Page semi-protected
Listen to this article


From Wikipedia, the free encyclopedia
  (Redirected from The Earth)
Jump to: navigation, search
This article is about the planet. For other uses, see Earth (disambiguation).
Earth Astronomical symbol of Earth
"The Blue Marble" photograph of Earth, taken by the Apollo 17 lunar mission. The Arabian peninsula, Africa and Madagascar lie in the upper half of the disc, whereas Antarctica is at the bottom.
"The Blue Marble" photograph of Earth, taken during the Apollo 17 lunar mission in 1972
Orbital characteristics
Epoch J2000[n 1]
152,100,000 km (94,500,000 mi)
(1.01673 AU) [n 2]
147,095,000 km (91,401,000 mi)
(0.9832687 AU) [n 2]
149,598,023 km (92,955,902 mi)
(1.000001018 AU) [1]
Eccentricity 0.0167086[1]
365.256363004 d[2]
(1.00001742096 yr)
(107,200 km/h (66,600 mph))
−11.26064°[3] to J2000 ecliptic
Physical characteristics
Mean radius
6,371.0 km (3,958.8 mi)[6]
Equatorial radius
6,378.1 km (3,963.2 mi)[7][8]
Polar radius
6,356.8 km (3,949.9 mi)[9]
Flattening 0.0033528[10]
1/298.257222101 (ETRS89)
  • 510,072,000 km2 (196,940,000 sq mi)[13][14][n 4]
  •  (148,940,000 km2 (57,510,000 sq mi) (29.2%) land
  •   361,132,000 km2 (139,434,000 sq mi) (70.8%) water)
Mass 5.97237×1024 kg (1.31668×1025 lb)[15]
(3.0×10−6 M)
Mean density
9.807 m/s2 (32.18 ft/s2)[16]
11.186 km/s (6.951 mi/s)[3]
0.99726968 d[18]
(23h 56m 4.100s)
Equatorial rotation velocity
1,674.4 km/h (1,040.4 mph)[19]
Surface temp. min mean max
Celsius −89.2 °C 15 °C 56.7 °C
Fahrenheit −128.5 °F 59 °F 134 °F
Surface pressure
101.325 kPa (at MSL)
Composition by volume


According to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago.[27][28][29] Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, Earth rotates about its axis 366.26 times, creating 365.26 solar days or one sidereal year.[n 7] 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 within a period of one tropical year (365.24 solar days).[30] The Moon is the Earth's only permanent natural satellite; their gravitational interaction causes ocean tides, stabilizes the orientation of Earth's rotational axis, and gradually slows Earth's rotational rate.[31]

Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. 71% of Earth's surface is covered with water.[32] The remaining 29% is land mass—consisting of continents and islands—that together has many lakes, rivers, and other sources of water that contribute to the hydrosphere. The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, and a convecting mantle that drives plate tectonics.

Within the first billion years of Earth's history, life appeared in the oceans and began to affect the atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms. Since then, the combination of Earth's distance from the Sun, physical properties, and geological history have allowed life to evolve and thrive. Life arose on Earth by 3.5 billion years ago, though some geological evidence indicates that life may have arisen as much as 4.1 billion years ago.[33][34] In the history of the Earth, biodiversity has gone through long durations of expansion but occasionally punctuated by mass extinction events. Over 99% of all species of life[35] that ever lived on Earth are extinct.[36][37] Estimates of the number of species on Earth today vary widely;[38][39][40] most species have not been described.[41] Over 7.3 billion humans[42] live on Earth and depend on its biosphere and minerals for their survival. Humanity has developed diverse societies and cultures; politically, the world is divided into about 200 sovereign states.

Name and etymology

The modern English word Earth developed from a wide variety of Middle English forms,[n 8] which derived from an Old English noun most often spelled eorðe.[43] It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was already being used to translate the many senses of Latin terra and Greek γῆ (): the ground,[n 9] its soil,[n 10] dry land,[n 11] the human world,[n 12] the surface of the world (including the sea),[n 13] and the globe itself.[n 14] As with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus,[52] and later Norse mythology included Jörð, a giantess often given as the mother of Thor.[53]



Main article: History of Earth


Artist's impression of the early Solar System's planetary disk

The oldest material found in the Solar System is dated to 4.5672±0.0006 billion years ago (Gya).[55] By 4.54±0.04 Gya[56] the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred along with those of the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that disk along with the Sun. A nebula contains gas, ice grains, and dust (including primordial nuclides). According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10–20 Ma to form.[57]

A subject of on-going research is the formation of the Moon, some 4.53 billion years ago.[58] A working hypothesis is that it formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, impacted Earth.[59] In this scenario, the mass of Theia was approximately 10% of that of Earth,[60] it impacted Earth with a glancing blow,[61] and some of its mass merged with Earth. Between approximately 4.1 and 3.8 Gya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to that of Earth.

