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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 Planet, or by its Latin name, Terra.HP 586006-321 Battery

Home to millions of species, including humans, Earth is currently the only astronomical body where life is known to exist.The planet formed 4.54 billion years ago, and life appeared on its surface within one billion years.HP 586006-361 Battery

Earth's biosphere has significantly altered the atmosphere and otherabiotic 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.HP 586028-341 Battery

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.

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.HP h588178-141 Battery

About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Liquid water, necessary for all known life, is not known to exist in equilibrium on any other planet's surface.HP 593553-001 Battery

Earth's poles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice cap). The planet's interiorremains 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. HP 593554-001 Battery

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. The Earth's axis of rotation is tilted 23.4° away from theperpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).HP HSTNN-CB0W Battery

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.HP HSTNN-CB0X Battery

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.HP HSTNN-I79C Battery

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.HP HSTNN-I81C Battery

Chronology

Scientists have been able to reconstruct detailed information about the planet's past. The earliest dated Solar System material was formed4.5672 ± 0.0006 billion years ago, and by 4.54 billion years ago (within an uncertainty of 1%) the Earth and the other planets in the Solar System had formed out of the solar nebula—a disk-shaped mass of dust and gas left over from the formation of the Sun.HP HSTNN-IB0N Battery

This assembly of the Earth through accretion was thus largely completed within 10–20 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.HP HSTNN-IB0X Battery

The current consensus model[30] 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 mass impacted the Earth in a glancing blow.HP HSTNN-LB0W Battery

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. HP HSTNN-OB0X Battery

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. The newly formed Sun was only 70% of its present luminosity, yet evidence shows that the early oceans remained liquid—a contradiction dubbed the faint young Sun paradox. HP HSTNN-OB0Y Battery

A combination of greenhouse gases and higher levels ofsolar activity served to raise the Earth's surface temperature, preventing the oceans from freezing over. 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. HP HSTNN-Q61C Battery

Two major models have been proposed for the rate of continental growth: steady growth to the present-day and rapid growth early in Earth history. Current research shows that the second option is most likely, with rapid initial growth of continental crust[39]followed by a long-term steady continental area.HP HSTNN-Q62C Battery

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. HP HSTNN-YB0X Battery

The continents later recombined to form Pannotia, 600–540 Ma, then finally Pangaea, which broke apart 180 Ma. 

Evolution of life

At present, Earth provides the only example of an environment that has given rise to the evolution of life.HP NBP6A174B1 Battery

Highly energetic chemistry is believed to have produced a self-replicating molecule around4 billion years ago and half a billion years later the last common ancestor of all life existed. The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms;HP HSTNN-YB0X Battery

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.HP NBP6A174 Battery

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.HP NBP6A174B1 Battery

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.HP NBP6A175 Battery

Following the Cambrian explosion, about 535 Ma, there have been five major mass extinctions. 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. HP NBP6A175B1 Battery

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. 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. HP WD548AA Battery

The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,[51] 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. HP WD549AA Battery

High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years ago.

Future

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.HP Envy 17 Notebook PC Battery

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.HP Envy 17T Battery

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 approximately500 million 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.HP HSTNN-Q61C Battery

After another billion years all surface water will have disappeared and the mean global temperature will reach 70 °C[55] (158 °F). The Earth is expected to be effectively habitable for about another 500 million years from that point,[24] although this may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere.HP GSTNN-Q62C Battery

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 volcanism,[57] and 35% of the water in the oceans would descend to the mantle due to reduced steam venting from mid-ocean ridges.HP HSTNN-IB1E Battery

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 (150,000,000 km). Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, HP HSTNN-IB0X Battery

without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 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,HP HSTNN-OB0X Battery

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.HP HSTNN-YB0X Battery

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 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. It also is the only terrestrial planet with active plate tectonics. HP 586006-361 Battery

Shape

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 theequator.[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.HP NBP6A174 Battery

The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/?, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.HP NBP6A174B1 Battery

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.HP 588178-141 Battery

The largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above local sea level) and the Mariana Trench (10,911 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 Huascarán in Peru.HP Pavilion dv5 Adapter

Chemical composition

The mass of the Earth is approximately 5.98×1024 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. Sony NP-F330 Battery

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.

