Difference between revisions of "Earth" - New World Encyclopedia

From New World Encyclopedia
 
(77 intermediate revisions by 12 users not shown)
Line 1: Line 1:
−
{{otheruses}}
+
[[Category:Public]]
 +
{{Ebapproved}}{{Copyedited}}{{Paid}}{{submitted}}{{images OK}}{{Approved}}
 
{{Planet Infobox/Earth}}
 
{{Planet Infobox/Earth}}
−
'''Earth''' (often referred to as "The Earth") is the third [[planet]] in the [[solar system]] in terms of distance from the [[Sun]], and the fifth in order of size. It is the largest of its [[planetary system]]'s [[terrestrial planet]]s and the only place in the [[universe]] currently known to support [[life]]. Scientists estimate that the Earth was formed around 4.57 [[billion]] (4.57{{e|9}})<ref>G.B. Dalrymple, 1991, "The Age of the Earth", ''Stanford University Press'', California, ISBN 0-8047-1569-6.</ref> [[year]]s ago (see [[Age of the Earth]]) and its largest [[natural satellite]], the [[Moon]], began orbiting it shortly thereafter, around 4.533 billion years ago. Since its formation, the Earth has been transformed through geologic and biological processes, so that any traces of the original conditions have been virtually eliminated.
+
'''Earth''' is the third [[planet]] in the [[solar system]] in terms of distance from the [[Sun]], and the fifth in order of size. It is the largest and densest of the solar system's [[terrestrial planet]]s. Scientists estimate that the Earth was formed around 4.57 billion (4.57{{e|9}}) [[year]]s ago.<ref>G.B. Dalrymple, ''The Age of the Earth'' (Stanford, CA: Stanford University Press, 1991, ISBN 0804715696).</ref> About 70 percent of the surface is presently covered by [[ocean]]s, and the remainder consists of [[continents]] and [[islands]]. The abundance of water is a unique feature that distinguishes our "blue planet" from others in the solar system.
  
−
The Earth's outer surface, or [[crust]], 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 molten [[mantle]] and an iron core that generates a [[magnetic field]]. The atmospheric conditions have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 70% of the surface is presently covered in salt water [[ocean]]s, and the remainder consists of [[continents]] and [[islands]].
+
Since its formation, the Earth has been transformed through various geological and biological processes. The Earth's upper layer ''([[lithosphere]])'' is divided into several [[#Plate tectonics|tectonic plate]]s that gradually migrate across the surface over geologic time spans. [[Earthquake]]s, [[Volcano|volcanic]] activity, [[mountain]]-building, and oceanic trench formation occur along plate boundaries. The interior of the planet is also active, with a thick layer ''(mantle)'' of highly viscous material and an iron core that generates a [[magnetic field]].
  
−
There is significant interaction between the Earth and its environment in space. The relatively large Moon provides ocean [[tide]]s and has gradually modified the length of the planet's rotation period. A [[comet]]ary bombardment during the early history of the planet is believed to have formed the oceans. Later, [[asteroid]] impacts are understood to have caused significant changes to the surface environment. Changes in the planet's orbit may also be responsible for the [[ice age]]s that have covered significant portions of the surface in glacial sheets.
+
The Earth is the only place in the [[universe]] known to support [[life]], and its position, structure, and composition make it ideally suited for this purpose. Orbiting the Sun at an appropriate distance, this planet receives the proper amounts of heat and light to sustain life, and its rotational and orbital movements allow for daily and seasonal cycles. Its natural satellite, the [[Moon]], is sufficiently large to produce regular tides and to stabilize the planet's axial tilt. In addition, the [[Earth's atmosphere|atmosphere]] provides useful gases, transports water vapor, moderates temperature extremes, absorbs harmful [[ultraviolet]] radiation, and burns up most meteors before they can strike the Earth's surface. The Earth's magnetosphere shields the surface by deflecting charged particles brought by the solar wind. The presence of water and the chemical composition of the Earth's surface play significant roles in the development and sustenance of living organisms. At the same time, the Earth's surface conditions have been significantly altered by the presence of life forms, which have created an ecological balance.
 +
{{toc}}
 +
Photographic images of the Earth from space reveal its awesome beauty, and at the same time highlight the fragility of our common home, often called "spaceship earth." That image has become  a contemporary icon. Unlike political maps, it has no borders. It calls for cooperation and peace among all Earth's inhabitants to assure a future for humanity and the planet.
  
−
==Lexicography==
+
== Historical descriptions of Earth ==
−
Terms that refer to the Earth can use the [[Latin (language)|Latin]] root ''terra-'', such as the word ''terrestrial''. There is also the alternative Latin root ''tellur-'', as used in words such as [[telluric]], [[tellurian]], [[tellurion]] and [[Tellurium]]. Both terms derive from the [[Roman mythology|Roman goddess]] [[Terra (mythology)|Terra Mater]], who was also called by the presumably more ancient name Tellūs Mater. Scientific terms such as [[geography]], [[geocentric]] and [[geothermal]] use the [[Greek language|Greek]] prefix ''geo-'', derived from Terra Mater's [[Greek mythology|Greek]] counterpart [[Gaia (mythology)|Gaia]].
+
In ancient cultures, the Earth was often personified as a [[deity]], particularly a [[goddess]] who, like a mother, gives birth to and nurtures life ''(see [[Gaea]])''. The [[China|Chinese]] Earth goddess [[Hou-Tu]] is similar to Gaia. 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]]. By contrast, in ancient Egyptian mythology, Earth was symbolized as a male, [[Geb]], and the sky was taken to be a female, [[Nut]].
  
−
The English word "earth" has [[cognate]]s in many modern and ancient languages. Examples in modern tongues include ''aarde'' in [[Dutch language|Dutch]] and ''Erde'' in [[German language|German]]. The root also has cognates in extinct languages such as ''ertha'' in [[Old Saxon]] and ''ert'' (meaning "ground") in [[Middle Irish]], derived from the [[Old English]] ''eor&#0240;e''. All of these words are derived from the [[Proto-Indo-European language|Proto-Indo-European]] base *er-. Given [[metathesis (linguistics)|metathesis]], we can find cognates of "earth" between ''terra'' and the modern [[Romance languages]], for instance ''tierra'' in [[Spanish language|Spanish]] or ''terra'' in [[Portuguese language|Portuguese]].
+
In terms of the Earth's shape, earlier belief systems included the idea of a [[flat Earth]]. On the other hand, ancient [[Greek philosophers]] and other thinkers, including [[Thomas Aquinas]] (1225&ndash;1274), believed the Earth was spherical. Arguments used to support the latter view took into account some of the secondary effects of the Earth's shape and parallels with the shape of other planets. Today there is sufficient evidence to indicate that the Earth is an [[oblate spheroid]]&mdash;it bulges slightly at the equator and is slightly flattened at the poles. The idea of a flat Earth is more frequently treated tongue-in-cheek, or with mockery.
  
−
Several [[Semitic languages]] have words for "earth" similar to those in [[Indo-European languages]], although evidence of a link is not overwhelming. Arabic has ''aard''; [[Akkadian language|Akkadian]], ''irtsitu''; [[Aramaic language|Aramaic]], ''araa''; [[Phoenician languages|Phoenician]], ''erets'' (which appears in the [[Mesha Stele]]); and [[Hebrew language|Hebrew]], ארץ (''arets'', or ''erets'' when followed by a [[Hebrew grammar#Nouns|noun modifier]]).
+
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>Forrest J. Ackerman, ''Forrest J Ackerman's World of Science Fiction'' (Los Angeles, CA: RR Donnelley & Sons Company, 1997, ISBN 1575440695), 116-117. </ref> Apollo 17's "Blue Marble" photograph of Earth, taken in 1972 from cislunar space (volume of space within the moon's orbit), became the current iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. A photo of a distant Earth taken by ''[[Voyager 1]]'' in 1990 inspired [[Carl Sagan]] to describe the planet as a "pale blue dot."<ref> Pale Blue Dot SETI@home.</ref> Earth has also been described as a massive [[spaceship]], with a [[life support system]] that requires maintenance, or as having a [[biosphere]] that forms one large [[organism]] ''(See [[Gaia hypothesis]].)''
 +
 
 +
Historically, the disciplines devoted to depicting the Earth's surface have been [[cartography]], the study and practice of mapmaking, and [[geography]]. [[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 information needed for describing the Earth's surface.
  
 
===Symbol===
 
===Symbol===
−
The [[astrology|astrological]] and [[astronomical symbol]] for Earth consists of a circled cross, the arms of the cross representing a [[meridian (geography)|meridian]] and the [[equator]] (<big>{{unicode|&#x2295;}}</big>). A variant puts the cross atop the circle (<big>{{unicode|&#x2641;}}</big>).
+
The [[astrology|astrological]] and [[Astronomy|astronomical]] symbol for Earth consists of a circled cross, the arms of the cross representing a [[meridian (geography)|meridian]] and the [[equator]] (<big>{{unicode|&#x2295;}}</big>). A variant puts the cross atop the circle (<big>{{unicode|&#x2641;}}</big>).
  
−
==History==
+
=== Earth-based terminology ===
−
{{main|History of Earth}}
+
Terms that refer to the Earth may use the [[Latin (language)|Latin]] root ''terra-'', such as the word ''terrestrial'', or the Latin root ''tellur-'', as in ''telluric'' and ''[[tellurium]]''. Both terms derive from the [[Roman mythology|Roman goddess]] [[Terra (mythology)|Terra Mater]], who was also called by the presumably more ancient name Tellūs Mater. Scientific terms such as geography, [[geocentric]], and [[geothermal]] use the [[Greek language|Greek]] prefix ''geo-'', derived from Terra Mater's [[Greek mythology|Greek]] counterpart, Gaia.
  
−
Based on the available evidence, scientists have been working to reconstruct some of the details of our planet's past. Earth is believed to have formed around 4.55 billion years ago out of a gaseous cloud called the "[[solar nebula]]," along with the Sun and other planets. The Moon formed soon after. Initially molten, the outer layer of the planet cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere; condensing [[water]] vapor, augmented by ice delivered by comets, produced the [[Origin of the world's oceans|oceans]].<ref>A. Morbidelli ''et al'', 2000, "[http://adsabs.harvard.edu/abs/2000M&PS...35.1309M Source Regions and Time Scales for the Delivery of Water to Earth]", ''Meteoritics & Planetary Science'', vol. 35, no. 6, pp. 1309-1320.</ref> Scientists speculate that a self-replicating molecule may have been formed around 4 billion years ago, and that the [[last universal common ancestor|last common ancestor of all life]] may have existed half a billion years later.<ref>W. Ford Doolitte,  "Uprooting the Tree of Life", ''Scientific American'', Feb. 2000.</ref>
+
==Physical characteristics==
 +
Earth is the largest of the "inner" planets of the [[solar system]], that is, the planets between the Sun and the asteroid belt. When viewed from [[outer space]], much of the Earth has a deep blue and white appearance, caused by the oceans and clouds in the atmosphere. It has an [[albedo]] (measure of light reflectivity) of 36.7 percent, which is exceeded only by [[Venus (planet)|Venus]] among the inner planets.
  