Geological history

Earth's atmosphere and oceans were formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, protoplanets, and comets.[62] In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity.[63] By 3.5 Gya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[64]

A crust formed when the molten outer layer of Earth cooled to form a solid as the accumulated water vapor began to act in the atmosphere.[citation needed] The two models[65] that explain land mass propose either a steady growth to the present-day forms[66] or, more likely, a rapid growth[67] early in Earth history[68] followed by a long-term steady continental area.[69][70][71] Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from Earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have assembled and broken apart. Roughly 750 mya (million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which also broke apart 180 mya.[72]

The present pattern of ice ages began about 40 mya and then intensified during the Pleistocene about 3 mya. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating about every 40,000–100000 years. The last continental glaciation ended 10,000 years ago.[73]

Origin of life and evolution

Chemical reactions led to the first self–replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all life arose.[74] The evolution of photosynthesis allowed the Sun's energy to be harvested directly by life forms. The resultant molecular oxygen (O2) accumulated in the atmosphere and due to interaction with ultraviolet solar radiation, formed a protective ozone layer (O3) in the upper atmosphere.[75] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[76] 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 Earth's surface.[77] Among the earliest fossil evidence for life is microbial mat fossils found in 3.48 billion-year-old sandstone in Western Australia,[78][79][80][81][82] biogenic graphite found in 3.7 billion-year-old metasedimentary rocks in Western Greenland,[83] remains of biotic material found in 4.1 billion-year-old rocks in Western Australia.[33][34]

Speculative phylogenetic tree of life on Earth based on rRNA analysis

During the Neoproterozoic, 750 to 580 mya, much of Earth might have been covered in ice. This hypothesis has been termed "Snowball Earth", and it is of particular interest because it preceded the Cambrian explosion, when multicellular life forms significantly increased in complexity.[84] Following the Cambrian explosion, 535 mya, there have been five major mass extinctions.[85] The most recent such event was 66 mya, 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 66 Ma, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.[86] This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of humans. The development of agriculture, and then civilization, led to humans having an influence on Earth and the nature and quantity of other life forms that continues today.[87]

Predicted future

Main article: Future of the Earth

Earth's long-term future is closely tied to that of the Sun. Over the next 1.1 Ga, solar luminosity will increase by 10%, and over the next 3.5 Ga by 40%.[88] 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 500–900 Ma.[89] 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.[90] After another billion years all surface water will have disappeared[91] and the mean global temperature will reach 70 °C[90] (158 °F). From that point, the Earth is expected to be habitable for another 500 Ma,[89] possibly up to 2.3 Ga if nitrogen is removed from the atmosphere.[92] Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the mantle in one billion years, due to reduced steam venting from mid-ocean ridges.[93]

The Sun will evolve to become a red giant in about 5 Ga. Models predict that the Sun will expand to roughly 1 AU (150,000,000 km), which is about 250 times its present radius.[88][94] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, Earth will move to an orbit 1.7 AU from the Sun when the star reaches its maximum radius. Most, if not all, remaining life will be destroyed by the Sun's increased luminosity (peaking at about 5,000 times its present level).[88] A 2008 simulation indicates that Earth's orbit will eventually decay due to tidal effects and drag, causing it to enter the Sun's atmosphere and be vaporized.[94]

Physical characteristics


Shape of planet Earth. Shown are distances between surface relief and the geocentre. The South American Andes summits are visible as elevated areas. Data from the Earth2014[95] global relief model.
Main article: Figure of the Earth

The shape of Earth is approximately oblate spheroidal. Due to rotation, the Earth is flattened along the geographic axis and bulging around the equator.[96] The diameter of the Earth at the equator is 43 kilometres (27 mi) larger than the pole-to-pole diameter.[97] Thus the point on the surface farthest from Earth's center of mass is the summit of the equatorial Chimborazo volcano in Ecuador.[98][99][100][101] The average diameter of the reference spheroid is 12,742 kilometres (7,918 mi). Local topography deviates from this idealized spheroid, although on a global scale these deviations are small compared to Earth's radius: The maximum deviation of only 0.17% is at the Mariana Trench (10,911 metres (35,797 ft) below local sea level), whereas Mount Everest (8,848 metres (29,029 ft) above local sea level) represents a deviation of 0.14%.[n 15]

Chemical composition

Chemical composition of the crust[103]
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%


A little more than 47% of Earth's crust consists of oxygen.[clarification needed] The most common rock constituents of the crust are nearly all oxides: chlorine, sulfur and fluorine are the 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 most common minerals of igneous rocks are of this nature. 99.22% of all rocks are composed of 11 oxides (see the table at right), with the other constituents occurring in minute quantities.[105][citation needed]

Internal structure

Earth's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. The outer layer 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 Mohorovičić discontinuity. The thickness of the crust varies from about km (kilometers) under the oceans to 30–50 km for 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 composed. 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 km 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.[106] The Earth's inner core might rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.[107] The radius of the inner core is about one fifth of that of Earth.

