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Earth is the fifth largest planet in the solar system.

Earth is a solar system planet. Earth contains an abundance of life. Earth, is often referred to as the Earth, or Planet Earth). Earth is the third planet in the solar system in terms of distance from the Sun. Earth is also the largest of its planetary system's terrestrial planets, making it the largest solid body in the solar system, and it is the only place in the universe known to support life. It is also the densest planet in the solar system. The widely accepted scientific theory states that the Earth was formed around 4.57 billion years ago and its natural satellite, the Moon, was orbiting it shortly thereafter, around 4.53 billion years ago.

Earth: planet. Earth symbol..
Earth.
Earth: The Blue Marble, taken from Apollo 17.
Orbital characteristics (Epoch J2000)
Aphelion 152,097,701 km
(1.016 710 333 5 AU)
Perihelion 147,098,074 km
(0.983 289 891 2 AU)
Semi-major axis 149,597,887.5 km
(1.000 000 112 4 AU)
Semi-minor axis 149,576,999.826 km
(0.999 860 486 9 AU)
Orbital circumference 924,375,700 km
( 6.179 069 900 7 AU)
Orbital eccentricity 0.016 710 219
Sidereal orbit period 365.256 366 d
(1.000 017 5 a)
Synodic period n/a
Max. orbital speed 30.287 km/s
(109,033 km/h)
Average orbital speed 29.783 km/s
(107,218 km/h)
Min. orbital speed 29.291 km/s
(105,448 km/h)
Orbital inclination to ecliptic 0
(7.25 to Sun's equator)
Longitude of the ascending node 348.739 36
Argument of the perihelion 114.207 83
Satellites 1 (the Moon)
(see also 3753 Cruithne)
Physical characteristics of Earth.
Aspect Ratio 0.996 647 1
Ellipticity 0.003 352 9
Equatorial radius 6,378.137 km
Polar radius 6,356.752 km
Mean radius 6,372.797 km
Equatorial circumference 40,075.02 km
Meridional circumference 40,007.86 km
Mean circumference 40,041.47 km
Surface area 510,065,600 km
Land area 148,939,100 km (29.2 %)
Water area 361,126,400 km (70.8 %)
Volume 1.083 207 31012 km
mass 5.97421024 kg
density 5,515.3 kg/m
Equatorial surface gravity 9.780 1 m/s
(0.997 32 g)
escape velocity 11.186 km/s
Sidereal rotation period 0.997 258 d (23.934 h)
Rotational velocity at equator 465.11 m/s
axial tilt 23.439 281
Right ascension of North pole 0 (0 h 0 min 0 s)
Declination +90
Albedo 0.367
Surface temperature 185 K (-88 C) min
287 K (14 C) mean
331 K (58 C) max
Surface pressure 101.3 kPa (MSL)
Adjective Terrestrial, Terran, Telluric, Tellurian, Earthly, Earthling (lifeforms)
Atmospheric constituents of Earth.
Nitrogen 78.08 %
Oxygen 20.94 %
Argon 0.93 %
carbon dioxide 0.038%
water vapor Trace (varies with climate)

Since it formed, the Earth has changed through geological and biological processes that have hidden traces of the original conditions. The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convecting yet solid Earth mantle and an iron core that generates a magnetic field. Its atmospheric conditions have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered in salt-water oceans, and the remainder consists of continents and islands.

There is significant interaction between the Earth and its space environment. The relatively large moon provides ocean tides and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet is believed to have played a role in the formation of the oceans. Later, asteroid impacts are understood to have caused significant changes to the surface environment. Changes in the orbit of the planet may also be responsible for the ice ages that have covered significant portions of the surface in glacial sheets.

The Earth's only natural orbiting body is the Moon, although the asteroid Cruithne has been erroneously described as such. Cruithne was discovered in 1986 and follows an elliptical orbit around the Sun at about the same average orbital radius as the Earth. However, from the point of view of the moving Earth, Cruithne follows a horseshoe orbit around the Sun that avoids close proximity with the Earth.

Earth Lexicography.

