This article is about the planet. For other uses, see Earth (disambiguation).
Earth
"The Blue Marble" photograph of Earth,
taken from Apollo 17
Designations
Alternate name(s)
Terra
Orbital characteristics
Epoch J2000.0[note 1]
Aphelion
152,098,232 km
1.01671388 AU[note 2]
Perihelion
147,098,290 km
0.98329134 AU[note 2]
Semi-major axis
149,598,261 km
1.00000261 AU[1]
Eccentricity
0.01671123[1]
Orbital period
365.256363004 days[2]
1.000017421 yr
Average
orbital speed
29.78 km/s[3]
Mean anomaly
357.51716°[3]
Inclination
107,200 km/h
7.155° to Sun's equator
1.57869°[4] to invariable plane
Longitude of
ascending node
348.73936°[3][note 3]
Argument of
perihelion
114.20783°[3][note 4]
Satellites
1 natural (The Moon)
8,300+ artificial (as of 1 March
2001)[5]
Physical characteristics
Mean radius
Equatorialradius
6,371.0 km[6]
6,378.1 km[7][8]
Polar radius
6,356.8 km[9]
Flattening
0.0033528[10]
Circumference
40,075.017 km (equatorial)[8]
40,007.86 km (meridional)[11]
Surface area
510,072,000 km2[12][13][note 5]
148,940,000 km2 land (29.2 %)
361,132,000 km2 water (70.8 %)
Volume
1.08321×1012 km3[3]
Mass
Mean density
5.9736×1024 kg[3]
5.515 g/cm3[3]
Equatorial
surface gravity
9.780327 m/s2[14]
Escape velocity
11.186 km/s[3]
Sidereal rotation
period
0.99726968 d[15]
Equatorial
rotation velocity
0.99732 g
23h 56m 4.100s
1,674.4 km/h (465.1 m/s)[16]
Axial tilt
23°26'21".4119[2]
Albedo
0.367 (geometric)[3]
0.306 (Bond)[3]
Surface temp.
Kelvin
Celsius
min
mean
[17]
184 K
287.2 K[18]
−89.2 °C 14 °C
max
331 K[19]
57.8 °C
Atmosphere
Surfacepressure
Composition
101.325 kPa (MSL)
78.08% nitrogen (N2)[3]
20.95% oxygen (O2)
0.93% argon
0.038% carbon dioxide
About 1% water vapor (varies
with climate)
Earth (or the Earth) is the third planet from the Sun, and the densest and fifth-largest of the eight planets
in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes
referred to as the World, the Blue Planet,[20] or by its Latin name, Terra.[note 6]
Earth formed 4.54 billion years ago, and life appeared on its surface within one billion years.[21] The planet
is home to millions of species, including humans.[22] Earth's biosphere has significantly altered the
atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as
well as the formation of the ozone layer which, together withEarth's magnetic field, blocks harmful solar
radiation, permitting life on land.[23] The physical properties of the Earth, as well as its geological history
and orbit, have allowed life to persist during this period. The planet is expected to continue supporting life
for at least another 500 million years.[24][25]
Earth's outer surface is divided into several rigid segments, or tectonic plates, that migrate across the
surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans,
with the remainder consisting of continents and islands which together have many lakes and other
sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with solid ice
(Antarctic ice sheet) or sea ice (Arctic ice cap). The planet's interior remains active, with a thick layer of
relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.
Earth interacts with other objects in space, especially the Sun and the Moon. At present, Earth orbits the
Sun once every 366.26 times it rotates about its own axis, which is equal to 365.26solar days, or
one sidereal year.[note 7] The Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital
plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24
solar days).[26] Earth's only known natural satellite, the Moon, which began orbiting it about 4.53
billion years ago, provides ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.
Between approximately 3.8 billion and 4.1 billion years ago, numerous asteroid impacts during the Late
Heavy Bombardment caused significant changes to the greater surface environment.
Both the mineral resources of the planet and the products of the biosphere contribute resources that are
used to support a global human population.[27] These inhabitants are grouped into about 200
independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human
cultures have developed many views of the planet, including personification as a deity, a belief in a flat
Earth or in the Earth as the center of the universe, and a modern perspective of the world as
an integrated environment that requires stewardship.
Contents
[hide]