Geologic layers of Earth[108]

Earth cutaway from core to exosphere. Not to scale.
Component layer Density
0–60 Lithosphere[n 16]
0–35 Crust[n 17] 2.2–2.9
35–60 Upper mantle 3.4–4.4
  35–2890 Mantle 3.4–5.6
100–700 Asthenosphere
2890–5100 Outer core 9.9–12.2
5100–6378 Inner core 12.8–13.1


Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[110] The major heat-producing isotopes within Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[111] At the center, the temperature may be up to 6,000 °C (10,830 °F),[112] and the pressure could reach 360 GPa.[113] Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's 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 Ga,[110] would have increased temperature gradients with radius,[clarification needed] increasing the rates of mantle convection and plate tectonics, and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today.[114]

Present-day major heat-producing isotopes[115]
Isotope Heat release
W/kg isotope
Mean mantle concentration
kg isotope/kg mantle
Heat release
W/kg mantle

The mean heat loss from Earth is 87 mW m−2, for a global heat loss of 4.42 × 1013 W.[116] 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.[117] More of the heat in 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 under the oceans because the crust there is much thinner than that of the continents.[118]

Tectonic plates

Earth's major plates[119]
Plate name Area
106 km2
Main article: Plate tectonics

The mechanically rigid outer layer of Earth, the lithosphere, is divided 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.[120] 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.[121]

As the tectonic plates migrate, oceanic crust 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 recycles the oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than 100 Ma old in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of 200 Ma.[122][123] By comparison, the oldest dated continental crust is 4030 Ma.[124]

The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include 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 mya. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/year[125] and the Pacific Plate moving 52–69 mm/year. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of 21 mm/year.[126]



The total surface area of the Earth is about 510 million km2 (197 million sq mi).[13] Of this, 70.8%,[13] or 361.13 million km2 (139.43 million sq mi), is below sea level and covered by ocean water.[127] Below the ocean's surface are much of the continental shelf, mountains, volcanoes,[97] oceanic trenches, submarine canyons, oceanic plateaus, abyssal plains, and a globe-spanning mid-ocean ridge system. The remaining 29.2% (148.94 million km2, or 57.51 million sq mi) not covered by water has terrain that varies greatly from place to place and consists of mountains, deserts, plains, plateaus, and other landforms. Tectonics and erosion, volcanic eruptions, flooding, weathering, glaciation, the growth of coral reefs, and meteorite impacts are among the processes that constantly reshaping and have reshaped the Earth's surface over geological time.[128][129]

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.[130] Sedimentary rock is formed from the accumulation of sediment that becomes buried and compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the crust.[131] 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 Earth's surface include quartz, feldspars, amphibole, mica, pyroxene and olivine.[132] Common carbonate minerals include calcite (found in limestone) and dolomite.[133]

The elevation of the land surface varies from the low point of −418 m at the Dead Sea, to a maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.[134]

The pedosphere is the outermost layer of Earth's continental surface and is composed of soil and subject to soil formation processes. The total arable land is 10.9% of the land surface, with 1.3% being permanent cropland.[135][136] Close to 40% of Earth's land surface is used for cropland and pasture, or an estimated 1.3×107 km2 of cropland and 3.4×107 km2 of pastureland.[137]


Main article: Hydrosphere
Elevation histogram of Earth's surface


The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of Earth's total mass. The oceans cover an area of 3.618×108 km2 with a mean depth of 3682 m, resulting in an estimated volume of 1.332×109 km3.[139] If all of Earth's crustal surface was at the same elevation as a smooth sphere, the depth of the resulting world ocean would be 2.7 to 2.8 km.[140][141]


The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5% salt).[143] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[144] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[145] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[146] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[147]


Main article: Atmosphere of Earth
A typhoon as seen from low Earth orbit

The atmospheric pressure on Earth's surface averages 101.325 kPa, with a scale height of about 8.5 km.[3] It has a composition of 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.[148]

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 Gya, forming the primarily nitrogen–oxygen atmosphere of today.[75] This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric O2 into O3. The ozone layer blocks ultraviolet solar radiation, permitting life on land.[149] Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[150] 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. Water vapor, carbon dioxide, methane and ozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C, in contrast to the current +15 °C, and life would likely not exist.[151]

Weather and climate

Main articles: Weather and Climate

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 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 thermal energy.[152]

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°.[153] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.[154]


The amount of solar energy reaching Earth's surface decreases with increasing latitude. At higher latitudes the sunlight reaches the surface at 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 (0.7 °F) per degree of latitude from the equator.[156] Earth's surface can be subdivided 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.[157]

This latitudinal rule has several anomalies:

  • Proximity to oceans moderates the climate. For example, the Scandinavian peninsula has more moderate climate than similarly northern latitudes of northern Canada.
  • The distance from the Earth to the Sun varies. The Earth is closest to the sun (at perihelion) in July, which is summer in the Northern Hemisphere. It is furthest away (at aphelion) in January, which is summer in the Southern Hemisphere, and only 93.55% of the solar radiation from the Sun falls on a given square area of land than at perihelion. Despite this, there are larger land masses in the Northern Hemisphere, which are easier to heat than the seas. Consequently, summers are 2.3 °C (4 °F) warmer in the Northern Hemisphere than in the Southern Hemisphere under similar conditions.[159]
  • Climate is colder at high altitudes than at sea level because of the decreased air density.

