This article is about the planet. For other uses see Earth (disambiguation). Earth   "The Blue Marble" photograph of Earth taken from Apollo 17 Designations Pronunciation i /r/ Adjective earthly tellurian telluric terran terrestrial. Orbital characteristics Epoch J2000.0note 1 Aphelion 152098232 km 1.01671388 AUnote 2 Perihelion 147098290 km 0.98329134 AUnote 2 Semi-major axis 149598261 km 1.00000261 AU1 Eccentricity 0.016711231 Orbital period 365.256363004 days2 1.000017421 yr Average orbital speed 29.78 km/s3 107200 km/h Mean anomaly 357.517163 Inclination 7.155 to Sun's equator 1.578694 to invariable plane Longitude of ascending node 348.739363note 3 Argument of perihelion 114.207833note 4 Satellites 1 natural (The Moon) 8300+ artificial (as of 1 March 2001 (2001 -03-01)update)5 Physical characteristics Mean radius 6371.0 km6 Equatorial radius 6378.1 km78 Polar radius 6356.8 km9 Flattening 0.003352810 Circumference 40075.017 km (equatorial)8 40007.86 km (meridional)11 Surface area 510072000 km21213note 5

148940000 km2 land (29.2 %) 361132000 km2 water (70.8 %) Volume 1.083211012 km33 Mass 5.97361024 kg3 Mean density 5.515 g/cm33 Equatorial surface gravity 9.780327 m/s214 0.99732 g Escape velocity 11.186 km/s3 Sidereal rotation period 0.99726968 d15 23h 56m 4.100s Equatorial rotation velocity 1674.4 km/h (465.1 m/s)16 Axial tilt 2326'21".41192 Albedo 0.367 (geometric)3 0.306 (Bond)3 Surface temp.    Kelvin    Celsius min mean max 184 K17 287.2 K18 331 K19 -89.2 C 14 C 57.8 C Atmosphere Surface pressure 101.325 kPa (MSL) Composition 78.08% nitrogen (N2)3 20.95% oxygen (O2) 0.93% argon 0.038% carbon dioxide About 1% water vapor (varies with climate)

Earth (or the Earth) is the third planet from the Sun and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the World the Blue Planet20 or by its Latin name Terra.note 6

Home to millions of species including humans Earth is currently the only astronomical body where life is known to exist.21 The planet formed 4.54 billion years ago and life appeared on its surface within one billion years.22 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 solar radiation permitting life on land.23 The physical properties of the Earth as well as its geological history and orbit have allowed life to persist during this period. The planet is expected to continue supporting life for at least another 500 million years.2425

Earth's outer surface is divided into several rigid segments or tectonic plates that 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 which together have many lakes and other sources of water contributing to the hydrosphere. Liquid water necessary for all known life is not known to exist in equilibrium on any other planet's surface.note 7 Earth's poles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice cap). The planet'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 space especially the Sun and the Moon. At present Earth orbits the Sun once every 366.26 times it rotates about its own axis which is equal to 365.26 solar days or one sidereal year.note 8 The Earth's axis of rotation is tilted 23.4 away from the perpendicular of its orbital plane producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).26 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. Between approximately 3.8 billion and 4.1 billion years ago numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment.

Both the mineral resources of the planet as well as the products of the biosphere contribute resources that are used to support a global human population. These inhabitants are grouped into about 200 independent sovereign states which interact through diplomacy travel trade and military action. Human cultures have developed many views of the planet including personification as a deity a belief in a flat Earth or in the Earth as the center of the universe and a modern perspective of the world as an integrated environment that requires stewardship. Contents 1 Chronology 1.1 Evolution of life 1.2 Future 2 Composition and structure 2.1 Shape 2.2 Chemical composition 2.3 Internal structure 2.4 Heat 2.5 Tectonic plates 2.6 Surface 2.7 Hydrosphere 2.8 Atmosphere 2.8.1 Weather and climate 2.8.2 Upper atmosphere 2.9 Magnetic field 3 Orbit and rotation 3.1 Rotation 3.2 Orbit 3.3 Axial tilt and seasons 4 Moon 5 Habitability 5.1 Biosphere 5.2 Natural resources and land use 5.3 Natural and environmental hazards 5.4 Human geography 6 Cultural viewpoint 7 See also 8 Notes 9 References 10 Further reading 11 External links Chronology Main article: History of the Earth See also: Geological history of Earth

Scientists have been able to reconstruct detailed information about the planet's past. The earliest dated Solar System material was formed 4.5672 0.0006 billion years ago27 and by 4.54 billion years ago (within an uncertainty of 1%)22 the Earth and the other planets in the Solar System had formed out of the solar nebulaa disk-shaped mass of dust and gas left over from the formation of the Sun. This assembly of the Earth through accretion was thus largely completed within 1020 million years.28 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 shortly thereafter 4.53 billion years ago.29

The current consensus model30 for the formation of the Moon is the giant impact hypothesis in which the Moon was created when a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass31 impacted the Earth in a glancing blow.32 In this model 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 coalesce into the Moon.

