians believe that the Earth is the perfect distance away from the Sun and take this to be evidence of God's existence. [1]
The Earth is approximately 8000 miles (13000 km) in diameter, so its circumference is approximately 8000·π = about 25,000 miles (40,000 km). Its surface area is approximately 4·π (4000 mi)2 = 200 million square miles (510 million km2). It has been said that Earth should really be called "water:" most of the Earth's surface, about 70%, is covered by water. In fact, the Pacific and Indian oceans alone cover about half of the Earth.
Estimates by geologists of the age of the Earth give about 4.55 billion years to an accuracy of about 1%. This estimate is primarily based on radioactive dating of meteorites. In addition, most scientists also believe that the Earth formed by natural processes instead of being supernaturally created. Indeed, astronomers have detected dust lanes around other stars that indicate planet formation is a common and ongoing process.
Bible scholars have also estimated the age of the earth based on the Creation account in Genesis and the genealogical accounts in Numbers and other books of the Pentateuch. One famous estimate was published in 1650 by R. B. Knox, James Ussher Archbishop of Armagh (usually referred to as Archbishop Ussher) in a book called Annals of the World, in which he estimated the Creation to have occurred on 23 October 4004 B.C. Other Biblical scholars maintain that there are possible gaps in the genealogies, often using the ideas of the 19th century Calvinist theologian Benjamin Warfield on the issue. [2] However, James Barr, regius professor of Hebrew at Oxford University, wrote in 1984 the following: "… probably, so far as I know, there is no professor of Hebrew or Old Testament at any world-class university who does not believe that the writer(s) of Genesis 1–11 intended to convey to their readers the ideas that: … the figures contained in the Genesis genealogies provided by simple addition a chronology from the beginning of the world up to later stages in the biblical story.’" [3] However, it should be noted that Barr himself rejects supernatural Christianity. [4]