In American English usage, the name can be capitalized or spelled in lowercase interchangeably, either when used absolutely or prefixed with "the" (i.e. Earth, the Earth, earth or the earth). Many deliberately spell the name of the planet with a capital, both as "Earth" or "the Earth". This is to distinguish it as a proper noun, distinct from the senses of the term as a count noun or verb (e.g. referring to soil, the ground, earthing in the electrical sense, etc.). Oxford Spelling recognizes the lowercase form as the most common, with the capitalized form as a variant of it. Another convention that is very common is to spell the name with a capital when occurring absolutely (e.g. Earth's atmosphere) and lowercase when preceded by "the" (e.g. the atmosphere of the earth). The term almost exclusively exists in lowercase when appearing in common phrases, even without "the" preceding it (e.g. it doesn't cost the earth; what on earth are you doing?).

Terms that refer to the Earth can use the Latin root terr-, as in terraform and terrestrial. An alternative Latin root is tellur-, which is used in words such as tellurian and tellurium. Such terms derive from Latin terra and tellus, which refer variously to the world, the element earth, the earth goddess and so forth. Scientific terms such as geography, geocentric and geothermal use the Greek prefix geo- (?a??-, gaio-), from ge (again meaning "earth"). In many science fiction books and video games, Earth is referred to as Terra or Gaia. Astronauts refer to the Earth as "Terra Firma".

The English word "earth" has cognates in many modern and ancient languages. Examples in modern tongues include aarde in Dutch and Erde in German. The root has cognates in extinct languages such as ertha in Old Saxon and ert (meaning "ground") in Middle Irish, derived from the Old English eore. All of these words derive from the Proto-Indo-European base *er-.

Several Semitic languages have words for "earth" similar to those in Indo-European languages. Arabic has ard; Akkadian, irtsitu; Aramaic, araa; Phoenician, erets (which appears in the Mesha Stele); and Hebrew, ??? (arets, or erets when not preceded by a definite article, or when followed by a noun modifier). The etymological connection between the words in Indo-European and Semitic languages are uncertain, though, and may simply be coincidence.

The standard name for people from Earth is Earthling, although Terran, Gaian, and Earther are alternate names that have been used in science fiction.

Words for Earth in other languages include: Terre (French), pr?thvi (Sanskrit), Maa (Finnish and Estonian), Pamnt (Romanian), Fld (Hungarian), Ziemia (Polish), Zemlja (Russian and Serbian), Tierra (Spanish), Terra (Italian), Diqiu (Mandarin), Deiqao (Cantonese), Jigu (Korean), Bumi (Malay), Chikyuu (Japanese), Jorden (Danish, Norwegian, Swedish), Gi, Choma (Greek), Dunia (Swahili), lem, Dünya (Arabic), Din (Kurdish), (Armenian), Jehun, Zamin (Persian), and Acun, Yeryüzü, Yerküre (Turkish)., (Hebrew)

Earth history.

Based on the available evidence, current scientists have been able to reconstruct detailed information about the planet's past. Earth is believed to have formed around 4.57 billion years ago out of the Solar nebula, along with the Sun and the other planets. Initially molten, the outer layer of the planet cooled when water began accumulating in the atmosphere when the planet was about half its current radius, resulting in the solid crust. The moon formed soon afterwards, possibly as the result of the impact with a Mars-sized object known as Theia. Outgassing and volcanic activity produced the primordial atmosphere; condensing water vapor, augmented by ice delivered by comets, produced the oceans. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life lived.

The development of photosynthesis allowed the sun's energy to be harvested directly; the resultant Oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes. Cells within colonies became increasingly specialized, resulting in true multicellular organisms. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.

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

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 lifeforms began to proliferate.

Since the Cambrian explosion, about 535 mya, there were five mass extinctions. The last occurred 65 mya, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several mya, a small African ape gained the ability to stand upright. This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short timespan as no other life form had, affecting both the nature and quantity of other life forms, and the global climate.

The shape of planet Earth.

Earth cutaway.
Earth cutaway from core to exosphere. Partially to scale.

The Earth's shape is very close to an oblate spheroid, although the precise shape (the geoid) varies from this by up to 100 meters (327 ft). The average diameter of the reference spheroid is approximately 12,742 km (more roughly, 40,000 km/p). The rotation of the Earth causes the Equator to bulge out slightly so that the equatorial diameter is 43 km larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,850 m above local sea level) and the Mariana Trench (10,924 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%. For comparison, this is less than the 0.22% tolerance allowed in billiard balls. Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.

Earth's composition.

