   #copyright

Earth

2007 Schools Wikipedia Selection. Related subjects: The Planets

   CAPTION: Earth Astronomical symbol of Earth

   A color image of Earth as seen from Apollo 17.
   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
   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 km²
   Land area 148,939,100 km² (29.2 %)
   Water area 361,126,400 km² (70.8 %)
   Volume 1.083 207 3×10^12 km³
   Mass 5.9742×10^24 kg
   Density 5,515.3 kg/m³
   Equatorial surface gravity 9.780 1 m/s²
   (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 (-88.3 °C) min
   287 K (14 °C) mean
   331 K (57.7 °C) max
   Surface pressure 101.3 kPa ( MSL)
   Adjective Terrestrial, Terran, Telluric, Tellurian, Earthly, Earthling
   (lifeforms)
   Atmospheric constituents
   Nitrogen 78.08 %
   Oxygen 20.94 %
   Argon 0.93 %
   Carbon dioxide 0.038 %
   Water vapor Trace (varies with climate)

   Earth ( IPA: /ˈəː(ɹ)θ/, often referred to as the Earth, Terra, the
   World or Planet Earth) is the third planet in the solar system in terms
   of distance from the Sun, and the fifth largest. It 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 humans to support life. It is also the densest planet
   in the solar system. Widely accepted scientific evidence indicates 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.

   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 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. Long term periodic 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.

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- (γαιο-,
   gaio-), from gē (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 Afrikaans and
   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 eorðe. 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,
   ארץ (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), पृथ्वी
   pr̥thvī (Sanskrit), Maa ( Finnish and Estonian), Pamînt ( Romanian),
   Föld ( 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),
   Âlem, Dünya الْمَسْكُونَة (Arabic), Dinê ( Kurdish), Ergir երկիր (
   Armenian), Jehun, Zamin ( Persian), and Acun, Yeryüzü, Yerküre (
   Turkish)., כדור הארץ (Hebrew), Bhoomi ( Telugu)

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 have been 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.

Shape

   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/π). 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 centre of
   the Earth is actually Mount Chimborazo in Ecuador.

Composition

   The mass of the Earth is approximately 5980 yottagrams (5.98 ×10^24
   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%).

Internal structure

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

   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:
          Depth                               Layer
   Kilometers   Miles
   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

Tectonic plates

          A map pointing out the Earth's major plates.

          Enlarge
          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
     * African Plate, covering Africa - Continental plate
     * Antarctic Plate, covering Antarctica - Continental plate
     * Australian Plate, covering Australia (fused with Indian Plate
       between 50 and 55 million years ago) - Continental plate
     * Eurasian Plate covering Asia and Europe - Continental plate
     * North American Plate covering North America and north-east Siberia
       - Continental plate
     * South American Plate covering South America - Continental plate
     * Pacific Plate, covering the Pacific Ocean - Oceanic plate

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

Surface

   Surface of the Earth, colors reflect changes in elevation
   Enlarge
   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
   × 10^9 acres of cropland and 8.4 × 10^9 acres of pastureland.

Extremes

   Elevation extremes: (measured relative to sea level)
     * Lowest point on land: Dead Sea −417 m
     * Lowest point overall: Challenger Deep of the Mariana Trench in the
       Pacific Ocean −10,924 m
     * Highest point: Mount Everest 8,844 m (2005 est.)

Hydrosphere

   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.

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.

   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.

Climate

   A part of the earth as it looks from high orbit.
   Enlarge
   A part of the earth as it looks from high orbit.

   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.

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.

Biosphere

   The planet's lifeforms are sometimes said to form a "biosphere". This
   biosphere is generally believed to have begun evolving about 3.5
   billion (3.5×10^9) 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.

Land use

   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:
     * Arable land: 13.13%
     * Permanent crops: 4.71%
     * Permanent pastures: 26%
     * Forests and woodland: 32%
     * Urban areas: 1.5%
     * Other: 30% (1993 est.)

   Irrigated land: 2,481,250 km² (1993 est.)

Natural and environmental hazards

   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.

Human geography

   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
   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.
   Enlarge
   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.

Solar system

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

Phases

   Earth and Moon from Mars, imaged by Mars Global Surveyor.
   Enlarge
   Earth and Moon from Mars, imaged by Mars Global Surveyor.

   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.

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 R[E]. 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.

Moon

   Name Diameter (km) Mass (kg) Semi-major axis (km) Orbital period
   Moon 3,474.8 7.349×10^22 384,400 27 days, 7 hours, 43.7 minutes
   Earthrise as seen from lunar orbit on Apollo 8, 24 December 1968.
   Enlarge
   Earthrise as seen from lunar orbit on Apollo 8, 24 December 1968.

   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.

   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 µs 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.

   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.
   The relative sizes of and distance between Earth and Moon, to scale
   Enlarge
   The relative sizes of and distance between Earth and Moon, 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 AA[29].

Descriptions

   The first time an "Earth-rise" was seen from the moon.
   Enlarge
   The first time an "Earth-rise" was seen from the moon.

   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 centre 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

   Artist's conception 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.
   Enlarge
   Artist's conception 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.
   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
   Enlarge
   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 CO[2] 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.

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