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.[153] The Köppen system rates regions of terrain based on observed temperature and precipitation.

The highest air temperature ever measured on Earth was 56.7 °C (134.1 °F) in Furnace Creek, California, in Death Valley, in 1913.[160] The lowest air temperature ever directly measured on Earth was −89.2 °C (−128.6 °F) at Vostok Station in 1983,[161] but satellites have used remote sensing to measure temperatures as low as −94.7 °C (−138.5 °F) in East Antarctica.[162] These temperature records are only measurements made with modern instruments from the 20th century onwards and likely do not reflect the full range of temperature on Earth.

Upper atmosphere

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

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[150] 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 geomagnetic fields interact with the solar wind.[163] 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 Earth's surface, is a working definition for the boundary between the atmosphere and outer space.[164]

Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth's gravity. This causes a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular mass, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gases.[165] The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface 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 thought to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[166] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[167] 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.[168]

Magnetic field

The main part of Earth's magnetic field is generated in the core, the site of a dynamo process that converts kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth's surface, where it is, to rough approximation, a dipole. The poles of the dipole are located close to Earth's geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is 3.05 × 10−5 T, with global magnetic dipole moment of 7.91 × 1015 T m3.[169] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes secular variation of the main field and field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[170][171]





Orbit and rotation


Main article: Earth's rotation
Earth rotation imaged by DSCOVR EPIC on 29 May 2016, a few weeks before the solstice


Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.098903691 seconds of mean solar time (UT1), or 23h 56m 4.098903691s.[2][n 20] Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86,164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s) as of 1982.[2] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[182] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[183] and 1962–2005.[184]



Main article: Earth's orbit
The historic Pale Blue Dot photo taken in 1990 by the Voyager 1 spacecraft showing Earth (center right) from nearly 6.4 billion kilometers (4×10^9 mi) away



The Hill sphere, or gravitational sphere of influence, of Earth is about 1.5 million kilometres (930,000 mi) in radius.[188][n 21] This is the maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.


Axial tilt and seasons

Main article: Axial tilt § Earth

The axial tilt of the Earth is approximately 23.439281°[2] with the axis of its orbit plane, pointing always towards the Celestial Poles. Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes seasonal change in climate, with summer in the northern hemisphere occurring when the Tropic of Cancer is facing the Sun, and winter taking place when the Tropic of Capricorn in the southern hemisphere faces the Sun. 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. In northern temperate latitudes, the Sun rises north of true east during the summer solstice, and sets north of true west, reversing in the winter. The Sun rises south of true east in the summer for the southern temperate zone, and sets south of true west.


By astronomical convention, the four seasons can be determined by the solstices—the points 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. In the northern hemisphere, winter solstice currently occurs around 21 December, summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the southern hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.[190]






Main article: Biosphere

A planet's life forms inhabit ecosystems, whose total is sometimes said to form a "biosphere". Earth's biosphere is thought to have begun evolving about 3.5 Gya.[75] 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.[196]


Natural resources and land use

Main articles: Natural resource and Land use
Estimated human land use, 2000[198]
Land use Mha
Cropland 1,510–1,611
Pastures 2,500–3,410
Natural forests 3,143–3,871
Planted forests 126–215
Urban areas 66–351
Unused, productive land 356–445

Earth has resources that have been exploited by humans. Those termed non-renewable resources, such as fossil fuels, only renew over geological timescales.

Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed within the crust through a process of ore genesis, resulting from actions of magmatism, erosion and plate tectonics.[199] These bodies form concentrated sources for many metals and other useful elements.

Earth's biosphere produces many useful biological products for humans, including 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.[200] In 1980, 5,053 Mha (50.53 million km2) of Earth's land surface consisted of forest and woodlands, 6,788 Mha (67.88 million km2) was grasslands and pasture, and 1,501 Mha (15.01 million km2) was cultivated as croplands.[201] The estimated amount of irrigated land in 1993 was 2,481,250 square kilometres (958,020 sq mi).[14] Humans also live on the land by using building materials to construct shelters.

Natural and environmental hazards

A volcano injecting hot ash into the atmosphere




Human geography

Main articles: Human geography and World
The seven continents of Earth[204]


Earth's human population reached approximately seven billion on 31 October 2011.[205] Projections indicate that the world's human population will reach 9.2 billion in 2050.[206] 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.[207]

It is estimated that one-eighth of Earth's surface is suitable for humans to live on – three-quarters of Earth's surface is covered by oceans, leaving one quarter as land. Half of that land area is desert (14%),[208] high mountains (27%),[209] or other unsuitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.[210] (82°28′N) The southernmost is the Amundsen–Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)


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.[212] 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.[213]

The first human to orbit Earth was Yuri Gagarin on 12 April 1961.[214] In total, about 487 people have visited outer space and reached orbit as of 30 July 2010, and, of these, twelve have walked on the Moon.[215][216][217] Normally, the only humans in space are those on the International Space Station. The station's crew, made up of six people, is usually replaced every six months.[218] The farthest that humans have travelled from Earth is 400,171 km, achieved during the Apollo 13 mission in 1970.[219]


Full moon as seen from Earth's Northern Hemisphere
Diameter 3,474.8 km
Mass 7.349×1022 kg
Semi-major axis 384,400 km
Orbital period 27 d 7 h 43.7 m
Main article: Moon




Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm/yr. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs/yr—add up to significant changes.[220] During the Devonian period, for example, (approximately 410 mya) there were 400 days in a year, with each day lasting 21.8 hours.[221]




Asteroids and artificial satellites

The International Space Station is an artificial satellite in orbit around Earth.