The mass of the Earth is approximately 5980 yottagrams (5.98 1024 kg). It is composed mostly of iron (35.0%), Oxygen (28.0%), silicon (17.0%), magnesium (15.7%), nickel (1.5%), calcium (1.4%) and aluminium (1.4%).

Earth's internal structure.

The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.

The geologic component layers of the Earth are at the following depths below the surface:

DepthLayer
KilometersMiles
0-60 0-37 Lithosphere (locally varies between 5 and 200 km)
0-35 0-22 ... crust (locally varies between 5 and 70 km)
35-60 22-37 ... Uppermost part of mantle
35-2890 22-1790 mantle
100-700 62-435 ... Asthenosphere
2890-5100 1790-3160 Outer core
5100-6378 3160-3954 inner core

Earth's Tectonic plates.

Earth's major plates.
A map pointing out the Earth's major plates.

According to plate tectonics theory currently accepted by the vast majority of scientists working in this area, the outermost part of the Earth's interior is made up of two layers: the Lithosphere comprising the crust, and the solidified uppermost part of the mantle. Below the lithosphere lies the Asthenosphere, which comprises the inner, viscous part of the mantle. The mantle behaves like a superheated and extremely viscous liquid.

The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates move in relation to one another at one of three types of plate boundaries: convergent, divergent, and transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries.

The main plates are:

Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate and the Scotia Plate.

Surface of planet Earth.

Surface of the Earth.
Surface of the Earth, colors reflect changes in elevation.

The Earth's terrain varies greatly from place to place. About 70% of the surface is covered by water, with much of the continental shelf below sea level. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2500 metres (approximately 8000 ft.). The remaining 30% not covered by water consists of mountains, deserts, plains, plateaus, etc.

Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 109 acres of cropland and 8.4 109 acres of pastureland.

Earth's Extremes. Elevation extremes: (measured relative to sea level).

Earth's Hydrosphere.

surface of the Earth.
Elevation histogram of the surface of the Earth.

The abundance of water on Earth is a unique feature that distinguishes the "Blue Planet" from others in the solar system. Approximately 70.8 percent of the Earth is covered by water and only 29.2 percent is terra firma.

The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1.35 10^18 tonnes, or about 1/4400 of the total mass of the Earth.

Earth's Atmosphere.

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Further up, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Beyond these, the exosphere thins out into the magnetosphere (where the Earth's magnetic fields interact with the solar wind). An important part of the atmosphere for life on Earth is the ozone layer.

Earth's atmosphere.
Source regions of global air masses.

The Atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 6 km. It is 78% Nitrogen and 21% Oxygen, with trace amounts of other gaseous molecules such as water vapor. The atmosphere protects the Earth's life forms by absorbing ultraviolet solar radiation, moderating temperature, transporting water vapor, and providing useful gases. The atmosphere is one of the principal components in determining weather and climate.

Because Hydrogen gas is light and based on Earth's mean temperature, achieves escape velocity, unfixed hydrogen leaves the Earth. For this reason, the Earth's environment is oxidizing, with consequences for the chemical nature of life which developed on the planet.

Earth's climate.

The most prominent features of the Earth's climate are its two large polar regions, two narrow temperate zones, and a wide Equatorial tropical region. Precipitation patterns vary widely, ranging from several metres of water per year to less than a millimetre.

Ocean currents are important factors in determining climate, particularly the spectacular thermohaline circulation which distributes heat energy from the equatorial oceans to the polar regions.

The Earth's pedosphere.

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the Lithosphere, atmosphere, hydrosphere and biosphere.

Earth's biosphere.

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

The planet's lifeforms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion (3.5109) years ago. Earth is the only place in the universe officially recognized by the communities of Earth where life is absolutely known to exist, and some scientists believe that biospheres might be rare.

The biosphere is divided into a number of biomes, inhabited by broadly similar flora and fauna. On land primarily latitude and height above the sea level separates biomes. Terrestrial biomes lying within the Arctic, Antarctic Circle or in high altitudes are relatively barren of plant and animal life, while most of the more populous biomes lie near the Equator.

Earth's natural resources.

Some of these resources, such as mineral fuels, are difficult to replenish on a short time scale, called non-renewable resources. The exploitation of non-renewable resources near the surface by human civilization has become a subject of significant controversy in modern environmentalism movements.

Land use on Earth.

Humans use the Earth's land to support themselves through the production of food, energy, and building material. They also live on the land by building shelters. Human use of land is approximately:

Earth irrigated land: 2,481,250 km (1993 est.) Natural and environmental hazards.

