Difference between revisions of "Earth"

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Template:Sprotect Template:Featured article Template:Two other uses Template:Infobox Planet Earth (Template:PronEng^[1]) is the third planet from the Sun and is the largest of the terrestrial planets in the Solar System, in both diameter and mass. It is also referred to as the Earth, Planet Earth, and the World, and in some contexts, Gaia and Terra.^[2]

Home to millions of species^[3] including humans, Earth is the only place in the universe where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years^[4]^[5]^[6]^[7] ago, and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land.^[8]

Earth's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's surface.^[9]^[10] Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.^[11] The Earth's axis of rotation is tilted 23.4°^[12] away from the perpendicular to its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans.^[13] Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the Earth's orbit, caused by the gravitational influence of other planets, are believed to have given rise to the ice ages that have intermittently covered significant portions of Earth's surface in glacial sheets.

History

For a more detailed treatment, see History of Earth.

Scientists have been able to reconstruct detailed information about the planet's past. Earth and the other planets in the Solar System formed 4.54 billion years ago^[4] out of the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass^[14] impacting the Earth in a glancing blow.^[15] Some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.

Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, produced the oceans.^[13] 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 existed.^[16]

The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and resulted in a layer of ozone (a form of molecular oxygen [O₃]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.^[17] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.^[18]

As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), the earliest known supercontinent, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which broke apart 180 mya.^[19]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.^[20]

Following the Cambrian explosion, about 535 mya, there have been five mass extinctions.^[21] The last extinction event occurred 65 mya, 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 past 65 million years, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright.^[22] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,^[23] affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 mya, then intensified during the Pleistocene about 3 mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.^[24]

Composition and structure

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity and the strongest magnetic field.^[25]

Shape

For a more detailed treatment, see Figure of the Earth.

File:Terrestrial planet size comparisons.jpg

Size comparison of inner planets (left to right): Mercury, Venus, Earth, and Mars

The Earth's shape is very close to an oblate spheroid—a rounded shape with a bulge around the equator—although the precise shape (the geoid) varies from this by up to 100 meters.^[26] The average diameter of the reference spheroid is about 12,742 km. More approximately the distance is 40,000 km/π because the meter was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France.^[27]

The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km larger than the pole to pole diameter.^[28] The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in billiard balls.^[29] Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.^[30]

Chemical composition

Template:Seealso

F. W. Clarke's Table of Crust Oxides
Compound	Formula	Composition
silica	SiO₂	59.71%
alumina	Al₂O₃	15.41%
lime	CaO	4.90%
Magnesia	MgO	4.36%
sodium oxide	Na₂O	3.55%
iron(II) oxide	FeO	3.52%
potassium oxide	K₂O	2.80%
iron(III) oxide	Fe₂O₃	2.63%
water	H₂O	1.52%
titanium dioxide	TiO₂	0.60%
phosphorus pentoxide	P₂O₅	0.22%
Total		99.22%

The mass of the Earth is approximately 5.98e kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.^[31]

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.^[32]

Internal structure

For a more detailed treatment, see Structure of the Earth.

File:Earth-crust-cutaway-english.svg

Earth cutaway from core to exosphere. Not to scale.

The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents.^[33]

The geologic component layers of the Earth^[34] are at the following depths below the surface:^[35]

Depth km	Layer	Density g/cm³
0–60	Lithosphere (locally varies between 5 and 200 km)	—
0–35	... Crust (locally varies between 5 and 70 km)	2.2–2.9
35–60	... Uppermost part of mantle	3.4–4.4
35–2890	Mantle	3.4–5.6
100–700	... Asthenosphere	—
2890–5100	Outer core	9.9–12.2
5100–6378	Inner core	12.8–13.1

The internal heat of the planet is most likely produced by the radioactive decay of potassium-40, uranium-238 and thorium-232 isotopes. All three have half-life decay periods of more than a billion years.^[36] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.^[37] A portion of the core's thermal energy is transported toward the crust by Mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.^[38]

Tectonic plates

For a more detailed treatment, see Plate tectonics.

File:Plates tect2 en.svg

A map illustrating the Earth's major plates.

According to plate tectonics theory, 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 forms the inner part of the upper mantle. The asthenosphere behaves like a superheated material that is in a semi-fluidic, plastic-like state.^[39]

The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: convergent, divergent and transform. The last occurs where two plates move laterally relative to each other, creating a strike-slip fault. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.^[40]

The main plates are:^[41]

Plate name	Area 10⁶ km²	Covering
African Plate	61.3	Africa
Antarctic Plate	60.9	Antarctica
Australian Plate	47.2	Australia
Eurasian Plate	67.8	Asia and Europe
North American Plate	75.9	North America and north-east Siberia
South American Plate	43.6	South America
Pacific Plate	103.3	Pacific Ocean

Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate actually fused with Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr^[42] (3.0 in/yr) and the Pacific Plate moving 52–69 mm/yr (2.1–2.7 in/yr). At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr (0.8 in/yr).^[43]

Surface

For more detailed treatments, see Landform and Extreme points of the World.

File:AYool topography 15min.png

Present day Earth altimetry and bathymetry. Data from the National Geophysical Data Center's TerrainBase Digital Terrain Model.

The Earth's terrain varies greatly from place to place. About 70.8%^[44] of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,^[28] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts^[45] also act to reshape the landscape.

As the continental plates migrate across the planet, the ocean floor is subducted under the leading edges. At the same time, upwellings of mantle material create a divergent boundary along mid-ocean ridges. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.^[46]^[47]

The continental plates consist of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.^[48] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.^[49] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.^[50] Common carbonate minerals include calcite (found in limestone), aragonite and dolomite.^[51]

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.^[52] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3e km² of cropland and 3.4e km² of pastureland.^[53]

File:Earth elevation histogram 2.svg

Elevation histogram of the surface of the Earth—approximately 71% of the Earth's surface is covered with water.

The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 686 m.^[54]

Hydrosphere

For a more detailed treatment, see Hydrosphere.

The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the solar system. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911 m.^[55]^[56] The average depth of the oceans is 3,794 m, more than five times the average height of the continents.^[54]

The mass of the oceans is approximately 1.35e metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386e km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.^[57] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.^[58]

About 3.5% of the total mass of the oceans consists of salt. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.^[59] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.^[60] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.^[61] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.^[62]

Atmosphere

For a more detailed treatment, see Earth's atmosphere.

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.^[63] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 7 km (23,000 ft) at the poles to 17 km (60,000 ft) at the equator, with some variation due to weather factors.^{[Citation Needed]}

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere that exists today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth's include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.^[64] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Carbon dioxide, water vapor, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C and life would likely not exist.^[44]

Weather and climate

For more detailed treatments, see Weather and Climate.
The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km (about 7 mi) of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.^[65]

The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.^[66] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.^[67]

File:Air masses 2.jpg

Source regions of global air masses.

Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.^[65] Most of the water is then transported back to lower elevations by river systems, usually returning to the oceans or being deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.^[68]

The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.^[69] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly-used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.^[66]

Upper atmosphere

File:Full moon partially obscured by atmosphere.jpg

This view from orbit shows the full Moon partially obscured by the Earth's atmosphere. NASA image.

Template:Seealso Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.^[64] Each of these layers has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere (where the Earth's magnetic fields interact with the solar wind).^[70] An important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.^[71]

Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can escape from the planet's gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate.^[72] For this reason, the Earth's current environment is oxidizing, rather than reducing, with consequences for the chemical nature of life which developed on the planet. The oxygen-rich atmosphere also preserves much of the surviving hydrogen by locking it up in water molecules.^[73]

File:Dipole field.jpg

The Earth's magnetic field, which approximates a dipole.

Magnetic field

For a more detailed treatment, see Earth's magnetic field.
The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.^[74]^[75]

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.^[76]

Orbit and rotation

For a more detailed treatment, see Earth's rotation.

File:Rotating earth (large).gif

An animation showing the rotation of the Earth.

Relative to the background stars, it takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds (one sidereal day) to rotate around the axis that connects the north and the south poles.^[77] From Earth, the main apparent motion of celestial bodies in the sky (except that of meteors within the atmosphere and low-orbiting satellites) is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes. (The apparent sizes of the Sun and the Moon are approximately the same.)

Earth orbits the Sun at an average distance of about 150 million kilometers (93.2 million miles) 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 (or a Sun or Moon diameter every 12 hours) eastward. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 30 km/s (108,000 km/h or 67,000 mi/h), which is fast enough to cover the planet's diameter (about 12,600 km [7,800 mi]) in seven minutes, and the distance to the Moon (384,000 km or 238,000 mi) in four hours.^[63]

File:PaleBlueDot.jpg

Earth seen as a tiny dot by the Voyager 1 spacecraft, more than 6 billion kilometers (4 billion miles) from Earth.

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. 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. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counter-clockwise. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.5 degrees from the perpendicular to the Earth–Sun plane (which causes the seasons); and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane (without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses).^[78]^[63]

Because of the axial tilt of the Earth, the position of the Sun in the sky (as seen by an observer on the surface) varies over the course of the year. For an observer at a northern latitude, when the northern pole is tilted toward the Sun the day lasts longer and the Sun climbs higher in the sky. This results in warmer average temperatures from the increase in solar radiation reaching the surface. When the northern pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the arctic circle, an extreme case is reached where there is no daylight at all for part of the year. (This is called a polar night.)

This variation in the climate (because of the direction of the Earth's axial tilt) results in the seasons. By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo a slight, irregular motion (known as nutation) with a main period of 18.6 years. The orientation (rather than the angle) of the Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.^[79]

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4 (for other eras, see precession and Milankovitch cycles). The changing Earth-Sun distance results in an increase of about 6.9%^[80] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.^[81]

The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (930,000 miles) in radius.^[82]^[83] This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

Moon

For a more detailed treatment, see Moon.

Name	Diameter	Mass	Semi-major axis	Orbital period
Moon	3,474.8 km	7.349e kg	384,400 km	27 days, 7 hours, 43.7 minutes
Moon	2,159.2 mi	8.1e (short) tons	238,700 mi	27 days, 7 hours, 43.7 minutes

The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's. It is the largest moon in the solar system relative to the size of its planet. (Charon is larger relative to the dwarf planet Pluto.) The natural satellites orbiting other planets are called "moons" after Earth's Moon.

The gravitational attraction between the Earth and Moon cause tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases: The dark part of the face is separated from the light part by the solar terminator.

Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm (1.5 in) a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs a year—add up to significant changes.^[84] During the Devonian period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.^[85]

The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.^[86] 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, extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life.^[87] However, this is a controversial subject, and further studies of Mars—which has a similar rotation period and axial tilt as Earth, but not its large Moon or liquid core—may settle the matter.

Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant. This allows total and annular eclipses to occur on Earth.

File:Earth-Moon2.jpg

A scale representation of the relative sizes of, and distance between, Earth and Moon.

The most widely accepted theory of the Moon's origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia 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.^[88]

Earth has at least two co-orbital satellites, the asteroids 3753 Cruithne and 2002 AA₂₉.^[89]

Habitability

Template:Seealso

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble, and sufficient energy to sustain metabolism.^[90] The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.^[91]

Biosphere

For a more detailed treatment, see Biosphere.

The planet's life forms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion years ago. Earth is the only place in the universe where life is known to exist. Some scientists believe that Earth-like biospheres might be rare.^[92]

The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land primarily latitude and height above the sea level separates biomes. Terrestrial biomes lying within the Arctic, Antarctic Circle or in high altitudes are relatively barren of plant and animal life, while the greatest latitudinal diversity of species is found at the Equator.^[93]

Natural resources and land use

For a more detailed treatment, see Natural resource.

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.

Large deposits of fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth's crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics.^[94] These bodies form concentrated sources for many metals and other useful elements.

The Earth's biosphere produces many useful biological products for humans, 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.^[95] Humans also live on the land by using building materials to construct shelters. In 1993, human use of land is approximately:

Land use	Percentage
Arable land:	13.13%^[52]
Permanent crops:	4.71%^[52]
Permanent pastures:	26%
Forests and woodland:	32%
Urban areas:	1.5%
Other:	30%

The estimated amount of irrigated land in 1993 was 2,481,250 km².^[52]

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.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.

A scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather conditions and a global rise in average sea levels.^[96]

Human geography

For a more detailed treatment, see Human geography.
Template:Earth Labelled Map

File:Earthlights dmsp.jpg

The Earth at night, a composite of DMSP/OLS ground illumination data on a simulated night-time image of the world. This image is not photographic and many features are brighter than they would appear to a direct observer.

Earth has approximately 6,671,226,000 human inhabitants as of July 2007.^[97] Projections indicate that the world's human population will reach seven billion in 2013 and 9.2 billion^[98] in 2050. Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.^[99]

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 either desert (14%),^[100] high mountains (27%),^[101] or other less suitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.^[102] (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)

Independent sovereign nations claim all of the planet's land surface, with the exception of some parts of Antarctica. As of 2007 there are 201 sovereign states, including the 192 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities. Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.

The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict. It is not, however, a world government. While the U.N. provides a mechanism for international law and, when the consensus of the membership permits, armed intervention,^[103] it serves primarily as a forum for international diplomacy.

In total, about 400 people have been outside the Earth's atmosphere as of 2004, and, of these, twelve have walked on the Moon. Normally the only humans in space are those on the International Space Station. The station's crew of three people is usually replaced every six months.

Cultural viewpoint

File:AS8-13-2329.jpg

The first photograph ever taken of an "Earthrise," on Apollo 8.

Etymology

The name Earth originates from the 8th century Anglo-Saxon word erda, which means ground or soil. In Old English the word became eorthe, then erthe in Middle English.^[104] Earth was first used as the name of the planet around 1400.^[105] It is the only planet whose name in English is not derived from Greco-Roman mythology.

The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle. This symbol is known as the wheel cross, sun cross, Odin's cross or Woden's cross. Although it has been used in various cultures for different purposes, it came to represent the compass points, earth and the land. Another version of the symbol is a cross on top of a circle; a stylized globus cruciger that was also used as an early astronomical symbol for the planet Earth.^[106]

Religious beliefs

Earth has often been personified as a deity, in particular a goddess. In many cultures the mother goddess, also called the Mother Earth, is also portrayed as a fertility deity. See also Graha.

To the Aztec, Earth was called Tonantzin—"our mother". The Chinese Earth goddess Hou-T'u^[107] is similar to Gaia, the Greek goddess personifying the Earth. To Hindus it is called Bhuma Devi, the Goddess of Earth. In Norse mythology, the Earth goddess Jord was the mother of Thor and the daughter of Annar. Ancient Egyptian mythology is different from that of other cultures because Earth is male, Geb, and sky is female, Nut.

In many religions, creation myths exist, recalling a story involving the creation of the Earth by a supernatural deity or deities.

Exploration and mapping

In the ancient past there were varying levels of belief in a flat Earth, with the Mesopotamian culture portraying the world as a flat disk afloat in an ocean. The spherical form of the Earth was suggested by early Greek philosophers; a belief espoused by Pythagoras. By the Middle Ages—as evidenced by thinkers such as Thomas Aquinas—European belief in a spherical Earth was widespread.^[108] Prior to circumnavigation of the planet and the introduction of space flight, belief in a spherical Earth was based on observations of the secondary effects of the Earth's shape and parallels drawn with the shape of other planets.^[109]

Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

Modern perspective

See also: Spaceship Earth, Gaia theory, and Geocentric orbit

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.^[110]

File:Earth and Moon from Mars PIA04531.jpg

Earth and Moon from Mars, imaged by Mars Global Surveyor. From space, the Earth can be seen to go through phases similar to the phases of the Moon.

Earth was first photographed from space by Explorer 6 in 1959.^[111] Yuri Gagarin became the first human to view Earth from space in 1961. The crew of the Apollo 8 was the first to view an Earth-rise from lunar orbit in 1968. In 1972 the crew of the Apollo 17 produced the famous "Blue Marble" photograph of the planet Earth from cislunar space (see top of page). This became an iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. NASA archivist Mike Gentry has speculated that "The Blue Marble" is the most widely distributed image in human history. A photo taken of a distant Earth by Voyager 1 in 1990 inspired Carl Sagan to describe the planet as a "Pale Blue Dot."^[112] Earth has also been described as a massive spaceship, with a life support system that requires maintenance,^[113] or as having a biosphere that forms one large organism.^[114]

Over the past two centuries a growing environmental movement has emerged that is concerned about humankind's effects on the Earth. The key issues of this socio-political movement are the conservation of natural resources, elimination of pollution, and the usage of land. Environmentalists advocate sustainable management of resources and stewardship of the natural environment through changes in public policy and individual behavior. Of particular concern is the large-scale exploitation of non-renewable resources. Changes sought by the environmental movements are sometimes in conflict with commercial interests due to the additional costs associated with managing the environmental impact of those interests.^[115]

Future

Template:Seealso The life cycle of the Sun.

