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= Earth =
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Introduction
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Earth is the third planet from the Sun and the only astronomical
object known to harbor life. This is enabled by Earth being an ocean
world, the only one in the Solar System sustaining liquid surface
water. Almost all of Earth's water is contained in its global ocean,
covering 70.8% of Earth's crust. The remaining 29.2% of Earth's crust
is land, most of which is located in the form of continental
landmasses within Earth's land hemisphere. Most of Earth's land is at
least somewhat humid and covered by vegetation, while large sheets of
ice at Earth's polar deserts retain more water than Earth's
groundwater, lakes, rivers, and atmospheric water combined. Earth's
crust consists of slowly moving tectonic plates, which interact to
produce mountain ranges, volcanoes, and earthquakes. Earth has a
liquid outer core that generates a magnetosphere capable of deflecting
most of the destructive solar winds and cosmic radiation.
Earth has a dynamic atmosphere, which sustains Earth's surface
conditions and protects it from most meteoroids and UV-light at entry.
It has a composition of primarily nitrogen and oxygen. Water vapor is
widely present in the atmosphere, forming clouds that cover most of
the planet. The water vapor acts as a greenhouse gas and, together
with other greenhouse gases in the atmosphere, particularly carbon
dioxide (CO2), creates the conditions for both liquid surface water
and water vapor to persist via the capturing of energy from the Sun's
light. This process maintains the current average surface temperature
of 14.76 C, at which water is liquid under normal atmospheric
pressure. Differences in the amount of captured energy between
geographic regions (as with the equatorial region receiving more
sunlight than the polar regions) drive atmospheric and ocean currents,
producing a global climate system with different climate regions, and
a range of weather phenomena such as precipitation, allowing
components such as nitrogen to cycle.
Earth is rounded into an ellipsoid with a circumference of about
40,000 km. It is the densest planet in the Solar System. Of the four
rocky planets, it is the largest and most massive. Earth is about
eight light-minutes (1 AU) away from the Sun and orbits it, taking a
year (about 365.25 days) to complete one revolution. Earth rotates
around its own axis in slightly less than a day (in about 23 hours and
56 minutes). Earth's axis of rotation is tilted with respect to the
perpendicular to its orbital plane around the Sun, producing seasons.
Earth is orbited by one permanent natural satellite, the Moon, which
orbits Earth at 384,400 km--1.28 light seconds--and is roughly a
quarter as wide as Earth. The Moon's gravity helps stabilize Earth's
axis, causes tides and gradually slows Earth's rotation. Likewise
Earth's gravitational pull has already made the Moon's rotation
tidally locked, keeping the same near side facing Earth.
Earth, like most other bodies in the Solar System, formed about 4.5
billion years ago from gas and dust in the early Solar System. During
the first billion years of Earth's history, the ocean formed and then
life developed within it. Life spread globally and has been altering
Earth's atmosphere and surface, leading to the Great Oxidation Event
two billion years ago. Humans emerged 300,000 years ago in Africa and
have spread across every continent on Earth. Humans depend on Earth's
biosphere and natural resources for their survival, but have
increasingly impacted the planet's environment. Humanity's current
impact on Earth's climate and biosphere is unsustainable, threatening
the livelihood of humans and many other forms of life, and causing
widespread extinctions.
Etymology
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The Modern English word 'Earth' developed, via Middle English, from
an Old English noun most often spelled '. It has cognates in every
Germanic language, from which has been reconstructed. In its earliest
attestation, the word ' was used to translate the many senses of Latin
' and Greek : the ground, its soil, dry land, the human world, the
surface of the world (including the sea), and the globe itself. As
with Roman (or ) and Greek , Earth may have been a personified
goddess in Germanic paganism: late Norse mythology included
('Earth'), a giantess often given as the mother of Thor.
Historically, 'Earth' has been written in lowercase. During the Early
Middle English period, its definite sense as "the globe" began being
expressed using the phrase 'the earth'. By the period of Early Modern
English, capitalization of nouns began to prevail, and 'the earth' was
also written 'the Earth', particularly when referenced along with
other heavenly bodies. More recently, the name is sometimes simply
given as 'Earth', by analogy with the names of the other planets,
though 'earth' and forms with 'the earth' remain common. House styles
now vary: Oxford spelling recognizes the lowercase form as the more
common, with the capitalized form an acceptable variant. Another
convention capitalizes 'Earth' when appearing as a name, such as a
description of the "Earth's atmosphere", but employs the lowercase
when it is preceded by 'the', such as "the atmosphere of the earth".
It almost always appears in lowercase in colloquial expressions such
as "what on earth are you doing?"
The name 'Terra' is occasionally used in scientific writing; it also
sees use in science fiction to distinguish humanity's inhabited planet
from others, while in poetry 'Tellus' has been used to denote
personification of the Earth. 'Terra' is also the name of the planet
in some Romance languages, languages that evolved from Latin, like
Italian and Portuguese, while in other Romance languages the word gave
rise to names with slightly altered spellings, like the Spanish and
the French . The Latinate form ( ) of the Greek poetic name ( or )
is rare, though the alternative spelling 'Gaia' has become common due
to the Gaia hypothesis, in which case its pronunciation is rather
than the more traditional English .
There are a number of adjectives for the planet Earth. The word
'earthly' is derived from 'Earth'. From the Latin comes 'terran' ,
'terrestrial' , and (via French) 'terrene' , and from the Latin
comes 'tellurian' and 'telluric'.
Formation
===========
The oldest material found in the Solar System is dated to Ga (billion
years) ago. By the primordial Earth had formed. The bodies in the
Solar System formed and evolved with the Sun. In theory, a solar
nebula partitions a volume out of a molecular cloud by gravitational
collapse, which begins to spin and flatten into a circumstellar disk,
and then the planets grow out of that disk with the Sun. A nebula
contains gas, ice grains, and dust (including primordial nuclides).
According to nebular theory, planetesimals formed by accretion, with
the primordial Earth being estimated as likely taking anywhere from 70
to 100 million years to form.