Earth map.

Antarctica
Australia
Africa
Asia
Europe
North
America
South
America
Pacific
Ocean
Pacific
Ocean
Atlantic
Ocean
Indian
Ocean
Southern Ocean
Arctic Ocean
Middle East
Caribbean
Central
Asia
East Asia
North Asia
South
Asia
Southeast
Asia
SW.
Asia
China
Australasia
Melanesia
Micronesia
Polynesia
Central
America
Latin
America
Northern
America
Americas
C.
Africa
E.
Africa
N.
Africa
Southern
Africa
W.
Africa
C.
Europe
E.
Europe
N.
Europe
S.
Europe
W.
Europe

Large areas are subject to extreme weather such as (tropical cyclones), hurricanes, or typhoons that dominate life in those areas. Many places are subject to Earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities and disasters.

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

Long-term climate alteration from enhancement of the greenhouse effect caused by the earth itself and human industrial carbon dioxide emissions is an increasing concern, the focus of intense study and debate.

Earth's Human geography.

The Earth at night.
The Earth at night, a composite of satellite photographs showing human made illumination on the Earth's surface. Taken between October 1994 and March 1995.

Earth has approximately 6,500,000,000 human inhabitants (February 24, 2006 estimate). Projections indicate that the world's human population will reach seven billion in 2013 and 9.1 billion in 2050 (2005 UN estimates). Most of the growth is expected to take place in developing nations. Human population density varies widely around the world.

It is estimated that only one eighth of the surface of the Earth is suitable for humans to live on - three-quarters is covered by oceans, and half of the land area is desert, high mountains or other unsuitable terrain.

The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada. The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole.

There are 267 administrative divisions, including nations, dependent areas, other, and miscellaneous entries. Earth does not have a sovereign government with planet-wide authority. Independent sovereign nations claim all of the land surface except for some segments of Antarctica. There is a worldwide general international organization, the United Nations. The United Nations is primarily an international discussion forum with only limited ability to pass and enforce laws.

In total, about 400 people have been outside the Earth's atmosphere as of 2004, and of these, twelve have walked on the Moon. Most of the time the only humans in space are those on the International Space Station, currently three people who are usually replaced every 6 months. See human spaceflight.

Earth's place in the solar system.

Earth's rotation.
An animation showing the rotation of the Earth.
Earth.
Earth seen as a tiny dot by the Voyager 1 spacecraft, four billion miles from Earth.

It takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds (one sidereal day) to rotate around the axis that connects the north and the south poles. From Earth, the main apparent motion of celestial bodies in the sky (except that of meteors within the atmosphere and low-orbiting satellites) is to the west at a rate of 15 /h = 15'/min, i.e., an apparent Sun or Moon diameter every two minutes.

Earth orbits the Sun every 365.2564 mean solar days (1 sidereal year). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1 /day, i.e., a Sun or Moon diameter every 12 hours, eastward. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is enough to cover the planet's diameter (~12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.

The Moon revolves with the Earth around a common barycenter, from fixed star to fixed star, every 27.32 days. When combined with the Earth-Moon system's common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (930,000 miles) in radius. Viewed from Earth's north pole, the motion of Earth, its moon and their axial rotations are all counterclockwise. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.5 degrees against the Earth-Sun plane (which causes the seasons); and the Earth-Moon plane is tilted about 5 degrees against the Earth-Sun plane (without a tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses).

In an inertial reference frame, the Earth's axis undergoes a slow precessional motion with a period of some 25,800 years, as well as a nutation with a main period of 18.6 years. These motions are caused by the differential attraction of Sun and Moon on the Earth's equatorial bulge because of its oblateness. In a reference frame attached to the solid body of the Earth, its rotation is also slightly irregular from polar motion. The polar motion is quasi-periodic, containing an annual component and a component with a 14-month period called the Chandler wobble. In addition, the rotational velocity varies, in a phenomenon known as length of day variation.

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4 (near the solstices, which are on about December 21 and June 21). For other eras, see precession and Milankovitch cycles.

Earth's phases.

Earth and Moon.
Earth and Moon from Mars, imaged by Mars Global Surveyor.
Earths magnetic field.
The Earth's magnetic field, which approximates a dipole.

