Page semi-protected
Listen to this article

Earth

From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about the planet. For other uses, see Earth (disambiguation).
Earth Astronomical symbol of Earth
Earth as seen from the Deep Space Climate Observatory on 6 July 2015.[1]
Orbital characteristics
Epoch J2000[n 1]
Aphelion
151930000 km
(1.01559 AU) [n 2]
Perihelion
147095000 km
(0.9832687 AU) [n 2]
149598261 km
(1.00000261 AU) [2]
Eccentricity 0.01671123[2]
365.256363004 d[3]
(1.00001742096 yr)
29.78 km/s[4]
(107200 km/h)
358.617 deg
Inclination
11.26064 deg[4] to J2000 ecliptic
102.94719 deg[4]
Satellites
Physical characteristics
Mean radius
6371.0 km[7]
Equatorial radius
6378.1 km[8][9]
Polar radius
6356.8 km[10]
Flattening 0.0033528[11]
1/298.257222101 (ETRS89)
Circumference
  • 510072000 km2[14][15][n 4]
  •  (148940000 km2 (29.2%) land
  •   361132000 km2 (70.8%) water)
Volume 1.08321×1012 km3[4]
Mass
5.97219×1024 kg[16]
(3.0×10-6 solar mass)
Mean density
5.514 g/cm3[4]
9.807 m/s2[17]
(1 g)
0.3307[18]
11.186 km/s[4]
0.99726968 d[19]
(23h 56m 4.100s)
Equatorial rotation velocity
1,674.4 km/h (465.1 m/s)[20]
23 deg 26 min 21.4119 s[3]
Albedo
Surface temp. min mean max
Kelvin 184 K[21] 288 K[22] 330 K[23]
Celsius −89.2 °C 15 °C 56.7 °C
Atmosphere
Surface pressure
101.325 kPa (at MSL)
Composition by volume

Earth, also called the world[n 5] and, less frequently, Gaia[n 6] (and Terra, from Latin, in some works of science fiction[27]) is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to accommodate life. The earliest life on Earth arose at least 3.5 billion years ago.[28][29][30] Earth's biodiversity has expanded continually except when interrupted by mass extinctions.[31] Although scholars estimate that over 99 percent of all species of life (over five billion)[32] that ever lived on Earth are extinct,[33][34] there are still an estimated 10–14 million extant species,[35][36] of which about 1.2 million have been documented and over 86 percent have not yet been described.[37] Over 7.3 billion humans[38] live on Earth and depend on its biosphere and minerals for their survival. Earth's human population is divided among about two hundred sovereign states which interact through diplomacy, conflict, travel, trade and communication media.

According to evidence from radiometric dating and other sources, Earth was formed about 4.54 billion years ago.[39][40][41] Within its first billion years,[42] life appeared in its oceans and began to affect its atmosphere and surface, promoting the proliferation of aerobic as well as anaerobic organisms and causing the formation of the atmosphere's ozone layer. This layer and the geomagnetic field blocked the most life-threatening parts of the Sun's radiation, so life was able to flourish on land as well as in water.[43] Since then, the combination of Earth's distance from the Sun, its physical properties and its geological history have allowed life to thrive and evolve.

Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. Seventy-one percent of Earth's surface is covered with water,[44] with the remainder consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere. Earth's polar regions are mostly covered with ice that includes the solid ice of the Antarctic ice sheet and the sea ice of the polar ice packs. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.

Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, Earth rotates about its own 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 with a period of one tropical year (365.24 solar days).[45] The Moon is Earth's only permanent natural satellite. It began orbiting Earth about 4.53 billion years ago. The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation.

Chronology

Formation

Main article: History of Earth
Artist's impression of the early Solar System's planetary disk

The earliest material found in the Solar System is dated to 4.5672±0.0006 billion years ago (bya);[46] therefore, it is inferred that Earth must have been formed by accretion around this time. By 4.54±0.04 bya[42] the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with 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 in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 Ma.[47] The Moon formed shortly thereafter, about 4.53 bya.[48]

The formation of the Moon remains a topic of debate. The working hypothesis is that it formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, impacted with Earth.[49] In this scenario, the mass of Theia is 10% of that of Earth,[50] it impacted Earth with a glancing blow,[51] and some of its mass merges with Earth. Between approximately 3.8 and 4.1 bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to Earth.

Geological history

Earth's atmosphere and oceans 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.[52] In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity.[53] By 3.5 bya, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[54] 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. The two models[55] that explain land mass propose either a steady growth to the present-day forms[56] or, more likely, a rapid growth[57] early in Earth history[58] followed by a long-term steady continental area.[59][60][61] 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 formed and broken up three times. 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.[62]

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 every 40–100000 years. The last continental glaciation ended 10,000 years ago.[63]

Evolution of life

Speculative phylogenetic tree of life on Earth based on rRNA analysis

Highly energetic chemical reactions are thought to have produced self–replicating molecules around four billion years ago. This was followed a half billion years later by the last common ancestor of all life.[64] The development 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 high energy solar radiation, formed a layer of protective ozone (O3) in the upper atmosphere.[65] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[66] 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.[67] The earliest fossil evidences for life are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[68] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[69][70]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.[71]

Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions.[72] 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.[73] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence Earth in a short time span as no other life form had,[74] affecting both the nature and quantity of other life forms.

Predicted future

Main article: Future of the Earth

Estimates on how much longer the planet will be able to continue to support life range from 500 million years (Myr), to as long as 2.3 billion years (Gyr).[75][76][77] The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 Ga and by 40% over the next 3.5 Ga.[78] Climate models indicate that the rise in radiation reaching Earth is likely to have dire consequences, including the loss of the planet's oceans.[79]

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.[75] 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.[80] After another billion years all surface water will have disappeared[76] and the mean global temperature will reach 70 °C[80] (158 °F). Earth is expected to be effectively habitable for about another 500 Ma from that point,[75] although this may be extended up to 2.3 Ga if the nitrogen is removed from the atmosphere.[77] 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.[81]

14 billion year timeline showing Sun's present age at 7017145164960000000♠4.6 Ga; from 7017189345600000000♠6 Ga Sun gradually warming, becoming a red dwarf at 7017315576000000000♠10 Ga, "soon" followed by its transformation into a white dwarf
Life cycle of the Sun

The Sun, as part of its evolution, will 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.[78][82] 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 (250,000,000 km) from the Sun, when the star reaches its maximum radius. The planet was, therefore, initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5,000 times its present level).[78] 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.[82]

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.[83] 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,[92] and later Norse mythology included Jörð, a giantess often given as the mother of Thor.[93]

Originally, earth was written in lowercase, and from early Middle English, its definite sense as "the globe" was expressed as the earth. By early Modern English, many nouns were capitalized, and the earth became (and often remained) the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets.[83] House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name (e.g. "Earth's atmosphere") but writes it in lowercase when preceded by the (e.g. "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"[94]

Composition and structure

Shape

Main article: Figure of the Earth
World map color-coded by relative height

The shape of Earth approximates an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator.[95] This bulge results from the rotation of Earth, and causes the diameter at the equator to be 43 kilometres (27 mi) larger than the pole-to-pole diameter.[96] Thus the point on the surface farthest from Earth's center of mass is the summit of the equatorial Chimborazo volcano in Ecuador.[97][98][99][100] The average diameter of the reference spheroid is about 12,742 kilometres (7,918 mi), which is approximately 40,000 km/π, because the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.[101]

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 (10911 m below local sea level), whereas Mount Everest (8,848 m above local sea level) represents a deviation of 0.14%. If Earth were shrunk to the size of a cue ball, some areas of Earth such as mountain ranges and oceanic trenches would feel like small imperfections, whereas much of the planet, including the Great Plains and the abyssal plains, would actually feel smoother than a cue ball.[102]

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%
Chimborazo, Ecuador. The point on Earth's surface farthest from its center.[97]

Chemical composition

Earth's mass is approximately 5.97×1024 kg (5,970 yg). It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[104]

The geochemist F. W. Clarke calculated that a little more than 47% of Earth's crust consists of oxygen. The more 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. 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), with the other constituents occurring in minute quantities.[105]

Internal structure

Earth's interior, 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 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, and the thickness of the crust varies: averaging 6 km (kilometers) under the oceans and 30–50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are 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 inner core may 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 Earth's.