Earth has at least five co-orbital asteroids, including 3753 Cruithne and 2002 AA29.[224][225] A trojan asteroid companion, 2010 TK7, is librating around the leading Lagrange triangular point, L4, in the Earth's orbit around the Sun.[226][227]



Cultural and historical viewpoint

Main article: Earth in culture
"Earthrise", the first photograph of Earth as a celestial body, taken by astronauts on board Apollo 8.

The standard astronomical symbol of Earth consists of a cross circumscribed by a circle, Earth symbol.svg,[229] representing the four quadrants of the world.

Human cultures have developed many views of the planet. Earth is sometimes personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity,[230] and by the mid-20th century the Gaia Principle compared Earth's environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability.[231][232][233] Creation myths in many religions involve the creation of Earth by a supernatural deity or deities.[230]

Scientific investigation has resulted in several culturally transformative shifts in our view of the planet. In the West, belief in a flat Earth[234] was displaced by the idea of spherical Earth, credited to Pythagoras in the 6th century BC.[235] Earth was further believed to be the center of the universe until the 16th century, when scientists first theorized that it was a moving object, comparable to the other planets in the Solar System.[236] Due to the efforts of influential Christian scholars and clerics such as James Ussher, who sought to determine the age of Earth through analysis of genealogies in Scripture, Westerners prior to the 19th century generally believed Earth to be a few thousand years old at most. It was only during the 19th century that geologists realized Earth's age was at least many millions of years.[237] Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864, sparking a vigorous debate on the subject; it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth's age was established, proving the planet to be billions of years old.[238][239] The perception of Earth shifted again in the 20th century when humans first viewed it from orbit, and especially with photographs of Earth returned by the Apollo program.[240]

See also


  3. ^ a b As of July 5, 2016, the United States Strategic Command tracked a total of 17,729 artificial objects, mostly debris. See: "Orbital Debris Quarterly News" (PDF). Vol. 20 no. 3. NASA. July 2016. p. 8. Retrieved 10 October 2016. 
  5. ^ Particularly as the setting for human civilization and experience.[24]
  6. ^ From the name of the Greek earth goddess, but now particularly used for the global ecosystem.[25]
  7. ^ The number of solar days is one less than the number of sidereal days because the orbital motion of Earth around the Sun causes one additional revolution of the planet about its axis.
  8. ^ Including eorþe, erþe, erde, and erthe.[43]
  9. ^ As in Beowulf (1531–33):
    Wearp ða wundelmæl   wrættum gebunden
    yrre oretta,   þæt hit on eorðan læg,
    stið ond stylecg.
    "Remove it. Why should it use up the soil?"[45]
  11. ^ As in Ælfric's Heptateuch (Gen. 1:10):
    Ond God gecygde ða drignysse eorðan ond ðære wætera gegaderunge he het sæ.[43][46]
  12. ^ As in the Wessex Gospels (Matt. 28:18):
    Me is geseald ælc anweald on heofonan & on eorðan.[43]
    "All authority in heaven and on earth has been given to me."[48]
  13. ^ As in the Codex Junius's Genesis (112–16):
    her ærest gesceop   ece drihten,
    helm eallwihta,   heofon and eorðan,
    rodor arærde   and þis rume land
    gestaþelode   strangum mihtum,
    frea ælmihtig.
    Seo eorðe stent on gelicnysse anre pinnhnyte, & seo sunne glit onbutan be Godes gesetnysse.[43]
  16. ^ Locally varies between 5 and 200 km.
  17. ^ Locally varies between 5 and 70 km.