−
With the development of [[photosynthesis|photosynthetic]] organisms, the Sun's energy could be harvested directly. As a result, [[oxygen]] accumulated in the atmosphere and gave rise to the [[ozone layer]].<ref>L. V. Berkner, L. C. Marshall, 1965, "On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere", ''Journal of the Atmospheric Sciences'', Vol. 22, No. 3, pp. 225–261.</ref> With the ozone layer absorbing harmful [[ultraviolet radiation]], life forms were able to colonize the Earth's surface.
+
As mentioned above, the Earth is shaped as an [[oblate]] [[spheroid]]. Its average diameter is approximately 12,742 km.<ref>"Geodetic Reference System 1980 (GRS80)," ''XVII General Assembly'', International Association of Geodesy.</ref> The planet's curvature is visible from some regions on the surface such as the [[Bonneville Salt Flats]] in the [[United States]].
  
−
It is thought that over hundreds of millions of years, the continents have migrated across the surface of the Earth, occasionally combining to form a [[supercontinent]] and then breaking apart. Roughly 750 million years ago (mya), the earliest known supercontinent, [[Rodinia]], began to break up. The continents later recombined to form [[Pannotia]], 600-540 mya, then finally [[Pangaea]], which broke apart 180 mya.<ref>J.B. Murphy, R.D. Nance, "[http://scienceweek.com/2004/sa040730-5.htm How do supercontinents assemble?]", ''American Scentist'', vol. 92, pp. 324-333.</ref>
+
Earth is also the densest planet in the entire solar system. Its average density is about 5,515 kilograms/meter<sup>3</sup> (kg/m<sup>3</sup>), while the average density of surface material is around 3,000 kg/m<sup>3</sup>. The [[mass]] of the Earth is approximately 5980 yottagrams (5.98 x 10<sup>24</sup> kg).
  
−
Since the 1960s, it has been hypothesized that severe [[Glacier|glacial]] action between [[1 E16 s|750 and 580 million years ago]], during the [[Neoproterozoic]], covered much of the planet in a sheet of ice. This hypothesis has been termed "[[Snowball Earth]]," and it is of particular interest as it precedes the [[Cambrian explosion]] when multicellular lifeforms began to proliferate.<ref> J.L. Kirschvink, 1992, "Late Proterozoic Low-Latitude Global Glaciation: The Snowball Earth", ''The Proterozoic Biosphere'', pp 51-52.</ref>
+
Taken as a whole, the Earth's composition by mass [http://earthref.org/cgi-bin/er.cgi?s=erda.cgi?n=547] is:
  
−
Since the Cambrian explosion, about 535 million years ago, there were five distinct [[Extinction event|mass extinctions]].<ref>D. Raup & J. Sepkoski, 1982, "Mass extinctions in the marine
+
{| border="2" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; border: 1px #aaa solid; border-collapse: collapse; font-size: 95%;"
−
fossil record", ''Science'', vol. 215, pp. 1501–1503.</ref> The last one occurred 65 million years ago, when a meteorite collision is thought to have triggered the extinction of the (non-avian) [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s, which seem to have resembled shrews. Over the last 65 million years, mammalian life diversified.
+
|[[iron]]:||align="right"|35||.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||%
 +
|-
 +
|[[aluminum]]:||align="right"|1||.5||%
 +
|-
 +
|[[sulfur]]:||align="right"|0||.70||%
 +
|-
 +
|[[sodium]]:||align="right"|0||.25||%
 +
|-
 +
|[[titanium]]:||align="right"|0||.071||%
 +
|-
 +
|[[potassium]]:||align="right"|0||.019&nbsp;||%
 +
|-
 +
|other elements: &nbsp; &nbsp;||align="right"|0||.53||%
 +
|}
  
−
Once humans appeared, they transformed the Earth in a short timespan like no other life form had before. Humans have affected both the nature and quantity of other life forms as well as global climate.
+
===Geologic layers===
 +
{{main|Structure of the Earth}}
 +
[[Image:Earth-crust-cutaway-english.png|thumb|right|250px|Earth cutaway from core to exosphere. Partially to scale]]
  
−
==Physical characteristics==
+
When considered in terms of its composition, the Earth consists of the following major layers: the [[Earth's atmosphere|atmosphere]], [[hydrosphere]], [[Crust (geology)|crust]], [[Mantle (geology)|mantle]], and [[core]]. Each of these has a number of sublayers.
−
Earth is the largest and densest of the "inner" planets of the [[solar system]], that is, the planets within the asteroid belt. When viewed from [[outer space]], much of the Earth has a deep blue and white appearance, caused by the oceans and clouds in the atmosphere. It has an [[albedo]] (measure of light reflectivity) of 36.7%, which is exceeded only by [[Venus (planet)|Venus]] among the inner planets.
 
  
−
===Atmosphere===
+
In addition, scientists use the term ''[[lithosphere]]'' when referring to the solid crust and the solidified uppermost part of the mantle. Below the lithosphere lies the ''[[asthenosphere]]'', which consists of the weak (or soft) zone of the upper mantle. The ''[[biosphere]]'', which refers to that part of the outer shell in which life occurs, is usually not considered one of the physical layers of the Earth.
−
{{main|Earth's atmosphere}}
 
  
−
The [[atmospheric pressure]] on the Earth's surface averages 101.3 kilopascals (kPa), with a [[scale height]] of about 6 kilometers (km). It is composed of 78% [[nitrogen]], 21% [[oxygen]], with trace amounts of other gaseous molecules such as water vapor. The atmosphere protects and sustains the Earth's life forms by absorbing [[ultraviolet]] [[solar radiation]], moderating temperature extremes, transporting water vapor, and providing useful gases.
+
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]," ''Procedings National Academy of Science'', 1979, Sept., 76(9): 4192&ndash;4200. Retrieved August 3, 2019.</ref> are located at the following depths below the surface:
  
−
===Geology===
+
{| border="2" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; border: 1px #aaa solid; border-collapse: collapse; font-size: 95%;"
−
{{main|Structure of the Earth}}
 
−
[[Image:Earth-crust-cutaway-english.png|thumb|left|Earth cutaway from core to exosphere. Partially to scale]]
 
−
 
 
−
<!-- This section has been moved to the article '''[[Structure of the Earth]]'''. A new 30-line summary section must be written from this main article to this "Physical characteristics" section. Help is welcome. —>
 
−
The Earth's shape is that of an [[oblate]] [[spheroid]], with an average diameter of approximately 12,742 km (~ 40,000 km / [[pi|π]]).<ref>"Geodetic Reference System 1980 (GRS80)", ''XVII General Assembly'', International Association of Geodesy.</ref> The planet's curvature is visible from some regions on the surface such as the [[Bonneville Salt Flats]] in the [[United States]]. The Earth consists of several [[atmospheric]], [[hydrologic]], and many [[geologic]] layers. Its components are the [[Earth's atmosphere|atmosphere]], the [[hydrosphere]], the [[Crust (geology)|crust]], the [[mantle (geology)|mantle]], and its [[core]]. The [[biosphere]] is a tiny layer in this composition and is usually not considered part of the physical layers of the Earth.
 
−
 
 
−
The geologic component layers of the Earth<ref>T. H. Jordan, "[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=411539 Structural Geology of the Earth's Interior]", ''Procedings National Academy of Science'', 1979, Sept., 76(9): 4192&ndash;4200.</ref> are located at the following depths below surface:
 
−
 
 
−
{| {{prettytable}}
 
 
!colspan=2|Depth
 
!colspan=2|Depth
 
!rowspan=2 valign="bottom"|Layer
 
!rowspan=2 valign="bottom"|Layer
 
|-
 
|-
−
!Kilometres
+
!Kilometers
 
!Miles
 
!Miles
 
|-
 
|-
Line 86: Line 106:
 
|}
 
|}
  
−
{{seealso|Geology}}
+
===Atmosphere===
 +
{{main|Earth's atmosphere}}
 +
 +
The Earth's atmosphere protects and sustains life forms by absorbing [[ultraviolet]] [[solar radiation]], moderating temperature extremes, transporting water vapor, and providing useful gases. It contains about 78 percent [[nitrogen]], 21 percent [[oxygen]], and trace amounts of other gases such as [[carbon dioxide]] and water vapor.
 +
 
 +
===Hydrosphere===
 +
{{main|hydrosphere}}
 +
 
 +
The hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. Approximately 70.8 percent of the Earth is covered by water and only 29.2 percent is terra firma. The abundance of water on Earth is a unique feature that distinguishes our "blue planet" from others in the solar system.
 +
 
 +
The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1.35 × 10<sup>18</sup> tons, or about 1/4400 of the total mass of the Earth.
  
−
==Earth in the solar system==
+
The movement of water within the hydrosphere is described by the [[hydrologic cycle]]. This cycle includes water beneath the Earth's surface and in rocks (lithosphere), the water in plants and animals (biosphere), the water covering the surface of the planet in liquid and solid forms, and the water in the atmosphere.
−
[[Image:Rotating earth (small).gif|thumb|left|150px|An image showing the rotation of the Earth.]]
+
 
−
It takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds ([[sidereal day|1 sidereal day]]) to rotate around the [[Axis_of_rotation|axis]] connecting the [[north pole]] and the [[south pole]]. From Earth the main apparent motion of celestial bodies in the sky (except [[meteor]]s which are within the atmosphere and low-orbiting satellites) is the movement to the west at a rate of 15 &deg;/h = 15'/min, i.e., a Sun or Moon diameter every two minutes.
+
===Crust===
 +
{{main|Crust (geology)}}
  
−
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 about 1 &deg;/day, i.e., a Sun or Moon diameter every 12 hours, eastward. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is enough to cover one Earth diameter (~12,600 km) in 7 minutes, and one distance to the Moon (384,000 km) in 4 hours.
+
The outer layer of the Earth is called its crust. It is composed mainly of [[basalt]] and [[granite]]. It is cooler and more rigid than the deeper layers of the mantle and core. The crust is a part of the lithosphere, which floats on fluid interior layers. Oceanic crust is different from that of the continents. Oceanic crust ([[Sima (geology)|sima]]) is 5&ndash;10 km thick and is composed primarily of basalt, a dark, dense rock. Continental crust ([[Sial|sial]]) is 20&ndash;70 km deep and is composed of a variety of less dense rocks.
  
−
Earth has one [[natural satellite]], the [[Moon]], which 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]] (sphere of influence) of the Earth is about 1.5 Gm (930,000 miles) in radius.
+
===Mantle===
 +
{{main|Mantle (geology)}}
  
−
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 Earth's mantle is the thick shell of rock that surrounds the outer core and lies directly beneath the thin crust. The mantle lies roughly between 30 and 2,900 km below the surface, and occupies about 70 percent of the Earth's volume. The mantle is composed of [[silicate]] rocks that are rich in [[iron]] and [[magnesium]] relative to the overlying crust. Temperatures in the mantle range between 100° C at the upper boundary to over 4,000° C at the boundary with the core. Because of the extremely high pressures within the mantle, it is thought to be a solid or highly viscous fluid. The high temperatures cause the silicate material to be sufficiently ductile that it can flow on very long timescales. [[Convection]] of the mantle is expressed at the surface through the motions of [[#Plate tectonics|tectonic plates]].
  