Estimates of the age of the Moon range from 4.5 Ga to significantly
younger. A leading hypothesis is that it was formed by accretion from
material loosed from Earth after a Mars-sized object with about 10% of
Earth's mass, named Theia, collided with Earth. It hit Earth with a
glancing blow and some of its mass merged with Earth. Between
approximately 4.0 and , numerous asteroid impacts during the Late
Heavy Bombardment caused significant changes to the greater surface
environment of the Moon and, by inference, to that of Earth.
After formation
=================
Earth's atmosphere and oceans were formed by volcanic activity and
outgassing. Water vapor from these sources condensed into the oceans,
augmented by water and ice from asteroids, protoplanets, and comets.
Sufficient water to fill the oceans may have been on Earth since it
formed. In this model, atmospheric greenhouse gases kept the oceans
from freezing when the newly forming Sun had only 70% of its current
luminosity. By , Earth's magnetic field was established, which helped
prevent the atmosphere from being stripped away by the solar wind.
As the molten outer layer of Earth cooled it formed the first solid
crust, which is thought to have been mafic in composition. The first
continental crust, which was more felsic in composition, formed by the
partial melting of this mafic crust. The presence of grains of the
mineral zircon of Hadean age in Eoarchean sedimentary rocks suggests
that at least some felsic crust existed as early as , only after
Earth's formation. There are two main models of how this initial small
volume of continental crust evolved to reach its current abundance:
(1) a relatively steady growth up to the present day, which is
supported by the radiometric dating of continental crust globally and
(2) an initial rapid growth in the volume of continental crust during
the Archean, forming the bulk of the continental crust that now
exists, which is supported by isotopic evidence from hafnium in
zircons and neodymium in sedimentary rocks. The two models and the
data that support them can be reconciled by large-scale recycling of
the continental crust, particularly during the early stages of Earth's
history.
New continental crust forms as a result of plate tectonics, a process
ultimately driven by the continuous loss of heat from Earth's
interior. Over the period of hundreds of millions of years, tectonic
forces have caused areas of continental crust to group together to
form supercontinents that have subsequently broken apart. At
approximately , one of the earliest known supercontinents, Rodinia,
began to break apart. The continents later recombined to form Pannotia
at , then finally Pangaea, which also began to break apart at .
The most recent pattern of ice ages began about , and then intensified
during the Pleistocene about . High- and middle-latitude regions have
since undergone repeated cycles of glaciation and thaw, repeating
about every 21,000, 41,000 and 100,000 years. The Last Glacial Period,
colloquially called the "last ice age", covered large parts of the
continents, to the middle latitudes, in ice and ended about 11,700
years ago.
Origin of life and evolution
==============================
Chemical reactions led to the first self-replicating molecules about
four billion years ago. A half billion years later, the last common
ancestor of all current life arose. The evolution of photosynthesis
allowed the Sun's energy to be harvested directly by life forms. The
resultant molecular oxygen () accumulated in the atmosphere and due to
interaction with ultraviolet solar radiation, formed a protective
ozone layer () in the upper atmosphere. The incorporation of smaller
cells within larger ones resulted in the development of complex cells
called eukaryotes. True multicellular organisms formed as cells within
colonies became increasingly specialized. Aided by the absorption of
harmful ultraviolet radiation by the ozone layer, life colonized
Earth's surface. Among the earliest fossil evidence for life is
microbial mat fossils found in 3.48 billion-year-old sandstone in
Western Australia, biogenic graphite found in 3.7 billion-year-old
metasedimentary rocks in Western Greenland, and remains of biotic
material found in 4.1 billion-year-old rocks in Western Australia. The
earliest direct evidence of life on Earth is contained in 3.45
billion-year-old Australian rocks showing fossils of microorganisms.
During the Neoproterozoic, , much of Earth might have been covered in
ice. This hypothesis has been termed "Snowball Earth", and it is of
particular interest because it preceded the Cambrian explosion, when
multicellular life forms significantly increased in complexity.
Following the Cambrian explosion, , there have been at least five
major mass extinctions and many minor ones. Apart from the proposed
current Holocene extinction event, the most recent was , when an
asteroid impact triggered the extinction of non-avian dinosaurs and
other large reptiles, but largely spared small animals such as
insects, mammals, lizards and birds. Mammalian life has diversified
over the past , and several million years ago, an African ape species
gained the ability to stand upright. This facilitated tool use and
encouraged communication that provided the nutrition and stimulation
needed for a larger brain, which led to the evolution of humans. The
development of agriculture, and then civilization, led to humans
having an influence on Earth and the nature and quantity of other life
forms that continues to this day.
Future
========
Earth's expected long-term future is tied to that of the Sun. Over the
next , solar luminosity will increase by 10%, and over the next by
40%. Earth's increasing surface temperature will accelerate the
inorganic carbon cycle, possibly reducing concentration to levels
lethally low for current plants ( for C4 photosynthesis) in
approximately . A lack of vegetation would result in the loss of
oxygen in the atmosphere, making current animal life impossible. Due
to the increased luminosity, Earth's mean temperature may reach 100 C
in 1.5 billion years, and all ocean water will evaporate and be lost
to space, which may trigger a runaway greenhouse effect, within an
estimated 1.6 to 3 billion years. Even if the Sun were stable and
eternal, a significant fraction of the water in the modern oceans
would descend into the mantle, due to reduced steam venting from
mid-ocean ridges as the core of the Earth slowly cools.
The Sun will evolve to become a red giant in about . Models predict
that the Sun will expand to roughly 1 AU, about 250 times its present
radius. Earth's fate is less clear. As a red giant, the Sun will lose
roughly 30% of its mass, so, without tidal effects, Earth will move to
an orbit 1.7 AU from the Sun when the star reaches its maximum radius,
otherwise, with tidal effects, it may enter the Sun's atmosphere and
be vaporized, with the heavier elements sinking to the core of the
dying sun.
Size and shape
================
Earth has a rounded shape, through hydrostatic equilibrium, with an
average diameter of 12742 km, making it the fifth largest planetary
sized and largest terrestrial object of the Solar System.
Due to Earth's rotation it has the shape of an ellipsoid, bulging at
its equator; its diameter is 43 km longer there than at its poles.