Outgassing and volcanic activity produced the primordial atmosphere of the Earth. Condensing water vapor augmented by ice and liquid water delivered by asteroids and the larger proto-planets comets and trans-Neptunian objects produced the oceans.33 The newly formed Sun was only 70% of its present luminosity yet evidence shows that the early oceans remained liquida contradiction dubbed the faint young Sun paradox. A combination of greenhouse gases and higher levels of solar activity served to raise the Earth's surface temperature preventing the oceans from freezing over.34 By 3.5 billion years ago the Earth's magnetic field was established which helped prevent the atmosphere from being stripped away by the solar wind.35 Two major models have been proposed for the rate of continental growth:36 steady growth to the present-day37 and rapid growth early in Earth history.38 Current research shows that the second option is most likely with rapid initial growth of continental crust39 followed by a long-term steady continental area.404142 On time scales lasting hundreds of millions of years the surface continually reshaped as continents formed and broke up. The continents migrated across the surface occasionally combining to form a supercontinent. Roughly 750 million years ago (Ma) one of the earliest known supercontinents Rodinia began to break apart. The continents later recombined to form Pannotia 600540 Ma then finally Pangaea which broke apart 180 Ma.43 Evolution of life Main article: Evolutionary history of life At present Earth provides the only example of an environment that has given rise to the evolution of life.44 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.45 The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed 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.46 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.47 Since the 1960s it has been hypothesized that severe glacial action between 750 and 580 Ma 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.48 Following the Cambrian explosion about 535 Ma there have been five major mass extinctions.49 The most recent such event was 65 Ma when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles but spared some small animals such as mammals which then resembled shrews. Over the past 65 million years mammalian life has diversified and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.50 This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain which allowed the evolution of the human race. The development of agriculture and then civilization allowed humans to influence the Earth in a short time span as no other life form had51 affecting both the nature and quantity of other life forms. The present pattern of ice ages began about 40 Ma and then intensified during the Pleistocene about 3 Ma. High-latitude regions have since undergone repeated cycles of glaciation and thaw repeating every 40100000 years. The last continental glaciation ended 10000 years ago.52 Future Main article: Future of the Earth See also: Risks to civilization humans and planet Earth 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 at the Sun's core the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 Gyr (1.1 billion years) and by 40% over the next 3.5 Gyr.53 Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences including the loss of the planet's oceans.54 The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle reducing its concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 500 million24 to 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere so animal life will become extinct within several million more years.55 After another billion years all surface water will have disappeared25 and the mean global temperature will reach 70 C55 (158 F). The Earth is expected to be effectively habitable for about another 500 million years from that point24 although this may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere.56 Even if the Sun were eternal and stable the continued internal cooling of the Earth would result in a loss of much of its CO2 due to reduced volcanism57 and 35% of the water in the oceans would descend to the mantle due to reduced steam venting from mid-ocean ridges.58 The Sun as part of its evolution will become a red giant in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present radius roughly 1 AU (150000000 km).5359 Earth's fate is less clear. As a red giant the Sun will lose roughly 30% of its mass so without tidal effects the Earth will move to an orbit 1.7 AU (250000000 km) from the Sun when the star reaches it maximum radius. The planet was therefore initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere though most if not all remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5000 times its present level).53 However a 2008 simulation indicates that Earth's orbit will decay due to tidal effects and drag causing it to enter the red giant Sun's atmosphere and be vaporized.59 Composition and structure Main article: Earth science Further information: Earth physical characteristics tables 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 in size and mass. Of these four planets Earth also has the highest density the highest surface gravity the strongest magnetic field and fastest rotation.60 It also is the only terrestrial planet with active plate tectonics.61 Shape Main article: Figure of the Earth Size