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium ash at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will increase by 10 percent over the next 1.1 billion years (1.1 Gyr), and by 40% over the next 3.5 Gyr.^[116] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet's oceans.^[117]

The Earth's increasing surface temperature will accelerate the inorganic CO₂ cycle, reducing its concentration to the lethal levels for plants (10 ppm for C4 photosynthesis) in 900 million years. But even if the Sun were eternal and stable, the continued internal cooling of the Earth would have resulted in a loss of much of its atmosphere and oceans (due to lower volcanism).^[118] After another billion years the surface water will have completely disappeared.^[119] The Earth is expected to be effectively habitable for another 500 million years or so.^[120]

The Sun, as part of its solar lifespan, will expand to a red giant in about 5 Gyr. Models predict that the Sun will expand out to about 99% of the distance to Earth's present orbit (1 astronomical unit, or AU). However, by that time, the orbit of Earth may have expanded to about 1.7 AU because of the diminished mass of the Sun. It was thought that the planet might thus escape envelopment by the expanded Sun's sparse outer atmosphere, though most (if not all) existing life would have been destroyed by the Sun's proximity to Earth.^[116] However, more recent simulations have shown that Earth's orbit will decay due to tidal effects, causing it to enter the Sun's atmosphere and be destroyed.^[121]

Notes

↑ Earth (PLANET), entry in the Cambridge Advanced Learner's Dictionary, Cambridge University Press, online. Accessed 14-II-2008.
↑ Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. C.f.:
Blue, Jennifer (July 5, 2007). Descriptor Terms (Feature Types). Gazetteer of Planetary Nomenclature. USGS. Retrieved on 2007-07-05.
↑ May, Robert M. (1999). "How many species are there on earth?". Science 241 (4872): 1441-1449. http://adsabs.harvard.edu/abs/1988Sci...241.1441M. Retrieved 2007-08-14.
↑ ^4.0 ^4.1 Dalrymple, G.B. (1991). The Age of the Earth. California: Stanford University Press. ISBN 0-8047-1569-6.
↑ Newman, William L. (July 9, 2007). Age of the Earth. Publications Services, USGS. Retrieved on 2007-09-20.
↑ Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Geological Society, London, Special Publications 190: 205-221. http://sp.lyellcollection.org/cgi/content/abstract/190/1/205. Retrieved 2007-09-20.
↑ Stassen, Chris (September 10, 2005). The Age of the Earth. The TalkOrigins Archive. Retrieved on 2007-09-20.
↑ Harrison, Roy M.; Hester, Ronald E. (2002). Causes and Environmental Implications of Increased UV-B Radiation. Royal Society of Chemistry. ISBN 0854042652.
↑ Other planets in the solar system are either too hot or too cold to support liquid water. However, it is confirmed to have existed on the surface of Mars in the past, and may still appear today. See: Msnbc. "Rover reveals Mars was once wet enough for life", NASA, March 2, 2007. Retrieved on 2007-08-28. Staff. "Simulations Show Liquid Water Could Exist on Mars", University of Arkansas, November 7, 2005. Retrieved on 2007-08-08.
↑ As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: G. Tinetti et al. (July, 2007). "Water vapour in the atmosphere of a transiting extrasolar planet". Nature 448: 169-171. http://www.nature.com/nature/journal/v448/n7150/abs/nature06002.html.
↑ The number of solar days is one less than the number of sidereal days because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis.
↑ Ahrens, Global Earth Physics: A Handbook of Physical Constants, p. 8.
↑ ^13.0 ^13.1 Morbidelli, A.; Chambers, J.; Lunine, J. I.; Petit, J. M.; Robert, F.; Valsecchi, G. B.; Cyr, K. E. (2000). "Source regions and time scales for the delivery of water to Earth". Meteoritics & Planetary Science 35 (6): 1309–1320. http://adsabs.harvard.edu/abs/2000M&PS...35.1309M. Retrieved 2007-03-06.
↑ Template:Cite conference
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↑ ^44.0 ^44.1 Pidwirny, Michael (2006). Fundamentals of Physical Geography. PhysicalGeography.net. Retrieved on 2007-03-19.
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↑ Staff. Layers of the Earth. Volcano World. Retrieved on 2007-03-11.
↑ Jessey, David. Weathering and Sedimentary Rocks. Cal Poly Pomona. Retrieved on 2007-03-20.
↑ Staff. Minerals. Museum of Natural History, Oregon. Retrieved on 2007-03-20.
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↑ Takuyo measurement; see Mariana Trench for details.
↑ The total volume of the Earth's oceans is: 1.4e km³. The total surface area of the Earth is 5.1e km². So, to first approximation, the average depth would be the ratio of the two, or 2.7 km.
↑ Igor A. Shiklomanov et al (1999). World Water Resources and their use Beginning of the 21st century" Prepared in the Framework of IHP UNESCO. State Hydrological Institute, St. Petersburg. Retrieved on 2006-08-10.
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↑ Fisher, Rick (January, 30, 1996). Astronomical Times. National Radio Astronomy Observatory. Retrieved on 2007-03-21.
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↑ Fisher, Rick (February 5, 1996). Earth Rotation and Equatorial Coordinates. National Radio Astronomy Observatory. Retrieved on 2007-03-21.
↑ Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% the energy at aphelion.
↑ Williams, Jack (December 20, 2005). Earth's tilt creates seasons. USAToday. Retrieved on 2007-03-17.
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↑ For the Earth, the Hill radius is
${\begin{smallmatrix}R_{H}=a\left({\frac {m}{3M}}\right)^{\frac {1}{3}}\end{smallmatrix}}$ ,
where m is the mass of the Earth, a is an Astronomical Unit, and M is the mass of the Sun. So the radius in A.U. is about: ${\begin{smallmatrix}\left({\frac {1}{3\cdot 332,946}}\right)^{\frac {1}{3}}=0.01\end{smallmatrix}}$ .
↑ Espenak, F.; Meeus, J. (February 7, 2007). Secular acceleration of the Moon. NASA. Retrieved on 2007-04-20.
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↑ Hillebrand, Helmut (2004). "On the Generality of the Latitudinal Gradient". American Naturalist 163 (2): 192–211.
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↑ Lovelock, James E. (1979). Gaia: A New Look at Life on Earth, First edition, Oxford: Oxford University Press. ISBN 0-19-286030-5.
↑ Meyer, Stephen M. (August 18, 2002). MIT Project on Environmental Politics & Policy. Massachusetts Institute of Technology. Retrieved on 2006-08-10.
↑ ^116.0 ^116.1 Sackmann, I.-J.; Boothroyd, A. I.; Kraemer, K. E. (1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457–468. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...418..457S. Retrieved 2007-03-31.
↑ Kasting, J.F. (1988). "Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus". Icarus 74: 472–494. http://adsabs.harvard.edu/abs/1988Icar...74..472K. Retrieved 2007-03-31.
↑ Guillemot, H.; Greffoz, V. (Mars 2002). "Ce que sera la fin du monde" (in French). Science et Vie N° 1014.
↑ Carrington, Damian. "Date set for desert Earth", BBC News, February 21, 2000. Retrieved on 2007-03-31.
↑ Britt, Robert (2000-02-25). Freeze, Fry or Dry: How Long Has the Earth Got?.
↑ Palmer, Jason. "Hope dims that Earth will survive Sun's death", NewScientist.com news service, February 22, 2008.

References

Yoder, Charles F. (1995). in T. J. Ahrens: Global Earth Physics: A Handbook of Physical Constants. Washington: American Geophysical Union. ISBN 0875908519. Retrieved on 2007-03-17.

Williams, David R. (September 1, 2004). Earth Fact Sheet. NASA. Retrieved on 2007-03-17.

Comins, Neil F. (2001). Discovering the Essential Universe, Second Edition, W. H. Freeman. ISBN 0-7167-5804-0. Retrieved on 2007-03-17.

Kirk Munsell:Solar System Exploration: Earth. NASA (October 19, 2006). Retrieved on 2007-03-17.

External links

Template:Sisterlinks Template:Portal Template:Portal

WikiSatellite view of Earth at WikiMapia
USGS Geomagnetism Program
NASA Earth Observatory
Earth Profile by NASA's Solar System Exploration
The size of Earth compared with other planets/stars
Climate changes cause Earth's shape to change - NASA
Geody Earth World's search engine that supports Google Earth, NASA World Wind, Celestia, GPS, and other applications.
Planet Earth From AOL Research & Learn: Photos, quizzes and info about Earth's climate, creatures and science.
"The age of the earth": an article by Richard A. Fortey in the TLS, January 9 2008

Template:Start box Template:Succession box Template:End box

Template:Footer SolarSystem

Template:Earth location Template:Nature nav Template:Featured article

Template:Link FA Template:Link FA Template:Link FA Template:Link FA Template:Link FA

[1] Earth (PLANET), entry in the Cambridge Advanced Learner's Dictionary, Cambridge University Press, online. Accessed 14-II-2008.

[2] Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. C.f.:
Blue, Jennifer (July 5, 2007). Descriptor Terms (Feature Types). Gazetteer of Planetary Nomenclature. USGS. Retrieved on 2007-07-05.

[3] May, Robert M. (1999). "How many species are there on earth?". Science 241 (4872): 1441-1449. http://adsabs.harvard.edu/abs/1988Sci...241.1441M. Retrieved 2007-08-14.

[age_earth-4] 4.0 ^4.1 Dalrymple, G.B. (1991). The Age of the Earth. California: Stanford University Press. ISBN 0-8047-1569-6.

[5] Newman, William L. (July 9, 2007). Age of the Earth. Publications Services, USGS. Retrieved on 2007-09-20.

[6] Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Geological Society, London, Special Publications 190: 205-221. http://sp.lyellcollection.org/cgi/content/abstract/190/1/205. Retrieved 2007-09-20.

[7] Stassen, Chris (September 10, 2005). The Age of the Earth. The TalkOrigins Archive. Retrieved on 2007-09-20.

[8] Harrison, Roy M.; Hester, Ronald E. (2002). Causes and Environmental Implications of Increased UV-B Radiation. Royal Society of Chemistry. ISBN 0854042652.

[9] Other planets in the solar system are either too hot or too cold to support liquid water. However, it is confirmed to have existed on the surface of Mars in the past, and may still appear today. See: Msnbc. "Rover reveals Mars was once wet enough for life", NASA, March 2, 2007. Retrieved on 2007-08-28. Staff. "Simulations Show Liquid Water Could Exist on Mars", University of Arkansas, November 7, 2005. Retrieved on 2007-08-08.

[10] As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: G. Tinetti et al. (July, 2007). "Water vapour in the atmosphere of a transiting extrasolar planet". Nature 448: 169-171. http://www.nature.com/nature/journal/v448/n7150/abs/nature06002.html.

[11] The number of solar days is one less than the number of sidereal days because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis.

[12] Ahrens, Global Earth Physics: A Handbook of Physical Constants, p. 8.

[comet-13] 13.0 ^13.1 Morbidelli, A.; Chambers, J.; Lunine, J. I.; Petit, J. M.; Robert, F.; Valsecchi, G. B.; Cyr, K. E. (2000). "Source regions and time scales for the delivery of water to Earth". Meteoritics & Planetary Science 35 (6): 1309–1320. http://adsabs.harvard.edu/abs/2000M&PS...35.1309M. Retrieved 2007-03-06.

[14] Template:Cite conference

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[22] Gould, Stephan J. (October , 1994). "The Evolution of Life on Earth". Scientific American. http://brembs.net/gould.html. Retrieved 2007-03-05.

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[ngdc2006-28] 28.0 ^28.1 Sandwell, D. T.; Smith, W. H. F. (Jul7 26, 2006). Exploring the Ocean Basins with Satellite Altimeter Data. NOAA/NGDC. Retrieved on 2007-04-21.

[29] Staff (November, 2001). WPA Tournament Table & Equipment Specifications. World Pool-Billiards Association. Retrieved on 2007-03-10.

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[EB1911-32] Template:1911

[33] Tanimoto, Toshiro (1995). in Thomas J. Ahrens: Crustal Structure of the Earth (PDF), Washington, DC: American Geophysical Union. ISBN 0-87590-851-9. Retrieved on 2007-02-03.

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[36] Sanders, Robert. "Radioactive potassium may be major heat source in Earth's core", UC Berkeley News, December 10, 2003. Retrieved on 2007-02-28.

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[38] Richards, M. A.; Duncan, R. A.; Courtillot, V. E. (1989). "Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails". Science 246 (4926): 103–107. http://www.sciencemag.org/cgi/content/abstract/246/4926/103. Retrieved 2007-04-21.

[39] Seligman, Courtney (2008). The Structure of the Terrestrial Planets. Online Astronomy eText Table of Contents. cseligman.com. Retrieved on 2008-02-28.

[40] Kious, W. J.; Tilling, R. I. (May 5, 1999). Understanding plate motions. USGS. Retrieved on 2007-03-02.

[41] Brown, W. K.; Wohletz, K. H. (2005). SFT and the Earth's Tectonic Plates. Los Alamos National Laboratory. Retrieved on 2007-03-02.

[42] Meschede, M.; Udo Barckhausen, U. (November 20, 2000). Plate Tectonic Evolution of the Cocos-Nazca Spreading Center. Proceedings of the Ocean Drilling Program. Texas A&M University. Retrieved on 2007-04-02.

[43] Staff. GPS Time Series. NASA JPL. Retrieved on 2007-04-02.

[Pidwirny2006-44] 44.0 ^44.1 Pidwirny, Michael (2006). Fundamentals of Physical Geography. PhysicalGeography.net. Retrieved on 2007-03-19.

[45] Kring, David A.. Terrestrial Impact Cratering and Its Environmental Effects. Lunar and Planetary Laboratory. Retrieved on 2007-03-22.

[46] Duennebier, Fred (August 12, 1999). Pacific Plate Motion. University of Hawaii. Retrieved on 2007-03-14.

[47] Mueller, R.D.; Roest, W.R.; Royer, J.-Y.; Gahagan, L.M.; Sclater, J.G. (March 7, 2007). Age of the Ocean Floor Poster. NOAA. Retrieved on 2007-03-14.

[48] Staff. Layers of the Earth. Volcano World. Retrieved on 2007-03-11.

[49] Jessey, David. Weathering and Sedimentary Rocks. Cal Poly Pomona. Retrieved on 2007-03-20.

[50] Staff. Minerals. Museum of Natural History, Oregon. Retrieved on 2007-03-20.

[51] Cox, Ronadh (2003). Carbonate sediments. Williams College. Retrieved on 2007-04-21.

[cia-52] 52.0 ^52.1 ^52.2 ^52.3 Staff (February 8, 2007). The World Factbook. U.S. C.I.A.. Retrieved on 2007-02-25.

[53] FAO Staff (1995). FAO Production Yearbook 1994, Volume 48, Rome, Italy: Food and Agriculture Organization of the United Nations. ISBN 9250038445.

[hr_mill-54] 54.0 ^54.1 Mill, Hugh Robert (1893). "The Permanence of Ocean Basins". The Geographical Journal 1 (3): 230–234. http://www.wku.edu/~smithch/wallace/S453.htm. Retrieved 2007-02-25.

[rain.org-55] Staff. "Deep Ocean Studies". Ocean Studies. RAIN National Public Internet and Community Technology Center. Retrieved on 2006-04-02.

[56] Takuyo measurement; see Mariana Trench for details.

[57] The total volume of the Earth's oceans is: 1.4e km³. The total surface area of the Earth is 5.1e km². So, to first approximation, the average depth would be the ratio of the two, or 2.7 km.

[58] Igor A. Shiklomanov et al (1999). World Water Resources and their use Beginning of the 21st century" Prepared in the Framework of IHP UNESCO. State Hydrological Institute, St. Petersburg. Retrieved on 2006-08-10.

[59] Mullen, Leslie (June 11, 2002). Salt of the Early Earth. NASA Astrobiology Magazine. Retrieved on 2007-03-14.

[60] Morris, Ron M.. Oceanic Processes. NASA Astrobiology Magazine. Retrieved on 2007-03-14.

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[earth_fact_sheet-63] 63.0 ^63.1 ^63.2 Williams, David R. (September 1, 2004). Earth Fact Sheet. NASA. Retrieved on 2007-03-17.

[atmosphere-64] 64.0 ^64.1 Staff (October 8, 2003). Earth's Atmosphere. NASA. Retrieved on 2007-03-21.

[moran2005-65] 65.0 ^65.1 Moran, Joseph M. (2005). Weather. World Book Online Reference Center. NASA/World Book, Inc.. Retrieved on 2007-03-17.

[berger2002-66] 66.0 ^66.1 Berger, Wolfgang H. (2002). The Earth's Climate System. University of California, San Diego. Retrieved on 2007-03-24.

[67] Rahmstorf, Stefan (2003). The Thermohaline Ocean Circulation. Potsdam Institute for Climate Impact Research. Retrieved on 2007-04-21.

[68] Various (July 21, 1997). The Hydrologic Cycle. University of Illinois. Retrieved on 2007-03-24.

[69] Staff. Climate Zones. UK Department for Environment, Food and Rural Affairs. Retrieved on 2007-03-24.

[70] Staff (2004). Stratosphere and Weather; Discovery of the Stratosphere. Science Week. Retrieved on 2007-03-14.

[71] Córdoba, S. Sanz Fernández (June 21, 2004). 100 km. Altitude Boundary for Astronautics. Fédération Aéronautique Internationale. Retrieved on 2007-04-21.

[72] Liu, S. C.; Donahue, T. M. (1974). "The Aeronomy of Hydrogen in the Atmosphere of the Earth". Journal of Atmospheric Sciences 31 (4): 1118–1136. http://adsabs.harvard.edu/abs/1974JAtS...31.1118L. Retrieved 2007-03-02.

[73] Abedon, Stephen T. (March 31, 1997). History of Earth. Ohio State University. Retrieved on 2007-03-19.

[74] Fitzpatrick, Richard (February 16, 2006). MHD dynamo theory. NASA WMAP. Retrieved on 2007-02-27.

[campbelwh-75] Campbell, Wallace Hall (2003). Introduction to Geomagnetic Fields. New York: Cambridge University Press, p57. ISBN 0521822068.

[76] Stern, David P. (July 8, 2005). Exploration of the Earth's Magnetosphere. NASA. Retrieved on 2007-03-21.

[77] Fisher, Rick (January, 30, 1996). Astronomical Times. National Radio Astronomy Observatory. Retrieved on 2007-03-21.