Earth's shape also has local topographic variations; the largest local
variations, like the Mariana Trench (10925 m below local sea level),
shortens Earth's average radius by 0.17% and Mount Everest (8848 m
above local sea level) lengthens it by 0.14%. Since Earth's surface is
farthest out from its center of mass at its equatorial bulge, the
summit of the volcano Chimborazo in Ecuador (6384.4 km) is its
farthest point out. Parallel to the rigid land topography the ocean
exhibits a more dynamic topography.
To measure the local variation of Earth's topography, geodesy employs
an idealized Earth producing a geoid shape. Such a shape is gained if
the ocean is idealized, covering Earth completely and without any
perturbations such as tides and winds. The result is a smooth but
irregular geoid surface, providing a mean sea level as a reference
level for topographic measurements.
Surface
=========
Earth's surface is the boundary between the atmosphere and the solid
Earth and oceans. Defined in this way, it has an area of about 510
e6km2. Earth can be divided into two hemispheres: by latitude into the
polar Northern and Southern hemispheres; or by longitude into the
continental Eastern and Western hemispheres.
Most of Earth's surface is ocean water: 70.8% or 361 e6km2. This vast
pool of salty water is often called the 'world ocean', and makes Earth
with its dynamic hydrosphere a water world or ocean world. Indeed, in
Earth's early history the ocean may have covered Earth completely. The
world ocean is commonly divided into the Pacific Ocean, Atlantic
Ocean, Indian Ocean, Antarctic or Southern Ocean, and Arctic Ocean,
from largest to smallest. The ocean covers Earth's oceanic crust, with
the shelf seas covering the shelves of the continental crust to a
lesser extent. The oceanic crust forms large oceanic basins with
features like abyssal plains, seamounts, submarine volcanoes, oceanic
trenches, submarine canyons, oceanic plateaus, and a globe-spanning
mid-ocean ridge system. At Earth's polar regions, the ocean surface is
covered by seasonally variable amounts of sea ice that often connects
with polar land, permafrost and ice sheets, forming polar ice caps.
Earth's land covers 29.2%, or 149 e6km2 of Earth's surface. The land
surface includes many islands around the globe, but most of the land
surface is taken by the four continental landmasses, which are (in
descending order): Africa-Eurasia, America (landmass), Antarctica, and
Australia (landmass). These landmasses are further broken down and
grouped into the continents. The terrain of the land surface varies
greatly and consists of mountains, deserts, plains, plateaus, and
other landforms. The elevation of the land surface varies from a low
point of -418 m at the Dead Sea, to a maximum altitude of 8848 m at
the top of Mount Everest. The mean height of land above sea level is
about 797 m.
Land can be covered by surface water, snow, ice, artificial structures
or vegetation. Most of Earth's land hosts vegetation, but considerable
amounts of land are ice sheets (10%, not including the equally large
area of land under permafrost) or deserts (33%).
The pedosphere is the outermost layer of Earth's land surface and is
composed of soil and subject to soil formation processes. Soil is
crucial for land to be arable. Earth's total arable land is 10.7% of
the land surface, with 1.3% being permanent cropland. Earth has an
estimated 16.7 e6km2 of cropland and 33.5 e6km2 of pastureland.
The land surface and the ocean floor form the top of Earth's crust,
which together with parts of the upper mantle form Earth's
lithosphere. Earth's crust may be divided into oceanic and continental
crust. Beneath the ocean-floor sediments, the oceanic crust is
predominantly basaltic, while the continental crust may include lower
density materials such as granite, sediments and metamorphic rocks.
Nearly 75% of the continental surfaces are covered by sedimentary
rocks, although they form about 5% of the mass of the crust.
Earth's surface topography comprises both the topography of the ocean
surface, and the shape of Earth's land surface. The submarine terrain
of the ocean floor has an average bathymetric depth of 4 km, and is as
varied as the terrain above sea level. Earth's surface is continually
being shaped by internal plate tectonic processes including
earthquakes and volcanism; by weathering and erosion driven by ice,
water, wind and temperature; and by biological processes including the
growth and decomposition of biomass into soil.
Tectonic plates
=================
Earth's mechanically rigid outer layer of Earth's crust and upper
mantle, the lithosphere, is divided into tectonic plates. These plates
are rigid segments that move relative to each other at one of three
boundaries types: at convergent boundaries, two plates come together;
at divergent boundaries, two plates are pulled apart; and at transform
boundaries, two plates slide past one another laterally. Along these
plate boundaries, earthquakes, volcanic activity, mountain-building,
and oceanic trench formation can occur. The tectonic plates ride on
top of the asthenosphere, the solid but less-viscous part of the upper
mantle that can flow and move along with the plates.
As the tectonic plates migrate, oceanic crust is subducted under the
leading edges of the plates at convergent boundaries. At the same
time, the upwelling of mantle material at divergent boundaries creates
mid-ocean ridges. The combination of these processes recycles the
oceanic crust back into the mantle. Due to this recycling, most of the
ocean floor is less than old. The oldest oceanic crust is located in
the Western Pacific and is estimated to be old. By comparison, the
oldest dated continental crust is , although zircons have been found
preserved as clasts within Eoarchean sedimentary rocks that give ages
up to , indicating that at least some continental crust existed at
that time.
The seven major plates are the Pacific, North American, Eurasian,
African, Antarctic, Indo-Australian, and South American. Other notable
plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate
off the west coast of South America and the Scotia Plate in the
southern Atlantic Ocean. The Australian Plate fused with the Indian
Plate between . The fastest-moving plates are the oceanic plates, with
the Cocos Plate advancing at a rate of 75 mm/year and the Pacific
Plate moving 52 -. At the other extreme, the slowest-moving plate is
the South American Plate, progressing at a typical rate of 10.6
mm/year.