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. Asus Eee PC 904HA Adapter

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. Nikon D80  Battery

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.Sony PCGA-AC19V9 Adapter

Internal structure

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 solidcrust, which is underlain by a highly viscous solid mantle. Dell T555C Battery

The crust is separated from the mantle by the Mohorovi?i? discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents. The crust and the cold, rigid, top of theupper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised.HP Pavilion DV6-3013SL Battery

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.HP Pavilion DV7-2085EL Battery

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 0.1–0.5° per year.

Heat

Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).HP Pavilion DV9700 Battery

The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa. Because much of the heat is provided by radioactive decay,HP Mini 702EA Battery

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 ago,would have increased temperature gradients within the Earth,HP Pavilion DV7-1120EO Battery

increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.

The mean heat loss from the Earth is 87 mW m?2, for a global heat loss of 4.42 × 1013 W.HP Compaq 491278-001 Battery

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.More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. HP Pavilion DV6-1030US Battery

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.

Tectonic plates

The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. HP Compaq 490306-001 Battery

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. HP Pavilion DV5-1211EM Battery

Earthquakes, volcanic activity,mountain-building, and oceanic trench formation can occur along these plate boundaries. 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 plates, and their motion is strongly coupled with convection patterns inside the Earth's mantle.HP EliteBook 2530P Battery

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. HP Pavilion DV4385EA Battery

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. By comparison, the oldest dated continental crust is4030 million years old.Compaq PP2100 Battery

Other notable plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and theScotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and 55 million years ago. HP Compaq KU532AA Battery

The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr[90] and the Pacific Plate moving 52–69 mm/yr. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr.HP Pavilion DV6-1270EQ Battery

Surface

The Earth's terrain varies greatly from place to place. About 70.8% of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes, oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. HP Pavilion DV4385EA Battery

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. HP Compaq Business  NC4400 Battery

The surface features built up or deformed through plate tectonics are subject to steadyweathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[93] also act to reshape the landscape.HP Pavilion ZD8227 Battery

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. Sedimentary rock is formed from the accumulation of sediment that becomes compacted together.Nikon D200 Battery

Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust. 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. Acer PA-1650-02 Adapter

The most abundant silicate minerals on the Earth's surface include quartz, thefeldspars, amphibole, mica, pyroxene and olivine.Common carbonate minerals include calcite (found in limestone) and dolomite.

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. Sony CYBER-SHOT DSC-T10  Battery

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. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3×107 km2 of cropland and 3.4×107 km2 of pastureland.HP Compaq Business 6715b Battery

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 840 m.Kodak KLIC-7004 Battery

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. Acer Aspire 5920G Adapter

The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of ?10,911.4 m.

The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332×109 km3.Canon Digital IXUS 90 IS Battery

If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km. 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.

The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (35 ‰).HP 2133 Mini-Note PC Adapter

Most of this salt was released from volcanic activity or extracted from cool, igneous rocks. The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms. Sea water has an important influence on the world's climate, with the oceans acting as a largeheat reservoir.Toshiba PA3536U-1BRS Battery

Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.

Atmosphere

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.HP Compaq Business NX6110 Battery

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.HP Pavilion DV9000 Adapter

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.Dell Studio 1537 Battery

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. This last phenomenon is known as the greenhouse effect:HP PPP009H Adapter

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.Sony VGP-BPS8 Battery

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 of the planet's surface. This lowest layer is called the troposphere. Dell PA-12 Adapter

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.Sony Cyber-shot DSC-T7/B  Battery

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°. 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.Dell LATITUDE M65 Battery

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. HP Compaq 6720s Adapter

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. Acer AS07B41 Battery

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.

The amount of solar energy reaching the Earth's decreases with increasing latitude. Dell Vostro 1310 Battery

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 0.4°C per per degree of latitude away from the equator.Sony VGP-BPL13 Battery

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. Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. Sony VGP-BPS13 Battery

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.Sony VGP-BPS13/B Battery

Upper atmosphere

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. 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.Sony VGP-BPS13/S Battery

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 Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.Sony VGP-BPS13A/B Battery

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 escapefrom the planet's gravity. This results in a slow but steady leakage of the atmosphere into space. Sony VGP-BPS13A/S Battery

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. The leakage of hydrogen into space contributes to the pushing of the Earth from an initially reducing state to its current oxidizing one. Sony VGP-BPS13AS Battery

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. Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet.Sony VGP-BPS13B/B Battery

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.