From space, the Earth can be seen to go through phases similar to the phases of the Moon and Venus. This appearance is caused by light that reflects off the Earth as it moves around the Sun. The phases seen depend upon the observer's location in space. The phases of the Earth can be simulated by shining light on a globe of the Earth.

From orbit around the Earth, one can see all of the phases of the Earth in progression from New Earth to New Earth. The speed at which one sees these phases is related to the orbit of the observer and the speed of the observer around the Earth.

A Martian observer can see the Earth go through phases similar to those that an Earth-bound observer sees the phases of Venus (as discovered be Galileo), going for the Martian's perspective from New Earth to Fat Crescent to wane to New Earth. It is can be shown that an imaginary observer on the Sun would not see the Earth going through phases. The sun observer would only be able to see the lit side of the earth.

Earth's magnetic field.

The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. The field forms the magnetosphere, which deflects particles in the solar wind. The bow shock is located about at 13.5 RE. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.

Earth and the Moon.

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

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

NameDiameter (km)Mass (kg)Semi-major axis (km)Orbital period
Moon 3,474.8 7.3491022 384,400 27 days, 7 hours, 43.7 minutes

Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications-and the lengthening of Earth's day by about 17 s a year-add up to significant changes. During the Devonian period, there were 400 days in a year, with each day lasting 21.8 hours.

Earth-Moon system.
Details of the Earth-Moon system. Besides the radius of each object, the radius to the Earth-Moon barycenter is shown. Photos from NASA. Data from NASA. The Moon's axis is located by Cassini's third law.

The Moon may dramatically affect the development of life by taming the weather. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon. Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, as it appears to be for Mars. If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life. However, this is a controversial subject, and further studies of Mars-which shares Earth's rotation period and axial tilt, but not its large moon or liquid core-may settle the matter.

Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent angular size as the Sun (the Sun is 400 times larger, and the Moon is 400 times closer). This allows total eclipses and annular eclipses to occur on Earth.

Earth and Moon.
The relative sizes of and distance between Earth and Moon, not to scale.

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

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

Earth descriptions.

Earth rise.
The first time an "Earth-rise" was seen from the moon.
Earthrise.
Earthrise as seen from lunar orbit on Apollo 8, 24 December 1968.

Earth has often been personified as a deity, in particular a goddess (see Gaia and Mother Earth). The Chinese Earth goddess Hou-Tu is similar to Gaia, the deification of the Earth. As the patroness of fertility, her element is Earth. In Norse mythology, the Earth goddess Jord was the mother of Thor and the daughter of Annar. Ancient Egyptian mythology is different from that of other cultures because Earth is male, Geb, and sky is female, Nut (goddess).

Although commonly thought to be a sphere, the Earth is actually an oblate spheroid. It bulges slightly at the equator and is slightly flattened at the poles. In the past there were varying levels of belief in a flat Earth, but ancient Greek philosophers and, in the Middle Ages, thinkers such as Thomas Aquinas believed that it was spherical. A 19th-century organization called the Flat Earth Society advocated the even-then discredited idea that the Earth was actually disc-shaped, with the north pole at its center and a 150 foot (50 m) high wall of ice at the outer edge. It and similar organizations continued to promote this idea, based on religious beliefs and conspiracy theories, through the 1970s. Today, the subject is more frequently treated tongue-in-cheek or with mockery.

Prior to the introduction of space flight, these inaccurate beliefs were countered with deductions based on observations of the secondary effects of the Earth's shape and parallels drawn with the shape of other planets. Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, to an lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

The technological developments of the latter half of the 20th century are widely considered to have altered the public's perception of the Earth. Before space flight, the popular image of Earth was of a green world. science fiction artist Frank R. Paul provided perhaps the first image of a cloudless blue planet (with sharply defined land masses) on the back cover of the July 1940 issue of Amazing Stories, a common depiction for several decades thereafter. Apollo 17's 1972 "Blue Marble" photograph of Earth from cislunar space became the current iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. A photo taken of a distant Earth by Voyager 1 in 1990 inspired Carl Sagan to describe the planet as a "Pale Blue Dot." Earth has also been described as a massive spaceship, with a life support system that requires maintenance, or as having a biosphere that forms one large organism. See Spaceship Earth and Gaia theory.

Future of planet Earth.

Earth Sun.
Artist's impression of the remains of artificial structures on the Earth after the Sun enters its red giant phase and swells to roughly 100 times its current size.
Sun.
Comparison between the red supergiant Antares and the Sun. The black circle is the size of the orbit of Mars. Arcturus is also included in the picture for comparison.