−
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 Earth's equatorial bulge, due to its 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.
+
===Core===
 +
{{main|Structure of the Earth}}
  
−
In modern times, Earth's [[perihelion]] is always about [[January 3]], and [[aphelion]] is about [[July 4]] (near the [[solstice]]s, which are on about [[December 21]] and [[June 21]]). For other eras, see [[precession]] and [[Milankovitch cycles]]. The Earth is sometimes referred to as the '''Third Planet from the Sun''' because, of the nine planets of our solar system, Earth is the third closest planet to the sun.
+
Seismic measurements indicate that the Earth's core is divided into two parts: a solid inner core with a radius of about 1,220 km, and a liquid outer core with a radius extending from 1,220 km to 3,480 km. The solid inner core is believed to be 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 liquid outer core surrounds the inner core and is thought to be composed of iron mixed with 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. The solid inner core is too hot to hold a permanent magnetic field, but it probably acts to stabilize the magnetic field generated by the liquid outer core.
  
 
===Magnetic field===
 
===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. The field forms the "[[Magnetosphere]]" that deflects particles in the [[solar wind]]. The [[bow shock]] is located approximately 13.5 R<sub>E</sub>. The collision between the magnetic field and the solar wind forms what are called the "[[Van Allen radiation belt]]s," which are a pair of concentric, [[torus]]-shaped regions of energetic [[charged particle]]s. When the [[plasma]] (composed of ionized particles) enters the Earth's atmosphere at the magnetic poles, it produced a phenomenon called the [[Aurora (astronomy)|Aurora]].
+
[[Image:Magnetosphere schematic.jpg|thumb|350px|The Earth's magnetosphere shields the planet's surface by deflecting charged particles coming from the solar wind.]]
 +
 
 +
The [[Earth's magnetic field]] is shaped roughly as a [[magnetic dipole]], with the poles currently located proximate to the planet's geographic poles. The field forms the "[[Magnetosphere]]," which shields the Earth's surface by deflecting charged particles coming from the [[solar wind]]. The collision between the magnetic field and solar wind forms what are called the "[[Van Allen radiation belt]]s," which are a pair of concentric, [[torus]]-shaped regions of energetic charged particles. When the [[plasma]] (composed of charged particles) enters the Earth's atmosphere at the magnetic poles, it produces a phenomenon called the [[Aurora (astronomy)|Aurora]].
 +
 
 +
==History of Earth==
 +
{{main|History of Earth}}
 +
 
 +
Based on the available evidence, scientists have been working to reconstruct some of the details of our planet's past. It is thought that the Earth was formed around 4.57 billion years ago (see [[Age of the Earth]]) out of a gaseous cloud called the "[[solar nebula]]," from which the [[solar system]] emerged. The moon formed soon after. Initially molten, the outer layer of the planet cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere; condensing [[water]] vapor, augmented by ice delivered by comets, produced the [[Origin of the world's oceans|oceans]].<ref>A. Morbidelli ''et al'', "[http://adsabs.harvard.edu/abs/2000M&PS...35.1309M Source Regions and Time Scales for the Delivery of Water to Earth]," ''Meteoritics & Planetary Science'', 35(6) (2000): 1309-1320. Retrieved August 3, 2019.</ref>
 +
 
 +
Scientists think that early living organisms may have existed more than 3.5 billion years ago.<ref>W. Ford Doolittle, "Uprooting the Tree of Life," ''Scientific American'', Feb. 2000.</ref> Later, as [[photosynthesis|photosynthetic]] organisms developed, they could harvest the Sun's energy directly. As a result, [[oxygen]] accumulated in the atmosphere and gave rise to the [[ozone layer]].<ref>L. V. Berkner, L. C. Marshall, "On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere," ''Journal of the Atmospheric Sciences'', 22(3) (1965): 225–261.</ref> With the ozone layer absorbing harmful [[ultraviolet radiation]], life forms were able to colonize the Earth's surface.
 +
 
 +
It is thought that over hundreds of millions of years, the continents migrated across the surface of the Earth, occasionally combining to form a [[supercontinent]] and then breaking apart. Roughly 750 million years ago (m.y.a.), the earliest known supercontinent, ''Rodinia'', began to break up. The continents later recombined to form ''Pannotia'', 600-540 m.y.a., then finally ''[[Pangaea]]'', which broke apart 180 m.y.a.<ref>J.B. Murphy and R.D. Nance, "How do supercontinents assemble?", ''American Scientist'', 92(4) (2004): 324-333.</ref> (See [[#Plate tectonics|Plate tectonics]] below.)
 +
 
 +
Since the 1960s, it has been hypothesized that severe [[Glacier|glacial]] action during the [[Neoproterozoic]], between 750 and 580 m.y.a., covered much of the planet in a sheet of ice. This hypothesis, termed "[[Snowball Earth]]," is of particular interest as it precedes the [[Cambrian explosion]] when multicellular life forms began to proliferate.<ref> J.L. Kirschvink, "Late Proterozoic Low-Latitude Global Glaciation: The Snowball Earth," ''The Proterozoic Biosphere'', 1992, 51-52.</ref>
 +
 
 +
It appears that from the time of the Cambrian explosion, about 535 million years ago, there were five distinct [[mass extinction]]s.<ref>D. Raup & J. Sepkoski, "Mass extinctions in the marine fossil record," ''Science'', 215 (1982): 1501–1503.</ref> The last one occurred 65 million years ago, when a meteorite collision is thought to have triggered the extinction of the (non-avian) [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s, which seem to have resembled shrews. Over the last 65 million years, mammalian life diversified.
 +
 
 +
Once humans appeared, they transformed the Earth in a short time span like no other life form had before. Humans have influenced both the nature and quantity of other life forms, as well as global climate.
 +
 
 +
==Plate tectonics==
 +
{{main|Plate tectonics}}
 +
 
 +
[[Image:Plate tectonics map.gif|thumb|right|380px|A map pointing out the Earth's major plates.]]
 +
 
 +
'''Plate tectonics''' (from the [[Greek language|Greek]] word τεκτων, ''tektoon'', meaning "one who constructs and destroys") is a [[theory]] that arose out of two separate geological observations: [[continental drift]], noticed in the early twentieth century, and [[seafloor spreading]], noticed in the 1960s. The theory was developed during the late 1960s and has since been accepted by virtually all scientists working in the field.
 +
 
 +
According to this theory, the lithosphere is broken up into what are called ''tectonic plates'', which essentially "float" on the asthenosphere. These plates move in relation to one another. [[Earthquake]]s, [[volcano|volcanic activity]], [[mountain]]-building, and [[ocean]]ic trench formation occur along plate boundaries. This theory has revolutionized [[earth science]] (geology) and is valued for its unifying and explanatory power for diverse geological phenomena.
 +
 
 +
<br clear="all">
 +
{|align="center"
 +
|[[Image:Oceanic-continental convergence Fig21oceancont.gif|thumb|250px|Oceanic / Continental]]
 +
|[[Image:Continental-continental convergence Fig21contcont.gif|thumb|250px|Continental / Continental]]
 +
|[[Image:Oceanic-oceanic convergence Fig21oceanocean.gif|thumb|250px|Oceanic / Oceanic]]
 +
|}
 +
 
 +
==Earth in the solar system==
 +
[[File:Rotating_earth_mini.gif|thumb|right|225px|An image showing the rotation of the Earth.]]
 +
 
 +
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]] connecting the North and South Poles. From Earth, the main apparent motion of celestial bodies in the sky (except [[meteor]]s, which are within the atmosphere, and low-orbiting satellites) is the movement to the west at a rate of 15 &deg;/h (= 15'/min), that is, a sun or moon diameter every two minutes.
 +
 
 +
Earth orbits the sun every 365.2564 mean solar days ([[sidereal year|one sidereal year]]). From Earth, this gives an apparent movement of the sun with respect to the stars at a rate of about one degree per day, that is, a sun or moon diameter every 12 hours, eastward. The Earth's orbital speed averages about 30 km/s (108,000 km/h), which is enough to cover one Earth diameter (~12,600 km) in seven minutes, and one distance to the moon (384,000 km) in four hours.
 +
 
 +
Earth has one [[natural satellite]], the [[moon]]. The diameter of the sun is about 400 times that of the moon, but the moon is 400 times closer to the Earth than the sun. Consequently, the sun and moon appear roughly equal in size when viewed from the Earth. Moreover, this apparent equality of size allows us to observe a total [[solar eclipse]], when the moon completely covers the sun, and an annular eclipse, when the sun appears slightly larger than the moon and produces a bright ring (annulus) around the moon.
 +
 
 +
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 axis is tilted some 23.5 degrees against the Earth-Sun plane. This tilt has significant effects, including the variations of [[season]]s. Also, the Earth-moon plane is tilted about 5 degrees against the Earth-Sun plane&mdash;otherwise, there would be an eclipse every month.
 +
 
 +
In modern times, the Earth reaches its [[perihelion]] (when the planet is nearest to the Sun) about January 3, and its [[aphelion]] (when it is farthest from the Sun) about July 4. (These dates are near the [[solstice]]s, which occur around December 21 and June 21).
  
 
===The Moon===
 
===The Moon===
Line 124: Line 202:
 
|}
 
|}
 
</center>
 
</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 (moon)|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, sometimes called "Luna," is a relatively large satellite that structurally resembles a terrestrial planet. The moon's diameter is about one-quarter that of the Earth. The natural satellites orbiting other planets are also called ''moons'', after the Earth's moon. With the exception of [[Pluto|Pluto's]] [[Charon (moon)|Charon]], it is the largest moon in the solar system relative to the size of its planet.
  
−
Due to their [[Tidal acceleration|tidal interaction]], the Moon recedes from Earth at the rate of approximately 38 [[Millimetre|mm]] per year. The Earth's day also lengthens by about 17 [[Microsecond|µs]] every year. Over millions of years these tiny modifications can add up to significant changes. So for example, during the [[Devonian]] period there were 400 days in a year, with each day lasting 21.8 hours.
+
The most widely accepted theory of the moon's origin, the "giant impact hypothesis," states that it was formed from the collision of a Mars-size [[protoplanet]] with the early Earth. This hypothesis explains (among other things) the moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.
  
−
[[Image:Earth-moon.jpg|thumb|right|''Earthrise'' as seen from lunar orbit on ''[[Apollo 8]]'', [[24 December]] [[1968]]. Due to [[tidal locking]], from any point on the Moon's surface, the Earth does not rise or set, but is always located in the same position in the sky.]]
+
[[Image:Earth-moon.jpg|thumb|right|250px|''Earthrise'' as seen from lunar orbit on ''Apollo 8'', December 24, 1968. The moon always presents the same face to the Earth. Consequently, our planet, when viewed from the moon, does not rise or set, but is always located in the same position in the sky.]]
−
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 stabilised 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. 261-285.</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 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.<ref>Williams, D.M., J.F. Kasting, 1997, "Habitable planets with high obliquities", ''Icarus'' 129, 254-268.</ref> This remains a controversial subject, however, and further studies of Mars&mdash;which shares Earth's [[sidereal day|rotation period]] and [[axial tilt]], but not its large moon or liquid core&mdash;may provide additional insight.
 