Internal structure
====================
Geologic layers of Earth |framelessIllustration of Earth's cutaway,
not to scale
last1=Robertson |first1=Eugene C. |date=26 July 2001
|url=
http://pubs.usgs.gov/gip/interior/ |title=The Interior of the
Earth |publisher=USGS |access-date=24 March 2007 |archive-date=28
August 2011
|archive-url=
https://web.archive.org/web/20110828015257/http://pubs.usgs.gov/gip/interior/
|url-status=live }} (km) Component layer name
|0-60 |Lithosphere |--
|0-35 |Crust |2.2-2.9
|35-660 |Upper mantle |3.4-4.4
|660-2890 |Lower mantle |3.4-5.6
|100-700 |Asthenosphere |--
|2890-5100 |Outer core |9.9-12.2
|5100-6378 |Inner core |12.8-13.1
Earth's interior, like that of the other terrestrial planets, is
divided into layers by their chemical or physical (rheological)
properties. The outer layer is a chemically distinct silicate solid
crust, which is underlain by a highly viscous solid mantle. The crust
is separated from the mantle by the Mohorovičić discontinuity. The
thickness of the crust varies from about 6 km under the oceans to 30 -
for the continents. The crust and the cold, rigid, top of the upper
mantle are collectively known as the lithosphere, which is divided
into independently moving tectonic plates.
Beneath the lithosphere is the asthenosphere, a relatively
low-viscosity layer on which the lithosphere rides. Important changes
in crystal structure within the mantle occur at 410 and below the
surface, spanning a transition zone that separates the upper and lower
mantle. Beneath the mantle, an extremely low viscosity liquid outer
core lies above a solid inner core. Earth's inner core may be rotating
at a slightly higher angular velocity than the remainder of the
planet, advancing by 0.1-0.5° per year, although both somewhat higher
and much lower rates have also been proposed. The radius of the inner
core is about one-fifth of that of Earth. The density increases with
depth. Among the Solar System's planetary-sized objects, Earth is the
object with the highest density.
Chemical composition
======================
Earth's mass is approximately (). It is composed mostly of iron
(32.1% by mass), oxygen (30.1%), silicon (15.1%), magnesium (13.9%),
sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%),
with the remaining 1.2% consisting of trace amounts of other elements.
Due to gravitational separation, the core is primarily composed of the
denser elements: iron (88.8%), with smaller amounts of nickel (5.8%),
sulfur (4.5%), and less than 1% trace elements. The most common rock
constituents of the crust are oxides. Over 99% of the crust is
composed of various oxides of eleven elements, principally oxides
containing silicon (the silicate minerals), aluminium, iron, calcium,
magnesium, potassium, or sodium.
Internal heat
===============
The major contributors to Earth's internal heat are primordial heat
(heat left over from Earth's formation) and radiogenic heat (heat
produced by radioactive decay). The major heat-producing isotopes
within Earth are potassium-40, uranium-238, and thorium-232. At the
center, the temperature may be up to 6000 C, and the pressure could
reach 360 GPa. Because much of the heat is provided by radioactive
decay, scientists postulate that early in Earth's history, before
isotopes with short half-lives were depleted, Earth's heat production
was much higher. At approximately , twice the present-day heat would
have been produced, increasing the rates of mantle convection and
plate tectonics, and allowing the production of uncommon igneous rocks
such as komatiites that are rarely formed today.
The mean heat loss from Earth is , for a global heat loss of . A
portion of the core's thermal energy is transported toward the crust
by mantle plumes, a form of convection consisting of upwellings of
higher-temperature rock. These plumes can produce hotspots and flood
basalts. More of the heat in Earth is lost through plate tectonics, by
mantle upwelling associated with mid-ocean ridges. The final major
mode of heat loss is through conduction through the lithosphere, the
majority of which occurs under the oceans.
Gravitational field
=====================
The gravity of Earth is the acceleration that is imparted to objects
due to the distribution of mass within Earth. Near Earth's surface,
gravitational acceleration is approximately 9.8 m/s2. Local
differences in topography, geology, and deeper tectonic structure
cause local and broad regional differences in Earth's gravitational
field, known as gravity anomalies.
Magnetic field
================
The main part of Earth's magnetic field is generated in the core, the
site of a dynamo process that converts the kinetic energy of thermally
and compositionally driven convection into electrical and magnetic
field energy. The field extends outwards from the core, through the
mantle, and up to Earth's surface, where it is, approximately, a
dipole. The poles of the dipole are located close to Earth's
geographic poles. At the equator of the magnetic field, the
magnetic-field strength at the surface is , with a magnetic dipole
moment of at epoch 2000, decreasing nearly 6% per century (although
it still remains stronger than its long time average). The convection
movements in the core are chaotic; the magnetic poles drift and
periodically change alignment. This causes secular variation of the
main field and field reversals at irregular intervals averaging a few
times every million years. The most recent reversal occurred
approximately 700,000 years ago.
The extent of Earth's magnetic field in space defines the
magnetosphere. Ions and electrons of the solar wind are deflected by
the magnetosphere; solar wind pressure compresses the day-side of the
magnetosphere, to about 10 Earth radii, and extends the night-side
magnetosphere into a long tail. Because the velocity of the solar wind
is greater than the speed at which waves propagate through the solar
wind, a supersonic bow shock precedes the day-side magnetosphere
within the solar wind. Charged particles are contained within the
magnetosphere; the plasmasphere is defined by low-energy particles
that essentially follow magnetic field lines as Earth rotates. The
ring current is defined by medium-energy particles that drift relative
to the geomagnetic field, but with paths that are still dominated by
the magnetic field, and the Van Allen radiation belts are formed by
high-energy particles whose motion is essentially random, but
contained in the magnetosphere. During magnetic storms and substorms,
charged particles can be deflected from the outer magnetosphere and
especially the magnetotail, directed along field lines into Earth's
ionosphere, where atmospheric atoms can be excited and ionized,
causing an aurora.
Rotation
==========
Earth's rotation period relative to the Sun--its mean solar day--is
of mean solar time (). Because Earth's solar day is now slightly
longer than it was during the 19th century due to tidal deceleration,
each day varies between longer than the mean solar day.
Earth's rotation period relative to the fixed stars, called its
'stellar day' by the International Earth Rotation and Reference
Systems Service (IERS), is of mean solar time (UT1), or Earth's
rotation period relative to the precessing or moving mean March
equinox (when the Sun is at 90° on the equator), is of mean solar
time (UT1) . Thus the sidereal day is shorter than the stellar day by
about 8.4 ms.
Apart from meteors within the atmosphere and low-orbiting satellites,
the main apparent motion of celestial bodies in Earth's sky is to the
west at a rate of 15°/h = 15'/min. For bodies near the celestial
equator, this is equivalent to an apparent diameter of the Sun or the
Moon every two minutes; from Earth's surface, the apparent sizes of
the Sun and the Moon are approximately the same.