The future of the planet is closely tied to that of the Sun. The luminosity of the Sun will continue to steadily increase, growing from the current luminosity by 10% in 1.1 billion years (1.1 Gyr) and up to 40% in 3.5 Gyr. Climate models indicate that the increase in radiation reaching the Earth is likely to have dire consequences, including possible loss of the oceans.

The Sun, as part of its solar lifespan, will expand to a red giant in 5 Gyr. Models predict that the Sun will expand out to about 99% of the distance to the Earth's present orbit (1 astronomical unit, or AU). However, by that time, the orbit of the Earth may have expanded to about 1.7 AUs because of the diminished mass of the Sun. The planet might thus escape envelopment.

The increased heat will accelerate the inorganic CO2 cycle,reducing its concentration to the lethal dose for plants (10 ppm for C4 photosynthesis) in 900 million years. But even if the Sun were eternal and stable, the continued internal cooling of the Earth would have resulted in a loss of much of its atmosphere and oceans (due to lower volcanism). More specifically, for Earth's oceans, the lower temperatures in the crust will permit their water to leak more deeply than today(at certain debt the water is evaporating) resulting in their total disappearance in 1 billion years.

Other things to consider about the Earth.

Subtopic.Earth topics of interest.
Astronomy Darwin (ESA) Terrestrial Planet Finder
Ecology Millennium Ecosystem Assessment
Economy World economy
Fiction Hollow Earth Journey to the Center of the Earth Earth in fiction The Core
Geography,
Geology
Continents Timezones Degree Confluence Project Earthquake Extremes on Earth plate tectonics Equatorial bulge
History Geologic time scale Human history Origin and evolution of the solar system Timeline of evolution
Law International law
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Solar system related pages. astronomical objects and the solar system's list of solar system objects.

More About The Earth.

Greenland is Melting Faster

Dec 21, 2005 - NASA's Grace Earth observation satellite has created the first, comprehensive survey of the entire Greenland ice sheet. The spacecraft found that the volume of ice is decreasing by 162 cubic kilometres per year (39 cubic miles), which is higher than all previously published estimates. This ice melt is contributing 0.4 millimeters (.016 inches) per year to global sea level rise. Grace was also able to measure detailed changes in the surface of the sea floor after the Sumatran earthquake and resulting tsunami that happened almost a year ago.

Northern Lights on the Move

Dec 11, 2005 - For more than 400 years, the Earth's magnetic North pole was in a roughly stable position, but now it's on the move, having drifted nearly 1,100 km (680 miles) in the last century. At this rate, it'll move out of Canada, and into Siberia in the next 50 years. If that happens, Alaska and Northern Canada may lose the beautiful Northern Lights, which are caused by the interaction of the magnetic pole and the solar wind. It could be that this is a normal oscillation of the magnetic pole, or it might be that the Earth's magnetic poles are getting ready to flip.

Smaller Ozone Hole This Year

Dec 7, 2005 - The ozone hole that developed above Antarctica looks smaller this year than previous years, based on observations from NASA's Aura satellite. The largest hole was measured in 1998; almost triple the size of 1985's hole. The temperature of the atmosphere above Antarctica seems to be one of the biggest factors deciding the size of the ozone hole - the colder it gets, the more ozone that's destroyed.

New View of Space Weather Cold Fronts

Dec 5, 2005 - Scientists from NASA and the National Science Foundation have created a new way to view the Earth's atmosphere during space storms. These large-scale storms resemble weather cold fronts that result from plumes of electrified plasma that flash across the Earth's ionosphere. These plumes used to seem like random events, but scientists have gotten pretty good at predicting them now, using a fleet of spacecraft. For the first time, they can now directly connect plasma observed in the atmosphere with these plumes that can extend thousands of kilometres into space.

Oxygen Levels on Earth Rose Gradually

Dec 5, 2005 - The rise of complex life on Earth matches the appearance of oxygen in the atmosphere, and new evidence from University of Maryland scientists suggests that the increase was more gradual than previously believed. According to microbial evidence, oxygen first appeared in our atmosphere 2.4 billion years ago, and a second large increase started 1.3 billion years ago, and reached its current levels about 600 million years ago.