  
−
The Moon is just far enough away to have, when seen from Earth, very nearly the same apparent angular size as the Sun (the Sun is 400 times larger, but the Moon is 400 times closer). This allows total [[eclipse]]s and annular eclipses to occur on Earth.
+
The gravitational attraction between the Earth and moon causes the [[tides]] on Earth. The same attraction has led to a phenomenon known as ''tidal locking'': The moon's rotation about its own axis takes the same time as its orbit around the Earth. As a result, the moon always presents the same face to our 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 what is called the ''solar terminator''.
  
−
<div style="clear: both" />
+
Based on a phenomenon called ''tidal acceleration'', the moon recedes from Earth at a rate of approximately 38 millimeters (mm) per year. The Earth's day also lengthens by about 17 microseconds (µs) every year. Over millions of years, these tiny modifications can add up to significant changes. During the [[Devonian]] period, for example, there were 400 days in a year, with each day lasting 21.8 hours.
−
[[Image:Earth-Moon.jpg|thumb|center|800px|The relative sizes of and distance between Earth and Moon, to scale.]]
+
 
 +
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., "[https://www.aanda.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 (2004): 261-285. Retrieved August 3, 2019.</ref> Some theorists believe that without this stabilization against the torques 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]].
  
−
The most widely accepted theory of the Moon's origin, the [[Giant impact hypothesis|giant impact theory]], states that it was formed from the collision of a Mars-size [[protoplanet]] with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.
+
If Earth's axis of rotation were to approach the plane of the ecliptic (the apparent path of the Sun as viewed from the Earth), seasonal differences could become extreme, resulting in severe [[weather]] conditions. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life.<ref>D.M. Williams, J.F. Kasting, "Habitable planets with high obliquities," ''Icarus'' 129 (1997): 254-268.</ref> This remains a controversial subject, however, and further studies of Mars&mdash;which shares Earth's rotation period and axial tilt, but not its large moon or liquid core&mdash;may provide added insights.
  
−
The Earth does not have another natural orbiting body other than the Moon, although the asteroid [[3753 Cruithne|Cruithne]] has been erroneously described as such. Cruithne was discovered in 1986 and follows an elliptical orbit around the Sun at about the same average orbital radius as the Earth. However, from the point of view of the moving Earth, Cruithne follows a horseshoe orbit around the Sun that avoids close proximity with the Earth.
+
<div style="clear: both" />
 +
[[Image:Earth-Moon.jpg|thumb|center|800px|The relative sizes of and distance between Earth and moon, to scale.]]
  
−
==Geography==
+
The Earth does not have a natural orbiting body other than the moon, although the asteroid [[3753 Cruithne|Cruithne]] has been erroneously described as such. Cruithne was discovered in 1986 and follows an elliptical orbit around the Sun at about the same average orbital radius as the Earth. However, from the point of view of the moving Earth, Cruithne follows a horseshoe orbit around the Sun that avoids close proximity with the Earth.
−
{{main|Geography}}
 
  
 +
==Geographical statistics==
 
[[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)]]]]
 
[[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:'''
+
'''Area:'''<ref name="cia">CIA, "World" ''The World Factbook''.</ref>
−
 
 
−
[[Time Zone]]s, [[Coordinate]]s.
 
−
 
 
−
'''Biggest geographic subdivision'''
 
−
 
 
−
[[Continent]]s, [[Ocean]]s
 
  
−
'''Area:'''<ref name="cia">CIA: The World Factbook, "[http://www.cia.gov/cia/publications/factbook/geos/xx.html World]".</ref>
+
* ''Total surface area:'' 510.072 million km<sup>2</sup>
−
 
 
−
* ''Total:'' [[1 E14 m²|510.072 million]]. [[square kilometre|km<sup>2</sup>]]
 
 
* ''[[Landform|Land]]:'' 148.94 million km<sup>2</sup>
 
* ''[[Landform|Land]]:'' 148.94 million km<sup>2</sup>
 
* ''[[Water]]:'' 361.132 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
+
* ''Note:'' 70.8% of the world's surface is covered by water, 29.2% is exposed land
  
−
'''Total water:''' 1.4 &times; 10<sup>9</sup> km<sup>3</sup>, of which 2.5% is freshwater.<ref>Shiklomanov, Igor A. 1993, "World fresh water resources", In ''Water in crisis: A guide to the world's fresh water resources'', ed. Peter H. Gleick, 13–24. New York: Oxford University Press, ISBN 0195076273.</ref>
+
'''Total water volume:'''
 +
 
 +
1.4 &times; 10<sup>9</sup> km<sup>3</sup>, of which 2.5% is freshwater.<ref>Igor A. Shiklomanov, "World fresh water resources," In ''Water in crisis: A guide to the world's fresh water resources'', ed. Peter H. Gleick (New York: Oxford University Press, 1993, ISBN 0195076273), 13–24. </ref>
  
 
'''Land boundaries:'''
 
'''Land boundaries:'''
−
the land boundaries in the world total 250,472 km<ref name="cia" /> (not counting shared boundaries twice)
+
 
 +
The land boundaries in the world total 250,472 km<ref name="cia" /> (not counting shared boundaries twice)
  
 
'''Coastline:'''
 
'''Coastline:'''
−
356,000 km.<ref name="cia" /> (other figures vary substantially depending on how precisely it is measured, tides etc)
 
  
−
'''Maritime claims:''' see [[United Nations Convention on the Law of the Sea]]
+
356,000 km.<ref name="cia" /> (Figures from different sources vary substantially, depending on precision of measurements, tides, and so forth.)
 +
 
 +
'''Maritime claims:''' (see [[United Nations Convention on the Law of the Sea]])
  
 
* ''Contiguous zone:'' 24 [[nautical mile]]s (44 km) claimed by most, but can vary
 
* ''Contiguous zone:'' 24 [[nautical mile]]s (44 km) claimed by most, but can vary
Line 179: Line 253:
 
* ''Note:'' boundary situations with neighboring states prevent many countries from extending their fishing or economic zones to a full 200 nautical miles (370 km)
 
* ''Note:'' boundary situations with neighboring states prevent many countries from extending their fishing or economic zones to a full 200 nautical miles (370 km)
 
* 42 nations and other areas are completely landlocked (see list of [[landlocked|landlocked countries]])
 
* 42 nations and other areas are completely landlocked (see list of [[landlocked|landlocked countries]])
−
 
−
==Plate tectonics==
 
−
{{main|Plate tectonics}}[[Image:Plate tectonics map.gif|thumb|right|380px|A map pointing out the Earth's major plates.]]
 
−
'''Plate tectonics''' (from the [[Greek language|Greek]] word for "one who constructs and destroys", τεκτων, ''tektoon'') is a [[theory]] of [[geology]] developed to explain the phenomenon of [[continental drift]] and is currently the theory accepted by the vast majority of scientists working in this area. In the theory of plate tectonics 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 viscous part of the mantle. The mantle behaves like a superheated and extremely viscous liquid.
 
−
 
−
 
−
The lithosphere essentially ''floats'' on the asthenosphere. The lithosphere 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.
 
−
 
−
<br clear="all">
 
−
{|align="center"
 
−
|[[Image:Oceanic-continental convergence Fig21oceancont.gif|thumb|Oceanic / Continental]]
 
−
|[[Image:Continental-continental convergence Fig21contcont.gif|thumb|Continental / Continental]]
 
−
|[[Image:Oceanic-oceanic convergence Fig21oceanocean.gif|thumb|Oceanic / Oceanic]]
 
−
|}
 
−
 
−
Plate tectonic theory arose out of two separate geological observations: [[continental drift]], noticed in the early 20th century, and [[seafloor spreading]], noticed in the 1960s. The theory itself was developed during the late 1960s and has since been universally accepted by virtually all scientists. The theory has revolutionized the [[earth science]]s comparable in its unifying and explanatory power for diverse geological phenomena as was the development of the [[periodic table]] for [[chemistry]], the discovery of the [[genetic code]] for [[biology]], and [[quantum mechanics]] in [[physics]].
 
  
 
== Environment and ecosystem ==
 
== Environment and ecosystem ==
 
{{main|Biosphere}}
 
{{main|Biosphere}}
  
−
The planet's lifeforms are sometimes said to form a "[[biosphere]]". This biosphere is generally believed to have begun [[evolution|evolving]] about 3.5 billion (3.5{{e|9}}) years ago. Earth is the only place in the universe where life is absolutely known to exist, and  some scientists believe that [[Rare Earth hypothesis|biospheres might be rare]].
+
Earth is the only place in the universe where life is absolutely known to exist. The various life forms on Earth are sometimes said to form a ''biosphere''.
 +
 
 +
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]].
  
−
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|250px|A familiar [[beach|scene]] on Earth simultaneously shows the lithosphere, hydrosphere, and atmosphere. Photo  by Fir0002/Flagstaffotos]]
−
[[Image:90 mile beach.jpg|thumb|right|220px|A familiar [[beach|scene]] on Earth which simultaneously shows the lithosphere, hydrosphere and atmosphere.]]
 
  
 
=== Climate ===
 
=== Climate ===
−
{{main|Climate}}
+
The most prominent features of the Earth's climate are its two large polar regions, two narrow [[temperate]] zones, and a wide, equatorial, [[tropical]] to [[subtropical]] region. [[precipitation (meteorology)|Precipitation]] patterns vary widely, ranging from several meters of water per year to less than a millimeter per year.
−
 
 
−
The most prominent features of the earth's climate are its two large polar regions, two narrow [[temperate]] zones, and a wide [[equator]]ial [[tropical]] to [[subtropical]] region. [[precipitation (meteorology)|Precipitation]] patterns vary widely, ranging from several metres of water per year to less than a millimetre.
 
  
−
Ocean currents are important factors in determining climate, particularly the spectacular [[thermohaline circulation]] which distributes heat energy from the equatorial oceans to the polar regions.
+
Ocean currents are important factors in determining climate, particularly the spectacular "thermohaline circulation" (the density-driven circulation of the oceans), which distributes heat energy from the equatorial oceans to the polar regions.
  
 
=== Terrain ===
 
=== Terrain ===
−
The Earth's [[terrain]] can vary greatly from place to place. Covered with about 70% water, much of the Earth's [[continental shelf]] is below sea level. If all of the land on Earth were spread evenly, then water would rise higher than the [[Statue of Liberty]].<ref>The average depth is, in fact, significantly greater than the statue of liberty. Letting the average depth be approximately equal to water volume divided by the Earth's surface area: the total volume of water is about 1.4 &times; 10<sup>9</sup> km<sup>3</sup>; the total area of Earth is about 5.1 &times; 10<sup>8</sup> km<sup>2</sup>. So the average depth would be roughly 2.8 km, whereas the statue of liberty is only 0.093 km, including the pedestal.</ref> The remaining 30% that is dry land has [[mountains]], [[deserts]], [[plain|plains]], [[plateau]]s, etc.
+
The Earth's terrain can vary greatly from place to place. Only about 30 percent of the Earth's surface consists of dry land, while the remainder is covered with water. The exposed land has [[mountains]], [[deserts]], [[plain|plains]], [[plateau]]s, and so forth.
  