[moon_fact_sheet-78] Williams, David R. (September 1, 2004). Moon Fact Sheet. NASA. Retrieved on 2007-03-21.

[79] Fisher, Rick (February 5, 1996). Earth Rotation and Equatorial Coordinates. National Radio Astronomy Observatory. Retrieved on 2007-03-21.

[80] Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% the energy at aphelion.

[81] Williams, Jack (December 20, 2005). Earth's tilt creates seasons. USAToday. Retrieved on 2007-03-17.

[82] Vázquez, M.; Montañés Rodríguez, P.; Palle, E. (2006). The Earth as an Object of Astrophysical Interest in the Search for Extrasolar Planets. Instituto de Astrofísica de Canarias. Retrieved on 2007-03-21.

[83] For the Earth, the Hill radius is
${\begin{smallmatrix}R_{H}=a\left({\frac {m}{3M}}\right)^{\frac {1}{3}}\end{smallmatrix}}$ ,
where m is the mass of the Earth, a is an Astronomical Unit, and M is the mass of the Sun. So the radius in A.U. is about: ${\begin{smallmatrix}\left({\frac {1}{3\cdot 332,946}}\right)^{\frac {1}{3}}=0.01\end{smallmatrix}}$ .

[84] Espenak, F.; Meeus, J. (February 7, 2007). Secular acceleration of the Moon. NASA. Retrieved on 2007-04-20.

[85] Poropudas, Hannu K. J. (December 16, 1991). Using Coral as a Clock. Skeptic Tank. Retrieved on 2007-04-20.

[86] Laskar, J.; Robutel, P.; Joutel, F.; Gastineau, M.; Correia, A.C.M.; Levrard, B. (2004). "A long-term numerical solution for the insolation quantities of the Earth". Astronomy and Astrophysics 428: 261–285. http://adsabs.harvard.edu/abs/2004A&A...428..261L. Retrieved 2007-03-31.

[87] Williams, D.M.; J.F. Kasting (1996). "Habitable planets with high obliquities". Lunar and Planetary Science 27: 1437–1438. http://adsabs.harvard.edu/abs/1996LPI....27.1437W. Retrieved 2007-03-31.

[88] R. Canup and E. Asphaug (2001). "Origin of the Moon in a giant impact near the end of the Earth's formation". Nature 412: 708–712.

[89] Whitehouse, David. "Earth's little brother found", BBC News, October 21, 2002. Retrieved on 2007-03-31.

[90] Staff (September, 2003). Astrobiology Roadmap. NASA, Lockheed Martin. Retrieved on 2007-03-10.

[91] Dole, Stephen H. (1970). Habitable Planets for Man, 2nd edition, American Elsevier Publishing Co.. ISBN 0-444-00092-5. Retrieved on 2007-03-11.

[92] Ward, P. D.; Brownlee, D. (January 14, 2000). Rare Earth: Why Complex Life is Uncommon in the Universe, 1st edition, New York: Springer-Verlag. ISBN 0387987010.

[93] Hillebrand, Helmut (2004). "On the Generality of the Latitudinal Gradient". American Naturalist 163 (2): 192–211.

[94] Staff (November 24, 2006). Mineral Genesis: How do minerals form?. Non-vertebrate Paleontology Laboratory, Texas Memorial Museum. Retrieved on 2007-04-01.

[95] Rona, Peter A. (2003). "Resources of the Sea Floor". Science 299 (5607): 673–674. http://www.sciencemag.org/cgi/content/full/299/5607/673?ijkey=AHVbRrqUsmdHY&keytype=ref&siteid=sci. Retrieved 2007-02-04.

[96] Staff (February 2, 2007). Evidence is now ‘unequivocal’ that humans are causing global warming – UN report. United Nations. Retrieved on 2007-03-07.

[LiveScience-97] David, Leonard. "Planet's Population Hit 6.5 Billion Saturday", Live Science, February 2, 2006. Retrieved on 2006-04-02.

[98] Staff. World Population Prospects: The 2006 Revision. United Nations. Retrieved on 2007-03-07.

[99] Staff (2007). Human Population: Fundamentals of Growth: Growth. Population Reference Bureau. Retrieved on 2007-03-31.

[100] Peel, M. C.; Finlayson, B. L.; McMahon, T. A. (2007). "Updated world map of the Köppen-Geiger climate classification". Hydrology and Earth System Sciences Discussions 4: 439–473. http://overview.sref.org/1812-2116/hessd/2007-4-439. Retrieved 2007-03-31.

[101] Staff. Themes & Issues. Secretariat of the Convention on Biological Diversity. Retrieved on 2007-03-29.

[102] Staff (2006-08-15). Canadian Forces Station (CFS) Alert. Information Management Group. Retrieved on 2007-03-31.

[103] Staff. International Law. United Nations. Retrieved on 2007-03-27.

[104] (July 2005) Random House Unabridged Dictionary. Random House. ISBN 0-375-42599-3.

[105] Harper, Douglas (November 2001). Earth. Online Etymology Dictionary. Retrieved on 2007-08-07.

[106] Liungman, Carl G. (2004). "Group 29: Multi-axes symmetric, both soft and straight-lined, closed signs with crossing lines", Symbols -- Encyclopedia of Western Signs and Ideograms. New York: Ionfox AB, pp. 281–282. ISBN 91-972705-0-4.

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[108] Russell, Jeffrey B.. The Myth of the Flat Earth. American Scientific Affiliation. Retrieved on 2007-03-14.; but see also Cosmas Indicopleustes

[109] Jacobs, James Q. (February 1, 1998). Archaeogeodesy, a Key to Prehistory. Retrieved on 2007-04-21.

[fja-110] Ackerman, Forrest J (1997). Forrest J Ackerman's World of Science Fiction. Los Angeles: RR Donnelley & Sons Company, 116–117. ISBN 1-57544-069-5.

[111] Staff (October, 1998). Explorers: Searching the Universe Forty Years Later (PDF). NASA/Goddard. Retrieved on 2007-03-05.

[seti-pbd-112] Staff. Pale Blue Dot. SETI@home. Retrieved on 2006-04-02.

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[114] Lovelock, James E. (1979). Gaia: A New Look at Life on Earth, First edition, Oxford: Oxford University Press. ISBN 0-19-286030-5.

[115] Meyer, Stephen M. (August 18, 2002). MIT Project on Environmental Politics & Policy. Massachusetts Institute of Technology. Retrieved on 2006-08-10.

[sun_future-116] 116.0 ^116.1 Sackmann, I.-J.; Boothroyd, A. I.; Kraemer, K. E. (1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457–468. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...418..457S. Retrieved 2007-03-31.

[117] Kasting, J.F. (1988). "Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus". Icarus 74: 472–494. http://adsabs.harvard.edu/abs/1988Icar...74..472K. Retrieved 2007-03-31.

[118] Guillemot, H.; Greffoz, V. (Mars 2002). "Ce que sera la fin du monde" (in French). Science et Vie N° 1014.

[119] Carrington, Damian. "Date set for desert Earth", BBC News, February 21, 2000. Retrieved on 2007-03-31.

[120] Britt, Robert (2000-02-25). Freeze, Fry or Dry: How Long Has the Earth Got?.

[121] Palmer, Jason. "Hope dims that Earth will survive Sun's death", NewScientist.com news service, February 22, 2008.