Geologic layers of Earth[108]
Earth-cutaway-schematic-english.svg

Earth cutaway from core to exosphere. Not to scale.
Depth[109]
km
Component Layer Density
g/cm3
0–60 Lithosphere[n 15]
0–35 Crust[n 16] 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

Heat

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 3 Ga,[110] would have increased temperature gradients with radius, 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
Half-life
years
Mean mantle concentration
kg isotope/kg mantle
Heat release
W/kg mantle
238U 94.6 × 10−6 4.47 × 109 30.8 × 10−9 2.91 × 10−12
235U 569 × 10−6 0.704 × 109 0.22 × 10−9 0.125 × 10−12
232Th 26.4 × 10−6 14.0 × 109 124 × 10−9 3.27 × 10−12
40K 29.2 × 10−6 1.25 × 109 36.9 × 10−9 1.08 × 10−12

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]
Shows the extent and boundaries of tectonic plates, with superimposed outlines of the continents they support
Plate name Area
106 km2
103.3
78.0
75.9
67.8
60.9
47.2
43.6
Main article: Plate tectonics

The mechanically rigid outer layer of Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: convergent boundaries, at which two plates come together, divergent boundaries, at which two plates are pulled apart, and transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.[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 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. 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 about 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 about 21 mm/year.[126]

Surface





Circle frame.svg

Features of Earth's solid surface shown as percentages of the planet's total surface area

  Oceanic ridges (22.1%)
  Ocean basin floors (29.8%)
  Continental mountains (10.3%)
  Continental lowlands (18.9%)
  Continental shelves and slopes (11.4%)
  Continental rise (3.8%)
  Volcanic island arcs, trenches, submarine volcanoes, and hills (3.7%)
An aerial view of Barringer Meteor Crater in Arizona.

Earth's terrain varies greatly from place to place. About 70.8%[14] of the surface is covered by water, with much of the continental shelf below sea level. This equates to 361.132 million km2 (139.43 million sq mi).[127] The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[96] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% (148.94 million km2, or 57.51 million sq mi) not covered by water consists of mountains, deserts, plains, plateaus, and other landforms.

The planetary surface undergoes reshaping over geological time periods due to tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering and erosion from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[128] also act to reshape the landscape.

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.[129] 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.[130] 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.[131] Common carbonate minerals include calcite (found in limestone) and dolomite.[132]

The pedosphere is the outermost layer of Earth's continental surface and is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The total arable land is 13.31% of the land surface, with 4.71% supporting permanent crops.[15] 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.[133]

The elevation of the land surface 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.[134]

Besides being divided logically into Northern and Southern hemispheres centered on the poles, Earth has been divided arbitrarily into Eastern and Western hemispheres. Earth's surface is traditionally divided into seven continents and various seas.

Hydrosphere

Main article: Hydrosphere
Elevation histogram of Earth's surface

The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from other planets in the Solar System. Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of 10,911.4 m.[n 18][135]

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.[136] 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.[137][138]

About 97.5% of the water is saline; the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.[139]

The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5% salt).[140] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[141] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[142] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[143] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[144]

Atmosphere

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.[4] 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.[145]

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 bya, forming the primarily nitrogen–oxygen atmosphere of today.[65] This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer, which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[146] 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 would be −18 °C, in contrast to the current +15 °C, and life would likely not exist.[147]

Weather and climate

Main articles: Weather and Climate
In this scene from Antarctica, Earth's south polar continent, ice ridges contrast with towering clouds

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.[148]

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°.[149] 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.[150]

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 falls to the surface as precipitation.[148] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topographic features and temperature differences determine the average precipitation that falls in each region.[151]

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.[152] 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.[153] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.[149]

Climate on Earth has latitudinal anomalies, namely the habitability of the Scandinavian peninsula very far north in sharp contrast to the polar climates of northern Canada as well as the cool summers expected at low latitudes in the Southern Hemisphere (for example on the west coast of South America). Another anomaly is the impact of landmass on temperature, manifested by the fact that Earth is much warmer at aphelion, where the planet is at a more distant position from the Sun.[154] When the Northern hemisphere is turned towards the sunlight even the increased distance to it does not hinder temperatures to be 2.3 °C (4 °F) warmer than at perihelion—when the marine southern hemisphere is turned towards the Sun.[155]

At high latitudes, the western sides of continents tend to be milder than the eastern sides—for example seen in North America and Western Europe where rough continental climates appear on the east coast on parallels with mild climates on the other side of the ocean.[156]

The highest air temperature ever measured on Earth was 56.7 °C (134.1 °F) in Furnace Creek, California, in Death Valley, in 1913.[157] The lowest air temperature ever directly measured on Earth was −89.2 °C (−128.6 °F) at Vostok Station in 1983,[158] but satellites have used remote sensing to measure temperatures as low as −94.7 °C (−138.5 °F) in East Antarctica.[159] 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.[146] 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.[160] 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.[161]

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.[162] 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 believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[163] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[164] 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.[165]

Magnetic field

Diagram showing the magnetic field lines of Earth's magnetosphere. The lines are swept back in the anti-solar direction under the influence of the solar wind.
Schematic of Earth's magnetosphere. The solar wind flows from left to right

The main part of the Earth's magnetic field is generated in the core, the site of a dynamo process that converts kinetic energy of fluid convective motion 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.[166] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[167][168]

Magnetosphere

The extent of Earth's magnetic field in space defines the magnetosphere. Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the dayside of the magnetosphere, to about 10 Earth radii, and extends the nightside magnetosphere into a long tail. Because the velocity of the solar wind is greater than the speed at which wave propagate through the solar wind, a supersonic bowshock precedes the dayside magnetosphere within the solar wind. Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates; the ring current is defined by medium-energy particles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field, and the Van Allen radiation belt are formed by high-energy particles whose motion is essentially random, but otherwise contained by the magnetosphere.

During a magnetic storm, charged particles can be deflected from the outer magnetosphere, directed along field lines into Earth's ionosphere, where atmospheric atoms can be excited and ionized, causing the aurora.[169]

Orbit and rotation

Rotation

Main article: Earth's rotation
Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit

Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).[170] Because Earth's solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 SI ms longer.[171][172]

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.[3][n 19] 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.[3] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[173] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[174] and 1962–2005.[175]

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.[176][177]

Orbit

Main article: Earth's orbit

Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. This gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to travel a distance equal to Earth's diameter, about 12,742 km, in seven minutes, and the distance to the Moon, 384,000 km, in about 3.5 hours.[4]

The Moon and Earth orbit a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common orbit 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 counterclockwise. Viewed from a vantage point above the north poles of both the Sun and Earth, Earth orbits 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 (the ecliptic), and the Earth–Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.[4][178]

The Hill sphere, or gravitational sphere of influence, of Earth is about 1.5 Gm or 1,500,000 km in radius.[179][n 20] 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.

Earth, along with the Solar System, is situated in the Milky Way and orbits about 28,000 light years from its center. It is about 20 light years above the galactic plane in the Orion Arm.[180]

Axial tilt and seasons

Main article: Axial tilt § Earth

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 North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. 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.

Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year, up to six months at the North Pole itself, a polar night. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole. Six months later, this pole will experience a midnight sun, a day of 24 hours, again reversing with the South 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. 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 swapped.[181]

The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years.[182] The orientation (rather than the angle) of Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth's equatorial bulge. The poles also migrate a few meters across Earth's surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.[183]

In modern times, Earth's perihelion occurs around January 3, and its aphelion around July 4. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth–Sun distance causes an increase of about 6.9%[n 21] in solar energy reaching Earth at perihelion relative to aphelion. Because the southern hemisphere is tilted toward the Sun at about the same time that Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. 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.[184]

Habitability

A planet that can sustain life is termed habitable, even if life did not originate there. Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.[185] The distance of 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 current climatic conditions at the surface.[186]

Biosphere

Main article: Biosphere
Coral reef and beach

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 bya.[65] 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.[187]

Natural resources and land use

Main articles: Natural resource and Land use
Estimated human land use, 2000[188]
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.[189] 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.[190] 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.[191] The estimated amount of irrigated land in 1993 was 2,481,250 square kilometres (958,020 sq mi).[15] 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

Large areas of Earth's surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. From 1980 to 2000, these events caused an average of 11,800 deaths per year.[192] Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, wildfires, and other calamities and disasters.