−
Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.<ref name="cia" /> Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 &times; 10<sup>9</sup> [[acre]]s of cropland and 8.4 &times; 10<sup>9</sup> acres of pastureland pastureland.<ref>FAO, 1995, "United Nations Food and Agricultural Organization Production Yearbook", 49.</ref>
+
Currently, the total arable land is 13.31 percent of the land surface, with only 4.71 percent supporting permanent crops.<ref name="cia" /> Close to 40 percent of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 &times; 10<sup>9</sup> acres of cropland and 8.4 &times; 10<sup>9</sup> acres of pastureland.<ref>FAO, "United Nations Food and Agricultural Organization Production Yearbook," 1995, 49.</ref>
  
−
==== Extremes ====
+
==== Extremes of elevation ====
−
{{main|Extreme points of the world}}
 
 
'''Elevation extremes:''' (measured relative to [[sea level]])
 
'''Elevation extremes:''' (measured relative to [[sea level]])
  
−
* Lowest point on land: [[Dead Sea]] [[1 E2 m|−417]] m
+
* Lowest point on land: [[Dead Sea]] −417 m (1,376 feet)
−
* Lowest point overall: Challenger Deep of the [[Mariana Trench]] in the [[Pacific Ocean]] [[1 E4 m|−10,924]] m <!-- Takuyo measurement; see Mariana Trench for details —><ref name="rain.org">{{cite web
+
* Lowest point overall: Challenger Deep of the [[Mariana Trench]] in the [[Pacific Ocean]] −10,924 m (36,000 feet or 6.8 miles)
−
| url = http://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html
+
* Highest point: [[Mount Everest]] 8,844 m (29,185 feet) (2005 estimate)
−
| title = "Deep Ocean Studies"
 
−
| work = Ocean Studies
 
−
| publisher = RAIN National Public Internet and Community Technology Center
 
−
| accessdate = 2006-04-02
 
−
}}
 
−
</ref>
 
−
* Highest point: [[Mount Everest]] [[1 E3 m|8,844]] m (2005 est.)
 
  
 
=== Natural resources ===
 
=== Natural resources ===
−
{{main|Natural resource}}
+
* The Earth's crust contains large deposits of [[fossil fuel]]s, including [[coal]], [[petroleum]], [[natural gas]], and methane clathrate. We tap these deposits for energy production and as feedstock for chemical production.
 +
* Mineral [[ore]] bodies have been formed in the 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.
 +
* Earth's biosphere produces many useful biological products, including (but not limited to) [[food]], [[wood]], [[pharmaceutical]]s, oxygen, and recycled 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.
  
−
* 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.
+
Some of these resources, such as mineral fuels, are difficult to replenish over a short time scale. They are therefore called ''non-renewable resources''. The exploitation of non-renewable resources by humans has become a subject of significant controversy.
−
* Mineral [[ore]] bodies have 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.
 
−
* 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 ===
 
=== Land use ===
−
 
+
* ''Arable land:'' 13.13 percent<ref name="cia"/>
−
* ''Arable land:'' 13.13%<ref name="cia">CIA: The World Factbook, "[http://www.cia.gov/cia/publications/factbook/geos/xx.html World]".</ref>
+
* ''Permanent crops:'' 4.71 percent<ref name="cia" />
−
* ''Permanent crops:'' 4.71%<ref name="cia" />
+
* ''Permanent pastures:'' 26 percent
−
* ''Permanent pastures:'' 26%
+
* ''Forests and woodland:'' 32 percent
−
* ''Forests and woodland:'' 32%
+
* ''Urban areas:'' 1.5 percent
−
* ''Urban areas:'' 1.5%
+
* ''Other:'' 30 percent (1993 est.)
−
* ''Other:'' 30% (1993 est.)
 
  
 
'''Irrigated land:'''
 
'''Irrigated land:'''
−
2,481,250 km<sup>2</sup> ([[1993]] est.)
+
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, [[blizzard]]s, [[flood]]s, [[drought]]s, and other calamities and [[disaster]]s.
 
−
 
 
−
Large 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]].
 
  
−
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.
+
=== Environmental hazards ===
 +
Many places experience natural hazards such as [[hurricane]]s, earthquakes, landslides, [[tsunami]]s, volcanic eruptions, [[tornado]]es, sinkholes, blizzards, floods, droughts, and other calamities. In addition, large areas are subject to human-made [[pollution]] of the air and water, loss of vegetation (as a result of [[overgrazing]], [[deforestation]], and [[desertification]]), loss of [[wildlife]], [[species]] [[extinction]], [[soils retrogression and degradation|soil degradation]], soil depletion, [[erosion]], and the introduction of [[invasive species]]. There is also concern about [[global warming]], based on a "greenhouse effect" caused by increasing emissions of carbon dioxide and particulate matter, and this subject is the focus of intense study and debate.
  
−
=== Human geography ===
+
=== Human population distribution ===
 
{{main|Human geography}}
 
{{main|Human geography}}
 
[[Image:Earthlights dmsp.jpg|333px|thumb|right|Earth at night, composite of pictures taken between October 1994 and March 1995.]]
 
[[Image:Earthlights dmsp.jpg|333px|thumb|right|Earth at night, composite of pictures taken between October 1994 and March 1995.]]
−
Earth has approximately 6,500,000,000 human inhabitants ([[February 24]] [[2006]] estimate).
+
The Earth has over 7 billion human inhabitants. Human population density varies widely around the world. It is estimated that only one-eighth of the Earth's surface is suitable for humans to dwell on. About 30 percent of the surface is exposed land (above the oceans), and about half of that is desert, high mountains, or other unsuitable terrain.
−
<ref name="LiveScience">
 
−
{{cite news
 
−
  | first= Leonard
 
−
  | last= David
 
−
  | url= http://www.livescience.com/othernews/060224_world_population.html
 
−
  | title= Planet's Population Hit 6.5 Billion Saturday
 
−
  | work= Live Science
 
−
  | date= [[2006-02-24]]
 
−
  | accessdate= 2006-04-02
 
−
}}
 
−
</ref>
 
−
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.
 
−
 
 
−
It is estimated that only one eighth of the surface of the Earth is suitable for [[human]]s to live on &mdash; three-quarters is covered by [[ocean]]s, and half of the land area is [[desert]], high [[mountain]]s or other unsuitable terrain.
 
−
 
 
−
The northernmost settlement in the world is [[Alert, Nunavut|Alert]], [[Ellesmere Island]], [[Canada]]. The southernmost is the [[Amundsen-Scott South Pole Station]], in [[Antarctica]], almost exactly at the [[South Pole]].
 
−
 
 
−
There are 267 administrative divisions, including nations, dependent areas, other, and miscellaneous entries. Earth does not have a [[sovereignty|sovereign]] [[government]] with planet-wide authority. Independent sovereign [[nation]]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.
 
−
 
 
−
In total, about four hundred people have been outside the Earth's atmosphere as of [[2004]], and of these, twelve have walked on the [[Moon]]. ''See [[space exploration]].''
 
−
 
 
−
== Descriptions of Earth ==
 
−
 
 
−
Earth has often been personified as a [[deity]], in particular a [[goddess]] (''see [[Gaia (mythology)|Gaia]] and [[Mother Earth]]''). The [[China|Chinese]] Earth goddess [[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 than that of other cultures because Earth is male, [[Geb]], and sky is female, [[Nut]].
 
−
 
 
−
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]].
 
  
−
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]] (50 [[meter]]) 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.
+
The northernmost settlement in the world is Alert, [[Nunavut]], on [[Ellesmere Island]] in [[Canada]]. The southernmost is the [[Amundsen-Scott South Pole Station]] in [[Antarctica]], almost exactly at the [[South Pole]].
  
−
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 mapmaking, and vicariously [[geography]], have historically been the disciplines devoted to depicting the Earth. [[Surveying]], the determination of locations and distances, and to a somewhat lesser extent [[navigation]], the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.
+
There are 267 administrative divisions, including nations, dependent areas, and other miscellaneous territories. Earth does not have a sovereign [[government]] with planet-wide authority. Independent sovereign [[nation]]s claim all the land surface except for some segments of [[Antarctica]]. The United Nations is a general international organization, but it is primarily a discussion forum with only limited ability to pass and enforce [[international law]]s.
  
−
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.
+
In total, about 400 people have been outside the Earth's atmosphere as of 2004, and of these, 12 have walked on the moon. (See [[space exploration]].)
−
<ref name="fja">
 
−
{{cite book
 
−
| last = Ackerman
 
−
| first = Forrest J
 
−
| authorlink = Forrest J Ackerman
 
−
| year = 1997
 
−
| title = Forrest J Ackerman's World of Science Fiction
 
−
| publisher = RR Donnelley & Sons Company
 
−
| location = Los Angeles
 
−
| id = ISBN 1-57544-069-5
 
−
| pages = 116-117
 
−
}}
 
−
</ref> [[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">
 
−
{{cite web
 
−
| url = http://gtrc911.quaker.org/pale_blue_dot.html
 
−
| title = Pale Blue Dot
 
−
| publisher = SETI@home
 
−
| accessdate = 2006-04-02
 
−
}}
 
−
</ref>
 
−
Earth has also been described as a massive [[spaceship]], with a [[life support system]] that requires maintenance, or as having a [[biosphere]] that forms one large [[organism]]. ''See [[Spaceship Earth]] and [[Gaia theory]].''
 
−
 
 
−
In the fictional ''[[Hitchhiker's Guide To The Galaxy]]'', Earth's entire entry consists of a single word — "harmless" — emphasizing its insignificance in the cosmos. Later in the eponymous science fiction series, this humbling consideration is alleviated slightly when the entry is updated to read "mostly harmless". The Earth is also a computer to search for the Ultimate Question of Life, the Universe and Everything.
 
  
 
==Earth's future==
 
==Earth's future==
−
<!-- Red giants can have different diameters depending on their mass and the stage of evolution. —>
+
[[Image:redgiants.jpg|thumb|225px|Comparison between the red giant [[Antares]] and the Sun. The black circle is the size of the orbit of Mars.]]
−
[[Image:redgiants.jpg|thumb|200px|Comparison between the red giant [[Antares]] and the Sun. The black circle is the size of the orbit of Mars.]]
 
−
The most probable cause for the Earth's destruction will take place when the [[Sun]] as part of its solar lifespan, expands to a [[Red Giant]]. Current 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 will expand to about 1.7 [[Astronomical Unit|AUs]] due to mass loss by the Sun, and so the planet will escape envelopment.<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> This event is estimated to take place in 5 billion years. (5&nbsp;[[Gigayear|Gyr]]).
 
  
−
Before reaching the Red Giant stage, however, the [[Solar luminosity|luminosity of the Sun]] will continue to steadily increase. It will grow from the current luminosity by 10% in 1.1 Gyr and up to 40% in 3.5 Gyr.<ref name="sun_future" /> Climate models show that a steady increase in radiation reaching the Earth are 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>
+
According to current models, the Earth's destruction will take place in about 5 billion years (Gigayears, Gyr), when the Sun (as part of its solar lifespan) expands to a form called a ''[[Red Giant]]''. It is thought that the Sun will expand out to about 99 percent of the distance to the Earth's present orbit (1 [[Astronomical Unit]], or AU). By that time, however, the Earth's orbit will expand to about 1.7 AU, due to mass loss by the Sun, and so the planet will escape envelopment.<ref name=sun>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'', 418 (1993): 457. Retrieved August 3, 2019.</ref>.
  
