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For alternate uses see: Earth (disambiguation).

Click image for description
Physical characteristics
Equatorial radius 6,378.14 km
Polar radius 6,356.78 km
Mean radius 6,371.3 km
Equatorial circumference 40,075 km
Volume 1.0832×1012 km³
Mass 5.9737×1024 kg
Density 5.515 g/cm³
Surface area 510,065,700 km²
Equatorial surface gravity 9.766 m/s², or 1 gee
Escape velocity 11,180 m/s
Sidereal rotation period 23.934 hours
Equatorial inclination to orbit 23.45°
Surface temperature
min mean max
185 K 287 K; 331 K
Surface Pressure 1 bar
Orbital characteristics
Average distance from the Sun 149,597,890 km (1.000 A.U)
Perihelion (closest) 147,100,000 km
Aphelion (farthest) 152,100,000 km
Sidereal orbit period 365.25636 days (1.0000174 Julian years)
Mean Orbit Velocity 29,785.9 m/s
Orbital eccentricity 0.01671022
Orbital inclination to Ecliptic 0.00005°
Orbital circumference 924,375,700 km
Satellitess 1 (the Moon), but see also 3753 Cruithne
Satellite of Sun
Atmospheric constituents
nitrogen 77%
oxygen 21%
argon 1%
carbon dioxide
water vapor

Earth, also known as the Earth and Terra, is the planet on which humans live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science can confirm as harbouring life. The planet has a single natural satellite, the Moon, and formed around 4.5 billion years ago.

Table of contents
1 Physical characteristics
2 Earth in the Solar System
3 Geography
4 Climate
5 Terrain
6 Natural resources
7 Land use
8 Natural hazards
9 Environment - current issues
10 Human population
11 Government
12 Descriptions of Earth
13 See also
14 Reference

Physical characteristics

Main article: Geophysics


The interior of Earth, like that of the other terrestrial planets, is chemically divided into an outer siliceous solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The liquid outer core gives rise to a weak magnetic field due to the convection of its electrically conductive material.

New material constantly finds its way to the surface through volcanoes and cracks in the ocean floors (see seafloor spreading). Much of Earth's surface is less than 100 million years old; the very oldest parts of the crust are as much as 4.4 billion years old [1].

Taken as a whole, Earth's composition by mass is:


The interior of Earth reaches temperatures of 5270 K. The planet's internal heat was originally released during its accretion (see gravitational binding energy), and since then additional heat has continued to be generated by the decay of radioactive elements such as uranium, thorium, and potassium. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun.

The core

The average density of Earth is 5,515 kg/m3, making it the densest planet in the Solar system. Since the average density of surface material is only around 3000 kg/m3, we must conclude that denser materials exist within the core of the Earth. In its earliest stages, about 4.5 billion years ago, the Earth was mostly molten, and as a result gravity would have caused denser substances to sink towards the center in a process called planetary differentiation, while less dense materials would have migrated to the crust. As a result, the core is largely composed of iron (80%), along with nickel and silicon; while other dense elements, such as lead and uranium, are either too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see: felsic materials).

The core is divided into two parts, a solid inner core with a radius of ~1250 km and a liquid outer core extending beyond it to a radius of ~3500 km. The inner core is generally believed to be solid and composed primarily of iron and some nickel. Some have argued that the inner core may be in the form of a single iron crystal. The outer core surrounds the inner core and is believed to be composed of liquid iron mixed with liquid nickel and trace amounts of lighter elements. It is generally believed that convection in the outer core, combined with stirring caused by the Earth's rotation (see: Coriolis forces), gives rise to the Earth's magnetic field through a process known as the dynamo theory. The solid inner core is too hot to hold a permanent magnetic field (see: Curie temperature) but probably acts to stabilise the magnetic field generated by the liquid outer core.

Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet, by ~2° per year (Comins DEU-p.82).


Earth's mantle extends to a depth of 2,900 km. The pressure, at the bottom of the mantle, is ~1.4 Matm (140 GPa). It is largely composed of substances rich in iron and magnesium. The melting point of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is plastic (semi-molten). The viscosity of the upper mantle ranges between 1021 and 1024 Pas, depending on depth [1]. Thus, the upper mantle can only flow very slowly.

Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting point of iron rich substances are higher than pure iron. The core is composed almost entirely of pure iron, while iron rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-molten (as it is hot and they are under relatively little pressure), lower mantle iron-substances are solid (as they are under tremendous pressure), outer core pure iron is liquid as it has a very low melting point (despite enormous pressure), and the inner core is solid due to the overwhelming pressure found at the center of the planet.


The crust ranges from 5 to 35 km in depth. It is composed of silicon-based rockss. The crust-mantle boundary occurs as two physically different events. Firstly, there is a discontinuity in the seismic velocity which is known as the Mohorovicic discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain none (below). The second event is a chemical discontinuity between ultramafic cumulates and tectonized hartzburgites which has been observed from parts of the oceanic crust that have been obducted.


Main Article: Life

Earth is the only place where life is known to exist. The planet's lifeforms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion years ago. The biosphere is divided into a number of biomes, inhabited by broadly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life, while most of the more populous biomes lie near the Equator.


Main article: Earth's atmosphere

Earth has a relatively thick atmosphere composed of 78% nitrogen, 21% oxygen, and 1% argon, plus traces of other gases including carbon dioxide and water. The atmosphere acts as a buffer between Earth and the Sun. The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the globe and in response to seasonal changes. This is sometimes described as the "third atmosphere" to distinguish it from earlier atmospheric compositions.


Main article: Ocean

Earth is the only planet in our solar system whose surface has liquid water. Water covers 71% of Earth's surface (97% of it being sea water and 3% fresh water [1]) and divides it into five oceans and seven continents. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make Earth a water planet.

Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, Earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (Archaea) had colonized the oceans, the greenhouse effect failed, and Earth froze solid for 10 to 100 million years in what is called a snowball Earth event.

On other planets, such as Venus, gaseous water is cracked by solar ultraviolet, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. It is believed that this is the reason why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.

On Earth, a shield of ozone absorbs most of this energetic ultraviolet high in the atmosphere, reducing the cracking effect. The magnetosphere also shields the ionosphere from direct scouring by the solar wind.

Finally, vulcanism, aided by the Moon's tidal effects, continuously emits water vapor from the interior. Earth's plate tectonics recycle carbon and water as limestone fields are subducted into magma and volcanically emitted as gaseous carbon dioxide and steam. It is estimated that the minerals in the mantle may contain as much as 10 times the water as in all of the current oceans, though most of this trapped water will never be released.

Earth also suffers from the Chandler wobble.

Earth in the Solar System

It takes Earth 23 hours, 56 minutes and 4.09 seconds (1 sidereal day) to rotate around the axis connecting the North Pole and the South Pole. It orbits the Sun every 365.2564 mean solar days (1 sidereal year). Earth has one natural satellite, "the Moon", which orbits around Earth every 27 1/3 days. 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 (otherwise there would be an eclipse every month).

The Hill sphere (sphere of influence) of the earth is about 1.5 Gm (930 thousand miles) in radius, within which one natural satellite (the Moon) comfortably orbits.

The Moon

Main article: Moon

Name Diameter (km) Mass (kg) Mean Orbital Radius (km) Orbital Period
Moon 3,474.8 7.349 × 1022 384,400 27 Days, 7 hours, 43.7 minutes

The Moon is a relatively large terrestrial planet-like satellite, about one quarter of Earth's diameter. The natural satellites orbiting other planets are called "moons", after Earth's Moon.

The Moon's gravity causes the tides on Earth. The same effect has lead 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 line.

The Moon may enable life by moderating the weather. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilised by tidal interactions with the Moon. Without this stabilization, the rotational axis might be chaotically unstable, as it is with a sphere, and appears to be with Mars. If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result as one pole was continually heated and the other cooled. Planetologists who have studied the effect claim that this might kill all large animal and higher plant life. This remains a controversial subject, however, and further studies of Mars - which shares Earth's rotation period and axial tilt, but not its large moon or liquid core - may provide additional information.

The Moon is just far enough away to have, when seen from Earth, the same apparent angular size as the Sun. This allows a total eclipse to occur on Earth. (The Sun is 400 times larger, but the Moon is 400 times closer.)

The Moon's origin is unknown, but one popular theory is that it was formed from the collision of a Mars-sized protoplanet with the early Earth. This theory explains (among other things) the Moon's relative lack of iron and volatile elements. See Giant impact theory.

Earth also has at least one known co-orbital asteroid, 3753 Cruithne.


Main article: Geography

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Map references:

Time Zones, Coordinates.

Biggest geographic subdivision

Continents, Oceans


Land boundaries: the land boundaries in the world total 251,480.24 km (not counting shared boundaries twice)

Coastline: 356,000 km

Maritime claims: see United Nations Convention on the Law of the Sea


Main article:
Climate model

Two large areas of polar climates separated by two rather narrow temperate zones from a wide equatorial band of tropical to subtropical climates. Precipitation patterns vary widely, ranging from several metres of water per year to less than a millimetre.


Elevation extremes: (measured relative to sea level)

Natural resources

Main article:
Natural resources

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

Land use

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

Natural 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, floods, droughts, and other calamities and disasters.

Environment - current issues

Main article: Environmentalism

Large areas are subject to overpopulation, industrial disasters such as pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, soil degradation, soil depletion, erosion, and introduction of invasive species.

Human population

Nearly all humans currently reside on Earth: 6,327,152,352 inhabitants (November 1 2003 est.)

A few humans are in orbit around Earth on board the International Space Station, with others traveling briefly above the atmosphere. In total, about 400 people (astronauts, cosmonauts and taikonauts) have been outside Earth (in space) as of 2004. Most of them have reported a heightened understanding of its value and importance, reverence for human life and amazement at its beauty, not usually achieved by those living on the surface.

See also space colonization.

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

Age structure:

Population growth rate: 1.3% (2000 est.)

Birth rate: 22 births/1,000 population (2000 est.)

Death rate: 9 deaths/1,000 population (2000 est.)

Sex ratio:

Infant mortality rate: 54 deaths/1,000 live births (2000 est.)

Life expectancy at birth:

Total fertility rate: 2.8 children born/woman (2000 est.)


The worldwide general international organization is the United Nations. The United Nations is primarily an international discussion forum with only limited ability to pass and enforce laws.

Administrative divisions: 267 nations, dependent areas, other, and miscellaneous entries

Descriptions of Earth

Earth has often been personified as a deity, in particular a goddess. See Gaea and Mother Earth. In Norse mythology, Earth was the son of Nott and Annar.

Earth has also been described as a massive spaceship, with a life support system that requires maintenance. See Spaceship Earth.

Since Earth is rather large, it is not immediately obvious to the naked eye that it is spherical. Because of this, in the past it was sometimes thought that Earth was in fact flat. See flat Earth.

In science fiction the Earth is frequently the capital or a major administrative center of a hypothetical galactic government (especially when that galactic government is postulated to be human-dominated), often a representative federal republic, though empires and dictatorships are definitely not unseen. Notable are Star Trek and Babylon 5.

See also


The Solar System
Sun | Mercury | Venus | Earth | Moon | Mars | Asteroids | Jupiter | Saturn | Uranus | Neptune | Pluto
(For other objects and regions, see: List of solar system objects, Astronomical objects)