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

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.[193]

Human geography

Main articles: Human geography and World
The seven continents of Earth[194]
A composite picture consisting of DMSP/OLS ground-illumination data for 2000 placed on a simulated night-time image of Earth.

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

Earth's human population reached approximately seven billion on October 31, 2011.[195] Projections indicate that the world's human population will reach 9.2 billion in 2050.[196] 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.[197]

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%),[198] high mountains (27%),[199] or other unsuitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.[200] (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, a few land parcels along the Danube river's western bank, and the odd unclaimed area of Bir Tawil between Egypt and Sudan. As of 2015, there are 193 sovereign states that are member states of the United Nations, plus two observer states and 72 dependent territories and states with limited recognition.[15] Historically, Earth has never had a sovereign government with authority over the entire globe although a number of nation-states have striven for world domination and failed.[201]

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.[202] 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.[203]

The first human to orbit Earth was Yuri Gagarin on April 12, 1961.[204] 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.[205][206][207] 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.[208] The farthest that humans have travelled from Earth is 400,171 km, achieved during the Apollo 13 mission in 1970.[209]

Moon

Characteristics
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
Full moon as seen from Earth's Northern Hemisphere
Details of the Earth–Moon system, showing the radius of each object and the Earth-Moon barycenter. The Moon's axis is located by Cassini's third law.

The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of 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 of other planets are also referred to as "moons", after Earth's.

The gravitational attraction between Earth and the 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 Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator.

Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm/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.[210] During the Devonian period, for example, (approximately 410 mya) there were 400 days in a year, with each day lasting 21.8 hours.[211]

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.[212] Some theorists believe that without this stabilization against the torques applied by the Sun and planets to Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.[213]

Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[177] This allows total and annular solar eclipses to occur on Earth.

The most widely accepted theory of the Moon's origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of Earth's crust.[214]

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.[215][216] A trojan asteroid companion, 2010 TK7, is librating around the leading Lagrange triangular point, L4, in the Earth's orbit around the Sun.[217][218]

The tiny near-Earth asteroid 2006 RH120 makes close approaches to the Earth–Moon system roughly every twenty years. During these approaches, it can orbit Earth for brief periods of time.[219]

As of March 2015, there were 1,265 operational, human-made satellites orbiting Earth.[220] There are also inoperative satellites, including Vanguard 1 the oldest satellite currently in orbit, and over 300,000 pieces of space debris. Earth's largest artificial satellite is the International Space Station.

Cultural and historical viewpoint

Main article: Earth in culture

The standard astronomical symbol of Earth consists of a cross circumscribed by a circle, Earth symbol.svg,[221] 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,[citation needed] 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.[222][223][224] Creation myths in many religions involve the creation of Earth by a supernatural deity or deities.[citation needed]

Scientific investigation has resulted in several culturally transformative shifts in our view of the planet. In the West, belief in a flat Earth[225] was displaced by the idea of spherical Earth, credited to Pythagoras in the 6th century BC.[226] 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.[227] Geologists realized the Earth's age was many millions of years in the 19th century.[how?][citation needed] 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.[228]

"Earthrise", the first photograph of Earth as an astronomical body, taken by astronauts on board Apollo 8
"The Blue Marble" photograph of Earth, taken by the Apollo 17 lunar mission in 1972

See also

Notes

  1. ^ All astronomical quantities vary, both secularly and periodically. The quantities given are the values at the instant J2000.0 of the secular variation, ignoring all periodic variations.
  2. ^ a b aphelion = a × (1 + e); perihelion = a × (1 – e), where a is the semi-major axis and e is the eccentricity. The difference between Earth's perihelion and aphelion is 5 million kilometers.
  3. ^ United States Strategic Command tracks about 15,000 other artificial objects, mostly debris. See: "USSTRATCOM Space Control and Space Surveillance". January 2014. Retrieved 17 July 2015. 
  4. ^ Due to natural fluctuations, ambiguities surrounding ice shelves, and mapping conventions for vertical datums, exact values for land and ocean coverage are not meaningful. Based on data from the Vector Map and Global Landcover datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of Earth's surface. The ice shields of Antarctica and Greenland are counted as land, even though much of the rock that supports them lies below sea level.
  5. ^ Particularly as the setting for human civilization and experience.[25]
  6. ^ From the name of the Greek earth goddess, but now particularly used for the global ecosystem.[26]
  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.[83]
  9. ^ As in Beowulf (1531–33):
    Wearp ða wundelmæl   wrættum gebunden
    yrre oretta,   þæt hit on eorðan læg,
    stið ond stylecg.
    [83][84]
    "He threw the artfully-wound sword so that it lay upon the earth, firm and sharp-edged."[84]
  10. ^ As in the Old English glosses of the Lindisfarne Gospels (Luke 13:7):
    Succidite ergo illam ut quid etiam terram occupat: hrendas uel scearfað forðon ðailca uel hia to huon uutedlice eorðo gionetað uel gemerras.[83]
    "Remove it. Why should it use up the soil?"[85]
  11. ^ As in Ælfric's Heptateuch (Gen. 1:10):
    Ond God gecygde ða drignysse eorðan ond ðære wætera gegaderunge he het sæ.[83][86]
    "And God called the dry land Earth; and the gathering together of the waters called he Seas."[87]
  12. ^ As in the Wessex Gospels (Matt. 28:18):
    Me is geseald ælc anweald on heofonan & on eorðan.[83]
    "All authority in heaven and on earth has been given to me."[88]
  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.
    [83][89]
    "Here first with mighty power the Everlasting Lord, the Helm of all created things, Almighty King, made earth and heaven, raised up the sky and founded the spacious land."[90]
  14. ^ As in Ælfric's On the Seasons of the Year (Ch. 6, §9):
    Seo eorðe stent on gelicnysse anre pinnhnyte, & seo sunne glit onbutan be Godes gesetnysse.[83]
    "The earth can be compared to a pine cone, and the Sun glides around it by God's decree.[91]
  15. ^ Locally varies between 5 and 200 km.
  16. ^ Locally varies between 5 and 70 km.
  17. ^ Including the Somali Plate, which is being formed out of the African Plate. See: Chorowicz, Jean (October 2005). "The East African rift system". Journal of African Earth Sciences 43 (1–3): 379–410. Bibcode:2005JAfES..43..379C. doi:10.1016/j.jafrearsci.2005.07.019. 
  18. ^ This is the measurement taken by the vessel Kaikō in March 1995 and is believed to be the most accurate measurement to date. See the Challenger Deep article for more details.
  19. ^ The ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".—Aoki, S.; Kinoshita, H.; Guinot, B.; Kaplan, G. H.; McCarthy, D. D.; Seidelmann, P. K. (1982). "The new definition of universal time". Astronomy and Astrophysics 105 (2): 359–61. Bibcode:1982A&A...105..359A. 
  20. ^ For Earth, the Hill radius is R_H = a\left ( \frac{m}{3M} \right )^{\frac{1}{3}}, where m is the mass of Earth, a is an astronomical unit, and M is the mass of the Sun. So the radius in AU is about \left ( \frac{1}{3 \cdot 332,946} \right )^{\frac{1}{3}} = 0.01.
  21. ^ Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% the energy at aphelion.