−
==See also==
+
Before reaching the Red Giant stage, however, the Sun's luminosity is predicted to increase steadily&mdash;by 10 percent in 1.1 Gyr and up to 40 percent in 3.5 Gyr.<ref name=sun/> Climate models show that a steady increase in radiation reaching the Earth is likely to have dire consequences, including possible loss of the oceans.<ref>J.F. Kasting, "[http://adsabs.harvard.edu/abs/1988Icar...74..472K Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus]," ''Icarus'', 74 (1988): 472-494. Retrieved August 3, 2019.</ref>
  
−
{| {{prettytable}}
+
==Notes==
−
!align=left| Subtopic !!align=left| Links
+
<references/>
−
|-
 
−
| Astronomy || [[Darwin (ESA)]] · [[Terrestrial Planet Finder]]
 
−
|-
 
−
| Ecology || [[Millennium Ecosystem Assessment]]
 
−
|-
 
−
| Economy || [[World economy]]
 
−
|-
 
−
| Fiction || [[Hollow Earth]] · [[Journey to the Center of the Earth]] · [[Destruction of Earth]] · [[Earth in fiction]]
 
−
|-
 
−
| Geography,<br />Geology || [[Degree Confluence Project]] · [[Earthquake]] · [[Extremes on Earth]] · [[Plate tectonics]] · [[Equatorial bulge]]
 
−
|-
 
−
| History || [[Geologic time scale]] · [[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]]
 
−
|-
 
−
| Law || [[International law]]
 
−
|-
 
−
| Mapping || [[Google Earth]] · [[World Wind]]
 
−
|-
 
−
| Politics || [[List of countries]]
 
−
|}
 
  
 
==References==
 
==References==
−
* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html NASA's Earth fact sheet]
+
* Ackerman, Forrest J. ''Forrest J Ackerman's World of Science Fiction''. Los Angeles, CA: RR Donnelley & Sons Company, 1997. ISBN 1575440695
−
* ''Discovering the Essential Universe'' (Second Edition) by Neil F. Comins (2001)
+
* Comins, Neil F. ''Discovering the Essential Universe.'' 2nd ed. New York: W.H. Freeman, 2001. ISBN 978-0716744382
−
* [http://space.about.com/od/earth/a/earthinfo_2.htm space.about.com - Earth - Pictures and Astronomy Facts]
+
* Dalrymple, G.B. ''The Age of the Earth''. Stanford, CA: Stanford University Press, 1991. ISBN 0804715696
−
 
+
* Gleick, Peter H. (ed.). ''Water in crisis: A guide to the world's fresh water resources''. New York: Oxford University Press, 1993. ISBN 0195076273
−
===Notes===
+
* Gonzalez, Guillermo, and Jay Wesley Richards. ''The Privileged Planet: How Our Place in the Cosmos Is Designed for Discovery.'' Washington, DC: Regnery Pub, 2004. ISBN 978-0895260659
−
<references/>
+
* Greene, Nick. [https://www.thoughtco.com/earth-our-home-planet-3071503 Explore Earth - Our Home Planet] ''ThoughtCo'', October 25, 2018. Retrieved September 30, 2020.
 +
* Ward, Peter Douglas, and Donald Brownlee. ''Rare Earth: Why Complex Life Is Uncommon in the Universe.'' New York: Copernicus, 2003. ISBN 978-0387952895
 +
* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html NASA's Earth fact sheet] Retrieved September 30, 2020.
  
 
==External links==
 
==External links==
 +
All links retrieved February 12, 2024.
  
 
* [http://geomag.usgs.gov USGS Geomagnetism Program]
 
* [http://geomag.usgs.gov USGS Geomagnetism Program]
−
* [http://www.geo.cornell.edu/geology/classes/isacks/velstruct.pdf Overview of the Seismic Structure of Earth] {{PDFlink}}
 
−
* [http://earthobservatory.nasa.gov/Newsroom/BlueMarble NASA Earth Observatory]
 
−
* [http://www.funonthenet.in/content/view/282/31/ Beautiful Views of Planet Earth] Pictures of Earth from space
 
−
* [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.projectshum.org/Planets/earth.html Projectshum.org's Earth fact file] (for younger folk)
 +
* [http://www.gutenberg.org/ebooks/search/?query=earth Earth] Project Gutenberg
 +
* [https://www.nationalgeographic.com/science/space/solar-system/earth Earth] ''National Geographic''
  
 
{{Footer_SolarSystem}}
 
{{Footer_SolarSystem}}
−
 
−
<!--Categories—>
 
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]
−
[[Category:Earth science]]
+
[[Category:Astronomy]]
−
[[Category:Earth| ]]
+
[[Category:Earth sciences]]
−
 
 
−
<!--Interwiki—>
 
−
{{Link FA|it}}
 
−
{{Link FA|sr}}
 
  
−
{{credit|54663066}}
+
{{credit5|Earth|54663066|Hydrosphere|54466592|Crust_(geology)|53204709|Mantle_(geology)|53211439|Structure_of_the_Earth|52753170}}

Latest revision as of 17:34, 12 February 2024


Earth  Astronomical symbol of Earth
A color image of Earth as seen from Apollo 17.
The Blue Marble, taken from Apollo 17
Orbital characteristics (Epoch J2000)
Aphelion 152,097,701 km
(1.016 710 333 5 AU)
Perihelion 147,098,074 km
(0.983 289 891 2 AU)
Semi-major axis 149,597,887.5 km
(1.000 000 112 4 AU)
Semi-minor axis 149,576,999.826 km
(0.999 860 486 9 AU)
Orbital circumference 924,375,700 km
(6.179 069 900 7 AU)
Orbital eccentricity 0.016 710 219
Sidereal orbit period 365.256 366 d
(1.000 017 5 a)
Synodic period n/a
Max. orbital speed 30.287 km/s
(109,033 km/h)
Average orbital speed 29.783 km/s
(107,218 km/h)
Min. orbital speed 29.291 km/s
(105,448 km/h)
Orbital inclination to ecliptic 0
(7.25° to Sun's equator)
Longitude of the ascending node 348.739 36°
Argument of the perihelion 114.207 83°
Satellites 1 (the Moon)
(see also 3753 Cruithne)
 
Physical characteristics
Aspect Ratio 0.996 647 1
Ellipticity 0.003 352 9
 
Radius:
Equatorial 6,378.137 km
Polar 6,356.752 km
Mean 6,372.797 km
 
Diameter:
Equatorial 12,756.274 km
Polar 12,713.504 km
Mean 12,745.594 km
 
Circumference:
Equatorial 40,075.02 km
Meridional 40,007.86 km
Mean 40,041.47 km
   
Surface Area 510,065,600 km²
 Land 148,939,100 km² (29.2 %)
 Water 361,126,400 km² (70.8 %)
 
Volume 1.083 207 3×1012 km³
Mass 5.9742×1024 kg
Density 5,515.3 kg/m³
Equatorial surface gravity 9.7801 m/s²
(0.997 32 g)
Escape velocity 11.186 km/s
Sidereal rotation period 0.997 258 d (23.934 h)
Rotational velocity
(at the equator) 		465.11 m/s
Axial tilt 23.439 281°
Right ascension
of North pole 		0° (0 h 0 min 0 s)
Declination 90°
Albedo 0.367
Surface temperature
- min
- mean
- max
185 K (-88 °C)
287 K (14 °C)
331 K (58 °C)
Surface pressure 100 kPa
Adjective Earthling, Terrestrial, Terran
 
Atmospheric constituents
nitrogen 77 %
oxygen 21 %
argon 1 %
carbon dioxide 0.038%
water vapor trace (varies with climate)

Earth is the third planet in the solar system in terms of distance from the Sun, and the fifth in order of size. It is the largest and densest of the solar system's terrestrial planets. Scientists estimate that the Earth was formed around 4.57 billion (4.57×109) years ago.[1] About 70 percent of the surface is presently covered by oceans, and the remainder consists of continents and islands. The abundance of water is a unique feature that distinguishes our "blue planet" from others in the solar system.

Since its formation, the Earth has been transformed through various geological and biological processes. The Earth's upper layer (lithosphere) is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The interior of the planet is also active, with a thick layer (mantle) of highly viscous material and an iron core that generates a magnetic field.

The Earth is the only place in the universe known to support life, and its position, structure, and composition make it ideally suited for this purpose. Orbiting the Sun at an appropriate distance, this planet receives the proper amounts of heat and light to sustain life, and its rotational and orbital movements allow for daily and seasonal cycles. Its natural satellite, the Moon, is sufficiently large to produce regular tides and to stabilize the planet's axial tilt. In addition, the atmosphere provides useful gases, transports water vapor, moderates temperature extremes, absorbs harmful ultraviolet radiation, and burns up most meteors before they can strike the Earth's surface. The Earth's magnetosphere shields the surface by deflecting charged particles brought by the solar wind. The presence of water and the chemical composition of the Earth's surface play significant roles in the development and sustenance of living organisms. At the same time, the Earth's surface conditions have been significantly altered by the presence of life forms, which have created an ecological balance.

Photographic images of the Earth from space reveal its awesome beauty, and at the same time highlight the fragility of our common home, often called "spaceship earth." That image has become a contemporary icon. Unlike political maps, it has no borders. It calls for cooperation and peace among all Earth's inhabitants to assure a future for humanity and the planet.

Historical descriptions of Earth

In ancient cultures, the Earth was often personified as a deity, particularly a goddess who, like a mother, gives birth to and nurtures life (see Gaea). The Chinese Earth goddess Hou-Tu is similar to Gaia. 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. By contrast, in ancient Egyptian mythology, Earth was symbolized as a male, Geb, and the sky was taken to be a female, Nut.

In terms of the Earth's shape, earlier belief systems included the idea of a flat Earth. On the other hand, ancient Greek philosophers and other thinkers, including Thomas Aquinas (1225–1274), believed the Earth was spherical. Arguments used to support the latter view took into account some of the secondary effects of the Earth's shape and parallels with the shape of other planets. Today there is sufficient evidence to indicate that the Earth is an oblate spheroid—it bulges slightly at the equator and is slightly flattened at the poles. The idea of a flat Earth is more frequently treated tongue-in-cheek, or with mockery.

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.[2] Apollo 17's "Blue Marble" photograph of Earth, taken in 1972 from cislunar space (volume of space within the moon's orbit), became the current iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. A photo of a distant Earth taken by Voyager 1 in 1990 inspired Carl Sagan to describe the planet as a "pale blue dot."[3] Earth has also been described as a massive spaceship, with a life support system that requires maintenance, or as having a biosphere that forms one large organism (See Gaia hypothesis.)

Historically, the disciplines devoted to depicting the Earth's surface have been cartography, the study and practice of mapmaking, and geography. 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 information needed for describing the Earth's surface.