Orbit
=======
Earth orbits the Sun, making Earth the third-closest planet to the Sun
and part of the inner Solar System. Earth's average orbital distance
is about 150 e6km, which is the basis for the astronomical unit (AU)
and is equal to roughly 8.3 light minutes or 380 times Earth's
distance to the Moon. Earth orbits the Sun every 365.2564 mean solar
days, or one sidereal year. With an apparent movement of the Sun in
Earth's sky at a rate of about 1°/day eastward, which is one apparent
Sun or Moon diameter every 12 hours. Due to this motion, on average it
takes 24 hours--a solar day--for Earth to complete a full rotation
about its axis so that the Sun returns to the meridian.
The orbital speed of Earth averages about 29.78 km/s, which is fast
enough to travel a distance equal to Earth's diameter, about 12742 km,
in seven minutes, and the distance from Earth to the Moon, 384400 km,
in about 3.5 hours.
The Moon and Earth orbit a common barycenter every 27.32 days relative
to the background stars. When combined with the Earth-Moon system's
common orbit around the Sun, the period of the synodic month, from new
moon to new moon, is 29.53 days. Viewed from the celestial north pole,
the motion of Earth, the Moon, and their axial rotations are all
counterclockwise. Viewed from a vantage point above the Sun and
Earth's north poles, Earth orbits in a counterclockwise direction
about the Sun. The orbital and axial planes are not precisely aligned:
Earth's axis is tilted some 23.44 degrees from the perpendicular to
the Earth-Sun plane (the ecliptic), and the Earth-Moon plane is tilted
up to ±5.1 degrees against the Earth-Sun plane. Without this tilt,
there would be an eclipse every two weeks, alternating between lunar
eclipses and solar eclipses.
The Hill sphere, or the sphere of gravitational influence, of Earth is
about 1.5 e6km in radius. This is the maximum distance at which
Earth's gravitational influence is stronger than that of the more
distant Sun and planets. Objects must orbit Earth within this radius,
or they can become unbound by the gravitational perturbation of the
Sun. Earth, along with the Solar System, is situated in the Milky Way
and orbits about 28,000 light-years from its center. It is about 20
light-years above the galactic plane in the Orion Arm.
Axial tilt and seasons
========================
The axial tilt of Earth is approximately 23.439281° with the axis of
the plane of the Earth's orbit by definition pointing always towards
the Celestial Poles. Due to Earth's axial tilt, the amount of sunlight
reaching any given point on the surface varies over the course of the
year. This causes the seasonal change in climate, with summer in the
Northern Hemisphere occurring when the Tropic of Cancer is facing the
Sun, and in the Southern Hemisphere when the Tropic of Capricorn faces
the Sun. In each instance, winter occurs simultaneously in the
opposite hemisphere.
During the summer, the day lasts longer, and the Sun climbs higher in
the sky. In winter, the climate becomes cooler and the days shorter.
Above the Arctic Circle and below the Antarctic Circle there is no
daylight at all for part of the year, causing a polar night, and this
night extends for several months at the poles themselves. These same
latitudes also experience a midnight sun, where the sun remains
visible all day.
By astronomical convention, the four seasons can be determined by the
solstices--the points in the orbit of maximum axial tilt toward or
away from the Sun--and the equinoxes, when Earth's rotational axis is
aligned with its orbital axis. In the Northern Hemisphere, winter
solstice currently occurs around 21 December; summer solstice is near
21 June, spring equinox is around 20 March and autumnal equinox is
about 22 or 23 September. In the Southern Hemisphere, the situation is
reversed, with the summer and winter solstices exchanged and the
spring and autumnal equinox dates swapped.
The angle of Earth's axial tilt is relatively stable over long periods
of time. Its axial tilt does undergo nutation; a slight, irregular
motion with a main period of 18.6 years. The orientation (rather than
the angle) of Earth's axis also changes over time, precessing around
in a complete circle over each 25,800-year cycle; this precession is
the reason for the difference between a sidereal year and a tropical
year. Both of these motions are caused by the varying attraction of
the Sun and the Moon on Earth's equatorial bulge. The poles also
migrate a few meters across Earth's surface. This polar motion has
multiple, cyclical components, which collectively are termed
quasiperiodic motion. In addition to an annual component to this
motion, there is a 14-month cycle called the Chandler wobble. Earth's
rotational velocity also varies in a phenomenon known as length-of-day
variation.
Earth's annual orbit is elliptical rather than circular, and its
closest approach to the Sun is called perihelion. In modern times,
Earth's perihelion occurs around 3 January, and its aphelion around 4
July. These dates shift over time due to precession and changes to the
orbit, the latter of which follows cyclical patterns known as
Milankovitch cycles. The annual change in the Earth-Sun distance
causes an increase of about 6.8% in solar energy reaching Earth at
perihelion relative to aphelion. Because the Southern Hemisphere is
tilted toward the Sun at about the same time that Earth reaches the
closest approach to the Sun, the Southern Hemisphere receives slightly
more energy from the Sun than does the northern over the course of a
year. This effect is much less significant than the total energy
change due to the axial tilt, and most of the excess energy is
absorbed by the higher proportion of water in the Southern Hemisphere.
Moon
======
The Moon is a relatively large, terrestrial, planet-like natural
satellite, with a diameter about one-quarter of Earth's. It is the
largest moon in the Solar System relative to the size of its planet,
although Charon is larger relative to the dwarf planet Pluto. The
natural satellites of other planets are also referred to as "moons",
after Earth's. The most widely accepted theory of the Moon's origin,
the giant-impact hypothesis, states that it formed from the collision
of a Mars-size protoplanet called Theia with the early Earth. This
hypothesis explains the Moon's relative lack of iron and volatile
elements and the fact that its composition is nearly identical to that
of Earth's crust. Computer simulations suggest that two blob-like
remnants of this protoplanet could be inside the Earth.