1 Chronology
o
1.1 Evolution of life
o
1.2 Future
2 Composition and structure
o
2.1 Shape
o
2.2 Chemical composition
o
2.3 Internal structure
o
2.4 Heat
o
2.5 Tectonic plates
o
2.6 Surface
o
2.7 Hydrosphere
o
2.8 Atmosphere
o


2.8.1 Weather and climate

2.8.2 Upper atmosphere
2.9 Magnetic field
3 Orbit and rotation
o
3.1 Rotation
o
3.2 Orbit
o
3.3 Axial tilt and seasons

4 Moon

5 Habitability
o
5.1 Biosphere
o
5.2 Natural resources and land use
o
5.3 Natural and environmental hazards
o
5.4 Human geography

6 Cultural viewpoint

7 See also

8 Notes

9 References

10 Further reading

11 External links
Chronology
Main article: History of the Earth
See also: Geological history of Earth
The earliest dated Solar System material was formed 4.5672 ± 0.0006 billion years ago,[28] and by
4.54 billion years ago (within an uncertainty of 1%)[21] the Earth and the other planets in the Solar System
had formed out of the solar nebula—a disk-shaped mass of dust and gas left over from the formation of
the Sun. This assembly of the Earth through accretion was thus largely completed within 10–
20 million years.[29] Initially molten, the outer layer of the planet Earth cooled to form a solid crust when
water began accumulating in the atmosphere. The Moon formed shortly thereafter, 4.53 billion
years ago.[30]
The current consensus model[31] for the formation of the Moon is the giant impact hypothesis, in which the
Moon was created when a Mars-sized object (sometimes called Theia) with about 10% of the Earth's
mass[32] impacted the Earth in a glancing blow.[33] In this model, some of this object's mass would have
merged with the Earth and a portion would have been ejected into space, but enough material would
have been sent into orbit to coalesce into the Moon.
Outgassing and volcanic activity produced the primordial atmosphere of the Earth. Condensing water
vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets,
and trans-Neptunian objects produced the oceans.[34] The newly formed Sun was only 70% of its
present luminosity, yet evidence shows that the early oceans remained liquid—a contradiction dubbed
the faint young Sun paradox. A combination of greenhouse gases and higher levels of solar
activity served to raise the Earth's surface temperature, preventing the oceans from freezing
over.[35] By 3.5 billion years ago, the Earth's magnetic field was established, which helped prevent the
atmosphere from being stripped away by the solar wind.[36]
Two major models have been proposed for the rate of continental growth:[37] steady growth to the presentday[38] and rapid growth early in Earth history.[39] Current research shows that the second option is most
likely, with rapid initial growth of continental crust[40] followed by a long-term steady continental
area.[41][42][43] On time scales lasting hundreds of millions of years, the surface continually reshaped as
continents formed and broke up. The continents migrated across the surface, occasionally combining to
form a supercontinent. Roughly 750 million years ago (Ma), one of the earliest known
supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–
540 Ma, then finally Pangaea, which broke apart 180 Ma.[44]
Evolution of life
Main article: Evolutionary history of life
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 existed.[45] The development
of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen
accumulated in the atmosphere and formed a layer of ozone (a form of molecular oxygen [O3]) in the
upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of
complex cells called eukaryotes.[46] True multicellular organisms formed as cells within colonies became
increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life
colonized the surface of Earth.[47]
Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 Ma, during
the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed
"Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when
multicellular life forms began to proliferate.[48]
Following the Cambrian explosion, about 535 Ma, there have been five major mass
extinctions.[49] The most recent such event w