Symbol

The astrological and astronomical symbol for Earth consists of a circled cross, the arms of the cross representing a meridian and the equator (⊕). A variant puts the cross atop the circle (♁).

Earth-based terminology

Terms that refer to the Earth may use the Latin root terra-, such as the word terrestrial, or the Latin root tellur-, as in telluric and tellurium. Both terms derive from the Roman goddess Terra Mater, who was also called by the presumably more ancient name Tellūs Mater. Scientific terms such as geography, geocentric, and geothermal use the Greek prefix geo-, derived from Terra Mater's Greek counterpart, Gaia.

Physical characteristics

Earth is the largest of the "inner" planets of the solar system, that is, the planets between the Sun and the asteroid belt. When viewed from outer space, much of the Earth has a deep blue and white appearance, caused by the oceans and clouds in the atmosphere. It has an albedo (measure of light reflectivity) of 36.7 percent, which is exceeded only by Venus among the inner planets.

As mentioned above, the Earth is shaped as an oblate spheroid. Its average diameter is approximately 12,742 km.[4] The planet's curvature is visible from some regions on the surface such as the Bonneville Salt Flats in the United States.

Earth is also the densest planet in the entire solar system. Its average density is about 5,515 kilograms/meter3 (kg/m3), while the average density of surface material is around 3,000 kg/m3. The mass of the Earth is approximately 5980 yottagrams (5.98 x 1024 kg).

Taken as a whole, the Earth's composition by mass [1] is:

iron: 35 .1 %
oxygen: 28 .2 %
silicon: 17 .2 %
magnesium: 15 .9 %
nickel: 1 .6 %
calcium: 1 .6 %
aluminum: 1 .5 %
sulfur: 0 .70 %
sodium: 0 .25 %
titanium: 0 .071 %
potassium: 0 .019  %
other elements:     0 .53 %

Geologic layers

Earth cutaway from core to exosphere. Partially to scale

When considered in terms of its composition, the Earth consists of the following major layers: the atmosphere, hydrosphere, crust, mantle, and core. Each of these has a number of sublayers.

In addition, scientists use the term lithosphere when referring to the solid crust and the solidified uppermost part of the mantle. Below the lithosphere lies the asthenosphere, which consists of the weak (or soft) zone of the upper mantle. The biosphere, which refers to that part of the outer shell in which life occurs, is usually not considered one of the physical layers of the Earth.

The geologic component layers of the Earth[5] are located at the following depths below the surface:

Depth Layer
Kilometers Miles
0–60 0–37 Lithosphere (locally varies between 5 and 200 km)
0–35 0–22 Crust (locally varies between 5 and 70 km)
35–60 22–37 Uppermost part of mantle
35–2890 22–1790 Mantle
100–700 62–435 Asthenosphere
2890–5100 1790–3160 Outer core
5100–6378 6378–3954 Inner core

Atmosphere

Main article: Earth's atmosphere

The Earth's atmosphere protects and sustains life forms by absorbing ultraviolet solar radiation, moderating temperature extremes, transporting water vapor, and providing useful gases. It contains about 78 percent nitrogen, 21 percent oxygen, and trace amounts of other gases such as carbon dioxide and water vapor.

Hydrosphere

Main article: hydrosphere

The hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. Approximately 70.8 percent of the Earth is covered by water and only 29.2 percent is terra firma. The abundance of water on Earth is a unique feature that distinguishes our "blue planet" from others in the solar system.

The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1.35 × 1018 tons, or about 1/4400 of the total mass of the Earth.

The movement of water within the hydrosphere is described by the hydrologic cycle. This cycle includes water beneath the Earth's surface and in rocks (lithosphere), the water in plants and animals (biosphere), the water covering the surface of the planet in liquid and solid forms, and the water in the atmosphere.

Crust

Main article: Crust (geology)

The outer layer of the Earth is called its crust. It is composed mainly of basalt and granite. It is cooler and more rigid than the deeper layers of the mantle and core. The crust is a part of the lithosphere, which floats on fluid interior layers. Oceanic crust is different from that of the continents. Oceanic crust (sima) is 5–10 km thick and is composed primarily of basalt, a dark, dense rock. Continental crust (sial) is 20–70 km deep and is composed of a variety of less dense rocks.

Mantle

Main article: Mantle (geology)

The Earth's mantle is the thick shell of rock that surrounds the outer core and lies directly beneath the thin crust. The mantle lies roughly between 30 and 2,900 km below the surface, and occupies about 70 percent of the Earth's volume. The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Temperatures in the mantle range between 100° C at the upper boundary to over 4,000° C at the boundary with the core. Because of the extremely high pressures within the mantle, it is thought to be a solid or highly viscous fluid. The high temperatures cause the silicate material to be sufficiently ductile that it can flow on very long timescales. Convection of the mantle is expressed at the surface through the motions of tectonic plates.

Core

Seismic measurements indicate that the Earth's core is divided into two parts: a solid inner core with a radius of about 1,220 km, and a liquid outer core with a radius extending from 1,220 km to 3,480 km. The solid inner core is believed to be 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 liquid outer core surrounds the inner core and is thought to be composed of iron mixed with 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. The solid inner core is too hot to hold a permanent magnetic field, but it probably acts to stabilize the magnetic field generated by the liquid outer core.

Magnetic field

The Earth's magnetosphere shields the planet's surface by deflecting charged particles coming from the solar wind.

The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. The field forms the "Magnetosphere," which shields the Earth's surface by deflecting charged particles coming from the solar wind. The collision between the magnetic field and solar wind forms what are called the "Van Allen radiation belts," which are a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma (composed of charged particles) enters the Earth's atmosphere at the magnetic poles, it produces a phenomenon called the Aurora.

History of Earth

Based on the available evidence, scientists have been working to reconstruct some of the details of our planet's past. It is thought that the Earth was formed around 4.57 billion years ago (see Age of the Earth) out of a gaseous cloud called the "solar nebula," from which the solar system emerged. The moon formed soon after. Initially molten, the outer layer of the planet cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere; condensing water vapor, augmented by ice delivered by comets, produced the oceans.[6]

Scientists think that early living organisms may have existed more than 3.5 billion years ago.[7] Later, as photosynthetic organisms developed, they could harvest the Sun's energy directly. As a result, oxygen accumulated in the atmosphere and gave rise to the ozone layer.[8] With the ozone layer absorbing harmful ultraviolet radiation, life forms were able to colonize the Earth's surface.

It is thought that over hundreds of millions of years, the continents migrated across the surface of the Earth, occasionally combining to form a supercontinent and then breaking apart. Roughly 750 million years ago (m.y.a.), the earliest known supercontinent, Rodinia, began to break up. The continents later recombined to form Pannotia, 600-540 m.y.a., then finally Pangaea, which broke apart 180 m.y.a.[9] (See Plate tectonics below.)

Since the 1960s, it has been hypothesized that severe glacial action during the Neoproterozoic, between 750 and 580 m.y.a., covered much of the planet in a sheet of ice. This hypothesis, termed "Snowball Earth," is of particular interest as it precedes the Cambrian explosion when multicellular life forms began to proliferate.[10]

It appears that from the time of the Cambrian explosion, about 535 million years ago, there were five distinct mass extinctions.[11] The last one occurred 65 million years ago, when a meteorite collision is thought to have triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which seem to have resembled shrews. Over the last 65 million years, mammalian life diversified.

Once humans appeared, they transformed the Earth in a short time span like no other life form had before. Humans have influenced both the nature and quantity of other life forms, as well as global climate.

Plate tectonics

Main article: Plate tectonics
A map pointing out the Earth's major plates.

Plate tectonics (from the Greek word τεκτων, tektoon, meaning "one who constructs and destroys") is a theory that arose out of two separate geological observations: continental drift, noticed in the early twentieth century, and seafloor spreading, noticed in the 1960s. The theory was developed during the late 1960s and has since been accepted by virtually all scientists working in the field.

According to this theory, the lithosphere is broken up into what are called tectonic plates, which essentially "float" on the asthenosphere. These plates move in relation to one another. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. This theory has revolutionized earth science (geology) and is valued for its unifying and explanatory power for diverse geological phenomena.


Oceanic / Continental
Continental / Continental
Oceanic / Oceanic

Earth in the solar system

An image showing the rotation of the Earth.

It takes the Earth, on average, 23 hours, 56 minutes, and 4.091 seconds (one sidereal day) to rotate around the axis connecting the North and South Poles. From Earth, the main apparent motion of celestial bodies in the sky (except meteors, which are within the atmosphere, and low-orbiting satellites) is the movement to the west at a rate of 15 °/h (= 15'/min), that is, a sun or moon diameter every two minutes.

Earth orbits the sun every 365.2564 mean solar days (one sidereal year). From Earth, this gives an apparent movement of the sun with respect to the stars at a rate of about one degree per day, that is, a sun or moon diameter every 12 hours, eastward. The Earth's orbital speed averages about 30 km/s (108,000 km/h), which is enough to cover one Earth diameter (~12,600 km) in seven minutes, and one distance to the moon (384,000 km) in four hours.

Earth has one natural satellite, the moon. The diameter of the sun is about 400 times that of the moon, but the moon is 400 times closer to the Earth than the sun. Consequently, the sun and moon appear roughly equal in size when viewed from the Earth. Moreover, this apparent equality of size allows us to observe a total solar eclipse, when the moon completely covers the sun, and an annular eclipse, when the sun appears slightly larger than the moon and produces a bright ring (annulus) around the moon.

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 axis is tilted some 23.5 degrees against the Earth-Sun plane. This tilt has significant effects, including the variations of seasons. Also, the Earth-moon plane is tilted about 5 degrees against the Earth-Sun plane—otherwise, there would be an eclipse every month.

In modern times, the Earth reaches its perihelion (when the planet is nearest to the Sun) about January 3, and its aphelion (when it is farthest from the Sun) about July 4. (These dates are near the solstices, which occur around December 21 and June 21).

The Moon

Main article: Moon
Name Diameter (km) Mass (kg) Semi-major axis (km) Orbital period
Moon 3,474.8 7.349×1022 384,400 27 Days, 7 hours, 43.7 minutes

The moon, sometimes called "Luna," is a relatively large satellite that structurally resembles a terrestrial planet. The moon's diameter is about one-quarter that of the Earth. The natural satellites orbiting other planets are also called moons, after the Earth's moon. With the exception of Pluto's Charon, it is the largest moon in the solar system relative to the size of its planet.

The most widely accepted theory of the moon's origin, the "giant impact hypothesis," states that it was formed from the collision of a Mars-size protoplanet with the early Earth. This hypothesis explains (among other things) the moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.

Earthrise as seen from lunar orbit on Apollo 8, December 24, 1968. The moon always presents the same face to the Earth. Consequently, our planet, when viewed from the moon, does not rise or set, but is always located in the same position in the sky.

The gravitational attraction between the Earth and moon causes the tides on Earth. The same attraction has led to a phenomenon known as tidal locking: The moon's rotation about its own axis takes the same time as its orbit around the Earth. As a result, the moon always presents the same face to our planet. As the moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases. The dark part of the face is separated from the light part by what is called the solar terminator.