References

  1. ^ Murphy, Heather (21 July 2015). "Comparing Two 'Blue Marble' Photos of Earth". New York Times. Retrieved 21 July 2015. 
  2. ^ a b Standish E.M. "Keplerian Elements for Approximate Positions of the Major Planets" (PDF). Retrieved 15 February 2015. 
  3. ^ a b c d Staff (2007-08-07). "Useful Constants". International Earth Rotation and Reference Systems Service. Retrieved 2008-09-23. 
  4. ^ a b c d e f g h i j k l Williams, David R. (2004-09-01). "Earth Fact Sheet". NASA. Retrieved 2010-08-09. 
  5. ^ Allen, Clabon Walter; Cox, Arthur N. (2000). Allen's Astrophysical Quantities. Springer. p. 294. ISBN 0-387-98746-0. Retrieved 2011-03-13. 
  6. ^ "UCS Satellite Database". Union of Concerned Scientists. 31 January 2015. Archived from the original on 3 July 2015. Retrieved 17 July 2015. 
  7. ^ Various (2000). David R. Lide, ed. Handbook of Chemistry and Physics (81st ed.). CRC. ISBN 0-8493-0481-4. 
  8. ^ "Selected Astronomical Constants, 2011". The Astronomical Almanac. Archived from the original on 2013-08-26. Retrieved 2011-02-25. 
  9. ^ a b World Geodetic System (WGS-84). Available online from National Geospatial-Intelligence Agency.
  10. ^ Cazenave, Anny (1995). "Geoid, Topography and Distribution of Landforms". In Ahrens, Thomas J. Global earth physics a handbook of physical constants (PDF). Washington, DC: American Geophysical Union. ISBN 0-87590-851-9. Archived from the original (PDF) on 2006-10-16. Retrieved 2008-08-03. 
  11. ^ IERS Working Groups (2003). McCarthy, Dennis D.; Petit, Gérard, ed. General Definitions and Numerical Standards. IERS Technical Note No. 32 (U.S. Naval Observatory and Bureau International des Poids et Mesures). Archived from the original on 2010-02-01. Retrieved 2008-08-03. 
  12. ^ Humerfelt, Sigurd (October 26, 2010). "How WGS 84 defines Earth". Retrieved 2011-04-29. 
  13. ^ Earth's circumference is almost exactly 40,000 km because the metre was calibrated on this measurement—more specifically, 1/10-millionth of the distance between the poles and the equator.
  14. ^ a b Pidwirny, Michael (2006-02-02). "Surface area of our planet covered by oceans and continents.(Table 8o-1)". University of British Columbia, Okanagan. Retrieved 2007-11-26. 
  15. ^ a b c d Staff (2008-07-24). "World". The World Factbook. Central Intelligence Agency. Retrieved 2008-08-05. 
  16. ^ "Solar System Exploration: Earth: Facts & Figures". NASA. 13 December 2012. Retrieved 22 January 2012. 
  17. ^ The international system of units (SI) (PDF) (2008 ed.). United States Department of Commerce, NIST Special Publication 330. p. 52. 
  18. ^ Williams, James G. (1994). "Contributions to the Earth's obliquity rate, precession, and nutation". The Astronomical Journal 108: 711. Bibcode:1994AJ....108..711W. doi:10.1086/117108. ISSN 0004-6256. 
  19. ^ Allen, Clabon Walter; Cox, Arthur N. (2000). Allen's Astrophysical Quantities. Springer. p. 296. ISBN 0-387-98746-0. Retrieved 2010-08-17. 
  20. ^ Arthur N. Cox, ed. (2000). Allen's Astrophysical Quantities (4th ed.). New York: AIP Press. p. 244. ISBN 0-387-98746-0. Retrieved 2010-08-17. 
  21. ^ "World: Lowest Temperature". WMO Weather and Climate Extremes Archive. Arizona State University. Retrieved 2010-08-07. 
  22. ^ Kinver, Mark (December 10, 2009). "Global average temperature may hit record level in 2010". BBC Online. Retrieved 2010-04-22. 
  23. ^ "World: Highest Temperature". WMO Weather and Climate Extremes Archive. Arizona State University. Retrieved 2010-08-07. 
  24. ^ National Oceanic and Atmospheric Administration (5 December 2014). "Trends in Atmospheric Carbon Dioxide". Earth System Research Laboratory. 
  25. ^ Oxford English Dictionary, 3rd ed. "world, n." Oxford University Press (Oxford), 2010.
  26. ^ Oxford English Dictionary, 3rd ed. "Gaia, n." Oxford University Press (Oxford), 2007.
  27. ^ Oxford English Dictionary, 1st ed. "terra, n." Oxford University Press (Oxford), 1911.
  28. ^ Schopf, JW, Kudryavtsev, AB, Czaja, AD, and Tripathi, AB. (2007). Evidence of Archean life: Stromatolites and microfossils. Precambrian Research 158:141–155.
  29. ^ Schopf, JW (2006). Fossil evidence of Archaean life. Philos Trans R Soc Lond B Biol Sci 29;361(1470) 869-85.
  30. ^ Hamilton Raven, Peter; Brooks Johnson, George (2002). Biology. McGraw-Hill Education. p. 68. ISBN 978-0-07-112261-0. Retrieved 7 July 2013. 
  31. ^ Sahney, S., Benton, M.J. and Ferry, P.A. (27 January 2010). "Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land" (PDF). Biology Letters 6 (4): 544–47. doi:10.1098/rsbl.2009.1024. PMC 2936204. PMID 20106856. 
  32. ^ Kunin, W.E.; Gaston, Kevin, eds. (31 December 1996). The Biology of Rarity: Causes and consequences of rare—common differences. ISBN 978-0412633805. Retrieved 26 May 2015. 
  33. ^ 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. 
  34. ^ Novacek, Michael J. (8 November 2014). "Prehistory's Brilliant Future". New York Times. Retrieved 25 December 2014. 
  35. ^ May, Robert M. (1988). "How many species are there on earth?". Science 241 (4872): 1441–1449. Bibcode:1988Sci...241.1441M. doi:10.1126/science.241.4872.1441. PMID 17790039. 
  36. ^ Miller, G.; Spoolman, Scott (1 January 2012). "Biodiversity and Evolution". Environmental Science. Cengage Learning. p. 62. ISBN 1-133-70787-4. Retrieved 27 December 2014. 
  37. ^ Mora, C.; Tittensor, D.P.; Adl, S.; Simpson, A.G.; Worm, B. (23 August 2011). "How many species are there on Earth and in the ocean?". PLOS Biology. doi:10.1371/journal.pbio.1001127. Retrieved 26 May 2015. 
  38. ^ "Current World Population". worldometers. n.d. Retrieved 30 March 2015. 
  39. ^ "Age of the Earth". U.S. Geological Survey. 1997. Archived from the original on 23 December 2005. Retrieved 2006-01-10. 
  40. ^ Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Special Publications, Geological Society of London 190 (1): 205–221. Bibcode:2001GSLSP.190..205D. doi:10.1144/GSL.SP.2001.190.01.14. 
  41. ^ Manhesa, Gérard; Allègre, Claude J.; Dupréa, Bernard & Hamelin, Bruno (1980). "Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics". Earth and Planetary Science Letters 47 (3): 370–382. Bibcode:1980E&PSL..47..370M. doi:10.1016/0012-821X(80)90024-2. 
  42. ^ a b See:
  43. ^ 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. 
  44. ^ National Oceanic and Atmospheric Administration. "Ocean". NOAA.gov. Retrieved 3 May 2013. 
  45. ^ Yoder, Charles F. (1995). T. J. Ahrens, ed. Global Earth Physics: A Handbook of Physical Constants. Washington: American Geophysical Union. p. 8. ISBN 0-87590-851-9. Retrieved 2007-03-17. 
  46. ^ Bowring, S.; Housh, T. (1995). "The Earth's early evolution". Science 269 (5230): 1535–40. Bibcode:1995Sci...269.1535B. doi:10.1126/science.7667634. PMID 7667634. 
  47. ^ Yin, Qingzhu; Jacobsen, S. B.; Yamashita, K.; Blichert-Toft, J.; Télouk, P.; Albarède, F. (2002). "A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites". Nature 418 (6901): 949–52. Bibcode:2002Natur.418..949Y. doi:10.1038/nature00995. PMID 12198540. 