  3. ^ a b c d e f g h i j k l Williams, David R. (1 September 2004). "Earth Fact Sheet". NASA. Retrieved 9 August 2010. 
  5. ^ a b "UCS Satellite Database". Nuclear Weapons & Global Security. Union of Concerned Scientists. 11 August 2016. Retrieved 10 October 2016. 
  9. ^ Cazenave, Anny (1995). "Geoid, Topography and Distribution of Landforms" (PDF). In Ahrens, Thomas J. Global Earth Physics: A Handbook of Physical Constants. Washington, DC: American Geophysical Union. ISBN 0-87590-851-9. Archived from the original (PDF) on 16 October 2006. Retrieved 3 August 2008. 
  10. ^ International Earth Rotation and Reference Systems Service (IERS) Working Group (2004). "General Definitions and Numerical Standards" (PDF). In McCarthy, Dennis D.; Petit, Gérard. IERS Conventions (2003) (PDF). IERS Technical Note No. 32. Frankfurt am Main: Verlag des Bundesamts für Kartographie und Geodäsie. p. 12. ISBN 3-89888-884-3. Retrieved 29 April 2016. 
  11. ^ Humerfelt, Sigurd (26 October 2010). "How WGS 84 defines Earth". Retrieved 29 April 2011. 
  19. ^ Arthur N. Cox, ed. (2000). Allen's Astrophysical Quantities (4th ed.). New York: AIP Press. p. 244. ISBN 0-387-98746-0. Retrieved 17 August 2010. 
  20. ^ "World: Lowest Temperature". WMO Weather and Climate Extremes Archive. Arizona State University. Retrieved 7 August 2010. 
  21. ^ Kinver, Mark (10 December 2009). "Global average temperature may hit record level in 2010". BBC Online. Retrieved 22 April 2010. 
  23. ^ National Oceanic and Atmospheric Administration (5 December 2014). "Trends in Atmospheric Carbon Dioxide". Earth System Research Laboratory. 
  24. ^ Oxford English Dictionary, 3rd ed. "world, n." Oxford University Press (Oxford), 2010.
  25. ^ Oxford English Dictionary, 3rd ed. "Gaia, n." Oxford University Press (Oxford), 2007.
  26. ^ Oxford English Dictionary, "terra, n., used in Science Fiction" Oxford University Press (Oxford), 1989.
  30. ^ Yoder, Charles F. (1995). "Astrometric and Geodetic Properties of Earth and the Solar System" (PDF). In T. J. Ahrens. Global Earth Physics: A Handbook of Physical Constants. Washington: American Geophysical Union. p. 8. ISBN 0-87590-851-9. 
  36. ^ Stearns, Beverly Peterson; Stearns, S. C.; Stearns, Stephen C. (1 August 2000). Watching, from the Edge of Extinction. Yale University Press. p. 1921. ISBN 978-0-300-08469-6. Retrieved 27 December 2014. 
  42. ^ "Current World Population". worldometers. n.d. Retrieved 30 March 2015. 
  44. ^ a b Beowulf. Trans. Chad Matlick in "Beowulf: Lines 1399 to 1799". West Virginia University. Accessed 5 August 2014. (Old English) & (English)
  45. ^ Mounce Reverse-Intralinear New Testament: "Luke 13:7". Hosted at Bible Gateway. 2014. Accessed 5 August 2014. (Ancient Greek) & (English)
  46. ^ Ælfric of Eynsham. Heptateuch. Reprinted by S.J. Crawford as The Old English Version of the Heptateuch, Ælfric’s Treatise on the Old and New Testament and his Preface to Genesis. Humphrey Milford (London), 1922. Hosted at Wordhord. Accessed 5 August 2014. (Old English)
  47. ^ King James Version of the Bible: "Genesis 1:10". Hosted at Bible Gateway. 2014. Accessed 5 August 2014.
  49. ^ "Genesis A". Hosted at the Dept. of Linguistic Studies at the University of Padua. Accessed 5 August 2014. (Old English)
  50. ^ Killings, Douglas. Codex Junius 11, I.ii. 1996. Hosted at Project Gutenberg. Accessed 5 August 2014.
  52. ^ Tacitus. Germania, Ch. 40.
  56. ^ See:
  73. ^ Staff. "Paleoclimatology – The Study of Ancient Climates". Page Paleontology Science Center. Retrieved 2 March 2007. 
  74. ^ Doolittle, W. Ford; Worm, Boris (February 2000). "Uprooting the tree of life". Scientific American. 282 (6): 90–95. doi:10.1038/scientificamerican0200-90. PMID 10710791. Archived from the original (PDF) on 31 January 2011. 
  84. ^ Kirschvink, J. L. (1992). Schopf, J.W.; Klein, C.; Des Maris, D., eds. Late Proterozoic low-latitude global glaciation: the Snowball Earth. The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press. pp. 51–52. ISBN 0-521-36615-1. 
  93. ^ Bounama, Christine; Franck, S.; Von Bloh, W. (2001). "The fate of Earth's ocean" (PDF). Hydrology and Earth System Sciences. Germany: Potsdam Institute for Climate Impact Research. 5 (4): 569–75. Bibcode:2001HESS....5..569B. doi:10.5194/hess-5-569-2001. Retrieved 3 July 2009. 