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@@ Line 1: / Line 1: @@
-{{Planet|image=Blue Marble.jpg
+{{sprotect|small=yes}}
-|symbol=Earth symbol.svg
+{{featured article}}
-|order=3
+{{two other uses|Earth as a planet|the Earth's geography|World}}
-|primary=Sun
+<!-- Note to editors: Scroll down to get beyond this long template and edit the contents of this page.
-|periapsis=147,098,074 km (0.983 AU)
-|apoapsis=152,097,701 km (1.017 AU)
+Don't bother with "MOSTLY HARMLESS" vandalism.  It's been done to death.  Find something to improve.
-|semimajor=149,597,887.5 km (1.0 AU)
-|bode=1.0 AU
+Please note the formatting and layout of this infobox has been matched with the other bodies of the solar system. Please do not arbitrarily change it without discussion.
-|eccentricity=0.016710219
-|sidereal=365.256366 da
+-->
-|synodic=365.256366 da
+{{Infobox Planet
-|orbitspeed=29.783 km/s
+| bgcolour=#c0c0ff
-|inclination=0°
+| name = Earth
-|reference=the ecliptic
+| symbol = [[Image:Earth symbol.svg|25px|Astronomical symbol of Earth]]
-|siderealday=23.9345 h
+| image = [[Image:The Earth seen from Apollo 17.jpg|240px|A color image of Earth as seen from Apollo 17.]]
-|solarday=24 h
+| flag = {{flagicon|world}}
-|rotatespeed=465.11 m/s
+| caption = Famous "[[The Blue Marble|Blue Marble]]" photograph of Earth, taken from [[Apollo 17]].
-|axialtilt=23.439281°
+| epoch = [[J2000]]
-|mass=5.97599 * 10<sup>24</sup> kg
+| aphelion = 152,097,701&nbsp;km <br />1.0167103335&nbsp;[[astronomical unit|AU]]
-|density=5,515.3 kg/m³
+| perihelion = 147,098,074&nbsp;km <br /> 0.9832898912&nbsp;AU
-|surfacegrav=9.780327 m/s²
+| semimajor = 149,597,887.5&nbsp;km <br /> 1.0000001124&nbsp;AU
-|escapespeed=11.186 km/s
+| eccentricity = 0.016710219
-|meanradius=6371 km<ref>http://www.solarviews.com/eng/earth.htm</ref>
+| inclination = Reference (0) <br /> 7.25° to [[Sun]]'s equator
-|equatorradius =6378.135 km
+| asc_node = 348.73936°
-|polarradius =6356.750 km
+| arg_peri = 114.20783°
-|surfacearea=510,065,600 km²
+| period = 365.256366&nbsp;days<br /> 1.0000175&nbsp;[[julian year (astronomy)|yr]]
-|landarea=148,939,100 km²
+| avg_speed = 29.783&nbsp;km/s <br /> 107,218&nbsp;km/h
-|waterarea=361,126,400 km²
+| satellites = 1&nbsp;(the&nbsp;[[Moon]])
-|mintemp=185 K
+| physical_characteristics = yes
-|meantemp=287 K
+| flattening = 0.0033528<ref name=wgs84/>
-|maxtemp=331 K
+| equatorial_radius = 6,378.1&nbsp;km<ref name=wgs84>The [[WGS 84]] [[reference ellipsoid]].</ref>
-|moons=1
+| polar_radius = 6,356.8&nbsp;km<ref name=wgs84/>
-|composition=Rock
+| mean_radius = 6,371.0&nbsp;km<ref name=wgs84sphere> This is the radius that gives a sphere with the same volume as the [[WGS 84]] [[reference ellipsoid]].</ref>
-|albedo=0.37
+| circumference = 40,075.02&nbsp;km&nbsp;([[equator]]ial)<br>40,007.86&nbsp;km&nbsp;([[meridional]])<br>40,041.47&nbsp;km&nbsp;(mean)
+| surface_area = 510,072,000&nbsp;km²<ref>{{cite journal
+ | last = Pidwirny | first = Michael
+ | date=[[February 2]], [[2006]]
+ | title=Surface area of our planet covered by oceans and continents.(Table 8o-1)
+ | publisher=University of British Columbia, Okanagan
+ | url=http://www.physicalgeography.net/fundamentals/8o.html
+ | accessdate=2007-11-26 }}</ref>
+,940,000&nbsp;km²&nbsp;land&nbsp;&nbsp;(29.2&nbsp;%)<br>
+,132,000&nbsp;km²&nbsp;water&nbsp;(70.8&nbsp;%)
+| volume = 1.0832073{{e|12}}&nbsp;km³
+| mass = 5.9736{{e|24}}&nbsp;kg
+| density = 5.5153&nbsp;g/cm³
+| surface_grav = [[Earth's gravity|9.780327]] [[metre per second squared|m/s²]]<ref name="yoder12" /><br />0.99732&nbsp;[[g-force|''g'']]
+| escape_velocity = 11.186&nbsp;km/s&nbsp;<br />40,270&nbsp;km/h<br />
+| sidereal_day = 0.997258&nbsp;d<br />23<sup>h</sup>&nbsp;56<sup>m</sup>&nbsp;04.09054<sup>s</sup><ref name="yoder12">Yoder, C. F. (1995) p. 12.</ref>
+| rot_velocity = 465.11&nbsp;m/s
+| axial_tilt = 23.439281°
+| albedo = 0.367
+| adjectives = Terrestrial, Terran, [[Terra (mythology)|Telluric]], Tellurian, Earthly
+| atmosphere = yes
+| temperatures = yes
+| temp_name1 = [[Kelvin]]
+| min_temp_1 = 184&nbsp;K
+| mean_temp_1 = 287&nbsp;K
+| max_temp_1 = 331&nbsp;K
+| temp_name2 = [[Celsius]]
+| min_temp_2 = −89&nbsp;°C
+| mean_temp_2 = 14&nbsp;°C
+| max_temp_2 = 57.7&nbsp;°C
+| surface_pressure = 101.3&nbsp;[[kPa]] ([[Sea Level|MSL]])
+| atmosphere_composition = 78.08%&nbsp;[[Nitrogen]] (N<sub>2</sub>)<br /> 20.95%&nbsp;[[Oxygen]] (O<sub>2</sub>)<br /> 0.93%&nbsp;[[Argon]]<br /> 0.038%&nbsp;[[Carbon dioxide]]<br />Trace [[water vapor]] (varies with [[climate]])
 }}
-The '''Earth''' is the only known planet in our '''Solar System''' which can support life. It contains water, reasonable levels of oxygen, and a stable temperature range. Geometrically speaking, Earth is the largest of the terrestrial (rocky) planets in the solar system.
+'''Earth''' ({{pronEng|ˈɝːθ}}<ref>[http://dictionary.cambridge.org/define.asp?dict=CALD&key=24538&ph=on Earth (PLANET)], entry in the ''Cambridge Advanced Learner's Dictionary'',  Cambridge University Press, online.  Accessed 14-II-2008.</ref>) is the third [[planet]] from the [[Sun]] and is the largest of the [[terrestrial planet]]s in the [[Solar System]], in both [[diameter]] and [[mass]]. It is also referred to as ''the Earth'', ''Planet Earth'', and ''the [[World]]'', and in some contexts, ''[[Gaia]]'' and ''[[Terra]]''.<ref>Note that by [[International Astronomical Union]] convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. C.f.:<br />{{cite web
+ | last=Blue | first=Jennifer | date=[[July 5]], [[2007]]
+ | url=http://planetarynames.wr.usgs.gov/jsp/append5.jsp
+ | title=Descriptor Terms (Feature Types)
+ | work=Gazetteer of Planetary Nomenclature
+ | publisher=USGS | accessdate=2007-07-05 }}</ref>
+Home to millions of [[species]]<ref>{{cite journal
+ | last = May | first = Robert M.
+ | title=How many species are there on earth?
+ | journal=Science | year=1999 | volume=241
+ | issue=4872 | pages=1441-1449
+ | url=http://adsabs.harvard.edu/abs/1988Sci...241.1441M
+ | accessdate=2007-08-14 }}</ref> including [[human]]s, Earth is the only place in the [[universe]] where [[life]] is known to exist.
+Scientific evidence indicates that the planet formed [[Age of the Earth|4.54 billion years]]<ref name="age_earth">{{cite book
+ | first=G.B. | last=Dalrymple | year=1991
+ | title=The Age of the Earth
+ | publisher=Stanford University Press | location=California
+ | id=ISBN 0-8047-1569-6
+}}</ref><ref>{{cite web
+ | last=Newman
+ | first=William L.
+ | date=[[July 9]], [[2007]]
+ | url=http://pubs.usgs.gov/gip/geotime/age.html
+ | title=Age of the Earth
+ | publisher=Publications Services, USGS
+ | accessdate=2007-09-20
+}}</ref><ref>{{cite journal
+ | last=Dalrymple
+ | first=G. Brent
+ | title=The age of the Earth in the twentieth century: a problem (mostly) solved
+ | journal=Geological Society, London, Special Publications
+ | year=2001
+ | volume=190
+ | pages=205-221
+ | url=http://sp.lyellcollection.org/cgi/content/abstract/190/1/205
+ | accessdate=2007-09-20
+}}</ref><ref>{{cite web
+ | last=Stassen
+ | first=Chris
+ | date=[[September 10]], [[2005]]
+ | url=http://www.talkorigins.org/faqs/faq-age-of-earth.html
+ | title=The Age of the Earth
+ | publisher=The TalkOrigins Archive
+ | accessdate=2007-09-20
+}}</ref> ago, and life appeared on its surface within a billion years. Since then, Earth's [[biosphere]] has significantly altered [[Earth's atmosphere|the atmosphere]] and other [[abiotic]] conditions on the planet, enabling the proliferation of [[aerobic organisms]] as well as the formation of the [[ozone layer]] which, together with Earth's [[magnetic field]], blocks harmful radiation, permitting life on land.<ref>{{cite book
+ | first=Roy M. | last=Harrison
+ | coauthors=Hester, Ronald E. | year=2002
+ | title=Causes and Environmental Implications of Increased UV-B Radiation
+ | publisher=Royal Society of Chemistry
+ | id=ISBN 0854042652 }}</ref>
+Earth's [[Crust (geology)|outer surface]] is divided into several rigid segments, or [[tectonic plate]]s, that gradually migrate across the surface over periods of [[Geologic time scale|many millions of years]]. About 71% of the surface is covered with [[seawater|salt-water]] [[ocean]]s, the remainder consisting of [[continent]]s and [[island]]s; liquid [[water]], necessary for all known life, is not known to exist on any other planet's surface.<ref>Other planets in the solar system are either too hot or too cold to support liquid water. However, it is confirmed to have existed on the surface of Mars in the past, and may still appear today. See: <cite>{{cite news
+ | author=Msnbc
+ | title=Rover reveals Mars was once wet enough for life
+ | publisher=NASA
+ | date=[[March 2]], [[2007]]
+ | url=http://www.msnbc.msn.com/id/4202901/
+ | accessdate=2007-08-28 }}</cite><cite>{{cite news
+ | author=Staff
+ | title=Simulations Show Liquid Water Could Exist on Mars
+ | publisher=University of Arkansas
+ | date=[[November 7]], [[2005]]
+ | url=http://dailyheadlines.uark.edu/5717.htm
+ | accessdate=2007-08-08 }}</cite></ref><ref>As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: {{cite journal
+ | last=G. Tinetti et al.
+ | title=Water vapour in the atmosphere of a transiting extrasolar planet
+ | journal=Nature | date=July, 2007 | volume=448
+ | pages=169-171 | url=http://www.nature.com/nature/journal/v448/n7150/abs/nature06002.html}}</ref> Earth's interior remains active, with a thick layer of relatively solid [[Mantle (geology)|mantle]], a liquid outer core that generates a magnetic field, and a solid iron [[inner core]].
+Earth interacts with other objects in [[outer space]], including the [[Sun]] and the [[Moon]]. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a [[sidereal year]], which is equal to 365.26 [[solar day]]s.<ref>The number of solar days is one less than the number of [[sidereal day]]s because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis.</ref> The Earth's axis of rotation is [[axial tilt|tilted]] 23.4°<ref>Ahrens, ''Global Earth Physics: A Handbook of Physical Constants'', p. 8.</ref> away from the [[perpendicular]] to its [[Orbital plane (astronomy)|orbital plane]], producing seasonal variations on the planet's surface with a period of one [[tropical year]] (365.24 solar days). Earth's only known [[natural satellite]], the Moon, which began orbiting it about 4.53 billion years ago, provides ocean [[tide]]s, stabilizes the axial tilt and gradually slows the planet's rotation. A [[comet]]ary bombardment during the early history of the planet played a role in the formation of the oceans.<ref name="comet">{{cite journal | author=Morbidelli, A.; Chambers, J.; Lunine, J. I.; Petit, J. M.; Robert, F.; Valsecchi, G. B.; Cyr, K. E. | title=Source regions and time scales for the delivery of water to Earth | journal=Meteoritics & Planetary Science | year=2000 | volume=35 | issue=6 | pages=1309–1320 | url=http://adsabs.harvard.edu/abs/2000M&PS...35.1309M | accessdate=2007-03-06 }}</ref> Later, [[asteroid]] impacts caused significant changes to the surface environment. Long term [[Milankovitch cycles|periodic changes]] in the Earth's orbit, caused by the gravitational influence of other planets, are believed to have given rise to the [[ice age]]s that have intermittently covered significant portions of Earth's surface in glacial sheets.
+==History==
+{{main|History of Earth}}
+Scientists have been able to reconstruct detailed information about the planet's past. Earth and the other planets in the [[Solar System]] formed 4.54&nbsp;billion&nbsp;years ago<ref name="age_earth" /> out of the [[solar nebula]], a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially [[molten]], the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object (sometimes called [[Giant impact hypothesis|Theia]]) with about 10% of the Earth's mass<ref>{{cite conference | author = Canup, R. M.; Asphaug, E. | title = An impact origin of the Earth-Moon system | booktitle = Abstract #U51A-02 | publisher = American Geophysical Union | date = Fall Meeting 2001 | url = http://adsabs.harvard.edu/abs/2001AGUFM.U51A..02C | accessdate = 2007-03-10 }}</ref> impacting the Earth in a glancing blow.<ref>{{cite journal | last = R. Canup and E. Asphaug | title = Origin of the Moon in a giant impact near the end of the Earth's formation | journal = Nature | volume = 412 | pages = 708–712 | date = 2001 | url = http://www.nature.com/nature/journal/v412/n6848/abs/412708a0.html }}</ref> Some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the [[Moon]].
+Outgassing and [[Volcano|volcanic]] activity produced the primordial atmosphere. Condensing [[water vapor]], augmented by ice delivered by [[comet]]s, [[Origin of the world's oceans|produced the oceans]].<ref name="comet"/> The highly energetic chemistry is believed to have produced a self-replicating molecule around 4&nbsp;billion&nbsp;years ago, and half a billion years later, the [[last universal common ancestor|last common ancestor of all life]] existed.<ref>{{cite journal | last=Doolittle | first=W. Ford | title=Uprooting the tree of life | journal=Scientific American | date=February, 2000 | volume=282 | issue=6 | pages=90–95 }}</ref>
+The development of [[photosynthesis]] allowed the Sun's energy to be harvested directly by life forms; the resultant [[oxygen]] accumulated in the atmosphere and resulted in a layer of [[ozone]] (a form of [[molecular oxygen]] [O<sub>3</sub>]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the [[endosymbiotic theory|development of complex cells]] called [[eukaryotes]].<ref>{{cite journal | author=Berkner, L. V.; Marshall, L. C. | title= On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere | journal=Journal of Atmospheric Sciences | year=1965 | volume=22 | issue=3 | pages=225–261 | url=http://adsabs.harvard.edu/abs/1965JAtS...22..225B | accessdate=2007-03-05 }}</ref> True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful [[ultraviolet radiation]] by the [[ozone layer]], life colonized the surface of Earth.<ref>{{cite web | last = Burton | first = Kathleen | date = [[November 29]], [[2000]] | url = http://www.nasa.gov/centers/ames/news/releases/2000/00_79AR.html | title = Astrobiologists Find Evidence of Early Life on Land | publisher = NASA | accessdate = 2007-03-05 }}</ref>
+As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a [[supercontinent]]. Roughly 750&nbsp;million&nbsp;years ago (mya), the earliest known supercontinent, [[Rodinia]], began to break apart. The continents later recombined to form [[Pannotia]], 600–540&nbsp;mya, then finally [[Pangaea]], which broke apart 180&nbsp;mya.<ref>{{cite journal | author=Murphy, J. B.; Nance, R. D. | title=How do supercontinents assemble? | journal=American Scientist | year=1965 | volume=92 | pages=324–33 | url=http://scienceweek.com/2004/sa040730-5.htm | accessdate=2007-03-05 }}</ref>
+Since the 1960s, it has been hypothesized that severe [[Glacier|glacial]] action between 750 and 580&nbsp;mya, during the [[Neoproterozoic]], covered much of the planet in a sheet of ice. This hypothesis has been termed "[[Snowball Earth]]", and is of particular interest because it preceded the [[Cambrian explosion]], when multicellular life forms began to proliferate.<ref>{{cite book | last=Kirschvink | first=J. L. | editors=Schopf, J.W.; Klein, C. | year=1992 | title=The Proterozoic Biosphere: A Multidisciplinary Study | pages=51–52 | publisher=Cambridge University Press | id=ISBN 0521366151 }}</ref>
+Following the [[Cambrian explosion]], about 535&nbsp;mya, there have been five [[Extinction event|mass extinctions]].<ref>{{cite journal | author=Raup, D. M.; Sepkoski, J. J. | title=Mass Extinctions in the Marine Fossil Record | journal=Science | year=1982 | volume=215 | issue=4539 | pages=1501–1503 | url=http://adsabs.harvard.edu/abs/1982Sci...215.1501R | accessdate=2007-03-05 }}</ref> The last extinction event occurred 65&nbsp;mya, when a meteorite collision probably triggered the extinction of the (non-avian) [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s, which then resembled shrews. Over the past 65&nbsp;million years, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright.<ref>{{cite journal | last = Gould | first = Stephan J. | title=The Evolution of Life on Earth | journal=Scientific American | date=October , 1994 | url=http://brembs.net/gould.html | accessdate=2007-03-05 }}</ref> This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,<ref>{{cite journal
+ | author=Wilkinson, B. H.; McElroy, B. J.
+ | title=The impact of humans on continental erosion and sedimentation
+ | journal=Bulletin of the Geological Society of America
+ | year=2007
+ | volume=119
+ | issue=1–2
+ | pages=140–156
+ | url=http://bulletin.geoscienceworld.org/cgi/content/abstract/119/1-2/140
+ | accessdate=2007-04-22 }}</ref> affecting both the nature and quantity of other life forms.
+The present pattern of [[ice age]]s began about 40&nbsp;mya, then intensified during the [[Pleistocene]] about 3&nbsp;mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40&ndash;100,000&nbsp;years. The last ice age ended 10,000&nbsp;years ago.<ref>{{cite web | author=Staff | url = http://www.lakepowell.net/sciencecenter/paleoclimate.htm | title = Paleoclimatology - The Study of Ancient Climates | publisher = Page Paleontology Science Center | accessdate = 2007-03-02 }}</ref>
+==Composition and structure==
+Earth is a [[terrestrial planet]], meaning that it is a rocky body, rather than a [[gas giant]] like [[Jupiter]]. It is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest [[surface gravity]] and the strongest [[magnetic field]].<ref>{{cite web
+ | last=Stern
+ | first=David P.
+ | date=[[November 25]], [[2001]]
+ | url= http://astrogeology.usgs.gov/HotTopics/index.php?/archives/147-Names-for-the-Columbia-astronauts-provisionally-approved.html
+ | title=Planetary Magnetism
+ | publisher=NASA
+ | accessdate=2007-04-01
+}}</ref>
+===Shape===
+{{main|Figure of the Earth}}
+[[Image:Terrestrial planet size comparisons.jpg|thumb|left|300px|Size comparison of inner planets (left to right): [[Mercury (planet)|Mercury]], [[Venus]], Earth, and [[Mars]]]]
+The Earth's shape is very close to an [[oblate spheroid]]&mdash;a rounded shape with a bulge around the [[equator]]&mdash;although the precise shape (the [[geoid]]) varies from this by up to 100&nbsp;meters.<ref>{{cite web | author=Milbert, D. G.; Smith, D. A. | url = http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html | title = Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model | publisher = National Geodetic Survey, NOAA | accessdate = 2007-03-07 }}</ref> The average diameter of the reference spheroid is about 12,742&nbsp;km. More approximately the distance is 40,000&nbsp;km/[[pi|π]] because the [[meter]] was originally defined as 1/10,000,000 of the distance from the equator to the [[north pole]] through [[Paris]], [[France]].<ref>{{cite web
+ | author=Mohr, P.J.; Taylor, B.N.
+ | date=October, 2000
+ | url=http://physics.nist.gov/cuu/Units/meter.html
+ | title=Unit of length (meter)
+ | work=NIST Reference on Constants, Units, and Uncertainty
+ | publisher=NIST Physics Laboratory