  48. ^ Kleine, Thorsten; Palme, Herbert; Mezger, Klaus; Halliday, Alex N. (2005-11-24). "Hf-W Chronometry of Lunar Metals and the Age and Early Differentiation of the Moon". Science 310 (5754): 1671–74. Bibcode:2005Sci...310.1671K. doi:10.1126/science.1118842. PMID 16308422. 
  49. ^ Reilly, Michael (October 22, 2009). "Controversial Moon Origin Theory Rewrites History". Archived from the original on 2010-01-09. Retrieved 2010-01-30. 
  50. ^ Canup, R. M.; Asphaug, E. (2001). An impact origin of the Earth-Moon system. American Geophysical Union, Fall Meeting 2001. Abstract #U51A-02. Bibcode:2001AGUFM.U51A..02C. 
  51. ^ Canup, R.; Asphaug, E. (2001). "Origin of the Moon in a giant impact near the end of the Earth's formation". Nature 412 (6848): 708–12. Bibcode:2001Natur.412..708C. doi:10.1038/35089010. PMID 11507633. 
  52. ^ Morbidelli, A. et al. (2000). "Source regions and time scales for the delivery of water to Earth". Meteoritics & Planetary Science 35 (6): 1309–20. Bibcode:2000M&PS...35.1309M. doi:10.1111/j.1945-5100.2000.tb01518.x. 
  53. ^ Guinan, E. F.; Ribas, I. Benjamin Montesinos, Alvaro Gimenez and Edward F. Guinan, ed. Our Changing Sun: The Role of Solar Nuclear Evolution and Magnetic Activity on Earth's Atmosphere and Climate. ASP Conference Proceedings: The Evolving Sun and its Influence on Planetary Environments (San Francisco: Astronomical Society of the Pacific). Bibcode:2002ASPC..269...85G. ISBN 1-58381-109-5. 
  54. ^ Staff (March 4, 2010). "Oldest measurement of Earth's magnetic field reveals battle between Sun and Earth for our atmosphere". Physorg.news. Retrieved 2010-03-27. 
  55. ^ Rogers, John James William; Santosh, M. (2004). Continents and Supercontinents. Oxford University Press US. p. 48. ISBN 0-19-516589-6. 
  56. ^ Hurley, P. M.; Rand, J. R. (Jun 1969). "Pre-drift continental nuclei". Science 164 (3885): 1229–42. Bibcode:1969Sci...164.1229H. doi:10.1126/science.164.3885.1229. PMID 17772560. 
  57. ^ De Smet, J.; Van Den Berg, A.P.; Vlaar, N.J. (2000). "Early formation and long-term stability of continents resulting from decompression melting in a convecting mantle". Tectonophysics 322 (1–2): 19. Bibcode:2000Tectp.322...19D. doi:10.1016/S0040-1951(00)00055-X. 
  58. ^ Armstrong, R. L. (1968). "A model for the evolution of strontium and lead isotopes in a dynamic earth". Reviews of Geophysics 6 (2): 175–99. Bibcode:1968RvGSP...6..175A. doi:10.1029/RG006i002p00175. 
  59. ^ Harrison, T. et al. (December 2005). "Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 ga". Science 310 (5756): 1947–50. Bibcode:2005Sci...310.1947H. doi:10.1126/science.1117926. PMID 16293721. 
  60. ^ Hong, D.; Zhang, Jisheng; Wang, Tao; Wang, Shiguang; Xie, Xilin (2004). "Continental crustal growth and the supercontinental cycle: evidence from the Central Asian Orogenic Belt". Journal of Asian Earth Sciences 23 (5): 799. Bibcode:2004JAESc..23..799H. doi:10.1016/S1367-9120(03)00134-2. 
  61. ^ Armstrong, R. L. (1991). "The persistent myth of crustal growth". Australian Journal of Earth Sciences 38 (5): 613–30. Bibcode:1991AuJES..38..613A. doi:10.1080/08120099108727995. 
  62. ^ Murphy, J. B.; Nance, R. D. (1965). "How do supercontinents assemble?". American Scientist 92 (4): 324–33. doi:10.1511/2004.4.324. 
  63. ^ Staff. "Paleoclimatology – The Study of Ancient Climates". Page Paleontology Science Center. Retrieved 2007-03-02. 
  64. ^ 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 2011-01-31. 
  65. ^ a b c Zimmer, Carl (3 October 2013). "Earth’s Oxygen: A Mystery Easy to Take for Granted". New York Times. Retrieved 3 October 2013. 
  66. ^ Berkner, L. V.; Marshall, L. C. (1965). "On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere". Journal of Atmospheric Sciences 22 (3): 225–61. Bibcode:1965JAtS...22..225B. doi:10.1175/1520-0469(1965)022<0225:OTOARO>2.0.CO;2. 
  67. ^ Burton, Kathleen (2002-11-29). "Astrobiologists Find Evidence of Early Life on Land". NASA. Retrieved 2007-03-05. 
  68. ^ Yoko Ohtomo, Takeshi Kakegawa, Akizumi Ishida, Toshiro Nagase, Minik T. Rosing (8 December 2013). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. doi:10.1038/ngeo2025. Retrieved 9 Dec 2013. 
  69. ^ Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". Associated Press. Retrieved 15 November 2013. 
  70. ^ Noffke, Nora; Christian, Daniel; Wacey, David; Hazen, Robert M. (8 November 2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia". Astrobiology (journal) 13 (12): 1103–24. Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030. PMC 3870916. PMID 24205812. Retrieved 15 November 2013. 
  71. ^ Kirschvink, J. L. (1992). Schopf, J.W.; Klein, C. and 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. 
  72. ^ Raup, D. M.; Sepkoski Jr, J. J. (1982). "Mass Extinctions in the Marine Fossil Record". Science 215 (4539): 1501–03. Bibcode:1982Sci...215.1501R. doi:10.1126/science.215.4539.1501. PMID 17788674. 
  73. ^ Gould, Stephan J. (October 1994). "The Evolution of Life on Earth". Scientific American 271 (4): 84–91. doi:10.1038/scientificamerican1094-84. PMID 7939569. Retrieved 2007-03-05. 
  74. ^ Wilkinson, B. H.; McElroy, B. J. (2007). "The impact of humans on continental erosion and sedimentation". Bulletin of the Geological Society of America 119 (1–2): 140–56. Bibcode:2007GSAB..119..140W. doi:10.1130/B25899.1. Retrieved 2007-04-22. 
  75. ^ a b c Britt, Robert (2000-02-25). "Freeze, Fry or Dry: How Long Has the Earth Got?". 
  76. ^ a b Carrington, Damian (2000-02-21). "Date set for desert Earth". BBC News. Retrieved 2007-03-31. 
  77. ^ a b Li, King-Fai; Pahlevan, Kaveh; Kirschvink, Joseph L.; Yung, Yuk L. (2009). "Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere" (PDF). Proceedings of the National Academy of Sciences 106 (24): 9576–79. Bibcode:2009PNAS..106.9576L. doi:10.1073/pnas.0809436106. PMC 2701016. PMID 19487662. Retrieved 2009-07-19. 
  78. ^ a b c Sackmann, I.-J.; Boothroyd, A. I.; Kraemer, K. E. (1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457–68. Bibcode:1993ApJ...418..457S. doi:10.1086/173407. 
  79. ^ Kasting, J.F. (1988). "Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus". Icarus 74 (3): 472–94. Bibcode:1988Icar...74..472K. doi:10.1016/0019-1035(88)90116-9. PMID 11538226. 
  80. ^ a b Ward, Peter D.; Brownlee, Donald (2002). The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of Our World. New York: Times Books, Henry Holt and Company. ISBN 0-8050-6781-7. 
  81. ^ 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 2009-07-03. 
  82. ^ 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. arXiv:0801.4031. Bibcode:2008MNRAS.386..155S. doi:10.1111/j.1365-2966.2008.13022.x. 
    See also Palmer, Jason (2008-02-22). "Hope dims that Earth will survive Sun's death". NewScientist.com news service. Archived from the original on 2012-04-15. Retrieved 2008-03-24. 
  83. ^ a b c d e f g h i Oxford English Dictionary, 3rd ed. "earth, n.¹" Oxford University Press (Oxford), 2010.