Based on a phenomenon called tidal acceleration, the moon recedes from Earth at a rate of approximately 38 millimeters (mm) per year. The Earth's day also lengthens by about 17 microseconds (µs) every year. Over millions of years, these tiny modifications can add up to significant changes. During the Devonian period, for example, there were 400 days in a year, with each day lasting 21.8 hours.

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.[12] Some theorists believe that without this stabilization against the torques 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.

If Earth's axis of rotation were to approach the plane of the ecliptic (the apparent path of the Sun as viewed from the Earth), seasonal differences could become extreme, resulting in severe weather conditions. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life.[13] This remains a controversial subject, however, and further studies of Mars—which shares Earth's rotation period and axial tilt, but not its large moon or liquid core—may provide added insights.

The relative sizes of and distance between Earth and moon, to scale.

The Earth does not have a natural orbiting body other than the moon, although the asteroid Cruithne has been erroneously described as such. Cruithne was discovered in 1986 and follows an elliptical orbit around the Sun at about the same average orbital radius as the Earth. However, from the point of view of the moving Earth, Cruithne follows a horseshoe orbit around the Sun that avoids close proximity with the Earth.

Geographical statistics

Physical map of the Earth (Medium) (Large 2 MB)

Area:[14]

  • Total surface area: 510.072 million km2
  • Land: 148.94 million km2
  • Water: 361.132 million km2
  • Note: 70.8% of the world's surface is covered by water, 29.2% is exposed land

Total water volume:

1.4 × 109 km3, of which 2.5% is freshwater.[15]

Land boundaries:

The land boundaries in the world total 250,472 km[14] (not counting shared boundaries twice)

Coastline:

356,000 km.[14] (Figures from different sources vary substantially, depending on precision of measurements, tides, and so forth.)

Maritime claims: (see United Nations Convention on the Law of the Sea)

  • Contiguous zone: 24 nautical miles (44 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 km) or to the edge of the continental margin
  • Exclusive fishing zone: 200 nautical miles (370 km) claimed by most, but can vary
  • Exclusive economic zone: 200 nautical miles (370 km) claimed by most, but can vary
  • Territorial sea: 12 nautical miles (22 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 km)
  • 42 nations and other areas are completely landlocked (see list of landlocked countries)

Environment and ecosystem

Main article: Biosphere

Earth is the only place in the universe where life is absolutely known to exist. The various life forms on Earth are sometimes said to form a biosphere.

The biosphere is divided into a number of biomes, inhabited by broadly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life, while most of the more populous biomes lie near the equator.

A familiar scene on Earth simultaneously shows the lithosphere, hydrosphere, and atmosphere. Photo by Fir0002/Flagstaffotos

Climate

The most prominent features of the Earth's climate are its two large polar regions, two narrow temperate zones, and a wide, equatorial, tropical to subtropical region. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter per year.

Ocean currents are important factors in determining climate, particularly the spectacular "thermohaline circulation" (the density-driven circulation of the oceans), which distributes heat energy from the equatorial oceans to the polar regions.

Terrain

The Earth's terrain can vary greatly from place to place. Only about 30 percent of the Earth's surface consists of dry land, while the remainder is covered with water. The exposed land has mountains, deserts, plains, plateaus, and so forth.

Currently, the total arable land is 13.31 percent of the land surface, with only 4.71 percent supporting permanent crops.[14] Close to 40 percent of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 × 109 acres of cropland and 8.4 × 109 acres of pastureland.[16]

Extremes of elevation

Elevation extremes: (measured relative to sea level)

  • Lowest point on land: Dead Sea −417 m (1,376 feet)
  • Lowest point overall: Challenger Deep of the Mariana Trench in the Pacific Ocean −10,924 m (36,000 feet or 6.8 miles)
  • Highest point: Mount Everest 8,844 m (29,185 feet) (2005 estimate)

Natural resources

  • The Earth's crust contains large deposits of fossil fuels, including coal, petroleum, natural gas, and methane clathrate. We tap these deposits for energy production and as feedstock for chemical production.
  • Mineral ore bodies have been formed in the Earth's crust by the action of erosion and plate tectonics. These bodies form concentrated sources for many metals and other useful elements.
  • Earth's biosphere produces many useful biological products, including (but not limited to) food, wood, pharmaceuticals, oxygen, and recycled 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 mineral fuels, are difficult to replenish over a short time scale. They are therefore called non-renewable resources. The exploitation of non-renewable resources by humans has become a subject of significant controversy.

Land use

  • Arable land: 13.13 percent[14]
  • Permanent crops: 4.71 percent[14]
  • Permanent pastures: 26 percent
  • Forests and woodland: 32 percent
  • Urban areas: 1.5 percent
  • Other: 30 percent (1993 est.)

Irrigated land: 2,481,250 km2 (1993 est.)

Environmental hazards

Many places experience natural hazards such as hurricanes, earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities. In addition, large areas are subject to human-made pollution of the air and water, loss of vegetation (as a result of overgrazing, deforestation, and desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and the introduction of invasive species. There is also concern about global warming, based on a "greenhouse effect" caused by increasing emissions of carbon dioxide and particulate matter, and this subject is the focus of intense study and debate.

Human population distribution

Earth at night, composite of pictures taken between October 1994 and March 1995.

The Earth has over 7 billion human inhabitants. Human population density varies widely around the world. It is estimated that only one-eighth of the Earth's surface is suitable for humans to dwell on. About 30 percent of the surface is exposed land (above the oceans), and about half of that is desert, high mountains, or other unsuitable terrain.

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

There are 267 administrative divisions, including nations, dependent areas, and other miscellaneous territories. Earth does not have a sovereign government with planet-wide authority. Independent sovereign nations claim all the land surface except for some segments of Antarctica. The United Nations is a general international organization, but it is primarily a discussion forum with only limited ability to pass and enforce international laws.

In total, about 400 people have been outside the Earth's atmosphere as of 2004, and of these, 12 have walked on the moon. (See space exploration.)

Earth's future

Comparison between the red giant Antares and the Sun. The black circle is the size of the orbit of Mars.

According to current models, the Earth's destruction will take place in about 5 billion years (Gigayears, Gyr), when the Sun (as part of its solar lifespan) expands to a form called a Red Giant. It is thought that the Sun will expand out to about 99 percent of the distance to the Earth's present orbit (1 Astronomical Unit, or AU). By that time, however, the Earth's orbit will expand to about 1.7 AU, due to mass loss by the Sun, and so the planet will escape envelopment.[17].

Before reaching the Red Giant stage, however, the Sun's luminosity is predicted to increase steadily—by 10 percent in 1.1 Gyr and up to 40 percent in 3.5 Gyr.[17] Climate models show that a steady increase in radiation reaching the Earth is likely to have dire consequences, including possible loss of the oceans.[18]

Notes

  1. ↑ G.B. Dalrymple, The Age of the Earth (Stanford, CA: Stanford University Press, 1991, ISBN 0804715696).
  2. ↑ Forrest J. Ackerman, Forrest J Ackerman's World of Science Fiction (Los Angeles, CA: RR Donnelley & Sons Company, 1997, ISBN 1575440695), 116-117.
  3. ↑ Pale Blue Dot SETI@home.
  4. ↑ "Geodetic Reference System 1980 (GRS80)," XVII General Assembly, International Association of Geodesy.
  5. ↑ T. H. Jordan, "Structural Geology of the Earth's Interior," Procedings National Academy of Science, 1979, Sept., 76(9): 4192–4200. Retrieved August 3, 2019.
  6. ↑ A. Morbidelli et al, "Source Regions and Time Scales for the Delivery of Water to Earth," Meteoritics & Planetary Science, 35(6) (2000): 1309-1320. Retrieved August 3, 2019.
  7. ↑ W. Ford Doolittle, "Uprooting the Tree of Life," Scientific American, Feb. 2000.
  8. ↑ L. V. Berkner, L. C. Marshall, "On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere," Journal of the Atmospheric Sciences, 22(3) (1965): 225–261.
  9. ↑ J.B. Murphy and R.D. Nance, "How do supercontinents assemble?", American Scientist, 92(4) (2004): 324-333.
  10. ↑ J.L. Kirschvink, "Late Proterozoic Low-Latitude Global Glaciation: The Snowball Earth," The Proterozoic Biosphere, 1992, 51-52.
  11. ↑ D. Raup & J. Sepkoski, "Mass extinctions in the marine fossil record," Science, 215 (1982): 1501–1503.
  12. ↑ Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A.C.M., Levrard, B., "A long term numerical solution for the insolation quantities of the Earth," Astronomy and Astrophysics, 428 (2004): 261-285. Retrieved August 3, 2019.
  13. ↑ D.M. Williams, J.F. Kasting, "Habitable planets with high obliquities," Icarus 129 (1997): 254-268.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 CIA, "World" The World Factbook.
  15. ↑ Igor A. Shiklomanov, "World fresh water resources," In Water in crisis: A guide to the world's fresh water resources, ed. Peter H. Gleick (New York: Oxford University Press, 1993, ISBN 0195076273), 13–24.
  16. ↑ FAO, "United Nations Food and Agricultural Organization Production Yearbook," 1995, 49.
  17. ↑ 17.0 17.1 I.J. Sackmann, A.I. Boothroyd, K.E. Kraemer, "Our Sun. III. Present and Future.," Astrophysical Journal, 418 (1993): 457. Retrieved August 3, 2019.
  18. ↑ J.F. Kasting, "Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus," Icarus, 74 (1988): 472-494. Retrieved August 3, 2019.

References
ISBN links support NWE through referral fees

  • Ackerman, Forrest J. Forrest J Ackerman's World of Science Fiction. Los Angeles, CA: RR Donnelley & Sons Company, 1997. ISBN 1575440695
  • Comins, Neil F. Discovering the Essential Universe. 2nd ed. New York: W.H. Freeman, 2001. ISBN 978-0716744382
  • Dalrymple, G.B. The Age of the Earth. Stanford, CA: Stanford University Press, 1991. ISBN 0804715696
  • Gleick, Peter H. (ed.). Water in crisis: A guide to the world's fresh water resources. New York: Oxford University Press, 1993. ISBN 0195076273
  • Gonzalez, Guillermo, and Jay Wesley Richards. The Privileged Planet: How Our Place in the Cosmos Is Designed for Discovery. Washington, DC: Regnery Pub, 2004. ISBN 978-0895260659
  • Greene, Nick. Explore Earth - Our Home Planet ThoughtCo, October 25, 2018. Retrieved September 30, 2020.
  • Ward, Peter Douglas, and Donald Brownlee. Rare Earth: Why Complex Life Is Uncommon in the Universe. New York: Copernicus, 2003. ISBN 978-0387952895
  • NASA's Earth fact sheet Retrieved September 30, 2020.

External links

All links retrieved February 12, 2024.

 The Solar System
Solar System Template Final.png
The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris
Planets · Dwarf planets · Moons: Terran · Martian · Asteroidal · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
SSSBs: Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.

Retrieved from https://www.newworldencyclopedia.org/p/index.php?title=Earth&oldid=1136104

Content is available under Creative Commons Attribution/Share-Alike License; additional terms may apply. See Terms of Use for details.
Copyright Logo
Powered by MediaWiki