+ | accessdate=2007-04-23
+}}</ref>
+The [[rotation]] of the Earth creates the [[equatorial bulge]] so that the equatorial diameter is 43&nbsp;km larger than the [[Geographical pole|pole]] to pole diameter.<ref name="ngdc2006">{{cite web
+ | author=Sandwell, D. T.; Smith, W. H. F.
+ | date = Jul7 26, 2006
+ | url =http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML
+ | title =Exploring the Ocean Basins with Satellite Altimeter Data
+ | publisher = NOAA/NGDC
+ | accessdate = 2007-04-21
+}}</ref> The largest local deviations in the rocky surface of the Earth are [[Mount Everest]] (8,848&nbsp;m above local [[sea level]]) and the [[Mariana Trench]] (10,911&nbsp;m below local sea level). Hence compared to a perfect [[ellipsoid]], the Earth has a [[tolerance (engineering)|tolerance]] of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in [[billiard ball]]s.<ref>{{cite web | author=Staff | date = November, 2001 | url = http://www.wpa-pool.com/index.asp?content=rules_spec | title = WPA Tournament Table & Equipment Specifications | publisher = World Pool-Billiards Association | accessdate = 2007-03-10 }}</ref> Because of the bulge, the feature farthest from the center of the Earth is actually [[Chimborazo (volcano)|Mount Chimborazo]] in [[Ecuador]].<ref>{{cite journal | last = Senne | first = Joseph H. | title=Did Edmund Hillary Climb the Wrong Mountain | journal=Professional Surveyor | year=2000 | volume=20 | issue=5 | url=http://www.profsurv.com/archive.php?issue=42&article=589 | accessdate=2007-02-04 }}</ref>
+===Chemical composition===
+{{seealso|Abundance of elements on Earth}}
+{| class="wikitable" style="float: right; clear: right; margin-left: 2em;"
+|+ F. W. Clarke's Table of Crust Oxides
+!Compound
+!Formula
+!Composition
+|-
+|[[silica]]
+|style="text-align: center;"|SiO<sub>2</sub>
+|style="text-align: right;"|59.71%
+|-
+|[[alumina]]
+|style="text-align: center;"|Al<sub>2</sub>O<sub>3</sub>
+|style="text-align: right;"|15.41%
+|-
+|[[Calcium oxide|lime]]
+|style="text-align: center;"|CaO
+|style="text-align: right;"|4.90%
+|-
+|[[Magnesia (mineral)|Magnesia]]
+|style="text-align: center;"|MgO
+|style="text-align: right;"|4.36%
+|-
+|[[sodium oxide]]
+|style="text-align: center;"|Na<sub>2</sub>O
+|style="text-align: right;"|3.55%
+|-
+|[[iron(II) oxide]]
+|style="text-align: center;"|FeO
+|style="text-align: right;"|3.52%
+|-
+|[[potassium oxide]]
+|style="text-align: center;"|K<sub>2</sub>O
+|style="text-align: right;"|2.80%
+|-
+|[[iron(III) oxide]]
+|style="text-align: center;"|Fe<sub>2</sub>O<sub>3</sub>
+|style="text-align: right;"|2.63%
+|-
+|[[water (molecule)|water]]
+|style="text-align: center;"|H<sub>2</sub>O
+|style="text-align: right;"|1.52%
+|-
+|[[titanium dioxide]]
+|style="text-align: center;"|TiO<sub>2</sub>
+|style="text-align: right;"|0.60%
+|-
+|[[phosphorus pentoxide]]
+|style="text-align: center;"|P<sub>2</sub>O<sub>5</sub>
+|style="text-align: right;"|0.22%
+|-
+!colspan="2"|Total
+!style="text-align: right;"|99.22%
+|}
+The [[mass]] of the Earth is approximately 5.98{{e|24}}&nbsp;kg. It is composed mostly of [[iron]] (32.1%), [[oxygen]] (30.1%), [[silicon]] (15.1%), [[magnesium]] (13.9%), [[sulfur]] (2.9%), [[nickel]] (1.8%), [[calcium]] (1.5%), and [[aluminium]] (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to [[mass segregation]], the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%),
+and less than 1% trace elements.<ref>{{cite journal | author=Morgan, J. W.; Anders, E. | title=Chemical composition of Earth, Venus, and Mercury | journal=Proceedings of the National Academy of Science | year=1980 | volume=71 | issue=12 | pages=6973–6977 | url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=350422 | accessdate=2007-02-04 }}</ref>
+The geochemist [[Frank Wigglesworth Clarke|F. W. Clarke]] calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the [[Earth's crust]] are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.<ref name=EB1911>{{1911|article=Petrology}}</ref>
+===Internal structure===
+{{main|Structure of the Earth}}
+[[Image:Earth-crust-cutaway-english.svg|thumb|300px|right|Earth cutaway from core to exosphere. Not to scale.]]
+The interior of the Earth, like that of the other [[terrestrial planets]], is [[chemical]]ly divided into layers. The Earth has an outer [[Silicate minerals|silicate]] solid [[Crust (geology)|crust]], a highly viscous [[Mantle (geology)|mantle]], a liquid [[outer core]] that is much less viscous than the mantle, and a solid [[inner core]]. The crust is separated from the mantle by the [[Mohorovičić discontinuity]], and the thickness of the crust varies: averaging 6&nbsp;km under the oceans and 30&ndash;50&nbsp;km on the continents.<ref>{{cite book
+ | first=Toshiro
+ | last=Tanimoto
+ | editor=Thomas J. Ahrens
+ | year=1995
+ | title=Crustal Structure of the Earth
+ | booktitle=Global Earth Physics: A Handbook of Physical Constants
+ | publisher=American Geophysical Union
+ | location=Washington, DC
+ | id=ISBN 0-87590-851-9
+ | url=http://www.agu.org/reference/gephys/15_tanimoto.pdf
+ | format=PDF
+ | accessdate=2007-02-03 }}</ref>
+The geologic component layers of the Earth<ref>{{cite journal
+ | last = Jordan
+ | first = T. H.
+ | title=Structural Geology of the Earth's Interior
+ | journal=Proceedings National Academy of Science
+ | year=1979
+ | volume=76
+ | issue=9
+ | pages=4192–4200
+ | url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=411539
+ | accessdate=2007-03-24 }}</ref> are at the following depths below the surface:<ref>{{cite web
+ | last = Robertson
+ | first = Eugene C.
+ | date = [[July 26]], [[2001]]
+ | url = http://pubs.usgs.gov/gip/interior/
+ | title = The Interior of the Earth
+ | publisher = USGS
+ | accessdate = 2007-03-24
+}}</ref>
+{| class="wikitable"
+!Depth<br><span style="font-size: smaller;">km</span>
+!style="vertical-align: bottom;"|Layer
+!Density<br><span style="font-size: smaller;">g/cm³</span>
+|-
+|style="text-align: center;"|0&ndash;60
+|[[Lithosphere]] (locally varies between 5 and 200&nbsp;km)
+|style="text-align: center;"| &mdash;
+|- style="background: #FEFEFE;"
+|style="text-align: center;"|0&ndash;35
+|... [[Crust (geology)|Crust]] (locally varies between 5 and 70&nbsp;km)
+|style="text-align: center;"| 2.2&ndash;2.9
+|- style="background: #FEFEFE;"
+|style="text-align: center;"|35&ndash;60
+|... Uppermost part of mantle
+|style="text-align: center;"| 3.4&ndash;4.4
+|-
+|style="text-align: center;"|35&ndash;2890
+|[[Mantle (geology)|Mantle]]
+|style="text-align: center;"| 3.4&ndash;5.6
+|- style="background: #FEFEFE;"
+|style="text-align: center;"|100&ndash;700
+|... [[Asthenosphere]]
+|style="text-align: center;"| &mdash;
+|-
+|style="text-align: center;"|2890&ndash;5100
+|[[Outer core]]
+|style="text-align: center;"| 9.9&ndash;12.2
+|-
+|style="text-align: center;"|5100&ndash;6378
+|[[Inner core]]
+|style="text-align: center;"| 12.8&ndash;13.1
+|}
+The internal heat of the planet is most likely produced by the radioactive decay of [[Potassium|potassium-40]], [[Uranium|uranium-238]] and [[Thorium|thorium-232]]
+[[isotope]]s. All three have [[half-life]] decay periods of more than a billion years.<ref>{{cite news | first=Robert | last=Sanders | title=Radioactive potassium may be major heat source in Earth's core | publisher=UC Berkeley News | date=[[December 10]], [[2003]] | url=http://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml | accessdate=2007-02-28 }}</ref> At the center of the planet, the temperature may be up to
+,000&nbsp;K and the pressure could reach 360&nbsp;[[GPa]].<ref>{{cite journal | author=Alfè, D.; Gillan, M. J.; Vocadlo, L.; Brodholt, J; Price, G. D. | title=The ''ab initio'' simulation of the Earth's core | journal= Philosophical Transaction of the Royal Society of London | year=2002 | volume=360 | issue=1795 | pages=1227–1244 | url=http://chianti.geol.ucl.ac.uk/~dario/pubblicazioni/PTRSA2002.pdf | format=PDF | accessdate=2007-02-28 }}</ref> A portion of the core's thermal energy is transported toward the crust by [[Mantle plume]]s; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce [[Hotspot (geology)|hotspots]] and [[flood basalt]]s.<ref>{{cite journal | author=Richards, M. A.; Duncan, R. A.; Courtillot, V. E. | title=Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails | journal=Science | year=1989 | volume=246 | issue=4926 | pages=103–107 | url=http://www.sciencemag.org/cgi/content/abstract/246/4926/103 | accessdate=2007-04-21 }}</ref>
+===Tectonic plates===
+{{main|Plate tectonics}}
+[[Image:Plates tect2 en.svg|thumb|right|300px|A map illustrating the Earth's major plates.]]
+According to plate tectonics theory, 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 forms the inner part of the upper mantle. The asthenosphere behaves like a superheated material that is in a semi-fluidic, plastic-like state.<ref>{{cite web
+ | first=Courtney | last=Seligman | year=2008
+ | url = http://cseligman.com/text/planets/innerstructure.htm
+ | title = The Structure of the Terrestrial Planets
+ | work = Online Astronomy eText Table of Contents
+ | publisher = cseligman.com | accessdate = 2008-02-28 }}</ref>
+The lithosphere essentially ''floats'' on the asthenosphere and is broken up into what are called [[List of tectonic plates|tectonic plates]]. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: [[Convergent boundary|convergent]], [[Divergent boundary|divergent]] and [[Transform boundary|transform]]. The last occurs where two plates move laterally relative to each other, creating a [[strike-slip fault]]. [[Earthquake]]s, [[volcano|volcanic activity]], [[Orogeny|mountain-building]], and [[oceanic trench]] formation can occur along these plate boundaries.<ref>{{cite web | author=Kious, W. J.; Tilling, R. I. | date = May 5, 1999 | url = http://pubs.usgs.gov/gip/dynamic/understanding.html | title = Understanding plate motions | publisher = USGS | accessdate = 2007-03-02 }}</ref>
+The main plates are:<ref>{{cite web | author=Brown, W. K.; Wohletz, K. H. | year = 2005 | url = http://www.ees1.lanl.gov/Wohletz/SFT-Tectonics.htm | title = SFT and the Earth's Tectonic Plates | publisher = Los Alamos National Laboratory | accessdate = 2007-03-02 }}</ref>
+{| class="wikitable"
+!Plate name
+!Area<br><span style="font-size: smaller;">10<sup>6</sup>&nbsp;km²</span>
+!Covering
+|-
+| [[African Plate]] ||style="text-align: center;"| 61.3 ||style="text-align: center;"| [[Africa]]
+|-
+| [[Antarctic Plate]] ||style="text-align: center;"| 60.9 ||style="text-align: center;"| [[Antarctica]]
+|-
+| [[Australian Plate]] ||style="text-align: center;"| 47.2 ||style="text-align: center;"| [[Australia]]
+|-
+| [[Eurasian Plate]] ||style="text-align: center;"| 67.8 ||style="text-align: center;"| [[Asia]] and [[Europe]]
+|-
+| [[North American Plate]] ||style="text-align: center;"| 75.9 ||style="text-align: center;"| [[North America]] and north-east [[Siberia]]
+|-
+| [[South American Plate]] ||style="text-align: center;"| 43.6 ||style="text-align: center;"| [[South America]]
+|-
+| [[Pacific Plate]] ||style="text-align: center;"| 103.3 ||style="text-align: center;"| [[Pacific Ocean]]
+|}
+Notable minor plates include the [[Indian Plate]], the [[Arabian Plate]], the [[Caribbean Plate]], the [[Nazca Plate]] off the west coast of [[South America]] and the [[Scotia Plate]] in the southern [[Atlantic Ocean]]. The Australian Plate actually fused with [[Indian Plate]] between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the [[Cocos Plate]] advancing at a rate of 75&nbsp;mm/yr<ref>{{cite web
+ | author=Meschede, M.; Udo Barckhausen, U.
+ | date=[[November 20]], [[2000]]
+ | url = http://www-odp.tamu.edu/publications/170_SR/chap_07/chap_07.htm
+ | title = Plate Tectonic Evolution of the Cocos-Nazca Spreading Center
+ | work=Proceedings of the Ocean Drilling Program
+ | publisher = Texas A&M University
+ | accessdate = 2007-04-02
+}}</ref> (3.0&nbsp;in/yr) and the [[Pacific Plate]] moving 52–69&nbsp;mm/yr (2.1&ndash;2.7 in/yr). At the other extreme, the slowest-moving plate is the [[Eurasian Plate]], progressing at a typical rate of about 21&nbsp;mm/yr (0.8&nbsp;in/yr).<ref>{{cite web
+ | author=Staff
+ | url = http://sideshow.jpl.nasa.gov/mbh/series.html
+ | title = GPS Time Series
+ | publisher = NASA JPL
+ | accessdate = 2007-04-02 }}</ref>
+===Surface===
+{{main|Landform|Extreme points of the World}}
+[[Image:AYool topography 15min.png|300px|left|thumb|Present day Earth [[terrain|altimetry]] and [[bathymetry]]. Data from the [[National Geophysical Data Center]]'s [http://www.ngdc.noaa.gov/seg/fliers/se-1104.shtml TerrainBase Digital Terrain Model].]]
+The Earth's [[terrain]] varies greatly from place to place. About 70.8%<ref name="Pidwirny2006">{{cite web
+ | last = Pidwirny
+ | first = Michael
+ | year = 2006
+ | url = http://www.physicalgeography.net/fundamentals/7h.html
+ | title = Fundamentals of Physical Geography
+ | edition = 2nd Edition
+ | publisher = PhysicalGeography.net
+ | accessdate = 2007-03-19 }}</ref> of the surface is covered by water, with much of the [[continental shelf]] below sea level. The submerged surface has mountainous features, including a globe-spanning [[mid-ocean ridge]] system, as well as undersea [[volcano]]es,<ref name="ngdc2006" /> [[oceanic trench]]es, [[submarine canyon]]s, [[oceanic plateau]]s and [[abyssal plain]]s. The remaining 29.2% not covered by water consists of [[mountains]], [[deserts]], [[plain]]s, [[plateau]]s, and other [[Geomorphology|geomorphologies]].
+The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and [[erosion]]. The surface features built up or deformed through plate tectonics are subject to steady [[weathering]] from [[Precipitation (meteorology)|precipitation]], thermal cycles, and chemical effects. [[Glaciation]], [[coastal erosion]], the build-up of [[coral reef]]s, and large meteorite impacts<ref>{{cite web | last = Kring | first = David A. | url = http://www.lpl.arizona.edu/SIC/impact_cratering/intro/ | title = Terrestrial Impact Cratering and Its Environmental Effects | publisher = Lunar and Planetary Laboratory | accessdate = 2007-03-22 }}</ref> also act to reshape the landscape.
+As the continental plates migrate across the planet, the ocean floor is [[Subduction|subducted]] under the leading edges. At the same time, upwellings of mantle material create a [[divergent boundary]] along [[mid-ocean ridge]]s. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.<ref>{{cite web | last = Duennebier | first = Fred | date = August 12, 1999 | url = http://www.soest.hawaii.edu/GG/ASK/plate-tectonics2.html | title = Pacific Plate Motion | publisher = University of Hawaii | accessdate = 2007-03-14 }}</ref><ref>{{cite web | author=Mueller, R.D.; Roest, W.R.; Royer, J.-Y.; Gahagan, L.M.; Sclater, J.G. | date = March 7, 2007 | url = http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html | title = Age of the Ocean Floor Poster | publisher = NOAA | accessdate = 2007-03-14 }}</ref>
+The continental plates consist of lower density material such as the [[igneous rock]]s [[granite]] and [[andesite]]. Less common is [[basalt]], a denser volcanic rock that is the primary constituent of the ocean floors.<ref>{{cite web | author=Staff | url = http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/part1.html | title = Layers of the Earth | publisher = Volcano World | accessdate = 2007-03-11 }}</ref> [[Sedimentary rock]]
+is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.<ref>{{cite web | last=Jessey | first=David | url = http://geology.csupomona.edu/drjessey/class/Gsc101/Weathering.html | title = Weathering and Sedimentary Rocks | publisher = Cal Poly Pomona | accessdate = 2007-03-20 }}</ref> The third form of rock material found on Earth is [[metamorphic rock]], which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include [[quartz]], the [[feldspar]]s, [[amphibole]], [[mica]], [[pyroxene]] and [[olivine]].<ref>{{cite web | author=Staff | url = http://natural-history.uoregon.edu/Pages/web/mineral.htm | title = Minerals | publisher = Museum of Natural History, Oregon | accessdate = 2007-03-20 }}</ref> Common carbonate minerals include [[calcite]] (found in [[limestone]]), [[aragonite]] and [[dolomite]].<ref>{{cite web
+ | last=Cox
+ | first=Ronadh
+ | year=2003
+ | url=http://madmonster.williams.edu/geos.302/L.08.html
+ | title=Carbonate sediments
+ | publisher=Williams College
+ | accessdate=2007-04-21
+}}</ref>
+The [[pedosphere]] is the outermost layer of the Earth that is composed of [[soil]] and subject to [[pedogenesis|soil formation processes]]. It exists at the interface of the [[lithosphere]], [[Earth's atmosphere|atmosphere]], [[hydrosphere]] and [[biosphere]]. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.<ref name="cia">{{cite web | author=Staff | date = February 8, 2007 | url = https://www.cia.gov/library/publications/the-world-factbook/geos/xx.html | title = The World Factbook | publisher = U.S. C.I.A. | accessdate = 2007-02-25 }}</ref> Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3{{e|7}}&nbsp;km² of cropland and 3.4{{e|7}}&nbsp;km² of pastureland.<ref>{{cite book
+ | author=FAO Staff
+ | year=1995
+ | title=FAO Production Yearbook 1994
+ | edition=Volume 48
+ | publisher=Food and Agriculture Organization of the United Nations
+ | location=Rome, Italy
+ | id=ISBN 9250038445 }}</ref>
+[[Image:Earth elevation histogram 2.svg|thumb|350px|Elevation [[histogram]] of the surface of the Earth&mdash;approximately 71% of the Earth's surface is covered with water.]]
+The elevation of the land surface of the Earth varies from the low point of −418&nbsp;m at the [[Dead Sea]], to a 2005-estimated maximum altitude of 8,848&nbsp;m at the top of [[Mount Everest]]. The mean height of land above sea level is 686&nbsp;m.<ref name="hr_mill">{{cite journal | last = Mill | first = Hugh Robert | title=The Permanence of Ocean Basins | journal=The Geographical Journal | year=1893 | volume=1 | issue=3 | pages=230–234 | url=http://www.wku.edu/~smithch/wallace/S453.htm | accessdate=2007-02-25 }}</ref>
+===Hydrosphere===
+{{main|Hydrosphere}}
+The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the solar system. The Earth's hydrosphere consists chiefly of the [[oceans]], but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000&nbsp;m. The deepest underwater location is Challenger Deep of the [[Mariana Trench]] in the [[Pacific Ocean]] with a depth of −10,911&nbsp;m.<ref name="rain.org">{{cite web | author=Staff | url = http://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html | title = "Deep Ocean Studies" | work = Ocean Studies | publisher = RAIN National Public Internet and Community Technology Center | accessdate = 2006-04-02
+}}</ref><ref>Takuyo measurement; see [[Mariana Trench]] for details.</ref> The average depth of the oceans is 3,794&nbsp;m, more than five times the average height of the continents.<ref name="hr_mill" />