  84. ^ a b Beowulf. Trans. Chad Matlick in "Beowulf: Lines 1399 to 1799". West Virginia University. Accessed 5 Aug 2014. (Old English) & (English)
  85. ^ Mounce Reverse-Intralinear New Testament: "Luke 13:7". Hosted at Bible Gateway. 2014. Accessed 5 Aug 2014. (Ancient Greek) & (English)
  86. ^ 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 Aug 2014. (Old English)
  87. ^ King James Version of the Bible: "Genesis 1:10". Hosted at Bible Gateway. 2014. Accessed 5 August 2014.
  88. ^ Mounce Reverse-Intralinear New Testament: "Matthew 28:18". Hosted at Bible Gateway. 2014. Accessed 5 Aug 2014. (Ancient Greek) & (English)
  89. ^ "Genesis A". Hosted at the Dept. of Linguistic Studies at the University of Padua. Accessed 5 August 2014. (Old English)
  90. ^ Killings, Douglas. Codex Junius 11, I.ii. 1996. Hosted at Project Gutenberg. Accessed 5 August 2014.
  91. ^ Ælfric, Abbot of Eynsham. "De temporibus annis" Trans. P. Baker as "On the Seasons of the Year". Hosted at Old English at the University of Virginia, 1998. Accessed 6 August 2014.
  92. ^ Tacitus. Germania, Ch. 40.
  93. ^ Simek, Rudolf. Trans. Angela Hall as Dictionary of Northern Mythology, p. 179. D.S. Brewer, 2007. ISBN 0-85991-513-1.
  94. ^ The New Oxford Dictionary of English, 1st ed. "earth". Oxford University Press (Oxford), 1998. ISBN 0-19-861263-X.
  95. ^ Milbert, D. G.; Smith, D. A. "Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model". National Geodetic Survey, NOAA. Retrieved 2007-03-07. 
  96. ^ a b Sandwell, D. T.; Smith, W. H. F. (2006-07-07). "Exploring the Ocean Basins with Satellite Altimeter Data". NOAA/NGDC. Retrieved 2007-04-21. 
  97. ^ a b "The 'Highest' Spot on Earth". Npr.org. 7 April 2007. Retrieved 31 July 2012. 
  98. ^ Senne, Joseph H. (2000). "Did Edmund Hillary Climb the Wrong Mountain". Professional Surveyor 20 (5): 16–21. 
  99. ^ Sharp, David (2005-03-05). "Chimborazo and the old kilogram". The Lancet 365 (9462): 831–32. doi:10.1016/S0140-6736(05)71021-7. PMID 15752514. 
  100. ^ "Tall Tales about Highest Peaks". Australian Broadcasting Corporation. Retrieved 2008-12-29. 
  101. ^ Mohr, P. J.; Taylor, B. N. (October 2000). "Unit of length (meter)". NIST Reference on Constants, Units, and Uncertainty. NIST Physics Laboratory. Retrieved 2007-04-23. 
  102. ^ "Is a Pool Ball Smoother than the Earth?" (PDF). Billiards Digest. 1 June 2013. Retrieved 26 November 2014. 
  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).
  104. ^ Morgan, J. W.; Anders, E. (1980). "Chemical composition of Earth, Venus, and Mercury". Proceedings of the National Academy of Sciences 77 (12): 6973–77. Bibcode:1980PNAS...77.6973M. doi:10.1073/pnas.77.12.6973. PMC 350422. PMID 16592930. 
  105. ^ Public Domain One or more of the preceding sentences incorporates text from a publication now in the public domainChisholm, Hugh, ed. (1911). "Petrology". Encyclopædia Britannica (11th ed.). Cambridge University Press. 
  106. ^ Tanimoto, Toshiro (1995). Thomas J. Ahrens, ed. Crustal Structure of the Earth (PDF). Washington, DC: American Geophysical Union. ISBN 0-87590-851-9. Archived from the original (PDF) on 2006-10-16. Retrieved 2007-02-03. 
  107. ^ Kerr, Richard A. (2005-09-26). "Earth's Inner Core Is Running a Tad Faster Than the Rest of the Planet". Science 309 (5739): 1313. doi:10.1126/science.309.5739.1313a. PMID 16123276. 
  108. ^ Jordan, T. H. (1979). "Structural geology of the Earth's interior". Proceedings of the National Academy of Sciences of the United States of America 76 (9): 4192–4200. Bibcode:1979PNAS...76.4192J. doi:10.1073/pnas.76.9.4192. PMC 411539. PMID 16592703. 
  109. ^ Robertson, Eugene C. (2001-07-26). "The Interior of the Earth". USGS. Retrieved 2007-03-24. 
  110. ^ a b Turcotte, D. L.; Schubert, G. (2002). "4". Geodynamics (2 ed.). Cambridge, England, UK: Cambridge University Press. pp. 136–37. ISBN 978-0-521-66624-4. 
  111. ^ Sanders, Robert (2003-12-10). "Radioactive potassium may be major heat source in Earth's core". UC Berkeley News. Retrieved 2007-02-28. 
  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. 
  113. ^ Alfè, D.; Gillan, M. J.; Vocadlo, L.; Brodholt, J.; Price, G. D. (2002). "The ab initio simulation of the Earth's core" (PDF). Philosophical Transactions of the Royal Society 360 (1795): 1227–44. Bibcode:2002RSPTA.360.1227A. doi:10.1098/rsta.2002.0992. Retrieved 2007-02-28. 
  114. ^ Vlaar, N; Vankeken, P.; Vandenberg, A. (1994). "Cooling of the Earth in the Archaean: Consequences of pressure-release melting in a hotter mantle" (PDF). Earth and Planetary Science Letters 121 (1–2): 1. Bibcode:1994E&PSL.121....1V. doi:10.1016/0012-821X(94)90028-0. 
  115. ^ Turcotte, D. L.; Schubert, G. (2002). "4". Geodynamics (2 ed.). Cambridge, England, UK: Cambridge University Press. p. 137. ISBN 978-0-521-66624-4. 
  116. ^ Pollack, Henry N.; Hurter, Suzanne J.; Johnson, Jeffrey R. (August 1993). "Heat flow from the Earth's interior: Analysis of the global data set". Reviews of Geophysics 31 (3): 267–80. Bibcode:1993RvGeo..31..267P. doi:10.1029/93RG01249. Archived from the original on 9 January 2014. 
  117. ^ Richards, M. A.; Duncan, R. A.; Courtillot, V. E. (1989). "Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails". Science 246 (4926): 103–07. Bibcode:1989Sci...246..103R. doi:10.1126/science.246.4926.103. PMID 17837768. 
  118. ^ Sclater, John G; Parsons, Barry; Jaupart, Claude (1981). "Oceans and Continents: Similarities and Differences in the Mechanisms of Heat Loss". Journal of Geophysical Research 86 (B12): 11535. Bibcode:1981JGR....8611535S. doi:10.1029/JB086iB12p11535. 
  119. ^ Brown, W. K.; Wohletz, K. H. (2005). "SFT and the Earth's Tectonic Plates". Los Alamos National Laboratory. Retrieved 2007-03-02. 
  120. ^ Kious, W. J.; Tilling, R. I. (1999-05-05). "Understanding plate motions". USGS. Retrieved 2007-03-02. 
  121. ^ Seligman, Courtney (2008). "The Structure of the Terrestrial Planets". Online Astronomy eText Table of Contents. cseligman.com. Retrieved 2008-02-28. 
  122. ^ Duennebier, Fred (1999-08-12). "Pacific Plate Motion". University of Hawaii. Retrieved 2007-03-14. 
  123. ^ Mueller, R. D. et al. (2007-03-07). "Age of the Ocean Floor Poster". NOAA. Retrieved 2007-03-14. 
  124. ^ Bowring, Samuel A.; Williams, Ian S. (1999). "Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada". Contributions to Mineralogy and Petrology 134 (1): 3. Bibcode:1999CoMP..134....3B. doi:10.1007/s004100050465. 
  125. ^ Meschede, Martin; Barckhausen, Udo (2000-11-20). "Plate Tectonic Evolution of the Cocos-Nazca Spreading Center". Proceedings of the Ocean Drilling Program. Texas A&M University. Retrieved 2007-04-02. 
  126. ^ Staff. "GPS Time Series". NASA JPL. Retrieved 2007-04-02. 
  127. ^ "CIA – The World Factbook". Cia.gov. Retrieved 2 November 2012. 
  128. ^ Kring, David A. "Terrestrial Impact Cratering and Its Environmental Effects". Lunar and Planetary Laboratory. Retrieved 2007-03-22. 