  94. ^ a b Schröder, K.-P.; Connon Smith, Robert (2008). "Distant future of the Sun and Earth revisited". Monthly Notices of the Royal Astronomical Society. 386 (1): 155–163. arXiv:0801.4031free to read. Bibcode:2008MNRAS.386..155S. doi:10.1111/j.1365-2966.2008.13022.x. 
  95. ^ "Earth2014 global topography (relief) model". Institut für Astronomische und Physikalische Geodäsie. Retrieved 4 March 2016. 
  100. ^ "Tall Tales about Highest Peaks". Australian Broadcasting Corporation. Retrieved 29 December 2008. 
  103. ^ Brown, Geoff C.; Mussett, Alan E. (1981). The Inaccessible Earth (2nd ed.). Taylor & Francis. p. 166. ISBN 0-04-550028-2.  Note: After Ronov and Yaroshevsky (1969).
  112. ^ "The Earth's Centre is 1000 Degrees Hotter than Previously Thought". The European Synchrotron (ESRF). 25 April 2013. Archived from the original on 12 June 2013. Retrieved 12 April 2015. 
  127. ^ "CIA – The World Factbook". Retrieved 2 November 2012. 
  130. ^ Staff. "Layers of the Earth". Volcano World. Archived from the original on 19 January 2013. Retrieved 11 March 2007. 
  131. ^ Jessey, David. "Weathering and Sedimentary Rocks". Cal Poly Pomona. Archived from the original on 21 July 2007. Retrieved 20 March 2007. 
  132. ^ de Pater, Imke; Lissauer, Jack J. (2010). Planetary Sciences (2nd ed.). Cambridge University Press. p. 154. ISBN 0-521-85371-0. 
  133. ^ Wenk, Hans-Rudolf; Bulakh, Andreĭ Glebovich (2004). Minerals: their constitution and origin. Cambridge University Press. p. 359. ISBN 0-521-52958-1. 
  134. ^ Sverdrup, H. U.; Fleming, Richard H. (1 January 1942). The oceans, their physics, chemistry, and general biology. Scripps Institution of Oceanography Archives. ISBN 0-13-630350-1. Retrieved 13 June 2008. 
  135. ^ "World Bank arable land". Retrieved 19 October 2015. 
  136. ^ "World Bank permanent cropland". Retrieved 19 October 2015. 
  137. ^ FAO Staff (1995). FAO Production Yearbook 1994 (Volume 48 ed.). Rome, Italy: Food and Agriculture Organization of the United Nations. ISBN 92-5-003844-5. 
  139. ^ Charette, Matthew A.; Smith, Walter H. F. (June 2010). "The Volume of Earth's Ocean". Oceanography. 23 (2): 112–14. doi:10.5670/oceanog.2010.51. Archived from the original (PDF) on 2 August 2013. Retrieved 6 June 2013. 
  140. ^ "sphere depth of the ocean – hydrology". Encyclopedia Britannica. Retrieved 12 April 2015. 
  144. ^ Mullen, Leslie (11 June 2002). "Salt of the Early Earth". NASA Astrobiology Magazine. Archived from the original on 22 July 2007. Retrieved 14 March 2007. 
  147. ^ Sample, Sharron (21 June 2005). "Sea Surface Temperature". NASA. Retrieved 21 April 2007. 
  149. ^ Harrison, Roy M.; Hester, Ronald E. (2002). Causes and Environmental Implications of Increased UV-B Radiation. Royal Society of Chemistry. ISBN 0-85404-265-2. 
  152. ^ a b Moran, Joseph M. (2005). "Weather". World Book Online Reference Center. NASA/World Book, Inc. Archived from the original on 10 March 2013. Retrieved 17 March 2007. 
  156. ^ Sadava, David E.; Heller, H. Craig; Orians, Gordon H. (2006). Life, the Science of Biology (8th ed.). MacMillan. p. 1114. ISBN 0-7167-7671-5. 
  161. ^ Lyons, Walter A (1997). The Handy Weather Answer Book (2nd ed.). Detroit, Michigan: Visible Ink Press. ISBN 0-7876-1034-8. 
  162. ^ "Coldest temperature ever recorded on Earth in Antarctica". The Guardian. 10 December 2013. Retrieved 12 July 2015. 
  164. ^ de Córdoba, S. Sanz Fernández (21 June 2004). "Presentation of the Karman separation line, used as the boundary separating Aeronautics and Astronautics". Fédération Aéronautique Internationale. Archived from the original on 17 January 2010. Retrieved 21 April 2007. 
  167. ^ Abedon, Stephen T. (31 March 1997). "History of Earth". Ohio State University. Archived from the original on 10 March 2013. Retrieved 19 March 2007. 
  169. ^ Lang, Kenneth R. (2003). The Cambridge guide to the solar system. Cambridge University Press. p. 92. ISBN 0-521-81306-9. 
  171. ^ Campbell, Wallace Hall (2003). Introduction to Geomagnetic Fields. New York: Cambridge University Press. p. 57. ISBN 0-521-82206-8. 
  172. ^ a b McElroy, Michael B. (2012). "Ionosphere and magnetosphere". Encyclopedia Britannica. Encyclopedia Britannica, Inc. 
  182. ^ Seidelmann, P. Kenneth (1992). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 48. ISBN 0-935702-68-7. 
  183. ^ Staff. "IERS Excess of the duration of the day to 86400s ... since 1623". International Earth Rotation and Reference Systems Service (IERS). Retrieved 23 September 2008. —Graph at end.