+The mass of the oceans is approximately 1.35{{e|18}}&nbsp;[[metric ton]]s, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386{{e|9}}&nbsp;km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7&nbsp;km.<ref>The total volume of the Earth's oceans is: 1.4{{e|9}}&nbsp;km³. The total surface area of the Earth is 5.1{{e|8}}&nbsp;km². So, to [[Orders of approximation|first approximation]], the average depth would be the ratio of the two, or 2.7&nbsp;km.</ref> About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.<ref>{{cite web | author = Igor A. Shiklomanov ''et al'' | year = 1999 | url = http://espejo.unesco.org.uy/ | title = World Water Resources and their use Beginning of the 21st century" Prepared in the Framework of IHP UNESCO | publisher = State Hydrological Institute, St. Petersburg | accessdate = 2006-08-10 }}</ref>
+About 3.5% of the total mass of the oceans consists of [[salt]]. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.<ref>{{cite web | last = Mullen | first = Leslie | date = June 11, 2002 | url = http://www.astrobio.net/news/article223.html | title = Salt of the Early Earth | publisher = NASA Astrobiology Magazine | accessdate = 2007-03-14 }}</ref> The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.<ref>{{cite web | last = Morris | first = Ron M. | url = http://seis.natsci.csulb.edu/rmorris/oxy/oxy4.html | title = Oceanic Processes | publisher = NASA Astrobiology Magazine | accessdate = 2007-03-14 }}</ref> Sea water has an important influence on the world's
+climate, with the oceans acting as a large [[heat reservoir]].<ref>{{cite web | last = Scott | first = Michon | date = [[April 24]], [[2006]] | url = http://earthobservatory.nasa.gov/Study/HeatBucket/ | title = Earth's Big heat Bucket | publisher = NASA Earth Observatory | accessdate = 2007-03-14
+}}</ref> Shifts in the oceanic temperature distribution
+can cause significant weather shifts, such as the
+[[El Niño-Southern Oscillation]].<ref>{{cite web
+ | first=Sharron
+ | last=Sample
+ | date =June 21, 2005
+ | url =http://science.hq.nasa.gov/oceans/physical/SST.html
+ | title =Sea Surface Temperature
+ | publisher =NASA
+ | accessdate = 2007-04-21
+}}</ref>
+===Atmosphere===
+{{main|Earth's atmosphere}}
+The [[atmospheric pressure]] on the surface of the Earth averages 101.325&nbsp;[[kPa]], with a [[scale height]] of about 8.5&nbsp;km.<ref name="earth_fact_sheet"/> It is 78% [[nitrogen]] and 21% [[oxygen]], with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the [[troposphere]] varies with [[latitude]], ranging between 7 km (23,000 ft) at the poles to 17 km (60,000 ft) at the equator, with some variation due to weather factors.{{Fact|date=January 2008}}
+Earth's [[biosphere]] has significantly altered its [[atmosphere]]. [[Oxygen evolution#Oxygen evolution in nature|Oxygenic photosynthesis]] evolved 2.7 billion years ago, [[Oxygen Catastrophe|forming]] the primarily nitrogen-oxygen [[atmosphere]] that exists today. This change enabled the proliferation of [[aerobic organisms]] as well as the formation of the [[ozone layer]] which, together with Earth's [[magnetic field]], blocks [[ultraviolet]] [[solar radiation]], permitting life on land. Other atmospheric functions important to life on Earth's include transporting water vapor, providing useful gases, causing small [[meteor]]s to burn up before they strike the surface, and moderating temperature.<ref name="atmosphere">{{cite web | author=Staff | date = October 8, 2003 | url = http://www.nasa.gov/audience/forstudents/9-12/features/912_liftoff_atm.html | title = Earth's Atmosphere | publisher = NASA | accessdate = 2007-03-21 }}</ref> This last phenomenon is known as the [[greenhouse effect]]: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Carbon dioxide, water vapor, methane and ozone are the primary [[greenhouse gas]]es in the Earth's atmosphere. Without this heat-retention effect, the average surface temperature would be −18&nbsp;°C and life would likely not exist.<ref name="Pidwirny2006" />
-==Physical Description==
+====Weather and climate====
+{{main|Weather|Climate}}
+The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11&nbsp;km (about 7&nbsp;mi) of the planet's surface. This lowest layer is called the [[troposphere]]. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is [[atmospheric circulation]] that drives the [[weather]] and [[climate]] through redistribution of heat energy.<ref name="moran2005">{{cite web | last=Moran | first=Joseph M. | year=2005 | url=http://www.nasa.gov/worldbook/weather_worldbook.html | title=Weather | work=World Book Online Reference Center | publisher=NASA/World Book, Inc. | accessdate=2007-03-17 }}</ref>
-Within the [[solar system]], Earth is the third planet from the Sun. It is approximately 8,000 miles (13,000 km) in diameter, which means that its equatorial circumference (measured around the equator) is 24,901 miles (40,075 kilometers), and the polar circumference is slightly less at 24,809 miles (40,008 km).<ref>http://geography.about.com/library/faq/blqzcircumference.htm</ref> The reason its metric circumference is so close to the "round number" of 40,000 is that the [[kilometer]] was defined (by the French) as 1/10,000th the distance from the equator to the [[North Pole]].
+The primary atmospheric circulation bands consist of the [[trade winds]] in the [[equator]]ial region below 30° latitude and the [[westerlies]] in the mid-latitudes between
+° and 60°.<ref name="berger2002">{{cite web
+ | last = Berger
+ | first = Wolfgang H.
+ | year=2002
+ | url = http://earthguide.ucsd.edu/virtualmuseum/climatechange1/cc1syllabus.shtml
+ | title = The Earth's Climate System
+ | publisher = University of California, San Diego
+ | accessdate = 2007-03-24
+}}</ref> Ocean currents are also important factors in determining climate, particularly the [[thermohaline circulation]] that distributes heat energy from the equatorial oceans to the polar regions.<ref>{{cite web
+ | first=Stefan
+ | last=Rahmstorf
+ | year=2003
+ | url =http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html
+ | title =The Thermohaline Ocean Circulation
+ | publisher =Potsdam Institute for Climate Impact Research
+ | accessdate = 2007-04-21
+}}</ref>
-Its surface area is approximately 4&middot;&pi;&nbsp;(4000 mi)<sup>2</sup> = 200 million square miles (510 million km<sup>2</sup>). It has been said that Earth should really be called "water", as the larger part of it&mdash;about 70%&mdash;is covered by water. In fact, the Pacific and Indian oceans alone cover about half of the Earth's surface.
+[[Image:Air masses 2.jpg|left|thumb|300px|Source regions of global [[air mass]]es.]]
+Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere.
+When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as [[Precipitation (meteorology)|precipitation]].<ref name="moran2005" /> Most of the water is then transported back to lower elevations by [[river]] systems, usually returning to the oceans or being deposited into [[lake]]s. This [[water cycle]] is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. [[Atmospheric circulation]], topological features and temperature differences determine the average precipitation that falls in each region.<ref>{{cite web
+ | author=Various
+ | date = [[July 21]], [[1997]]
+ | url = http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml
+ | title = The Hydrologic Cycle
+ | publisher = University of Illinois
+ | accessdate = 2007-03-24
+}}</ref>
-The Earth orbits at an average distance of about 93 million miles (150 million km) from the Sun in an almost circular orbit. It takes light (and other forms of electromagnetic radiation) approximately 500 seconds to travel from the Sun to the Earth, i.e. the distance can also be stated as "500 light-seconds." The distance to nearby stars is made by measuring the stellar [[parallax]] between observations when the earth is at opposite ends of its orbit, so the earth's orbit itself is a measuring stick for astronomical distances, and is known as the ''astronomical unit'' (A. U.)  It is worth noting that if the Earth were slightly closer to the Sun, it would be too hot for life while if Earth were slightly further away from the Sun, water would freeze and life would be impossible.
+The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the [[equator]] to the polar regions, these are the [[tropics|tropical]] (or equatorial), [[Subtropics|subtropical]], [[temperate]] and [[Polar region|polar]] climates.<ref>{{cite web
+ | author=Staff
+ | url = http://www.ace.mmu.ac.uk/eae/Climate/Older/Climate_Zones.html
+ | title = Climate Zones
+ | publisher = UK Department for Environment, Food and Rural Affairs
+ | accessdate = 2007-03-24
+}}</ref> Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform [[air mass]]es. The commonly-used [[Köppen climate classification]] system (as modified by [[Wladimir Köppen]]'s student Rudolph Geiger) has five broad groups (humid tropics, [[Desert|arid]], humid middle latitudes, [[Continental climate|continental]] and cold polar), which are further divided into more specific subtypes.<ref name="berger2002" />
-==Origin==
+====Upper atmosphere====
-People disagree about when and how the Earth was created. The three most popular ideas are:
+[[Image:Full moon partially obscured by atmosphere.jpg|thumb|right|300px|This view from orbit shows the full Moon partially obscured by the Earth's atmosphere. ''NASA image.]]
-#That God created the Earth several hundred generations ago, as recounted in [[Genesis]]. By adding years and other time indications in the [[Old Testament]] chronologies, one can calculate the age of the Earth as approximately 6,000 years (see [[Date of creation]] and [[Young Earth Creationism]]).
+{{seealso|Outer space}}
-#That God created the Earth billions of years ago (see [[Old Earth Creationism]] and [[Theistic Evolution]]).
+Above the troposphere, the atmosphere is usually divided into the [[stratosphere]], [[mesosphere]], and [[thermosphere]].<ref name="atmosphere" /> Each of these layers has a different [[lapse rate]], defining the rate of change in temperature with height. Beyond these, the [[exosphere]] thins out into the [[magnetosphere]] (where the Earth's magnetic fields interact with the [[solar wind]]).<ref>{{cite web | author=Staff | year = 2004 | url = http://scienceweek.com/2004/rmps-23.htm | title = Stratosphere and Weather; Discovery of the Stratosphere | publisher = Science Week | accessdate = 2007-03-14 }}</ref> An important part of the atmosphere for [[life on Earth]] is the [[ozone layer]], a component of the stratosphere that partially shields the surface from ultraviolet light. The [[Kármán line]], defined as 100&nbsp;km above the Earth's surface, is a working definition for the boundary between atmosphere and space.<ref>{{cite web
-#That the Earth came into existence billions of years ago, but entirely through natural processes and without any intervention by God
+ | first=S. Sanz Fernández
+ | last=de Córdoba
+ | date =June 21, 2004
+ | url =http://www.un.org/members/list.shtml
+ | title =100&nbsp;km. Altitude Boundary for Astronautics
+ | publisher =Fédération Aéronautique Internationale
+ | accessdate = 2007-04-21
+}}</ref>
-Clashes between adherents of these ideas have gone on since time immemorial (see [[origins debate]]), though the last theory has only been fully developed in the last 200 years.
+Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can [[escape velocity|escape]] from the planet's gravity. This results in a slow but steady [[Atmospheric escape|leakage of the atmosphere into space]]. Because unfixed [[hydrogen]] has a low molecular weight, it can achieve [[escape velocity]] more readily and it leaks into outer space at a greater rate.<ref>{{cite journal | author=Liu, S. C.; Donahue, T. M. | title=The Aeronomy of Hydrogen in the Atmosphere of the Earth | journal=Journal of Atmospheric Sciences | year=1974 | volume=31 | issue=4 | pages=1118–1136 | url=http://adsabs.harvard.edu/abs/1974JAtS...31.1118L | accessdate=2007-03-02 }}</ref> For this reason, the Earth's current environment is [[Redox|oxidizing]], rather than [[Redox|reducing]], with consequences for the [[chemical]] nature of [[life]] which developed on the planet. The oxygen-rich atmosphere also preserves much of the surviving hydrogen by locking it up in water molecules.<ref>{{cite web | last = Abedon | first = Stephen T. | date = March 31, 1997 | url = http://www.mansfield.ohio-state.edu/~sabedon/biol1010.htm | title = History of Earth | publisher = Ohio State University | accessdate = 2007-03-19 }}</ref>
+[[Image:Dipole field.jpg|thumb|right|300px|The [[Earth's magnetic field]], which approximates a dipole.]]
+===Magnetic field===
+{{main|Earth's magnetic field}}
+The [[Earth's magnetic field]] is shaped roughly as a [[magnetic dipole]], with the poles currently located proximate to the planet's geographic poles. According to [[dynamo theory]], the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in [[geomagnetic reversal|field reversals]] at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.<ref>{{cite web | last = Fitzpatrick | first = Richard | date = [[February 16]], [[2006]] | url = http://farside.ph.utexas.edu/teaching/plasma/lectures/node69.html | title = MHD dynamo theory | publisher = NASA WMAP | accessdate = 2007-02-27 }}</ref><ref name=campbelwh>{{cite book
+ | last =Campbell
+ | first =Wallace Hall
+ | authorlink =
+ | coauthors =
+ | title =Introduction to Geomagnetic Fields
+ | publisher =Cambridge University Press
+ | date =2003
+ | location =New York
+ | pages =p57
+ | url =
+ | doi =
+ | id =
+ | isbn = 0521822068}}</ref>
-== Young Earth Creationist view ==
+The field forms the [[magnetosphere]], which deflects particles in the [[solar wind]]. The sunward edge of the [[bow shock]] is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the [[Van Allen radiation belt]]s, a pair of concentric, [[torus]]-shaped regions of energetic [[charged particle]]s. When the [[plasma (physics)|plasma]] enters the Earth's atmosphere at the magnetic poles, it forms the [[Aurora (astronomy)|aurora]].<ref>{{cite web | last = Stern | first = David P. | date = July 8, 2005 | url = http://www-spof.gsfc.nasa.gov/Education/wmap.html | title = Exploration of the Earth's Magnetosphere | publisher = NASA | accessdate = 2007-03-21 }}</ref>
+{{-}}
-===Formation and Age===
+==Orbit and rotation==
+{{main|Earth's rotation}}
-Young Earth creationists believe, on the basis of the biblical account in Genesis and biblical geochronologies, that the entire Earth, including animal, plant, and human life, was formed in six days, around 4000 B.C.
+[[Image:Rotating earth (large).gif|thumb|right|175px|An animation showing the rotation of the Earth.]]
-Mainstream scientific journals, committed to a [[naturalism|naturalistic]] worldview, contend this view.<ref>http://www.answersingenesis.org/docs2005/0822sternberg.asp 1</ref><ref>http://www.answersingenesis.org/docs/538.asp 2</ref><ref>http://worldnetdaily.com/news/article.asp?ARTICLE_ID=53400 3</ref><ref>http://www.discovery.org/scripts/viewDB/index.php?command=view&id=3833&program=DI%20Main%20Page%20-%20News&callingPage=discoMainPage 4</ref>
+Relative to the background stars, it takes the Earth, on average, 23&nbsp;hours, 56&nbsp;minutes and 4.091&nbsp;seconds ([[sidereal day|one sidereal day]]) to rotate around the [[Axis of rotation|axis]] that connects the [[north pole|north]] and the [[south pole]]s.<ref>{{cite web | last = Fisher | first = Rick | date = January, 30, 1996 | url = http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html | title = Astronomical Times | publisher = National Radio Astronomy Observatory | accessdate = 2007-03-21 }}</ref> From Earth, the main apparent motion of celestial bodies in the sky (except that of [[meteor]]s within the atmosphere and low-orbiting satellites) is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes. (The apparent sizes of the Sun and the Moon are approximately the same.)
-Most scientists believe believe that the Earth formed by natural processes instead of being supernaturally created. However, as one scientist noted, “...&nbsp;most every prediction by theorists about planetary formation has been wrong.” <ref>http://www.creationscience.com/onlinebook/ReferencesandNotes43.html</ref>
+Earth orbits the Sun at an average distance of about 150&nbsp;million kilometers (93.2&nbsp;million miles) every 365.2564&nbsp;mean&nbsp;solar&nbsp;days ([[sidereal year|1 sidereal&nbsp;year]]). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1°/day (or a Sun or Moon diameter every 12&nbsp;hours) eastward. Because of this motion, on average it takes 24 hours&mdash;a [[Solar time|solar day]]&mdash;for Earth to complete a full rotation about its axis so that the Sun returns to the [[Meridian (astronomy)|meridian]]. The orbital speed of the Earth averages about 30&nbsp;km/s (108,000&nbsp;km/h or 67,000&nbsp;mi/h), which is fast enough to cover the planet's diameter (about 12,600&nbsp;km [7,800&nbsp;mi]) in seven minutes, and the distance to the Moon (384,000&nbsp;km or 238,000&nbsp;mi) in four hours.<ref name="earth_fact_sheet">{{cite web | last = Williams | first = David R. | date = September 1, 2004 | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html | title = Earth Fact Sheet | publisher = NASA | accessdate = 2007-03-17 }}</ref>
-===Shape of the Earth===
+[[Image:PaleBlueDot.jpg|thumb|left|195px|[[Pale Blue Dot|Earth seen as a tiny dot]] by the [[Voyager 1]] spacecraft, more than 6 billion kilometers (4 billion miles) from Earth.]]
+The [[Moon]] revolves with the Earth around a common [[barycenter]] every 27.32&nbsp;days relative to the background stars. 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&nbsp;days. Viewed from the [[celestial pole|celestial north pole]], the motion of Earth, the Moon and their axial rotations are all [[counter-clockwise]]. The orbital and axial planes are not precisely aligned: Earth's [[axial tilt|axis is tilted]] some 23.5&nbsp;degrees from the perpendicular to the Earth–Sun plane (which causes the [[season]]s); and the Earth–Moon plane is tilted about 5&nbsp;degrees against the Earth-Sun plane (without this tilt, there would be an eclipse every two weeks, alternating between [[lunar eclipse]]s and [[solar eclipse]]s).<ref name="moon_fact_sheet">{{cite web | last = Williams | first = David R. | date = September 1, 2004 | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html | title = Moon Fact Sheet | publisher = NASA | accessdate = 2007-03-21 }}</ref><ref name="earth_fact_sheet" />
-Some people dispute the shape of the earth, saying that according to the Bible, the earth is flat rather than spherical (presumably square, given that it is described as having "four corners" (Revelations 7:1)). However, no credible organization has ever expressed support for this theory.<ref>http://www.lhup.edu/~dsimanek/crea-fe.htm</ref> Some have disputed the idea that the earth rotates around the sun.<ref>http://www.lhup.edu/~dsimanek/febible.htm</ref>
+Because of the axial tilt of the Earth, the position of the Sun in the sky (as seen by an observer on the surface) varies over the course of the year. For an observer at a northern latitude, when the northern pole is tilted toward the Sun the day lasts longer and the Sun climbs higher in the sky. This results in warmer average temperatures from the increase in solar radiation reaching the surface. When the northern pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the [[arctic circle]], an extreme case is reached where there is no daylight at all for part of the year. (This is called a [[polar night]].)
