Earth

For other uses, see Earth (disambiguation).

Template: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 planets and the only place in the universe currently known to support life. The Earth was formed around 4.57 billion (4.57×10⁹) years ago (see Age of the Earth) and its largest natural satellite, the Moon, was orbiting it shortly thereafter, around 4.533 billion years ago.

Since it formed, the Earth has evolved through geologic and biological processes so that any traces of the original conditions have been virtually eliminated. The outer surface is divided into several tectonic plates 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 oceans, and the remainder consists of continents and islands.

There is significant interaction between the Earth and the space environment. The relatively large Moon provides ocean tides and has gradually modified the length of the planet's rotation period. A cometary 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 orbit of the planet may also be responsible for the ice ages that have covered significant portions of the surface in glacial sheets.

The Earth does not have another natural orbiting body other than the Moon, although 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.

Lexicography

Terms that refer to the Earth can use the 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 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.

The English word "earth" has cognates in many modern and ancient languages. Examples in modern tongues include aarde in Dutch and Erde in German. The root also has cognates in extinct languages such as ertha in Old Saxon and ert (meaning "ground") in Middle Irish, derived from the Old English eorðe. All of these words are derived from the Proto-Indo-European base *er-. Given metathesis, we can find cognates of "earth" between terra and the modern Romance languages, for instance tierra in Spanish or terra in Portuguese.

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, irtsitu; Aramaic, araa; Phoenician, erets (which appears in the Mesha Stele); and Hebrew, ארץ (arets, or erets when followed by a noun modifier).

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 (♁).

History

Main article: History of Earth

Based on the available evidence, scientists have been able to reconstruct detailed information about the planet's past. Earth is believed to have formed around 4.55 billion years ago out of the solar nebula, along with the Sun and other planets. The moon formed soon afterwards. 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. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life lived.

The development of photosynthesis allowed the sun's energy to be harvested directly; the resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes. Cells within colonies became increasingly specialized, resulting in true multicellular organisms. With the ozone layer absorbing harmful ultraviolet radiation, life colonized the surface of Earth.

Over hundreds of millions of years, continents formed and broke up as the surface of Earth continually reshaped itself. The continents have migrated across the surface of the Earth, occasionally combining to form a supercontinent. Roughly 750 million years ago, the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, 600-540 mya, then finally Pangaea, which broke apart 180 mya.

Since the 1960s it has been hypothesized that a severe glacial action between 750 and 580 million years ago, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed the "Snowball Earth", and it is of particular interest as it precedes the Cambrian explosion when multi-cellular lifeforms began to proliferate.

Since the Cambrian explosion, about 535 million years ago, there were 5 distinct mass extinctions. The last one occurred 65 million years ago, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the last 65 million years, mammalian life diversified, and several million years ago, a small African ape gained the ability to stand upright. As brain size increased, these hominids developed the use of tools and language. The development of agriculture, and then civilization allowed humans to affect the Earth in a short timespan like no other life form had before, affecting both the nature and quantity of other life forms as well as global climate.

Physical characteristics

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 of 36.7%, which is exceeded only by Venus among the inner planets of the solar system. It is also the largest and densest of the inner planets.

Atmosphere

Main article: Earth's atmosphere

The atmospheric pressure on the surface of the Earth averages 101.3 kPa, with a scale height of about 6 km. It is 78% nitrogen, 21% oxygen, with trace amounts of other gaseous molecules such as water vapor. The atmosphere protects the Earth's life forms by absorbing ultraviolet solar radiation, moderating temperature extremes, transporting water vapor, and providing useful gases.

Geology

Main article: Structure of the Earth

Earth cutaway from core to exosphere. Partially to scale

The Earth's shape is that of an oblate spheroid, with an average diameter of approximately 12,742 km (~ 40,000 km / π). 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 atmosphere, the hydrosphere, the crust, the mantle, and its core. The biosphere is a tiny layer in this composition and is usually not considered part of the physical layers of the Earth.

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

Depth		Layer
Kilometres	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

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 theories, through the 1970s. Today, the subject is more frequently treated tongue-in-cheek or with mockery.

See also: Geology

Earth in the solar system

File:Rotating earth (small).gif

An image showing the rotation of the Earth.

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

Earth orbits the Sun every 365.2564 mean solar days (1 sidereal year). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1 °/day, i.e., a Sun or Moon diameter every 12 hours, eastward. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), 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.

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.

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 (which causes the seasons); and the Earth-Moon plane is tilted about 5 degrees against the Earth-Sun plane (otherwise there would be an eclipse every month).

In an inertial reference frame, the Earth's axis undergoes a slow precessional 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 length of day variation.

In modern times, Earth's perihelion is always about January 3, and aphelion is about July 4 (near the solstices, 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.

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 terrestrial planet-like satellite, whose diameter is about one-quarter of the Earth's. With the exception of Pluto's Charon, it is the largest moon in the Solar system relative to the size of its planet. The natural satellites 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 phases: The dark part of the face is separated from the light part by the solar terminator.

Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm per year. The Earth's day also lengthens by about 17 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.

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.

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. 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 with Mars. If Earth's axis of rotation were to approach the 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 scientists who have studied the effect claim that this might kill all large animal and higher plant life. This remains a controversial subject, however, and further studies of Mars—which shares Earth's rotation period and axial tilt, but not its large moon or liquid core—may provide additional insight.