The gravitational attraction between Earth and the Moon causes lunar
tides on Earth. The same effect on the Moon has led to its tidal
locking: its rotation period is the same as the time it takes to orbit
Earth. As a result, it always presents the same face to the planet. As
the Moon orbits Earth, different parts of its face are illuminated by
the Sun, leading to the lunar phases. Due to their tidal interaction,
the Moon recedes from Earth at the rate of approximately 38 mm/yr.
Over millions of years, these tiny modifications--and the lengthening
of Earth's day by about 23 μs/yr--add up to significant changes.
During the Ediacaran period, for example, (approximately ) there were
400±7 days in a year, with each day lasting 21.9±0.4 hours.
The Moon may have dramatically affected the development of life by
moderating the planet's climate. Paleontological evidence and computer
simulations show that Earth's axial tilt is stabilized by tidal
interactions with the Moon. Some theorists think that without this
stabilization against the torques applied by the Sun and planets to
Earth's equatorial bulge, the rotational axis might be chaotically
unstable, exhibiting large changes over millions of years, as is the
case for Mars, though this is disputed.
Viewed from Earth, the Moon is just far enough away to have almost the
same apparent-sized disk as the Sun. The angular size (or solid angle)
of these two bodies match because, although the Sun's diameter is
about 400 times as large as the Moon's, it is also 400 times more
distant. This allows total and annular solar eclipses to occur on
Earth.
Asteroids and artificial satellites
=====================================
Earth's co-orbital asteroids population consists of quasi-satellites,
objects with a horseshoe orbit and trojans. There are at least seven
quasi-satellites, including 469219 Kamoʻoalewa, ranging in diameter
from 10 m to 5000 m. A trojan asteroid companion, , is librating
around the leading Lagrange triangular point, L4, in Earth's orbit
around the Sun. The tiny near-Earth asteroid makes close approaches
to the Earth-Moon system roughly every twenty years. During these
approaches, it can orbit Earth for brief periods of time.
, there are 4,550 operational, human-made satellites orbiting Earth.
There are also inoperative satellites, including Vanguard 1, the
oldest satellite currently in orbit, and over 16,000 pieces of tracked
space debris. Earth's largest artificial satellite is the
International Space Station (ISS).
Hydrosphere
======================================================================
Earth's hydrosphere is the sum of Earth's water and its distribution.
Most of Earth's hydrosphere consists of Earth's global ocean. Earth's
hydrosphere also consists of water in the atmosphere and on land,
including clouds, inland seas, lakes, rivers, and underground waters.
The mass of the oceans is approximately 1.35 metric tons or about
1/4400 of Earth's total mass. The oceans cover an area of 361.8 e6km2
with a mean depth of 3682 m, resulting in an estimated volume of 1.332
e9km3.
If all of Earth's crustal surface were at the same elevation as a
smooth sphere, the depth of the resulting world ocean would be 2.7 to.
About 97.5% of the water is saline; the remaining 2.5% is fresh water.
Most fresh water, about 68.7%, is present as ice in ice caps and
glaciers. The remaining 30% is ground water, 1% surface water
(covering only 2.8% of Earth's land) and other small forms of fresh
water deposits such as permafrost, water vapor in the atmosphere,
biological binding, etc.
In Earth's coldest regions, snow survives over the summer and changes
into ice. This accumulated snow and ice eventually forms into
glaciers, bodies of ice that flow under the influence of their own
gravity. Alpine glaciers form in mountainous areas, whereas vast ice
sheets form over land in polar regions. The flow of glaciers erodes
the surface, changing it dramatically, with the formation of U-shaped
valleys and other landforms. Sea ice in the Arctic covers an area
about as big as the United States, although it is quickly retreating
as a consequence of climate change.
The average salinity of Earth's oceans is about 35 grams of salt per
kilogram of seawater (3.5% salt). Most of this salt was released from
volcanic activity or extracted from cool igneous rocks. The oceans are
also a reservoir of dissolved atmospheric gases, which are essential
for the survival of many aquatic life forms. Sea water has an
important influence on the world's climate, with the oceans acting as
a large heat reservoir. Shifts in the oceanic temperature distribution
can cause significant weather shifts, such as the El Niño-Southern
Oscillation.
The abundance of water, particularly liquid water, on Earth's surface
is a unique feature that distinguishes it from other planets in the
Solar System. Solar System planets with considerable atmospheres do
partly host atmospheric water vapor, but they lack surface conditions
for stable surface water. Despite some moons showing signs of large
reservoirs of extraterrestrial liquid water, with possibly even more
volume than Earth's ocean, all of them are large bodies of water under
a kilometers thick frozen surface layer.
Atmosphere
======================================================================
The atmospheric pressure at Earth's sea level averages 101.325 kPa,
with a scale height of about 8.5 km. A dry atmosphere is composed of
78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of
carbon dioxide and other gaseous molecules. Water vapor content varies
between 0.01% and 4% but averages about 1%. Clouds cover around
two-thirds of Earth's surface, more so over oceans than land. The
height of the troposphere varies with latitude, ranging between 8 km
at the poles to 17 km at the equator, with some variation resulting
from weather and seasonal factors.
Earth's biosphere has significantly altered its atmosphere. Oxygenic
photosynthesis evolved , forming the primarily nitrogen-oxygen
atmosphere of today. This change enabled the proliferation of aerobic
organisms and, indirectly, the formation of the ozone layer due to the
subsequent conversion of atmospheric into . The ozone layer blocks
ultraviolet solar radiation, permitting life on land. Other
atmospheric functions important to life include transporting water
vapor, providing useful gases, causing small meteors to burn up before
they strike the surface, and moderating temperature. This last
phenomenon is the greenhouse effect: trace molecules within the
atmosphere serve to capture thermal energy emitted from the surface,
thereby raising the average temperature. Water vapor, carbon dioxide,
methane, nitrous oxide, and ozone are the primary greenhouse gases in
the atmosphere. Without this heat-retention effect, the average
surface temperature would be -18 C, in contrast to the current +15 C,
and life on Earth probably would not exist in its current form.
Weather and climate
=====================
Earth's atmosphere has no definite boundary, gradually becoming
thinner and fading into outer space. Three-quarters of the
atmosphere's mass is contained within the first 11 km of the surface;
this lowest layer is called the troposphere. Energy from the Sun heats
this layer, and the surface below, causing expansion of the air. This
lower-density air then rises and is replaced by cooler, higher-density
air. The result is atmospheric circulation that drives the weather and
climate through redistribution of thermal energy.