comparison of inner planets (left to right): Mercury Venus Earth and Mars The shape of the Earth is very close to that of an oblate spheroid a sphere flattened along the axis from pole to pole such that there is a bulge around the equator.62 This bulge results from the rotation of the Earth and causes the diameter at the equator to be 43 km larger than the pole to pole diameter.63 The average diameter of the reference spheroid is about 12742 km which is approximately 40000 km/ as the meter was originally defined as 1/10000000 of the distance from the equator to the North Pole through Paris France.64 Local topography deviates from this idealized spheroid though on a global scale these deviations are very small: Earth has a tolerance of about one part in about 584 or 0.17% from the reference spheroid which is less than the 0.22% tolerance allowed in billiard balls.65 The largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above local sea level) and the Mariana Trench (10911 m below local sea level). Because of the equatorial bulge the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarn in Peru.666768 Chemical composition of the crust69 Compound Formula Composition Continental Oceanic silica SiO2 60.2% 48.6% alumina Al2O3 15.2% 16.5% lime CaO 5.5% 12.3% magnesia MgO 3.1% 6.8% iron(II) oxide FeO 3.8% 6.2% sodium oxide Na2O 3.0% 2.6% potassium oxide K2O 2.8% 0.4% iron(III) oxide Fe2O3 2.5% 2.3% water H2O 1.4% 1.1% carbon dioxide CO2 1.2% 1.4% titanium dioxide TiO2 0.7% 1.4% phosphorus pentoxide P2O5 0.2% 0.3% Total 99.6% 99.9% Chemical composition See also: Abundance of elements on Earth The mass of the Earth is approximately 5.981024 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.70 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 1672 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.71 Internal structure Main article: Structure of the Earth The interior of the Earth like that of the other terrestrial planets is divided into layers by their chemical or physical (rheological) properties but unlike the other terrestrial planets it has a distinct outer and inner core. The outer layer of the Earth is a chemically distinct silicate solid crust which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovii discontinuity and the thickness of the crust varies: averaging 6 km under the oceans and 3050 km on the continents. The crust and the cold rigid top of the upper mantle are collectively known as the lithosphere and it is of the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 kilometers below the surface spanning a transition zone that separates the upper and lower mantle. Beneath the mantle an extremely low viscosity liquid outer core lies above a solid inner core.72 The inner core may rotate at a slightly higher angular velocity than the remainder of the planet advancing by 0.10.5 per year.73 Geologic layers of the Earth74 Earth cutaway from core to exosphere. Not to scale. Depth75 km Component Layer Density g/cm3 060 Lithospherenote 9 035 Crustnote 10 2.22.9 3560 Upper mantle 3.44.4   352890 Mantle 3.45.6 100700 Asthenosphere 28905100 Outer core 9.912.2 51006378 Inner core 12.813.1 Heat Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).76 The major heat-producing isotopes in the Earth are potassium-40 uranium-238 uranium-235 and thorium-232.77 At the center of the planet the temperature may be up to 7000 K and the pressure could reach 360 GPa.78 Because much of the heat is provided by radioactive decay scientists believe that early in Earth history before isotopes with short half-lives had been depleted Earth's heat production would have been much higher. This extra heat production twice present-day at approximately 3 billion years ago76 would have increased temperature gradients within the Earth increasing the rates of mantle convection and plate tectonics and allowing the production of igneous rocks such as komatiites that are not formed today.79 Present-day major heat-producing isotopes80 Isotope Heat release W/kg isotope Half-life years Mean mantle concentration kg isotope/kg mantle Heat release W/kg mantle 238U 9.46 105 4.47 109 30.8 109 2.91 1012 235U 5.69 104 7.04 108 0.22 109 1.25 1013 232Th 2.64 105 1.40 1010 124 109 3.27 1012 40K 2.92 105 1.25 109 36.9 109 1.08 1012 The mean heat loss from the Earth is 87 mW m2 for a global heat loss of 4.42 1013 W.81 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.82 More of the heat in the Earth is lost through plate tectonics by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere the majority of which occurs in the oceans because the crust there is much thinner than that of the continents.83 Tectonic plates Earth's main plates84 Plate name Area 106 km2 African Platenote 11 78.0 Antarctic Plate 60.9 Indo-Australian Plate 47.2 Eurasian Plate 67.8 North American Plate 75.9 South American Plate 43.6 Pacific Plate 103.3 Main article: Plate tectonics The mechanically rigid outer layer of the Earth the lithosphere is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries at which two plates come together Divergent boundaries at which two plates are pulled apart and Transform boundaries in which two plates slide past one another laterally. Earthquakes volcanic activity mountain-building and oceanic trench formation can occur along these plate boundaries.85 The tectonic plates ride on top of the asthenosphere the solid but less-viscous part of the