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

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

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

Earth also has at least two co-orbital satellites, the asteroids 3753 Cruithne and 2002 AA₂₉.

Geography

Main article: Geography

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

Map references:

Time Zones, Coordinates.

Biggest geographic subdivision

Continents, Oceans

Area:

• Total: 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: 1.4 × 10⁹ km³, of which 2.5% is freshwater.

Land boundaries: the land boundaries in the world total 250,472 km (not counting shared boundaries twice)

Coastline: 356,000 km. (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

• 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)

Plate tectonics

Main article: Plate tectonics

A map pointing out the Earth's major plates.

Plate tectonics (from the 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 and the solidified uppermost part of the 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 plates. These plates move in relation to one another at one of three types of plate boundaries: convergent, divergent, and transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries.

File:Oceanic-continental convergence Fig21oceancont.svg Oceanic / Continental

Continental / Continental

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 sciences 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.

Pangaea

Main article: Pangaea

At one time the planet's plate tectonics, were fused into one super continent. The name of this supercontinent is called Pangaea. Pangaea or Pangea (derived from Παγγαία, Greek for "all earth") is the name given to the supercontinent that existed during the Paleozoic and Mesozoic eras, before the process of plate tectonics separated each of the component continents into their current configuration. The name was coined by Alfred Wegener, chief proponent of Continental Drift, in 1915.

In configuration, Pangaea was a C-shaped landmass that spread across the equator. The body of water that was enclosed within the resulting crescent has been named the Tethys Sea. Owing to Pangaea's massive size, the inland regions appear to have been very dry due to the lack of precipitation. The large supercontinent would potentially have allowed terrestrial animals to migrate freely all the way from the South Pole to the North Pole.

The vast ocean that once surrounded the supercontinent of Pangaea has been named Panthalassa. Pangaea, it seems, broke up about 180 mya in the Jurassic Period, first into two supercontinents (Gondwana to the south and Laurasia to the north), thereafter into the continents as we understand them today.

Pangaea was not the first supercontinent. From the evidence available, scientists have reconstructed a predecessor, termed Pannotia, that formed about 600 mya, before dividing again some 50 million years later. Another, Rodinia, apparently formed approximately 1,100 mya and divided 750 mya.

Environment and ecosystem

Main article: Biosphere

The planet's lifeforms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion (3.5×10⁹) years ago. Earth is the only place in the universe where life is absolutely known to exist, and some scientists believe that biospheres might be rare.

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 which simultaneously shows the lithosphere, hydrosphere and atmosphere.

Climate

Main article: 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 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.

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. The remaining 30% that is dry land has mountains, deserts, plains, plateaus, etc.

Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. Close to 40% 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 pastureland.

Extremes

Main article: Extreme points of the world

Elevation extremes: (measured relative to sea level)

• Lowest point on land: Dead Sea −417 m
• Lowest point overall: Challenger Deep of the Mariana Trench in the Pacific Ocean −10,924 m
• Highest point: Mount Everest 8,844 m (2005 est.)

Natural resources

Main article: Natural resource

• Earth's crust contains large deposits of fossil fuels: (coal, petroleum, natural gas, methane clathrate). These deposits are used by humans both for energy production and as feedstock for chemical production.
• Mineral ore bodies have been formed in Earth's crust by the action of erosion and plate tectonics. These bodies form concentrated sources for many metals and other useful elements.
• Earth's biosphere produces many useful biological products, including (but far from limited to) food, wood, pharmaceuticals, 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 mineral fuels, are difficult to replenish on a short time scale, called non-renewable resources. The exploitation of non-renewable resources by human civilization has become a subject of significant controversy in modern environmentalism movements.

Land use

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

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

Natural and environmental hazards

Large areas are subject to extreme weather such as (tropical cyclones), hurricanes, or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities and disasters.

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, soil degradation, soil depletion, erosion, and introduction of invasive species.

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

Human geography

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

Main article: Human

Earth has approximately 6,500,000,000 human inhabitants (February 24 2006 estimate). Projections indicate that the world's human population will reach seven billion in 2013 and 9.1 billion in 2050 (2005 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 humans to live on — three-quarters is covered by oceans, and half of the land area is desert, high mountains or other unsuitable terrain.

The northernmost settlement in the world is 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 sovereign government with planet-wide authority. Independent sovereign nations 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 laws.

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 and Mother Earth). The 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 it was spherical.

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.

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. Apollo 17's 1972 "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". 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

File:Redgiants.jpg

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 AUs due to mass loss by the Sun, and so the planet will escape envelopment. This event is estimated to take place in 5 billion years. (5 Gyr).

Before reaching the Red Giant stage, however, the 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. Climate models show that a steady increase in radiation reaching the Earth are likely to have dire consequences, including possible loss of the oceans.

See also

References

• NASA's Earth fact sheet
• Discovering the Essential Universe (Second Edition) by Neil F. Comins (2001)
• space.about.com - Earth - Pictures and Astronomy Facts

Notes

External links

• USGS Geomagnetism Program
• Overview of the Seismic Structure of Earth Template:PDFlink
• NASA Earth Observatory
• Beautiful Views of Planet Earth Pictures of Earth from space
• Java 3D Earth's Globe

• Projectshum.org's Earth fact file (for younger folk)