-In Revelations 7:1 it is stated:
+This variation in the climate (because of the direction of the Earth's axial tilt) results in the [[season]]s. By astronomical convention, the four seasons are determined by the [[solstice]]s&mdash;the point in the orbit of maximum axial tilt toward or away from the Sun&mdash;and the [[equinox]]es, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about [[December 21]], summer solstice is near [[June 21]], spring equinox is around [[March 20]] and autumnal equinox is about [[September 23]]. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.
-"And after these things I saw four angels standing on the four corners of the earth..." (Revelations 7:1)
+The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo a slight, irregular motion (known as [[nutation]]) with a main period of 18.6&nbsp;years. The orientation (rather than the angle) of the Earth's axis also changes over time, [[precession|precessing]] around in a complete circle over each 25,800&nbsp;year cycle; this precession is the reason for the difference between a sidereal year and a [[tropical year]]. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's [[equatorial bulge]]. From the perspective of the Earth, the poles also migrate a few meters across the surface. This [[polar motion]] has multiple, cyclical components, which collectively are termed [[quasiperiodic motion]]. In addition to an annual component to this motion, there is a 14-month cycle called the [[Chandler wobble]]. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.<ref>{{cite web | last = Fisher | first = Rick | date = February 5, 1996 | url = http://www.cv.nrao.edu/~rfisher/Ephemerides/earth_rot.html | title = Earth Rotation and Equatorial Coordinates | publisher = National Radio Astronomy Observatory | accessdate = 2007-03-21 }}</ref>
-A sphere has no corners. The four corners are also mentioned in Isaiah 11:12. This is often accepted to be a figure of speech.
+In modern times, Earth's [[perihelion]] occurs around [[January 3]], and the [[aphelion]] around [[July 4]] (for other eras, see [[precession (astronomy)|precession]] and [[Milankovitch cycles]]). The changing Earth-Sun distance results in an increase of about 6.9%<ref>Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% the energy at aphelion.</ref> in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.<ref>{{cite web | last = Williams | first = Jack | date = December 20, 2005 | url = http://www.usatoday.com/weather/tg/wseason/wseason.htm | title = Earth's tilt creates seasons | publisher = USAToday | accessdate = 2007-03-17 }}</ref>
-It is also stated:
+The [[Hill sphere]] ([[gravity|gravitational]] sphere of influence) of the Earth is about 1.5&nbsp;Gm (930,000&nbsp;miles) in radius.<ref>{{cite web | author=Vázquez, M.; Montañés Rodríguez, P.; Palle, E. | year=2006 | url =http://www.iac.es/folleto/research/preprints/files/PP06024.pdf | title = The Earth as an Object of Astrophysical Interest in the Search for Extrasolar Planets | publisher = Instituto de Astrofísica de Canarias | accessdate = 2007-03-21 }}</ref><ref>For the Earth, the Hill radius is
-"Then was the iron, the clay, the brass, the silver, and the gold, broken to pieces together, and became like the chaff of the summer threshingfloors; and the wind carried them away, that no place was found for them: and the stone that smote the image became a great mountain, and filled the whole earth." (Daniel 2:35)
+:<math>\begin{smallmatrix} R_H = a\left ( \frac{m}{3M} \right )^{\frac{1}{3}} \end{smallmatrix}</math>,
-AND
+where ''m'' is the mass of the Earth, ''a'' is an Astronomical Unit, and ''M'' is the mass of the Sun. So the radius in A.U. is about:
-"Thus were the visions of mine head in my bed; I saw, and behold a tree in the midst of the earth, and the height thereof was great. The tree grew, and was strong, and the height thereof reached unto heaven, and the sight thereof to the end of all the earth:" (Daniel 4:10-11)
+<math>\begin{smallmatrix} \left ( \frac{1}{3 \cdot 332,946} \right )^{\frac{1}{3}} = 0.01 \end{smallmatrix}</math>.</ref> This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.
-AND
+==Moon==
+{{main|Moon}}
-"Behold, [Jesus] cometh with clouds; and every eye shall see him" (Revelations 1:7)
+{| class="wikitable"
+!Name || Diameter || Mass || [[Semi-major axis]] || Orbital period
+|-
+|rowspan="2"|[[Moon]]
+| style="text-align: center" | 3,474.8&nbsp;km
+| style="text-align: center" | 7.349{{e|22}}&nbsp;kg
+| style="text-align: center" | 384,400&nbsp;km
+|rowspan="2"|27&nbsp;days, 7&nbsp;hours, 43.7&nbsp;minutes
+|-
+| style="text-align: center" | 2,159.2&nbsp;mi
+| style="text-align: center" | 8.1{{e|19}}&nbsp;(short) tons
+| style="text-align: center" | 238,700&nbsp;mi
+|}
+The Moon is a relatively large, [[Terrestrial planet|terrestrial]], planet-like satellite, with a diameter about one-quarter of the Earth's. It is the largest moon in the solar system relative to the size of its planet. ([[Charon (moon)|Charon]] is larger relative to the [[dwarf planet]] [[Pluto]].) The [[natural satellite]]s orbiting other planets are called "moons" after Earth's Moon.
-AND
+The gravitational attraction between the Earth and Moon cause [[tides]] on Earth. The same effect on the Moon has led to its [[tidal locking]]: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the [[lunar phase]]s: The dark part of the face is separated from the light part by the [[terminator (solar)|solar terminator]].
-"Once again, the devil took him to a very high mountain, and showed him all the kingdoms of the world [cosmos] in their glory." (Matthew 4:8)
+Because of their [[Tidal acceleration|tidal interaction]], the Moon recedes from Earth at the rate of approximately 38&nbsp;[[Millimetre|mm]] (1.5 in) a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 [[Microsecond|µs]] a year—add up to significant changes.<ref>{{cite web
+ | author=Espenak, F.; Meeus, J.
+ | date = February 7, 2007
+ | url = http://sunearth.gsfc.nasa.gov/eclipse/SEcat5/secular.html
+ | title = Secular acceleration of the Moon
+ | publisher = NASA
+ | accessdate = 2007-04-20
+}}</ref> During the [[Devonian]] period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.<ref>{{cite web
+ | first=Hannu K. J.
+ | last=Poropudas
+ | date =December 16, 1991
+ | url = http://www.skepticfiles.org/origins/coralclo.htm
+ | title = Using Coral as a Clock
+ | publisher = Skeptic Tank
+ | accessdate = 2007-04-20
+}}</ref>
-Apologist [[JP Holding]]'s take on this is that this may have been a vision. <ref>http://www.tektonics.org/af/earthshape.html#globe</ref>
+The Moon may have dramatically affected the development of life by moderating the planet's climate. [[Paleontology|Paleontological]] evidence and computer simulations show that Earth's [[axial tilt]] is stabilized by tidal interactions with the Moon.<ref>{{cite journal
+ | author=Laskar, J.; Robutel, P.; Joutel, F.; Gastineau, M.; Correia, A.C.M.;
+Levrard, B.
+ | title=A long-term numerical solution for the insolation quantities of the Earth
+ | journal=Astronomy and Astrophysics
+ | year=2004
+ | volume=428
+ | pages=261–285
+ | url=http://adsabs.harvard.edu/abs/2004A&A...428..261L
+ | accessdate=2007-03-31 }}</ref> Some theorists believe that without this stabilization against the [[torque]]s applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, as it appears to be for [[Mars (planet)|Mars]]. If Earth's axis of rotation were to approach the [[ecliptic|plane of the ecliptic]], extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during ''summer'' and directly away during ''winter''. [[Planetary science|Planetary scientists]] who have studied the effect claim that this might kill all large animal and higher plant life.<ref>{{cite journal
+ | author=Williams, D.M.; J.F. Kasting
+ | title=Habitable planets with high obliquities
+ | journal=Lunar and Planetary Science
+ | year=1996
+ | volume=27
+ | pages=1437–1438
+ | url=http://adsabs.harvard.edu/abs/1996LPI....27.1437W
+ | accessdate=2007-03-31 }}</ref> However, this is a controversial subject, and further studies of Mars—which has a similar [[sidereal day|rotation period]] and [[axial tilt]] as Earth, but not its large Moon or liquid core—may settle the matter.
-===Position of the Earth===
+Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. The [[angular size]] (or [[solid angle]]) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant. This allows total and annular [[eclipse]]s to occur on Earth.
-In the view of the Bible the Earth is fixed, and the sun revolves around it. This can be found in several passages:
+{{-}}
+[[Image:Earth-Moon2.jpg|thumb|center|800px| A scale representation of the relative sizes of, and distance between, Earth and Moon.]]
-"He has fixed the earth firm, immovable." (1 Chronicles 16:30)
+The most widely accepted theory of the Moon's origin, the [[Giant impact hypothesis|giant impact theory]], states that it formed from the collision of a Mars-size [[protoplanet]] called Theia 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.<ref>{{cite journal
+ | last = R. Canup and E. Asphaug
+ | title = Origin of the Moon in a giant impact near the end of the Earth's formation
+ | journal = Nature
+ | volume = 412
+ | pages = 708–712
+ | date = 2001 }}</ref>
-"Thou hast fixed the earth immovable and firm ..." (Psalm 93:1)
+Earth has at least two [[co-orbital satellite]]s, the [[asteroid]]s [[3753 Cruithne]] and [[2002 AA29|2002 AA<sub>29</sub>]].<ref>{{cite news
+ | first=David
+ | last=Whitehouse
+ | title=Earth's little brother found
+ | publisher=BBC News
+ | date=[[October 21]], [[2002]]
+ | url=http://news.bbc.co.uk/1/hi/sci/tech/2347663.stm
+ | accessdate=2007-03-31 }}</ref>
-"Thou didst fix the earth on its foundation so that it never can be shaken." (Psalm 104:5)
+==Habitability==
+{{seealso|Planetary habitability}}
-"...who made the earth and fashioned it, and himself fixed it fast..." (Isaiah 45:18)
+A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble, and sufficient energy to sustain [[metabolism]].<ref>{{cite web | author=Staff | date = September, 2003 | url = http://astrobiology.arc.nasa.gov/roadmap/g1.html | title = Astrobiology Roadmap | publisher = NASA, Lockheed Martin | accessdate = 2007-03-10 }}</ref> The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.<ref>{{cite book | first=Stephen H. | last=Dole | year=1970 | title=Habitable Planets for Man | edition=2nd edition | publisher=American Elsevier Publishing Co. | url=http://www.rand.org/pubs/reports/R414/ | accessdate=2007-03-11 | id=ISBN 0-444-00092-5 }}</ref>
-See Also: [[Geocentric theory]]
+===Biosphere===
+{{main|Biosphere}}
-==Christianity and the Earth==
+The planet's life forms are sometimes said to form a "[[biosphere]]". This biosphere is generally believed to have begun [[evolution|evolving]] about 3.5&nbsp;billion years ago. Earth is the only place in the universe where life is known to exist. Some scientists believe that Earth-like biospheres might be [[Rare Earth hypothesis|rare]].<ref>{{cite book
+ | author=Ward, P. D.; Brownlee, D.
+ | date=[[January 14]], [[2000]]
+ | title=Rare Earth: Why Complex Life is Uncommon in the Universe
+ | edition=1st edition
+ | publisher=Springer-Verlag
+ | location=New York
+ | id=ISBN 0387987010 }}</ref>
-===Christian Historical-Grammatical Bible Exegesis or Bible Literalism===
+The biosphere is divided into a number of [[biome]]s, inhabited by broadly similar [[plants]] and [[animals]]. On land primarily [[latitude]] and height above the sea level separates biomes. Terrestrial biomes lying within the [[Arctic Circle|Arctic]], [[Antarctic Circle]] or in high altitudes are relatively barren of [[plant]] and [[animal]] life, while the greatest [[Latitudinal gradients in species diversity|latitudinal diversity of species]] is found at the [[Equator]].<ref>{{cite journal
+ | last = Hillebrand
+ | first = Helmut
+ | title=On the Generality of the Latitudinal Gradient
+ | journal=American Naturalist
+ | year=2004
+ | volume=163
+ | issue=2
+ | pages=192–211 }}</ref>
-According to [[Creation Ministries International]], most young earth creationists use a [[hermeneutic]] "best described as the [[Historical-grammatical exegesis|historical-grammatical method]] in which historical narrative (such as the book of Genesis) is interpreted as literal history, prophecy is interpreted as prophecy, poetry is interpreted as poetry, etc."<ref>http://www.creationontheweb.com/images/pdfs/tj/tjv16v2_forster.pdf</ref> Creation Ministries International further states that "Historical-grammatical exegesis involves a systematic approach to analyzing in detail the historical situation, events and circumstances surrounding the text, and the semantics and syntactical relationships of the words which comprise the text."<ref>http://www.creationontheweb.com/content/view/4880/</ref>
+===Natural resources and land use===
+{{main|Natural resource}}
-Bible scholars have estimated the age of the earth based on the Creation account in Genesis and the genealogical accounts in Numbers and other books of the Pentateuch. One famous estimate was published in 1650 by [[James Ussher|James Ussher Archbishop of Armagh]] in a book called ''Annals of the World,'' in which he estimated the Creation to have occurred on 23 October 4004 B.C. Other Biblical scholars maintain that there are possible gaps in the genealogies, often using the ideas of the 19th century Calvinist theologian [[Benjamin Warfield]] on the issue. <ref>http://www.reasons.org/resources/apologetics/primeval_chronology.shtml</ref> However, [[James Barr]], regius professor of Hebrew at [[Oxford University]], wrote in 1984 the following: "… probably, so far as I know, there is no professor of Hebrew or Old Testament at any world-class university who does not believe that the writer(s) of Genesis 1–11 intended to convey to their readers the ideas that: … the figures contained in the Genesis genealogies provided by simple addition a chronology from the beginning of the world up to later stages in the biblical story.’"  <ref>http://www.creationontheweb.com/content/view/1606/</ref> Furthermore, it should be noted that Barr himself rejects supernatural Christianity, and so is not a priori biased in favor of creationism. <ref>http://www.reasons.org/resources/apologetics/pca_creation_study_committee_report.shtml</ref>
+The Earth provides resources that are exploitable by humans for useful purposes. Some of these are [[non-renewable resources]], such as [[fossil fuel|mineral fuels]], that are difficult to replenish on a short time scale.
-Many [[Christian|Christians]] believe that the Earth is the perfect distance away from the Sun and  take this to be evidence of [[God]]'s existence. <ref>http://www.everystudent.com/features/isthere.html</ref> Secular scientists, however, reject this reasoning using the anthropic principle<ref>http://www.anthropic-principle.com</ref>.
+Large deposits of [[fossil fuel]]s are obtained from the Earth's crust, consisting of [[coal]], [[petroleum]], [[natural gas]] and [[methane clathrate]]. These deposits are used by [[human]]s both for energy production and as feedstock for chemical production. Mineral [[ore]] bodies have also been formed in Earth's crust through a process of [[Ore genesis]], resulting from actions of [[erosion]] and [[plate tectonics]].<ref>{{cite web
+ | author=Staff
+ | date=[[November 24]], [[2006]]
+ | url=http://www.utexas.edu/tmm/npl/mineralogy/Mineral_Genesis/
+ | title=Mineral Genesis: How do minerals form?
+ | publisher=Non-vertebrate Paleontology Laboratory, Texas Memorial Museum
+ | accessdate=2007-04-01
+}}</ref> These bodies form concentrated sources for many [[metal]]s and other useful [[chemical element|elements]].
-===Christian Non-Literalism===
+The Earth's [[biosphere]] produces many useful biological products for humans, including (but far from limited to) [[food]], [[wood]], [[pharmaceutical]]s, oxygen, and the recycling of many organic wastes. The land-based [[ecosystem]] depends upon [[topsoil]] and fresh water, and the oceanic [[ecosystem]] depends upon dissolved nutrients washed down from the land.<ref>{{cite journal
+ | last = Rona
+ | first = Peter A.
+ | title=Resources of the Sea Floor
+ | journal=Science
+ | year=2003
+ | volume=299
+ | issue=5607
+ | pages=673–674
+ | url=http://www.sciencemag.org/cgi/content/full/299/5607/673?ijkey=AHVbRrqUsmdHY&keytype=ref&siteid=sci
+ | accessdate=2007-02-04 }}</ref> Humans also live on the [[Ecological footprint|land]] by using [[building material]]s to construct [[home|shelters]]. In 1993, human use of land is approximately:
-Many mainstream Christian denominations believe the story of Genesis is not meant to be read literally, and believe that the age of the Earth is on the order of millions or billions of years, not thousands of years. <ref>http://webusers.xula.edu/cporter/2000n/evolution_and_religion.htm</ref>
+{| class="wikitable"
+!Land use
+!Percentage
+|-
+| ''Arable land:'' ||style="text-align: right;"| 13.13%<ref name="cia" />
+|-
+| ''Permanent crops:'' ||style="text-align: right;"| 4.71%<ref name="cia" />
+|-
+| ''Permanent pastures:'' ||style="text-align: right;"| 26%
+|-
+| ''Forests and woodland:'' ||style="text-align: right;"| 32%
+|-
+| ''Urban areas:'' ||style="text-align: right;"| 1.5%
+|-
+| ''Other:'' ||style="text-align: right;"| 30%
+|}
-===Scientific Uniformitarianism===
+The estimated amount of irrigated land in 1993 was 2,481,250&nbsp;km².<ref name="cia" />
-Uniformitarian scientists believe that the earth is beyond 4 billion years old.<ref>http://pubs.usgs.gov/gip/geotime/age.html</ref>  They also refute that the Earth is only 6,000 years old by quoting older human societies dated by their dating method as older than that.<ref>http://www.ancientegypt.co.uk/</ref>
+===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.
-== Evolutionary view ==
+Many localized areas are subject to human-made [[pollution]] of the air and water, [[acid rain]] and toxic substances, loss of vegetation ([[overgrazing]], [[deforestation]], [[desertification]]), loss of [[wildlife]], [[species]] [[extinction]], [[soils retrogression and degradation|soil degradation]], soil depletion, [[erosion]], and introduction of [[invasive species]].
-Estimates by uniformitarian geologists of the age of the Earth and the beginning of life give about 4.55 billion years and 3.5 billion years ago respectively. These estimates are primarily based on radioactive dating of meteorites and fossil specimens. The majority of scientists conclude that the Earth formed by natural processes, specifically by the accumulation of debris orbiting the sun billions of years in the past.{{fact}}
+A [[scientific consensus]] exists linking human activities to [[global warming]] due to industrial [[carbon dioxide]] emissions. This is predicted to produce changes such as the melting of [[glacier]]s and [[ice sheet]]s, more extreme temperature ranges, significant changes in weather conditions and a [[Sea level rise|global rise in average sea levels]].<ref>{{cite web