upper mantle that can flow and move along with the plates86 and their motion is strongly coupled with convection patterns inside the Earth's mantle. As the tectonic plates migrate across the planet the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes continually recycles the oceanic crust back into the mantle. Because of this recycling most of the ocean floor is less than 100 million years in age. The oldest oceanic crust is located in the Western Pacific and has an estimated age of about 200 million years.8788 By comparison the oldest dated continental crust is 4030 million years old.89 Other notable 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 fused with the 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/yr90 and the Pacific Plate moving 5269 mm/yr. At the other extreme the slowest-moving plate is the Eurasian Plate progressing at a typical rate of about 21 mm/yr.91 Surface Main articles: Landform and Extreme points of Earth The Earth's terrain varies greatly from place to place. About 70.8%92 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 volcanoes63 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 because 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 impacts93 also act to reshape the landscape. Present day Earth altimetry and bathymetry. Data from the National Geophysical Data Center's TerrainBase Digital Terrain Model. The continental crust consists 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.94 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.95 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.96 Common carbonate minerals include calcite (found in limestone) and dolomite.97 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.13 Close to 40% of the Earth's land surface is presently used for cropland and pasture or an estimated 1.3107 km2 of cropland and 3.4107 km2 of pastureland.98 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 8848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.99 Hydrosphere Main article: Hydrosphere Elevation histogram of the surface of the Earth 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 2000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of 10911.4 m.note 12100 The mass of the oceans is approximately 1.351018 metric tons or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618108 km2 with a mean depth of 3682 m resulting in an estimated volume of 1.332109 km3.101 If all the land on Earth were spread evenly water would rise to an altitude of more than 2.7 km.note 13 About 97.5% of the water is saline while the remaining 2.5% is fresh water. Most fresh water about 68.7% is currently ice.102 The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (35 ).103 Most of this salt was released from volcanic activity or extracted from cool igneous rocks.104 The oceans are also a reservoir of dissolved atmospheric gases which are essential for the survival of many aquatic life forms.105 Sea water has an important influence on the world's climate with the oceans acting as a large heat reservoir.106 Shifts in the oceanic temperature distribution can cause significant weather shifts such as the El Nio-Southern Oscillation.107 Atmosphere Main article: Atmosphere of Earth The atmospheric pressure on the surface of the Earth averages 101.325 kPa with a scale height of about 8.5 km.3 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 8 km at the poles to 17 km at the equator with some variation resulting from weather and seasonal factors.108 Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago forming the primarily nitrogen-oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which blocks ultraviolet solar radiation permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor providing useful gases causing small meteors to burn up before they strike the surface and moderating temperature.109 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.92 Weather and climate Main articles: Weather and Climate Satellite cloud cover image of Earth using NASA's Moderate-Resolution Imaging Spectroradiometer 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 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.110 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 60.111 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.112 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.110 Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or 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.113 The amount of solar energy reaching the Earth's decreases with increasing latitude. At higher latitudes the sunlight reaches the surface at a lower angles and it must pass through thicker columns of the atmosphere. As a result the mean annual air temperature at sea level decreases by about 0.4C per per degree of latitude away from the equator.114 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.115 Climate can also be classified based on the temperature and precipitation with the climate regions characterized by fairly uniform air masses. The commonly used Kppen climate classification system (as modified by Wladimir Kppen'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.111 Upper atmosphere This view from orbit shows the full Moon partially obscured and deformed by the Earth's atmosphere. NASA image See also: Outer space Above the troposphere the atmosphere is usually divided into the stratosphere mesosphere and