Venus Express Photographs the Earth and Moon

Nov 25, 2005 - Now on its way to our nearest planetary neighbour, Venus Express tested its VIRTIS optics system by taking pictures of the Earth and the Moon. ESA controllers ran the spacecraft through a commissioning phase to test all of its scientific instruments. It took pictures of the Earth and the Moon when it was 3.5 million kilometres away. The VIRTIS instrument is also on board the Rosetta spacecraft, which also took images of our planet. ESA scientists will be able to compare the images to ensure the instrument is working perfectly.

Early Earth Had Continents

Nov 23, 2005 - Researchers from the University of Colorado at Boulder think they've found evidence that the very early Earth had continents soon after the planet formed, overturning theories that the planet was Moon-like, or covered with oceans. The team analyzed a rare element called hafnium in ancient minerals from the Jack Hills in Western Australia. It showed that continental crusts had formed 4.4 - 4.5 billion years ago, and were then recycled into the Earth's mantle.

Early Earth Wasn't So Hellish

Nov 18, 2005 - Most geologists believe that the early history of our planet was an extreme, "hellish" environment, under constant bombardment from asteroids, and completely devoid of modern formations, like continents. Researchers from ANU disagree, and think they've found evidence that continents had already formed within the first 500 million years, and there was liquid water interacting with rocks. The Earth at that time might have looked remarkably similar to our current planet, complete with continents and oceans.

Massive B-15A Iceberg Breaks Up

Nov 7, 2005 - After 5 years afloat, the gigantic B-15A iceberg has broken up off the coast of Antarctica's Cape Adare. This image of the iceberg was taken using ESA's Envisat satellite Advanced Synthetic Aperture Radar (ASAR). The bottle-shaped iceberg had run aground, and probably flexed and strained until it broke up into 9 pieces along fault lines on October 27. The largest pieces have been named B-15M, B-15N and B-15P.

Greenland's Ice Sheet is Growing

Nov 4, 2005 - After gathering data on Greenland for more than a decade, ESA scientists have reported that the island's ice sheet is actually growing at its interior. Data collection began in 1991 with the radar altimeter instrument on board ESA's ERS-1, followed by ERS-2, and most recently Envisat, which has 10 instruments to measure various properties of the Earth from orbit. Greenland's ice sheet seems to be thickening at a rate of 6.4 cm (2.6 inches) a year above altitudes of 1,500 metres (5000 feet). Below that altitude, the ice sheets are decreasing in thickness.

Hurricane Wilma Becomes a Record Setting Cat 5

Oct 19, 2005 - Just a few days ago, Wilma was a tropical storm, but now it has exploded into a Category 5 hurricane - with the lowest pressure ever recorded in an Atlantic storm. Wilma's pressure was measured early Wednesday at just 882 millibars, breaking the previous record of 888 set in 1988 by Hurricane Gilbert. Wilma is currently off the coast Mexico's Yucatan peninsula , but it's expected to take a sharp right turn and move up through the Florida panhandle on Saturday.

Studying the Health of the Great Barrier Reef from Space

Oct 4, 2005 - Australian researchers are using ESA's Envisat Earth Observation Satellite to peer down and help judge the health of the Great Barrier Reef. Envisat's MERIS sensor can detect coral bleaching down to 10 metres below the surface of the water. This bleaching occurs when the symbiotic algae living with the coral are expelled when ocean temperatures rise. Since Envisat images the entire planet every three days, scientists will be able to watch this bleaching process on a weekly basis to see how the reefs are doing.

Was There a Slushball Earth?

Sep 29, 2005 - With the "Snowball Earth" hypothesis, scientists have proposed that our planet was once encased under a thick layer of ice and snow. Life could only survive huddled around hot vents deep under water. But now scientists have found fossil evidence of creatures that lived during this period, but were photosynthesizing. This means they needed to live under thin enough ice for sunlight to get through. It's possible that the entire planet wasn't encased in ice, instead there were large patches of thin ice, or even open water near the equator.

Satellite Picture of Hurricane Rita

Sep 23, 2005 - The European Space Agency's Envisat satellite took this photo of Hurricane Rita on September 22, 2005 as it was passing Southern Florida. Envisat can use its radar instruments to peer through a hurricane's clouds and measure the roughness of the ocean beneath it. This is how scientists can estimate the wind speed of the storm at various points. Rita is expected to slam into Texas or Louisiana early Saturday morning.