The primary atmospheric circulation bands consist of the trade winds
in the equatorial region below 30° latitude and the westerlies in the
mid-latitudes between 30° and 60°. Ocean heat content and currents are
also important factors in determining climate, particularly the
thermohaline circulation that distributes thermal energy from the
equatorial oceans to the polar regions.
Earth receives 1361 W/m2 of solar irradiance. The amount of solar
energy that reaches Earth's surface decreases with increasing
latitude. At higher latitudes, the sunlight reaches the surface at
lower angles, and it must pass through thicker columns of the
atmosphere. As a result, the mean annual air temperature at sea level
decreases by about 0.4 C-change per degree of latitude from the
equator. Earth's surface can be subdivided into specific latitudinal
belts of approximately homogeneous climate. Ranging from the equator
to the polar regions, these are the tropical (or equatorial),
subtropical, temperate and polar climates.
Further factors that affect a location's climates are its proximity to
oceans, the oceanic and atmospheric circulation, and topology. Places
close to oceans typically have colder summers and warmer winters, due
to the fact that oceans can store large amounts of heat. The wind
transports the cold or the heat of the ocean to the land. Atmospheric
circulation also plays an important role: San Francisco and Washington
DC are both coastal cities at about the same latitude. San Francisco's
climate is significantly more moderate as the prevailing wind
direction is from sea to land. Finally, temperatures decrease with
height causing mountainous areas to be colder than low-lying areas.
Water vapor generated through surface evaporation is transported by
circulatory patterns in the atmosphere. When atmospheric conditions
permit an uplift of warm, humid air, this water condenses and falls to
the surface as precipitation. Most of the water is then transported to
lower elevations by river systems and usually returned to the oceans
or deposited into lakes. This water cycle is a vital mechanism for
supporting life on land and is a primary factor in the erosion of
surface features over geological periods. Precipitation patterns vary
widely, ranging from several meters of water per year to less than a
millimeter. Atmospheric circulation, topographic features, and
temperature differences determine the average precipitation that falls
in each region.
The commonly used Köppen climate classification system has five broad
groups (humid tropics, arid, humid middle latitudes, continental and
cold polar), which are further divided into more specific subtypes.
The Köppen system rates regions based on observed temperature and
precipitation. Surface air temperature can rise to around 55 C in hot
deserts, such as Death Valley, and can fall as low as -89 C in
Antarctica.
Upper atmosphere
==================
The upper atmosphere, the atmosphere above the troposphere, is usually
divided into the stratosphere, mesosphere, and thermosphere. Each
layer has a different lapse rate, defining the rate of change in
temperature with height. Beyond these, the exosphere thins out into
the magnetosphere, where the geomagnetic fields interact with the
solar wind. Within the stratosphere is the ozone layer, a component
that partially shields the surface from ultraviolet light and thus is
important for life on Earth. The Kármán line, defined as 100 km above
Earth's surface, is a working definition for the boundary between the
atmosphere and outer space.
Thermal energy causes some of the molecules at the outer edge of the
atmosphere to increase their velocity to the point where they can
escape from Earth's gravity. This causes a slow but steady loss of the
atmosphere into space. Because unfixed hydrogen has a low molecular
mass, it can achieve escape velocity more readily, and it leaks into
outer space at a greater rate than other gases. The leakage of
hydrogen into space contributes to the shifting of Earth's atmosphere
and surface from an initially reducing state to its current oxidizing
one. Photosynthesis provided a source of free oxygen, but the loss of
reducing agents such as hydrogen is thought to have been a necessary
precondition for the widespread accumulation of oxygen in the
atmosphere. Hence the ability of hydrogen to escape from the
atmosphere may have influenced the nature of life that developed on
Earth. In the current, oxygen-rich atmosphere most hydrogen is
converted into water before it has an opportunity to escape. Instead,
most of the hydrogen loss comes from the destruction of methane in the
upper atmosphere.
Life on Earth
======================================================================
Earth is the only known place that has ever been habitable for life.
Earth's life developed in Earth's early bodies of water some hundred
million years after Earth formed. Earth's life has been shaping and
inhabiting many particular ecosystems on Earth and has eventually
expanded globally forming an overarching biosphere.
Therefore, life has impacted Earth, significantly altering Earth's
atmosphere and surface over long periods of time, causing changes like
the Great Oxidation Event. Earth's life has also over time greatly
diversified, allowing the biosphere to have different biomes, which
are inhabited by comparatively similar plants and animals. The
different biomes developed at distinct elevations or water depths,
planetary temperature latitudes and on land also with different
humidity. Earth's species diversity and biomass reaches a peak in
shallow waters and with forests, particularly in equatorial, warm and
humid conditions. While freezing polar regions and high altitudes, or
extremely arid areas are relatively barren of plant and animal life.
Extreme weather, such as tropical cyclones (including hurricanes and
typhoons), occurs over most of Earth's surface and has a large impact
on life in those areas. From 1980 to 2000, these events caused an
average of 11,800 human deaths per year. Many places are subject to
earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes,
blizzards, floods, droughts, wildfires, and other calamities and
disasters. Human impact is felt in many areas due to pollution of the
air and water, acid rain, loss of vegetation (overgrazing,
deforestation, desertification), loss of wildlife, species extinction,
soil degradation, soil depletion and erosion. Human activities release
greenhouse gases into the atmosphere which cause global warming. This
is driving changes such as the melting of glaciers and ice sheets, a
global rise in average sea levels, increased risk of drought and
wildfires, and migration of species to colder areas.
Human geography
======================================================================
Humans, who originated from earlier primates in Eastern Africa
300,000years ago, have since been migrating around Earth, and with the
advent of agriculture in the 10th millennium BC, have been
increasingly settling Earth's land. In the 20th century, Antarctica
became the last continent to be explored and settled by humans,
although human presence there remains limited.
Since the 19th century, human population has grown exponentially to
eight billion in the 2020s, and is projected to peak at around ten
billion in the second half of the 21st century. Most of the growth is
expected to take place in sub-Saharan Africa.