  129. ^ Staff. "Layers of the Earth". Volcano World. Archived from the original on 2013-01-19. Retrieved 2007-03-11. 
  130. ^ Jessey, David. "Weathering and Sedimentary Rocks". Cal Poly Pomona. Archived from the original on 2007-07-21. Retrieved 2007-03-20. 
  131. ^ de Pater, Imke; Lissauer, Jack J. (2010). Planetary Sciences (2nd ed.). Cambridge University Press. p. 154. ISBN 0-521-85371-0. 
  132. ^ Wenk, Hans-Rudolf; Bulakh, Andreĭ Glebovich (2004). Minerals: their constitution and origin. Cambridge University Press. p. 359. ISBN 0-521-52958-1. 
  133. ^ 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. 
  134. ^ Sverdrup, H. U.; Fleming, Richard H. (1942-01-01). The oceans, their physics, chemistry, and general biology. Scripps Institution of Oceanography Archives. ISBN 0-13-630350-1. Retrieved 2008-06-13. 
  135. ^ "7,000 m Class Remotely Operated Vehicle KAIKO 7000". Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Retrieved 2008-06-07. 
  136. ^ Charette, Matthew A.; Smith, Walter H. F. (June 2010). "The Volume of Earth's Ocean" (PDF). Oceanography 23 (2): 112–14. doi:10.5670/oceanog.2010.51. Retrieved 2013-06-06. 
  137. ^ "sphere depth of the ocean – hydrology". Encyclopedia Britannica. Retrieved 12 April 2015. 
  138. ^ "Third rock from the Sun – restless Earth". NASA's Cosmos. Retrieved 12 April 2015. 
  139. ^ Perlman, Howard (17 March 2014). "The World's Water". USGS Water-Science School. Retrieved 12 April 2015. 
  140. ^ Kennish, Michael J. (2001). Practical handbook of marine science. Marine science series (3rd ed.). CRC Press. p. 35. ISBN 0-8493-2391-6. 
  141. ^ Mullen, Leslie (2002-06-11). "Salt of the Early Earth". NASA Astrobiology Magazine. Archived from the original on 2007-07-22. Retrieved 2007-03-14. 
  142. ^ Morris, Ron M. "Oceanic Processes". NASA Astrobiology Magazine. Retrieved 2007-03-14. 
  143. ^ Scott, Michon (2006-04-24). "Earth's Big heat Bucket". NASA Earth Observatory. Retrieved 2007-03-14. 
  144. ^ Sample, Sharron (2005-06-21). "Sea Surface Temperature". NASA. Retrieved 2007-04-21. 
  145. ^ Geerts, B.; Linacre, E. (November 1997). "The height of the tropopause". Resources in Atmospheric Sciences. University of Wyoming. Retrieved 2006-08-10. 
  146. ^ a b Staff (2003-10-08). "Earth's Atmosphere". NASA. Retrieved 2007-03-21. 
  147. ^ Pidwirny, Michael (2006). "Fundamentals of Physical Geography (2nd Edition)". PhysicalGeography.net. Retrieved 2007-03-19. 
  148. ^ a b Moran, Joseph M. (2005). "Weather". World Book Online Reference Center. NASA/World Book, Inc. Archived from the original on 2013-03-10. Retrieved 2007-03-17. 
  149. ^ a b Berger, Wolfgang H. (2002). "The Earth's Climate System". University of California, San Diego. Retrieved 2007-03-24. 
  150. ^ Rahmstorf, Stefan (2003). "The Thermohaline Ocean Circulation". Potsdam Institute for Climate Impact Research. Retrieved 2007-04-21. 
  151. ^ Various (1997-07-21). "The Hydrologic Cycle". University of Illinois. Retrieved 2007-03-24. 
  152. ^ 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. 
  153. ^ Staff. "Climate Zones". UK Department for Environment, Food and Rural Affairs. Archived from the original on 2010-08-08. Retrieved 2007-03-24. 
  154. ^ "Earth at Aphelion". Space Weather. July 2008. Retrieved 7 July 2015. 
  155. ^ "Earth at Aphelion". Space Weather. July 2008. Retrieved 7 July 2015. 
  156. ^ "Why U.S. East Coast is colder than Europe's West Coast". Live Science. 5 April 2011. Retrieved 7 July 2015. 
  157. ^ "Highest recorded temperature". Guinness World Records. Retrieved 12 July 2015. 
  158. ^ Lyons, Walter A (1997). The Handy Weather Answer Book (2nd ed.). Detroit, Michigan: Visible Ink Press. ISBN 0-7876-1034-8. 
  159. ^ "Coldest temperature ever recorded on Earth in Antarctica". The Guardian. 10 December 2013. Retrieved 12 July 2015. 
  160. ^ Staff (2004). "Stratosphere and Weather; Discovery of the Stratosphere". Science Week. Archived from the original on 2007-07-13. Retrieved 2007-03-14. 
  161. ^ de Córdoba, S. Sanz Fernández (2004-06-21). "Presentation of the Karman separation line, used as the boundary separating Aeronautics and Astronautics". Fédération Aéronautique Internationale. Archived from the original on 2010-01-17. Retrieved 2007-04-21. 
  162. ^ Liu, S. C.; Donahue, T. M. (1974). "The Aeronomy of Hydrogen in the Atmosphere of the Earth". Journal of Atmospheric Sciences 31 (4): 1118–36. Bibcode:1974JAtS...31.1118L. doi:10.1175/1520-0469(1974)031<1118:TAOHIT>2.0.CO;2. 
  163. ^ Catling, David C.; Zahnle, Kevin J.; McKay, Christopher P. (2001). "Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth". Science 293 (5531): 839–43. Bibcode:2001Sci...293..839C. doi:10.1126/science.1061976. PMID 11486082. 
  164. ^ Abedon, Stephen T. (1997-03-31). "History of Earth". Ohio State University. Archived from the original on 2013-03-10. Retrieved 2007-03-19. 
  165. ^ Hunten, D. M.; Donahue, T. M (1976). "Hydrogen loss from the terrestrial planets". Annual review of earth and planetary sciences 4 (1): 265–92. Bibcode:1976AREPS...4..265H. doi:10.1146/annurev.ea.04.050176.001405. 
  166. ^ Lang, Kenneth R. (2003). The Cambridge guide to the solar system. Cambridge University Press. p. 92. ISBN 0-521-81306-9. 
  167. ^ Fitzpatrick, Richard (2006-02-16). "MHD dynamo theory". NASA WMAP. Retrieved 2007-02-27. 
  168. ^ Campbell, Wallace Hall (2003). Introduction to Geomagnetic Fields. New York: Cambridge University Press. p. 57. ISBN 0-521-82206-8. 
  169. ^ Stern, David P. (2005-07-08). "Exploration of the Earth's Magnetosphere". NASA. Retrieved 2007-03-21. 
  170. ^ McCarthy, Dennis D.; Hackman, Christine; Nelson, Robert A. (November 2008). "The Physical Basis of the Leap Second". The Astronomical Journal 136 (5): 1906–08. Bibcode:2008AJ....136.1906M. doi:10.1088/0004-6256/136/5/1906. 
  171. ^ "Leap seconds". Time Service Department, USNO. Archived from the original on 2015-03-12. Retrieved 2008-09-23. 
  172. ^ "Rapid Service/Prediction of Earth Orientation" (.DAT file (displays as plaintext in browser)). IERS Bulletin-A 28 (15). 9 April 2015. Retrieved 12 April 2015. 
  173. ^ Seidelmann, P. Kenneth (1992). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 48. ISBN 0-935702-68-7. 
  174. ^ Staff. "IERS Excess of the duration of the day to 86400s ... since 1623". International Earth Rotation and Reference Systems Service (IERS). Retrieved 2008-09-23. —Graph at end.
  175. ^ Staff. "IERS Variations in the duration of the day 1962–2005". International Earth Rotation and Reference Systems Service (IERS). Archived from the original on 2007-08-13. Retrieved 2008-09-23. 
  176. ^ Zeilik, M.; Gregory, S. A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. p. 56. ISBN 0-03-006228-4. 
  177. ^ a b Williams, David R. (2006-02-10). "Planetary Fact Sheets". NASA. Retrieved 2008-09-28. —See the apparent diameters on the Sun and Moon pages.
  178. ^ Williams, David R. (2004-09-01). "Moon Fact Sheet". NASA. Retrieved 2007-03-21. 