+ | author=Staff
+ | date = [[February 2]], [[2007]]
+ | url = http://www.un.org/apps/news/story.asp?NewsID=21429&Cr=climate&Cr1=change
+ | title = Evidence is now ‘unequivocal’ that humans are causing global warming – UN report
+ | publisher = United Nations
+ | accessdate = 2007-03-07 }}</ref>
+===Human geography===
+{{main|Human geography}}
+{{Earth Labelled Map|width=400|float=right}}
+[[Image:Earthlights dmsp.jpg|400px|right|thumb|The Earth at night, a composite of [[Defense Meteorological Satellite Program|DMSP]]/OLS ground illumination data on a simulated night-time image of the world. This image is not [[Photography|photographic]] and many features are brighter than they would appear to a direct observer.]]
+Earth has approximately 6,671,226,000 human inhabitants as of July 2007.<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= [[February 2]], [[2006]]
+ | accessdate= 2006-04-02
+}}
+</ref> Projections indicate that the [[world population|world's human population]] will reach seven billion in 2013 and 9.2&nbsp;billion<ref>{{cite web
+ | author=Staff
+ | url = http://www.un.org/esa/population/publications/wpp2006/wpp2006.htm
+ | title = World Population Prospects: The 2006 Revision
+ | publisher = United Nations
+ | accessdate = 2007-03-07 }}</ref> in 2050. Most of the growth is expected to take place in [[developing nations]]. Human [[population density]] varies widely around the world, but a majority live in [[Asia]]. By 2020, 60% of the world's population is expected to be living in [[Urban area|urban]], rather than [[rural]], areas.<ref>{{cite web
+ | author = Staff
+ | year = 2007
+ | url = http://www.prb.org/Educators/TeachersGuides/HumanPopulation/PopulationGrowth/QuestionAnswer.aspx
+ | title = Human Population: Fundamentals of Growth: Growth
+ | publisher = Population Reference Bureau
+ | accessdate = 2007-03-31
+}}</ref>
+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 either [[desert]] (14%),<ref>{{cite journal
+ | author=Peel, M. C.; Finlayson, B. L.; McMahon, T. A.
+ | title=Updated world map of the Köppen-Geiger climate classification
+ | journal=Hydrology and Earth System Sciences Discussions
+ | year=2007
+ | volume=4
+ | pages=439–473
+ | url=http://overview.sref.org/1812-2116/hessd/2007-4-439
+ | accessdate=2007-03-31 }}</ref> high [[mountain]]s (27%),<ref>{{cite web
+ | author=Staff
+ | url = http://www.biodiv.org/programmes/default.shtml
+ | title = Themes & Issues
+ | publisher = Secretariat of the Convention on Biological Diversity
+ | accessdate = 2007-03-29
+}}</ref> or other less suitable terrain. The northernmost permanent settlement in the world is [[Alert, Nunavut|Alert]], on [[Ellesmere Island]] in [[Nunavut]], [[Canada]].<ref>{{cite web
+ | author = Staff
+ | date = 2006-08-15
+ | url = http://www.img.forces.gc.ca/org/cfiog/alert_e.asp
+ | title = Canadian Forces Station (CFS) Alert
+ | publisher = Information Management Group
+ | accessdate = 2007-03-31
+}}</ref> (82°28′N) The southernmost is the [[Amundsen-Scott South Pole Station]], in [[Antarctica]], almost exactly at the [[South Pole]]. (90°S)
+Independent sovereign [[nation]]s claim all of the planet's land surface, with the exception of some parts of [[Antarctica]]. As of 2007 there are [[List of sovereign states|201 sovereign states]], including the 192 [[United Nations member states]]. In addition, there are 59 [[Dependent territory|dependent territories]], and a number of [[List of autonomous areas by country|autonomous areas]], [[List of territorial disputes|territories under dispute]] and other entities. Historically, Earth has never had a [[sovereignty|sovereign]] [[government]] with authority over the entire globe, although a number of nation-states have striven for [[world domination]] and failed.
+The [[United Nations]] is a worldwide [[international organization|intergovernmental organization]] that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict. It is not, however, a world government. While the U.N. provides a mechanism for [[international law]] and, when the consensus of the membership permits, armed intervention,<ref>{{cite web | author=Staff | url = http://www.un.org/law/ | title = International Law | publisher = United Nations | accessdate = 2007-03-27 }}</ref> it serves primarily as a forum for international diplomacy.
+In total, about 400 people have been outside the Earth's atmosphere as of 2004, and, of these, [[Apollo program|twelve]] have walked on the [[Moon]]. Normally the only humans in space are those on the [[International Space Station]]. The station's crew of three people is usually replaced every six months.{{-}}
+==Cultural viewpoint==
+[[Image:AS8-13-2329.jpg|thumb|left|The first photograph ever taken of an "Earthrise," on [[Apollo 8]].]]
+===Etymology===
+The name ''Earth'' originates from the 8th century [[Old English language|Anglo-Saxon]] word ''erda'', which means ground or soil. In [[Old English]] the word became ''eorthe'', then ''erthe'' in [[Middle English]].<ref>{{cite book
+ | date=July 2005 | title=Random House Unabridged Dictionary
+ | publisher=Random House | id=ISBN 0-375-42599-3 }}</ref> Earth was first used as the name of the planet around 1400.<ref>{{cite web
+ | last = Harper| first = Douglas | date = November 2001
+ | url = http://www.etymonline.com/index.php?term=earth
+ | title = Earth | publisher = Online Etymology Dictionary
+ | accessdate = 2007-08-07 }}</ref> It is the only planet whose name in English is not derived from [[Greek mythology|Greco]]-[[Roman mythology]].
+The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle. This symbol is known as the wheel cross, sun cross, Odin's cross or Woden's cross. Although it has been used in various cultures for different purposes, it came to represent the compass points, earth and the land. Another version of the symbol is a cross on top of a circle; a stylized [[globus cruciger]] that was also used as an early astronomical symbol for the planet Earth.<ref>{{cite book
+ | first=Carl G. | last=Liungman | year=2004
+ | chapter=Group 29: Multi-axes symmetric, both soft and straight-lined, closed signs with crossing lines
+ | title=Symbols -- Encyclopedia of Western Signs and Ideograms
+ | pages=pp. 281&ndash;282 | publisher=Ionfox AB
+ | location=New York | id=ISBN 91-972705-0-4 }}</ref>
+===Religious beliefs===
+Earth has often been personified as a [[deity]], in particular a [[goddess]]. In many cultures the [[mother goddess]], also called the Mother Earth, is also portrayed as a [[Fertility god|fertility deity]].  See also ''[[Graha]]''.
+To the [[Aztec]], Earth was called [[Tonantzin]]&mdash;"our mother". The [[China|Chinese]] Earth goddess [[Hou-T'u]]<ref>{{cite book | first=E. T. C. | last=Werner | year=1922 | title=Myths & Legends of China | publisher=George G. Harrap & Co. Ltd. | location=New York | url=http://www.gutenberg.org/etext/15250 | accessdate=2007-03-14 }}</ref> is similar to [[Gaia (mythology)|Gaia]], the Greek goddess personifying the Earth. To [[Hindu]]s it is called [[Bhuma Devi]], the Goddess of Earth. In [[Norse mythology]], the Earth goddess [[Jord]] was the mother of [[Thor]] and the daughter of [[Annar]]. [[Ancient Egyptian religion|Ancient Egyptian mythology]] is different from that of other cultures because Earth is male, [[Geb]], and sky is female, [[Nut (goddess)|Nut]].
+In many [[religion]]s, [[creation myth]]s exist, recalling a story involving the creation of the Earth by a supernatural [[deity]] or deities.
+===Exploration and mapping===
+In the ancient past there were varying levels of belief in a [[flat Earth]], with the [[Mesopotamian mythology|Mesopotamian]] culture portraying the world as a flat disk afloat in an ocean. The spherical form of the Earth was suggested by early [[Greek philosophers]]; a belief espoused by [[Pythagoras]]. By the [[Middle Ages]]&mdash;as evidenced by thinkers such as [[Thomas Aquinas]]&mdash;European belief in a [[spherical Earth]] was widespread.<ref>{{cite web | last = Russell | first = Jeffrey B. | url = http://www.asa3.org/ASA/topics/history/1997Russell.html | title = The Myth of the Flat Earth | publisher = American Scientific Affiliation | accessdate = 2007-03-14 }}; but see also [[Cosmas Indicopleustes]]</ref> Prior to circumnavigation of the planet and the introduction of [[space flight]], belief in a spherical Earth was based on observations of the secondary effects of the Earth's shape and parallels drawn with the shape of other planets.<ref>{{cite web
+ | last = Jacobs
+ | first = James Q.
+ | date =February 1, 1998
+ | url =http://www.jqjacobs.net/astro/aegeo.html
+ | title =Archaeogeodesy, a Key to Prehistory
+ | accessdate = 2007-04-21
+}}</ref>
+[[Cartography]], the study and practice of map making, and vicariously [[geography]], have historically been the disciplines devoted to depicting the Earth. [[Surveying]], the determination of locations and distances, to a lesser extent [[navigation]], the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.
+===Modern perspective===
+{{see also|Spaceship Earth|Gaia theory|Geocentric orbit}}
+The technological developments of the latter half of the 20th century are widely considered to have altered the public's perception of the Earth. Before space flight, the popular image of Earth was of a green world. [[Science fiction]] artist [[Frank R. Paul]] provided perhaps the first image of a cloudless ''blue'' planet (with sharply defined land masses) on the back cover of the July 1940 issue of ''[[Amazing Stories]]'', a common depiction for several decades thereafter.<ref name="fja">
+{{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>
+[[Image:Earth and Moon from Mars PIA04531.jpg|200px|thumb|right|Earth and Moon from Mars, imaged by [[Mars Global Surveyor]]. From [[Outer space|space]], the Earth can be seen to go through phases similar to the [[lunar phases|phases of the Moon]].]]
+Earth was first photographed from space by [[Explorer 6]] in 1959.<ref>{{cite web | author=Staff | date = October, 1998 | url = http://www.nasa.gov/centers/goddard/pdf/106420main_explorers.pdf | format=PDF | title = Explorers: Searching the Universe Forty Years Later | publisher = NASA/Goddard | accessdate = 2007-03-05 }}</ref> [[Yuri Gagarin]] became the first human to view Earth from space in 1961. The crew of the [[Apollo 8]] was the first to view an Earth-rise from lunar orbit in 1968. In 1972 the crew of the [[Apollo 17]] produced the famous "[[The Blue Marble|Blue Marble]]" photograph of the planet Earth from [[cislunar space]] (see [[#top|top of page]]). This became an iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. NASA archivist Mike Gentry has speculated that "The Blue Marble" is the most widely distributed image in human history. 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
+ | author=Staff
+ | 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 [[Spacecraft|spaceship]], with a [[life support system]] that requires maintenance,<ref>{{cite book
+ | first=R. Buckminster | last=Fuller
+ | authorlink=Buckminster Fuller | year=1963
+ | title=[[Operating Manual for Spaceship Earth]]
+ | edition=First edition | publisher=E.P. Dutton & Co.
+ | location=New York | id=ISBN 0-525-47433-1
+ | url=http://www.futurehi.net/docs/OperatingManual.html
+ | accessdate=2007-04-21 }}</ref> or as having a [[biosphere]] that forms one large [[organism]].<ref>{{cite book
+ | first=James E. | last=Lovelock
+ | authorlink=James Lovelock | year=1979
+ | title=Gaia: A New Look at Life on Earth
+ | edition=First edition | publisher=Oxford University Press
+ | location=Oxford | id=ISBN 0-19-286030-5 }}</ref>
+Over the past two centuries a growing [[environmental movement]] has emerged that is concerned about humankind's effects on the Earth. The key issues of this socio-political movement are the [[conservation movement|conservation]] of [[natural resource]]s, elimination of [[pollution]], and the usage of land. Environmentalists advocate [[sustainability|sustainable]] management of resources and [[stewardship]] of the [[natural environment]] through changes in public policy and individual behavior. Of particular concern is the large-scale exploitation of [[non-renewable resources]]. Changes sought by the environmental movements are sometimes in conflict with commercial interests due to the additional costs associated with managing the environmental impact of those interests.<ref>{{cite web
+ | last = Meyer | first = Stephen M.
+ | date = [[August 18]], [[2002]]
+ | url = http://web.mit.edu/polisci/mpepp/
+ | title = MIT Project on Environmental Politics & Policy
+ | publisher = Massachusetts Institute of Technology
+ | accessdate = 2006-08-10 }}</ref>
+==Future==
+{{seealso|Risks to civilization, humans and planet Earth}}
+[[Image:Sun Life.png|550px|The life cycle of the Sun.]]
+The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium ash at the Sun's core, the [[Solar luminosity|star's total luminosity]] will slowly increase. The luminosity of the Sun will increase by 10 percent over the next 1.1 billion years (1.1&nbsp;[[Gigayear|Gyr]]), and by 40% over the next 3.5&nbsp;Gyr.<ref name="sun_future">{{cite journal
+ | author=Sackmann, I.-J.; Boothroyd, A. I.; Kraemer, K. E.
+ | title=Our Sun. III. Present and Future
+ | journal=Astrophysical Journal
+ | year=1993
+ | volume=418
+ | pages=457–468
+ | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...418..457S
+ | accessdate=2007-03-31 }}</ref> Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet's oceans.<ref>{{cite journal
+ | last = Kasting
+ | first = J.F.
+ | title=Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus
+ | journal=Icarus
+ | year=1988
+ | volume=74
+ | pages=472–494
+ | url=http://adsabs.harvard.edu/abs/1988Icar...74..472K
+ | accessdate=2007-03-31 }}</ref>
+The Earth's increasing surface temperature will accelerate the [[inorganic]] [[Carbon cycle|CO<sub>2</sub> cycle]], reducing its concentration to the lethal levels for plants (10 [[Parts-per notation|ppm]] for [[C4 carbon fixation|C4 photosynthesis]]) in 900 million years. But even if the Sun were eternal and stable, the continued internal cooling of the Earth would have resulted in a loss of much of its atmosphere and oceans (due to lower [[volcanism]]).<ref>{{cite journal
+ | author=Guillemot, H.; Greffoz, V.
+ | title=Ce que sera la fin du monde
+ | journal=Science et Vie
+ | date=Mars 2002
+ | volume=N° 1014
+ | language=French }}</ref> After another billion years the surface water will have completely disappeared.<ref>{{cite news
+ | first=Damian
+ | last=Carrington
+ | title=Date set for desert Earth
+ | publisher=BBC News
+ | date=[[February 21]], [[2000]]
+ | url=http://news.bbc.co.uk/1/hi/sci/tech/specials/washington_2000/649913.stm
+ | accessdate=2007-03-31 }}</ref>
+The Earth is expected to be effectively habitable for another 500 million years or so.<ref>{{cite web | first=Robert | last=Britt | url=http://www.space.com/scienceastronomy/solarsystem/death_of_earth_000224.html | title=Freeze, Fry or Dry: How Long Has the Earth Got? | date=2000-02-25}}</ref>
+The Sun, as part of its solar lifespan, will expand to a [[red giant]] in about 5 Gyr. Models predict that the Sun will expand out to about 99% of the distance to Earth's present orbit (1&nbsp;[[astronomical unit]], or AU). However, by that time, the orbit of Earth may have expanded to about 1.7&nbsp;AU because of the diminished mass of the Sun. It was thought that the planet might thus escape envelopment by the expanded Sun's sparse outer atmosphere, though most (if not all) existing life would have been destroyed by the Sun's proximity to Earth.<ref name="sun_future" /> However, more recent simulations have shown that Earth's orbit will decay due to tidal effects, causing it to enter the Sun's atmosphere and be destroyed.<ref>{{cite news
+ | url=http://space.newscientist.com/article/dn13369-hope-dims-that-earth-will-survive-suns-death.html?feedId=online-news_rss20
+ | title=Hope dims that Earth will survive Sun's death
+ | date=[[February 22]], [[2008]]
+ | publisher= NewScientist.com news service
+ | first= Jason| last=Palmer}}</ref>
+{{-}}
+==Notes==
+{{Reflist|2}}
 ==References==
-{{reflist|2}}
+* {{cite book | first=Charles F. | last=Yoder | editor=T. J. Ahrens | year=1995 | title=Global Earth Physics: A Handbook of Physical Constants | publisher=American Geophysical Union | location=Washington | url=http://www.agu.org/reference/gephys.html | accessdate=2007-03-17 | id=ISBN 0875908519 }}
+* {{cite web | last = Williams | first = David R. | date = [[September 1]], [[2004]] | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html | title = Earth Fact Sheet | publisher = NASA | accessdate = 2007-03-17 }}
+* {{cite book | first=Neil F. | last=Comins | year=2001 | title=Discovering the Essential Universe | edition=Second Edition | publisher=W. H. Freeman | url=http://adsabs.harvard.edu/abs/2003deu..book.....C | accessdate=2007-03-17 | id=ISBN 0-7167-5804-0 }}
+* {{cite web | editor=Kirk Munsell | date = [[October 19]], [[2006]] | url = http://sse.jpl.nasa.gov/planets/profile.cfm?Object=Earth&Display=OverviewLong| title = Solar System Exploration: Earth | publisher = NASA | accessdate = 2007-03-17 }}
+==See also==
+{{Earth}}
+==External links==
+{{sisterlinks|Earth}}
+{{portal|Solar System|Solar system.jpg}}
+{{portal|Earth sciences|Terra.png}}
+* [http://www.wikimapia.org/ WikiSatellite view of Earth at WikiMapia]
+* [http://geomag.usgs.gov USGS Geomagnetism Program]
+* [http://earthobservatory.nasa.gov/Newsroom/BlueMarble NASA Earth Observatory]
+* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Earth Earth Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
+* [http://www.co-intelligence.org/newsletter/comparisons.html The size of Earth compared with other planets/stars]
+* [http://www.nasa.gov/centers/goddard/earthandsun/earthshape.html Climate changes cause Earth's shape to change - NASA]
+* [http://www.geody.com/?world=terra Geody Earth] World's search engine that supports [[Google Earth]], [[NASA World Wind]], [[Celestia]], GPS, and other applications.
+* [http://reference.aol.com/earth Planet Earth] From AOL Research & Learn: Photos, quizzes and info about Earth's climate, creatures and science.
+* [http://entertainment.timesonline.co.uk/tol/arts_and_entertainment/the_tls/article3159404.ece "The age of the earth"]: an article by [[Richard Fortey|Richard A. Fortey]] in the [[The Times Literary Supplement|TLS]], [[January 9]] [[2008]]
+{{start box}}
+{{succession box|
+title=[[Person of the Year|Time's Person of the Year <br />The Endangered Earth]]|
+before=[[Mikhail Gorbachev]]|
+after=Mikhail Gorbachev|
+years=1988|
+}}
+{{end box}}
+{{Footer_SolarSystem}}
+{{Earth location}}
+{{Nature nav}}
+{{featured article}}
+[[Category:Earth|Earth]]
+[[Category:Geography]]
+[[Category:Geology]]
+[[Category:Terrestrial planets]]
+[[Category:Planets of the Solar System]]
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