Distribution and density of human population varies greatly around the
world with the majority living in south to eastern Asia and 90%
inhabiting the Northern Hemisphere of Earth, partly due to the
hemispherical predominance of the world's land mass, with 68% of the
world's land mass being in the Northern Hemisphere. Furthermore, since
the 19th century humans have increasingly converged into urban areas,
with the majority living in urban areas by the 21st century.
Beyond Earth's surface, humans have lived only in a few
special-purpose deep underground and underwater presences and a few
space stations. The human population virtually completely remains on
Earth's surface, fully depending on Earth and the environment it
sustains. Since the second half of the 20th century, some hundreds of
humans have temporarily stayed beyond Earth, a tiny fraction of whom
have reached another celestial body, the Moon.
Earth has been subject to extensive human settlement, and humans have
developed diverse societies and cultures. Most of Earth's land has
been territorially claimed since the 19th century by sovereign states
(countries) separated by political borders, and 205 such states exist
today, with only parts of Antarctica and a few small regions remaining
unclaimed. Together, most of these states form the United Nations, the
leading worldwide intergovernmental organization, which extends human
governance over the ocean and Antarctica, and therefore all of Earth.
Natural resources and land use
================================
Earth has resources that have been exploited by humans. Those termed
non-renewable resources, such as fossil fuels, are only replenished
over geological timescales. Large deposits of fossil fuels are
obtained from Earth's crust, consisting of coal, petroleum, and
natural gas. These deposits are used by humans both for energy
production and as feedstock for chemical production. Mineral ore
bodies have also been formed within the crust through a process of ore
genesis, resulting from actions of magmatism, erosion, and plate
tectonics. These metals and other elements are extracted by mining, a
process which often brings environmental and health damage.
Earth's biosphere produces many useful biological products for humans,
including food, wood, pharmaceuticals, oxygen, and the recycling of
organic waste. The land-based ecosystem depends upon topsoil and fresh
water, and the oceanic ecosystem depends on dissolved nutrients washed
down from the land. In 2019, 39 e6km2 of Earth's land surface
consisted of forest and woodlands, 12 e6km2 was shrub and grassland,
40 e6km2 were used for animal feed production and grazing, and 11
e6km2 were cultivated as croplands. Of the 1214% of ice-free land that
is used for croplands, 2 percentage points were irrigated in 2015.
Humans use building materials to construct shelters.
Humans and the environment
============================
Human activities have impacted Earth's environments. Through
activities such as the burning of fossil fuels, humans have been
increasing the amount of greenhouse gases in the atmosphere, altering
Earth's energy budget and climate. It is estimated that global
temperatures in the year 2020 were 1.2 C-change warmer than the
preindustrial baseline. This increase in temperature, known as global
warming, has contributed to the melting of glaciers, rising sea
levels, increased risk of drought and wildfires, and migration of
species to colder areas.
The concept of planetary boundaries was introduced to quantify
humanity's impact on Earth. Of the nine identified boundaries, five
have been crossed: Biosphere integrity, climate change, chemical
pollution, destruction of wild habitats and the nitrogen cycle are
thought to have passed the safe threshold. As of 2018, no country
meets the basic needs of its population without transgressing
planetary boundaries. It is thought possible to provide all basic
physical needs globally within sustainable levels of resource use.
Cultural and historical viewpoint
======================================================================
Human cultures have developed many views of the planet. The standard
astronomical symbols of Earth are a quartered circle, 🜨, representing
the four corners of the world, and a globus cruciger, ♁. Earth is
sometimes personified as a deity. In many cultures it is a mother
goddess that is also the primary fertility deity. Creation myths in
many religions involve the creation of Earth by a supernatural deity
or deities. The Gaia hypothesis, developed in the mid-20th century,
compared Earth's environments and life as a single self-regulating
organism leading to broad stabilization of the conditions of
habitability.
Images of Earth taken from space, particularly during the Apollo
program, have been credited with altering the way that people viewed
the planet that they lived on, called the overview effect, emphasizing
its beauty, uniqueness and apparent fragility. In particular, this
caused a realization of the scope of effects from human activity on
Earth's environment. Enabled by science, particularly Earth
observation, humans have started to take action on environmental
issues globally, acknowledging the impact of humans and the
interconnectedness of Earth's environments.
Scientific investigation has resulted in several culturally
transformative shifts in people's view of the planet. Initial belief
in a flat Earth was gradually displaced in Ancient Greece by the idea
of a spherical Earth, which was attributed to both the philosophers
Pythagoras and Parmenides. Earth was generally believed to be the
center of the universe until the 16th century, when scientists first
concluded that it was a moving object, one of the planets of the Solar
System.
It was only during the 19th century that geologists realized Earth's
age was at least many millions of years. Lord Kelvin used
thermodynamics to estimate the age of Earth to be between 20 million
and 400 million years in 1864, sparking a vigorous debate on the
subject; it was only when radioactivity and radioactive dating were
discovered in the late 19th and early 20th centuries that a reliable
mechanism for determining Earth's age was established, proving the
planet to be billions of years old.
Notes
======================================================================
{{reflist |30em |group="n" |refs=
}}
References
======================================================================
{{reflist|refs=
See:
*
*
*
See also
}}
External links
======================================================================
* [
https://solarsystem.nasa.gov/planets/earth/overview/ Earth -
Profile] - Solar System Exploration - NASA
* [
http://earthobservatory.nasa.gov/ Earth Observatory] - NASA
* Earth - Videos - International Space Station:
** [
https://www.youtube.com/watch?v=74mhQyuyELQ Video (01:02)] on
YouTube - Earth (time-lapse)
** [
https://www.youtube.com/watch?v=l6ahFFFQBZY Video (00:27)] on
YouTube - Earth and auroras (time-lapse)
*
[
https://www.google.com/maps/@36.6233227,-44.9959756,5662076m/data=!3m1!1e3
Google Earth 3D], interactive map
*
[
https://thehappykoala.github.io/Harmony-of-the-Spheres/#/category/Solar%20System/scenario/The%20Earth%20and%20Moon%20System
Interactive 3D visualization of the Sun, Earth and Moon system]
* [
http://portal.gplates.org/ GPlates Portal] (University of Sydney)
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Earth