thermosphere.109 Each layer 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.116 Within the stratosphere is the ozone layer a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Krmn line defined as 100 km above the Earth's surface is a working definition for the boundary between atmosphere and space.117 Thermal energy causes 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 than other gasses.118 The leakage of hydrogen into space contributes to the pushing of the Earth from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.119 Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet.120 In the current oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.121 Magnetic field Schematic of Earth's magnetosphere. The solar wind flows from left to right Main article: 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. At the equator of the magnetic field the magnetic field strength at the planet's surface is 3.05 105 T with global magnetic dipole moment of 7.91 1015 T m3.122 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; the magnetic poles drift 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 700000 years ago.123124 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.125 Orbit and rotation Rotation Main article: Earth's rotation Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit Earth's rotation period relative to the Sunits mean solar dayis 86400 seconds of mean solar time (86400.0025 SI seconds).126 As the Earth's solar day is now slightly longer than it was during the 19th century because of tidal acceleration each day varies between 0 and 2 SI ms longer.127128 Earth's rotation period relative to the fixed stars called its stellar day by the International Earth Rotation and Reference Systems Service (IERS) is 86164.098903691 seconds of mean solar time (UT1) or 23h 56m 4.098903691s.2note 14 Earth's rotation period relative to the precessing or moving mean vernal equinox misnamed its sidereal day is 86164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s).2 Thus the sidereal day is shorter than the stellar day by about 8.4 ms.129 The length of the mean solar day in SI seconds is available from the IERS for the periods 16232005130 and 19622005.131 Apart from meteors within the atmosphere and low-orbiting satellites the main apparent motion of celestial bodies in the Earth's sky is to the west at a rate of 15/h 15'/min. For bodies near the celestial equator this is equivalent to an apparent diameter of the Sun or Moon every two minutes; from the planet's surface the apparent sizes of the Sun and the Moon are approximately the same.132133 Orbit Main article: Earth's orbit Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days or one sidereal year. From Earth this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1/day or a Sun or Moon diameter every 12 hours. Because of this motion on average it takes 24 hoursa solar dayfor 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 29.8 km/s (107000 km/h) which is fast enough to cover the planet's diameter (about 12600 km) in seven minutes and the distance to the Moon (384000 km) in four hours.3 The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the EarthMoon 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. Viewed from a vantage point above the north poles of both the Sun and the Earth the Earth appears to revolve in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.4 degrees from the perpendicular to the EarthSun plane and the EarthMoon 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.3134 The Hill sphere or gravitational sphere of influence of the Earth is about 1.5 Gm (or 1500000 kilometers) in radius.135note 15 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. Illustration of the Milky Way Galaxy showing the location of the Sun Earth along with the Solar System is situated in the Milky Way galaxy orbiting about 28000 light years from the center of the galaxy. It is currently about 20 light years above the galaxy's equatorial plane in the Orion spiral arm.136 Axial tilt and seasons Main article: Axial tilt Because of the axial tilt of the Earth the amount of sunlight reaching any given point on the surface varies over the course of the year. This results in seasonal change in climate with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun and winter taking place when the pole is pointed away. During the summer the day lasts longer and the Sun climbs higher in the sky. In winter the climate becomes generally cooler and the days shorter. Above the Arctic Circle an extreme case is reached where there is no daylight at all for part of the yeara polar night. In the southern hemisphere the situation is exactly reversed with the South Pole oriented opposite the direction of the North Pole. Earth and Moon from Mars imaged by Mars Reconnaissance Orbiter. From space the Earth can be seen to go through phases similar to the phases of the Moon. By astronomical convention the four seasons are determined by the solsticesthe point in the orbit of maximum axial tilt toward or away from the Sunand the equinoxes when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere 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. In the Southern hemisphere the situation is reversed with the Summer and Winter Solstices exchanged and the Spring and Autumnal Equinox dates switched.137 The angle of the Earth's tilt is relatively stable over