Methane Release Raised Earth Temperatures 180 Million Years Ago

Sep 19, 2005 - Researchers from Open University have uncovered that the Earth suffered a sudden, severe period of global warming approximately 180 million years ago. During this period, vast quantities of methane gas were released in three huge pulses when underwater stores of gas hydrate melted. This greenhouse gas warmed the Earth by 10 degrees C and resulted in the extinction of many species on land and in the oceans.

Two Weather Satellites About to Launch

Sep 16, 2005 - NASA has two new Earth Observation satellites in the final stages of preparation before their launch: CloudSat and Calipso. The two satellites will be launched together by a Boeing Delta II rocket from Vandenberg Air Force Base in California. They will be launched into a polar orbit, and maintain a close formation. CloudSat has an extremely powerful cloud-profiling radar, which can distinguish between cloud particles and precipitation. Calipso will be able to detect aerosol particles in the air, and can tell the difference between these particles and clouds to measure the amount of air pollution. They may launch as soon as October 26.

Early Atmosphere Looked Very Different From Today

Sep 14, 2005 - Researchers at Washington University in St. Louis have used primitive meteorites called chondrites to develop a model of the Earth's early atmosphere. And it looked nothing like what we have today. Instead of the familiar oxygen and nitrogen, our early atmosphere would have been a toxic mixture of methane, ammonia, hydrogen and water vapour. Simulating this early environment was actually quite difficult to calculate because the minerals of the early Earth reacted to the hot environment in hard-to-predict ways.

South Ozone Hole Returns

Aug 30, 2005 - The ozone hole above the South Pole has returned, and it's on track to be one of the biggest on record. At this time, the hole is the size of Europe, but it will probably continue growing during September - bigger holes appeared in 1996 and 2000. The size of the ozone hole and the time of its appearance depends on the weather conditions in the southern hemisphere.

Earth's Core Rotates Faster Than Its Crust

Aug 29, 2005 - According to new research from geologists, the Earth's core rotates just a little bit faster - about 1 degree per year - than the crust of the planet. The scientists took advantage of historical records for "earthquake twins" near the South Sandwich Islands. These are quakes that occurred in virtually the same spot with the same magnitude, but were years apart. As the seismic waves passed through the Earth, they were bent as they passed through the Earth's iron core. The shape of this bending has changed over time, indicating the core's faster rotation.

Earth's Climate During the Permian Extinction

Aug 25, 2005 - Around 251 million years ago, something happened to the Earth's climate that wiped out 90-95% of marine life and 70% of terrestrial life. Scientists at the National Center for Atmospheric Research (NCAR) have developed a computer model that demonstrates that rapid increases in carbon dioxide belched out of volcanoes did the trick. Temperatures were 10 to 30 degrees Celsius (18 to 54 degrees Fahrenheit) higher than they are today, which broke a cycle that pulled carbon dioxide out of the atmosphere.

Links For The Earth.

Centre for Remote Imaging, Sensing and Processing - Remote Sensing satellite ground receiving station in Singapore
CSIRO Office of Space Science and Applications - co-ordinates and conducts research in Earth Observation science.
Earthdial - Earthdial #15, as part of the "Two Worlds, One Sun" Mars lander project, sponsered by the Planetary Society.
Earth Station 2000 - Real Time Earth Disaster/Alert links. Picture menu of select links to planetary, cosmological, and theoretical science sites.
ARC Science Simulation
Aviris
Canadian Center for Remote Sensing
Center for Earth Observation
Center for Earth Observing and Space Research
Defense Meteorological Satellite Program
Earth Changes TV
Earth Observation Center
Earth Observations Science
Earth Observatory
Earth Observing One
Earth Observing System
Earth Science and solar system Exploration Division
Earth Sensing Resources
Earthrise
Earthwatch
EROS Data Center
EUMETSAT
For Kids Only
GSFC Earth Sciences
HALOE
IERS Sub-bureau for Rapid Service & Predictions
Imagelinks
Infoterra
Landsat 7 Homepage
NASA Earth From Space
NASA Earth Science Enterprise
NPA
Ocean World
Orbimage
Space Imaging
Spot Image
Sunsat Homepage
Topex Poseidon
Total Ozone Mapping Spectrometer
Understanding The Universe
University Corporation for Atmospheric Research
Vexcel
Visualization of Remote Sensing Data
Winds



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