  185. ^ Zeilik, M.; Gregory, S. A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. p. 56. ISBN 0-03-006228-4. 
  190. ^ Bromberg, Irv (1 May 2008). "The Lengths of the Seasons (on Earth)". University of Toronto. Retrieved 8 November 2008. 
  193. ^ Williams, Jack (20 December 2005). "Earth's tilt creates seasons". USAToday. Retrieved 17 March 2007. 
  194. ^ Staff (September 2003). "Astrobiology Roadmap". NASA, Lockheed Martin. Archived from the original on 11 March 2012. Retrieved 10 March 2007. 
  195. ^ Dole, Stephen H. (1970). Habitable Planets for Man (2nd ed.). American Elsevier Publishing Co. ISBN 0-444-00092-5. Retrieved 11 March 2007. 
  196. ^ Hillebrand, Helmut (2004). "On the Generality of the Latitudinal Gradient". American Naturalist. 163 (2): 192–211. doi:10.1086/381004. PMID 14970922. 
  198. ^ Lambin, Eric F.; Meyfroidt, Patrick (1 March 2011). "Global land use change, economic globalization, and the looming land scarcity" (PDF). Proceedings of the National Academy of Sciences of the United States of America. National Academy of Sciences. 108 (9): 3465–72. Bibcode:2011PNAS..108.3465L. doi:10.1073/pnas.1100480108. Retrieved 30 August 2014.  See Table 1.
  199. ^ Ramdohr, Paul (1969). The Ore Minerals and their Intergrowths. AKADEMIE-VERLAG GmbH. Elsevier Ltd. doi:10.1016/B978-0-08-011635-8.50004-8. ISBN 978-0-08-011635-8. Retrieved 29 April 2016. 
  202. ^ Walsh, Patrick J. (16 May 1997). Sharon L. Smith; Lora E. Fleming, eds. Oceans and human health: risks and remedies from the seas. Academic Press, 2008. p. 212. ISBN 0-12-372584-4. 
  205. ^ "Various '7 billionth' babies celebrated worldwide". Retrieved 31 October 2011. 
  207. ^ Staff (2007). "Human Population: Fundamentals of Growth: Growth". Population Reference Bureau. Archived from the original on 10 February 2013. Retrieved 31 March 2007. 
  209. ^ Staff. "Themes & Issues". Secretariat of the Convention on Biological Diversity. Retrieved 29 March 2007. 
  211. ^ Kennedy, Paul (1989). The Rise and Fall of the Great Powers (1st ed.). Vintage. ISBN 0-679-72019-7. 
  214. ^ Kuhn, Betsy (2006). The race for space: the United States and the Soviet Union compete for the new frontier. Twenty-First Century Books. p. 34. ISBN 0-8225-5984-6. 
  215. ^ Ellis, Lee (2004). Who's who of NASA Astronauts. Americana Group Publishing. ISBN 0-9667961-4-4. 
  216. ^ Shayler, David; Vis, Bert (2005). Russia's Cosmonauts: Inside the Yuri Gagarin Training Center. Birkhäuser. ISBN 0-387-21894-7. 
  220. ^ Espenak, F.; Meeus, J. (7 February 2007). "Secular acceleration of the Moon". NASA. Archived from the original on 22 August 2011. Retrieved 20 April 2007. 
  222. ^ Murray, N.; Holman, M. (2001). "The role of chaotic resonances in the solar system". Nature. 410 (6830): 773–79. arXiv:astro-ph/0111602free to read. doi:10.1038/35071000. PMID 11298438. 
  225. ^ Christou, Apostolos A.; Asher, David J. (31 March 2011). "A long-lived horseshoe companion to the Earth". arXiv:1104.0036free to read [astro-ph.EP].  See table 2, p. 5.
  230. ^ a b Stookey, Lorena Laura (2004). Thematic Guide to World Mythology. Westport, Conn.: Greenwood Press. pp. 114–115. ISBN 978-0-313-31505-3. 
  232. ^ Lovelock, J.E. (1972). "Gaia as seen through the atmosphere". Atmospheric Environment. Elsevier. 6 (8): 579–580. Bibcode:1972AtmEn...6..579L. doi:10.1016/0004-6981(72)90076-5. ISSN 1352-2310. 
  233. ^ Lovelock, J.E.; Margulis, L. (1974). "Atmospheric homeostasis by and for the biosphere: the Gaia hypothesis". Tellus. Series A. Stockholm: International Meteorological Institute. 26 (1–2): 2–10. Bibcode:1974Tell...26....2L. doi:10.1111/j.2153-3490.1974.tb01946.x. ISSN 1600-0870. Retrieved 20 October 2012. 
  235. ^ Godwin, William (1876). "Lives of the Necromancers". p. 49. 
  237. ^ Monroe, James; Wicander, Reed; Hazlett, Richard (2007). Physical Geology: Exploring the Earth. Thomson Brooks/Cole. pp. 263–265. ISBN 978-0-495-01148-4. 
  238. ^ Henshaw, John M. (2014). An Equation for Every Occasion: Fifty-Two Formulas and Why They Matter. Johns Hopkins University Press. pp. 117–118. ISBN 978-1-421-41491-1. 

Further reading

External links

Listen to this article (4 parts) · (info)
Part 1 • Part 2 • Part 3 • Part 4
This audio file was created from a revision of the "Earth" article dated 2012-06-13, and does not reflect subsequent edits to the article. (Audio help)
More spoken articles