long periods of time. However the tilt does undergo nutation; a slight irregular motion with a main period of 18.6 years.138 The orientation (rather than the angle) of the Earth's axis also changes over time precessing around in a complete circle over each 25800 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.139 In modern times Earth's perihelion occurs around January 3 and the aphelion around July 4. However these dates change over time due to precession and other orbital factors which follow cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance results in an increase of about 6.9%note 16 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.140 Moon Characteristics Diameter 3474.8 km Mass 7.3491022 kg Semi-major axis 384400 km Orbital period 27 d 7 h 43.7 m Main article: Moon 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 although 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 causes 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 a year. Over millions of years these tiny modificationsand the lengthening of Earth's day by about 23 s a yearadd up to significant changes.141 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.142 Details of the Earth-Moon system. Besides the radius of each object the radius to the Earth-Moon barycenter is shown. Photos from NASA. Data from NASA. The Moon's axis is located by Cassini's third law. 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.143 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 exhibiting chaotic changes over millions of years as appears to be the case for Mars.144 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.133 This allows total and annular solar eclipses to occur on Earth. 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.145 Earth has at least five co-orbital asteroids including 3753 Cruithne and 2002 AA29.146147 As of 2011 there are 931 operational man-made satellites orbiting the Earth.148 A scale representation of the relative sizes of and average distance between Earth and Moon Habitability See also: Planetary habitability 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.149 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 believed necessary to originate and sustain life on this planet.150 Biosphere Main article: 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 where life is known to exist. The biosphere is divided into a number of biomes inhabited by broadly similar plants and animals. On land biomes are separated primarily by differences in latitude height above sea level and humidity. Terrestrial biomes lying within the Arctic or Antarctic Circles at high altitudes or in extremely arid areas are relatively barren of plant and animal life; species diversity reaches a peak in humid lowlands at equatorial latitudes.151 Natural resources and land use Main article: 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.152 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.153 Humans also live on the land by using building materials to construct shelters. In 1993 human use of land is approximately: Land use Arable land Permanent crops Permanent pastures Forests and woodland Urban areas Other Percentage 13.13%13 4.71%13 26% 32% 1.5% 30% The estimated amount of irrigated land in 1993 was 2481250 km2.13 Natural and environmental hazards Large areas of the Earth's surface 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. According to the United Nations 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 and a global rise in average sea levels.154 Human geography Main article: Human geography See also: World 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 and to a lesser extent navigation the determination of position and direction have developed alongside cartography and geography providing and suitably quantifying the requisite information. Earth has approximately 6910000000 human inhabitants as of April 25 2011.155 Projections indicate that the world's human population will reach 7 billion in early 2012 and 9.2 billion in 2050.156 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.157 It is estimated that only one-eighth of the surface of the Earth is suitable for humans to live onthree-quarters is covered by oceans and half of the land area is either desert (14%)158 high mountains (27%)159 or other less suitable terrain. The northernmost permanent settlement in the world is Alert on Ellesmere Island in Nunavut Canada.160 (8228N) The southernmost is the Amundsen-Scott South Pole Station in Antarctica almost exactly at the South Pole. (90S) 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. Independent sovereign nations claim the planet's entire land surface except for some parts of Antarctica and the odd unclaimed area of Bir Tawil between Egypt and Sudan. As of 2011 there are 203 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.13 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.161 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.162 It is not however a world government. The U.N. serves primarily as a forum for international diplomacy and international law. When the consensus of the membership permits it provides a mechanism for armed intervention.163 The first human to orbit the Earth was Yuri Gagarin on April 12 1961.164 In total about 400 people visited outer space and reached Earth orbit as of 2004 and of these twelve have walked on the Moon.165166167 Normally