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.Sony VGP-BPS13B/Q Battery

At the equator of the magnetic field, the magnetic field strength at the planet's surface is 3.05 × 10?5 T, with global magnetic dipole moment of7.91 × 1015 T m3. 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. Sony VGP-BPS13B/S Battery

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 700,000 years ago.Sony VGP-BPS13Q Battery

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. Sony VGP-BPS13S Battery

When the plasma enters the Earth's atmosphere at the magnetic poles, it forms theaurora.

Rotation

Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).Sony VGP-BPL21 Battery

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 mslonger.

Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is86164.098903691 seconds of mean solar time (UT1), or 23h 56m 4.098903691s.Sony VGP-BPS21 Battery

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.Sony VGP-BPS21/S Battery

The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[130] and 1962–2005.

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. Sony VGP-BPS21A Battery

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.

Orbit

Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year.Sony VGP-BPS21A/B Battery

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 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. Sony VGP-BPS21B Battery

The orbital speed of the Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. Sony VGP-BPL8 Battery

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. Sony VGP-BPL8A Battery

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 Earth–Sun plane, and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane. Sony VGP-BPL8B Battery

Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.

The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius. This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Sony VGP-BPS8 Battery

Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about 28,000 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.Sony VGP-BPS8A Battery

Axial tilt and seasons

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 inseasonal 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. Sony VGP-BPS8B Battery

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 year—a polar night.HP Compaq Business 6910p Battery

In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole.

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.Panasonic LUMIX DMC-TZ4S Battery

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.Sony NP-FH100 Battery

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.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; Toshiba Equium M50-192 Battery

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. Apple MacBook Pro Battery

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. Apple PowerBook G4 Battery

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. Acer BATBL50L6 Battery

The changing Earth-Sun distance results in an increase of about 6.9% 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,Canon EOS 450D Battery

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.Acer Travelmate 2400 Battery

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.Dell Inspiron 1525 Adapter

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. Compaq Presario 2100 Adapter

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. Acer Aspire 1362 Battery

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. 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.Toshiba Equium M50-192 Battery

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. Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, Dell XPS M1330 Battery

the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.

Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. Acer Travelmate 2200 Adapter

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 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 protoplanetcalled Theia with the early Earth. HP Compaq 6735b Adapter

This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.

Earth has at least five co-orbital asteroids, including 3753 Cruithne and 2002 AA29. As of 2011, there are 931 operational, man-madesatellites orbiting the Earth.Dell Vostro 1000 Battery

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.HP Compaq nx7300 Adapter

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.Asus f82 Battery

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. Dell XPS M1330 Battery

On land, biomes are separated primarily by differences in latitude, height above sea level andhumidity. 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.HP Compaq Business 6910p Battery

Natural resources and land use

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.Compaq PRESARIO R3000 Battery

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, HP Compaq Business NX6110 Battery

resulting from actions of erosion and plate tectonics. 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.Dell Latitude D610 Adapter

The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land. Humans also live on the land by using building materials to construct shelters. In 1993, human use of land is approximately:Asus Eee PC 701 Adapter

The estimated amount of irrigated land in 1993 was 2,481,250 km2.

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. HP Compaq Business NX6325 Battery

Many places are subject to earthquakes,landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities and disasters.Dell Studio XPS 16 Battery

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.Acer Aspire 5520 Battery

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.Acer Aspire 5520 Battery

Human geography

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,HP Compaq 6735s Adapter

have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

Earth has approximately 6,910,000,000 human inhabitants as of April 25, 2011. Projections indicate that the world's human population will reach 7 billion in early 2012 and 9.2 billion in 2050.Dell XPS M1330 Battery

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.HP Compaq nc6230 Adapter

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%),[158] high mountains (27%), or other less suitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.Dell T117C Battery

(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 the planet's entire land surface, except for some parts of Antarctica and the odd unclaimed area of Bir Tawil between Egypt and Sudan.Asus x5dij-sx039c Battery

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. Historically, Earth has never had a sovereign government with authority over the entire globe, Dell Inspiron 1464 Battery

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.HP Compaq 6720s Adapter

It is not, however, a world government. The U.N. serves primarily as a forum for international diplomacy andinternational law. When the consensus of the membership permits, it provides a mechanism for armed intervention.

The first human to orbit the Earth was Yuri Gagarin on April 12, 1961.Dell Latitude D630 Adapter

In total, about 400 people visited outer space and reached Earth orbit 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, currently six people, is usually replaced every six months.HP Pavilion dm4 Battery

The furthest humans have travelled from Earth is 400,171 km, achieved during the 1970 Apollo 13 mission.

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