  179. ^ Vázquez, M.; Rodríguez, P. Montañés; Palle, E. (2006). "The Earth as an Object of Astrophysical Interest in the Search for Extrasolar Planets" (PDF). Instituto de Astrofísica de Canarias. Retrieved 2007-03-21. 
  180. ^ Astrophysicist team (2005-12-01). "Earth's location in the Milky Way". NASA. Retrieved 2008-06-11. 
  181. ^ Bromberg, Irv (2008-05-01). "The Lengths of the Seasons (on Earth)". University of Toronto. Retrieved 2008-11-08. 
  182. ^ Lin, Haosheng (2006). "Animation of precession of moon orbit". Survey of Astronomy AST110-6. University of Hawaii at Manoa. Retrieved 2010-09-10. 
  183. ^ Fisher, Rick (1996-02-05). "Earth Rotation and Equatorial Coordinates". National Radio Astronomy Observatory. Retrieved 2007-03-21. 
  184. ^ Williams, Jack (2005-12-20). "Earth's tilt creates seasons". USAToday. Retrieved 2007-03-17. 
  185. ^ Staff (September 2003). "Astrobiology Roadmap". NASA, Lockheed Martin. Archived from the original on 2012-03-11. Retrieved 2007-03-10. 
  186. ^ Dole, Stephen H. (1970). Habitable Planets for Man (2nd ed.). American Elsevier Publishing Co. ISBN 0-444-00092-5. Retrieved 2007-03-11. 
  187. ^ Hillebrand, Helmut (2004). "On the Generality of the Latitudinal Gradient". American Naturalist 163 (2): 192–211. doi:10.1086/381004. PMID 14970922. 
  188. ^ 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.
  189. ^ Staff (2006-11-24). "Mineral Genesis: How do minerals form?". Non-vertebrate Paleontology Laboratory, Texas Memorial Museum. Retrieved 2007-04-01. 
  190. ^ Rona, Peter A. (2003). "Resources of the Sea Floor". Science 299 (5607): 673–74. doi:10.1126/science.1080679. PMID 12560541. Retrieved 2007-02-04. 
  191. ^ Turner, B. L., II (1990). The Earth As Transformed by Human Action: Global And Regional Changes in the Biosphere Over the Past 300 Years. CUP Archive. p. 164. ISBN 0521363578. 
  192. ^ Walsh, Patrick J. (1997-05-16). 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. 
  193. ^ Staff (2007-02-02). "Evidence is now 'unequivocal' that humans are causing global warming – UN report". United Nations. Archived from the original on 21 December 2008. Retrieved 2007-03-07. 
  194. ^ World at the Xpeditions Atlas, National Geographic Society, Washington D.C., 2006.
  195. ^ "Various '7 billionth' babies celebrated worldwide". Retrieved 2011-10-31. 
  196. ^ Staff. "World Population Prospects: The 2006 Revision". United Nations. Archived from the original on 5 September 2009. Retrieved 2007-03-07. 
  197. ^ Staff (2007). "Human Population: Fundamentals of Growth: Growth". Population Reference Bureau. Archived from the original on 2013-02-10. Retrieved 2007-03-31. 
  198. ^ Peel, M. C.; Finlayson, B. L.; McMahon, T. A. (2007). "Updated world map of the Köppen-Geiger climate classification". Hydrology and Earth System Sciences Discussions 4 (2): 439–73. doi:10.5194/hessd-4-439-2007. Retrieved 2007-03-31. 
  199. ^ Staff. "Themes & Issues". Secretariat of the Convention on Biological Diversity. Retrieved 2007-03-29. 
  200. ^ Staff (2006-08-15). "Canadian Forces Station (CFS) Alert". Information Management Group. Retrieved 2007-03-31. 
  201. ^ Kennedy, Paul (1989). The Rise and Fall of the Great Powers (1st ed.). Vintage. ISBN 0-679-72019-7. 
  202. ^ "U.N. Charter Index". United Nations. Archived from the original on 20 February 2009. Retrieved 2008-12-23. 
  203. ^ Staff. "International Law". United Nations. Archived from the original on 31 December 2009. Retrieved 2007-03-27. 
  204. ^ 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. 
  205. ^ Ellis, Lee (2004). Who's who of NASA Astronauts. Americana Group Publishing. ISBN 0-9667961-4-4. 
  206. ^ Shayler, David; Vis, Bert (2005). Russia's Cosmonauts: Inside the Yuri Gagarin Training Center. Birkhäuser. ISBN 0-387-21894-7. 
  207. ^ Wade, Mark (2008-06-30). "Astronaut Statistics". Encyclopedia Astronautica. Retrieved 2008-12-23. 
  208. ^ "Reference Guide to the International Space Station". NASA. 2007-01-16. Retrieved 2008-12-23. 
  209. ^ Cramb, Auslan (2007-10-28). "Nasa's Discovery extends space station". Telegraph. Retrieved 2009-03-23. 
  210. ^ Espenak, F.; Meeus, J. (2007-02-07). "Secular acceleration of the Moon". NASA. Archived from the original on 2011-08-22. Retrieved 2007-04-20. 
  211. ^ Poropudas, Hannu K. J. (1991-12-16). "Using Coral as a Clock". Skeptic Tank. Retrieved 2007-04-20. 
  212. ^ Laskar, J. et al. (2004). "A long-term numerical solution for the insolation quantities of the Earth". Astronomy and Astrophysics 428 (1): 261–85. Bibcode:2004A&A...428..261L. doi:10.1051/0004-6361:20041335. 
  213. ^ Murray, N.; Holman, M. (2001). "The role of chaotic resonances in the solar system". Nature 410 (6830): 773–79. arXiv:astro-ph/0111602. doi:10.1038/35071000. PMID 11298438. 
  214. ^ Canup, R.; Asphaug, E. (2001). "Origin of the Moon in a giant impact near the end of the Earth's formation". Nature 412 (6848): 708–12. Bibcode:2001Natur.412..708C. doi:10.1038/35089010. PMID 11507633. 
  215. ^ Whitehouse, David (2002-10-21). "Earth's little brother found". BBC News. Retrieved 2007-03-31. 
  216. ^ Christou, Apostolos A.; Asher, David J. (March 31, 2011). "A long-lived horseshoe companion to the Earth". arXiv:1104.0036 [astro-ph.EP].  See table 2, p. 5.
  217. ^ Connors, Martin; Wiegert, Paul; Veillet, Christian (July 27, 2011). "Earth's Trojan asteroid". Nature 475 (7357): 481–83. Bibcode:2011Natur.475..481C. doi:10.1038/nature10233. PMID 21796207. Retrieved 2011-07-27. 
  218. ^ Choi, Charles Q. (July 27, 2011). "First Asteroid Companion of Earth Discovered at Last". Space.com. Retrieved 2011-07-27. 
  219. ^ "2006 RH120 ( = 6R10DB9) (A second moon for the Earth?)". Great Shefford Observatory. Great Shefford Observatory. Archived from the original on 6 February 2015. Retrieved 17 July 2015. 
  220. ^ "UCS Satellite Database". Nuclear Weapons & Global Security. Union of Concerned Scientists. January 31, 2011. Retrieved 2011-05-12. 
  221. ^ Liungman, Carl G. (2004). "Group 29: Multi-axes symmetric, both soft and straight-lined, closed signs with crossing lines". Symbols – Encyclopedia of Western Signs and Ideograms. New York: Ionfox AB. pp. 281–82. ISBN 91-972705-0-4. 
  222. ^ Lovelock, James. The Vanishing Face of Gaia. Basic Books, 2009, p. 255. ISBN 978-0-465-01549-8
  223. ^ Lovelock, J.E. (1972). "Gaia as seen through the atmosphere". Atmospheric Environment (Elsevier) 6 (8): 579–580. doi:10.1016/0004-6981(72)90076-5. ISSN 1352-2310. 
  224. ^ 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. 
  225. ^ Russell, Jeffrey B. "The Myth of the Flat Earth". American Scientific Affiliation. Retrieved 2007-03-14. ; but see also Cosmas Indicopleustes.
  226. ^ Godwin, William (1876). "Lives of the Necromancers". p. 49. 
  227. ^ Arnett, Bill (July 16, 2006). "Earth". The Nine Planets, A Multimedia Tour of the Solar System: one star, eight planets, and more. Retrieved 2010-03-09. 
  228. ^ Neil deGrasse Tyson (2012) Why Space Matters

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