the only humans in space are those on the International Space Station. The station's crew currently six people is usually replaced every six months.168 The furthest humans have travelled from Earth is 400171 km achieved during the 1970 Apollo 13 mission.169 Cultural viewpoint Main article: Earth in culture The first photograph ever taken by astronauts of an "Earthrise" from Apollo 8 The name "Earth" derives from the Anglo-Saxon word erda which means ground or soil and is related to the German word Erde. It became eorthe later and then erthe in Middle English.170 The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.171 Unlike the rest of the planets in the Solar System humankind did not begin to view the Earth as a moving object in orbit around the Sun until the 16th century.172 Earth has often been personified as a deity in particular a goddess. In many cultures the mother goddess is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups often associated with fundamentalist branches of Protestantism173 or Islam174 assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life.175 Such assertions are opposed by the scientific community176177 and by other religious groups.178179180 A prominent example is the creation-evolution controversy. In the past there were varying levels of belief in a flat Earth181 but this was displaced by the concept of a spherical Earth due to observation and circumnavigation.182 The human perspective regarding the Earth has changed following the advent of spaceflight and the biosphere is now widely viewed from a globally integrated perspective.183184 This is reflected in a growing environmental movement that is concerned about humankind's effects on the planet.185 See also Earth sciences portal Book: Solar System Wikipedia Books are collections of articles that can be downloaded or ordered in print. Geodesy Geology Notes All astronomical quantities vary both secularly and periodically. The quantities given are the values at the instant J2000.0 of the secular variation ignoring all periodic variations. a b aphelion a (1 + e); perihelion a (1 - e) where a is the semi-major axis and e is the eccentricity. The reference lists the longitude of the ascending node as -11.26064 which is equivalent to 348.73936 by the fact that any angle is equal to itself plus 360. The reference lists the longitude of perihelion which is the sum of the longitude of the ascending node and the argument of perihelion. That is 114.20783 + (-11.26064) 102.94719. Due to natural fluctuations ambiguities surrounding ice shelves and mapping conventions for vertical datums exact values for land and ocean coverage are not meaningful. Based on data from the Vector Map and Global Landcover datasets extreme values for coverage of lakes and streams are 0.6% and 1.0% of the Earth's surface. The ice shields of Antarctica and Greenland are counted as land even though much of the rock which supports them lies below sea level. By International Astronomical Union convention the term terra is used only for naming extensive land masses on celestial bodies other than the Earth. Cf. Blue Jennifer (2007-07-05). "Descriptor Terms (Feature Types)". Gazetteer of Planetary Nomenclature. USGS. http://planetarynames.wr.usgs.gov/jsp/append5.jsp. Retrieved 2007-07-05.  At present the other planets in the Solar System are either too hot or too cold to support liquid water on the surface in vapor-liquid equilibrium. As of 2007 water vapor has been detected in the atmosphere of only one extrasolar planet and it is a gas giant. See: Tinetti G.; Vidal-Madjar A.; Liang M.C.; Beaulieu J. P.; Yung Y.; Carey S.; Barber R. J.; Tennyson J.; Ribas I (July 2007). "Water vapour in the atmosphere of a transiting extrasolar planet". Nature 448 (7150): 169171. Bibcode 2007Natur.448..169T. doi:10.1038/nature06002. PMID 17625559. 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. Locally varies between 5 and 200 km. Locally varies between 5 and 70 km. Including the Somali Plate which is currently in the process of formation out of the African Plate. See: Chorowicz Jean (October 2005). "The East African rift system". Journal of African Earth Sciences 43 (13): 379410. Bibcode 2005JAfES..43..379C. doi:10.1016/j.jafrearsci.2005.07.019.  This is the measurement taken by the vessel Kaik in March 1995 and is believed to be the most accurate measurement to date. See the Challenger Deep article for more details. The total surface area of the Earth is 5.1108 km2. To first approximation the average depth would be the ratio of the two or 2.7 km. Aoki the ultimate source of these figures uses the term "seconds of UT1" instead of "seconds of mean solar time".Aoki S. (1982). "The new definition of universal time". Astronomy and Astrophysics 105 (2): 359361. 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ISBN 0-525-47433-1. http://www.futurehi.net/docs/OperatingManual.html. Retrieved 2007-04-21.  Lovelock James E. (1979). Gaia: A New Look at Life on Earth (First ed.). Oxford: Oxford University Press. ISBN 0-19-286030-5.  For example: McMichael Anthony J. (1993). Planetary Overload: Global Environmental Change and the Health of the Human Species. Cambridge University Press. ISBN 0-521-45759-9.  Further reading Comins Neil F. (2001). Discovering the Essential Universe (2nd ed.). W. H. Freeman. Bibcode 2003deu..book.....C. 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