Earth is the third
planet from the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
and the only
astronomical object
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often u ...
known to harbor
life. While large
volumes of water can be found throughout the
Solar System, only
Earth sustains liquid surface water. About 71% of Earth's surface is made up of the
ocean, dwarfing Earth's polar ice, lakes, and rivers. The remaining 29% of Earth's surface is
land
Land, also known as dry land, ground, or earth, is the solid terrestrial surface of the planet Earth that is not submerged by the ocean or other bodies of water. It makes up 29% of Earth's surface and includes the continents and various islan ...
, consisting of continents and islands. Earth's surface layer is formed of several slowly moving
tectonic plates
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
, which interact to produce mountain ranges,
volcanoes, and earthquakes. Earth's liquid
outer core
Earth's outer core is a fluid layer about thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. The outer core begins approximately beneath Earth's surface at the core-mantle boundary and e ...
generates the magnetic field that shapes the
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dyna ...
of the Earth, deflecting destructive
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
s.
The atmosphere of the Earth consists mostly of
nitrogen and
oxygen.
Greenhouse gases in the atmosphere like
carbon dioxide (CO
2) trap a part of the
energy from the Sun close to the surface.
Water vapor is widely present in the atmosphere and
forms clouds that cover most of the planet. More
solar energy is received by tropical regions than polar regions and is redistributed by
atmospheric
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
and
ocean circulation. A region's climate is governed not only by latitude but also by elevation and proximity to moderating oceans. In most areas, severe weather, such as tropical cyclones, thunderstorms, and heatwaves, occurs and greatly impacts life.
Earth is 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 minute
The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly .
Just as the second forms the basis fo ...
s away from the Sun and
orbits it, taking a year (about 365.25 days) to complete one revolution. The
Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes). The
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 380,000 km (1.3 light seconds) and is roughly a quarter as wide as Earth. Through tidal locking, the Moon always faces the Earth with the same side, which causes
tides, stabilizes Earth's axis, and
gradually slows its rotation.
Earth, like most other bodies in the Solar System,
formed 4.5 billion years ago from gas in the
early Solar System. During the first
billion years
A billion years or giga-annum (109 years) is a unit of time on the petasecond scale, more precisely equal to seconds (or simply 1,000,000,000 years).
It is sometimes abbreviated Gy, Ga ("giga-annum"), Byr and variants. The abbreviations Gya or ...
of
Earth's history
The history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geologi ...
, the ocean formed and then
life developed within it. Life spread globally and began to affect Earth's atmosphere and surface, leading to the
Great Oxidation Event two billion years ago.
Humans
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
emerged 300,000 years ago, and have reached a population of 8 billion today. Humans depend on Earth's
biosphere
The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be ...
and natural resources for their survival, but have
increasingly impacted the planet's environment. Today, humanity's impact on Earth's climate, soils, waters, and ecosystems is
unsustainable, threatening people's lives and
causing widespread extinctions of other life.
Etymology
The
Modern English word ''Earth'' developed, via
Middle English, from an
Old English noun most often spelled '.
It has cognates in every
Germanic language, and their
ancestral root has been reconstructed as
*''erþō''. In its earliest attestation, the word ''eorðe'' was already being used to translate the many senses of
Latin ' and
Greek ''gē'': the ground, its
soil, dry land, the human world, the surface of the world (including the sea), and the globe itself. As with Roman
Terra
Terra may often refer to:
* Terra (mythology), primeval Roman goddess
* An alternate name for planet Earth, as well as the Latin name for the planet
Terra may also refer to: Geography Astronomy
* Terra (satellite), a multi-national NASA scienti ...
/Tellūs and Greek
Gaia
In Greek mythology, Gaia (; from Ancient Greek , a poetical form of , 'land' or 'earth'),, , . also spelled Gaea , is the personification of the Earth and one of the Greek primordial deities. Gaia is the ancestral mother—sometimes parth ...
, Earth may have been a
personified goddess in
Germanic paganism: late
Norse mythology included
Jörð ('Earth'), a giantess often given as the mother of
Thor.
Historically, ''earth'' has been written in lowercase. From
early Middle English, its
definite sense as "the globe" was expressed as ''the'' earth. By the era of
Early Modern English
Early Modern English or Early New English (sometimes abbreviated EModE, EMnE, or ENE) is the stage of the English language from the beginning of the Tudor period to the English Interregnum and Restoration, or from the transition from Middle ...
,
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'' remain common.
House styles now vary:
Oxford spelling
Oxford spelling (also ''Oxford English Dictionary'' spelling, Oxford style, or Oxford English spelling) is a spelling standard, named after its use by the University of Oxford, that prescribes the use of British spelling in combination with th ...
recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name (for example, "Earth's atmosphere") but writes it in lowercase when preceded by ''the'' (for example, "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"
Occasionally, the name ''Terra'' is used in scientific writing and especially 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
Italian(s) may refer to:
* Anything of, from, or related to the people of Italy over the centuries
** Italians, an ethnic group or simply a citizen of the Italian Republic or Italian Kingdom
** Italian language, a Romance language
*** Regional It ...
and
Portuguese
Portuguese may refer to:
* anything of, from, or related to the country and nation of Portugal
** Portuguese cuisine, traditional foods
** Portuguese language, a Romance language
*** Portuguese dialects, variants of the Portuguese language
** Portu ...
, while in other Romance languages the word gave rise to names with slightly altered spellings (like the
Spanish
Spanish might refer to:
* Items from or related to Spain:
**Spaniards are a nation and ethnic group indigenous to Spain
**Spanish language, spoken in Spain and many Latin American countries
**Spanish cuisine
Other places
* Spanish, Ontario, Can ...
''Tierra'' and the
French
French (french: français(e), link=no) may refer to:
* Something of, from, or related to France
** French language, which originated in France, and its various dialects and accents
** French people, a nation and ethnic group identified with Franc ...
''Terre''). The Latinate form ''Gæa'' or ''Gaea'' () of the Greek poetic name ''
Gaia
In Greek mythology, Gaia (; from Ancient Greek , a poetical form of , 'land' or 'earth'),, , . also spelled Gaea , is the personification of the Earth and one of the Greek primordial deities. Gaia is the ancestral mother—sometimes parth ...
'' (; 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 classical English .
There are a number of adjectives for the planet Earth. From ''Earth'' itself comes ''earthly''. From the Latin ''Terra'' comes ''terran'' , terrestrial , and (via French) ''terrene'' , and from the Latin ''Tellus'' comes ''tellurian'' and ''telluric''.
Chronology
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
A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reser ...
, and then the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, and
dust (including
primordial nuclide
In geochemistry, geophysics and nuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the ...
s). According to
nebular theory
The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting t ...
,
planetesimals formed by
accretion
Accretion may refer to:
Science
* Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity
* Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...
, 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 4.1 and , numerous
asteroid impacts during the
Late Heavy Bombardment
The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypot ...
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
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
and
outgassing. Water vapor from these sources
condensed into the oceans, augmented by water and ice from asteroids,
protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitation ...
s, and
comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are ...
s.
Sufficient water to fill the oceans may have been on Earth since it formed. In this model, atmospheric
greenhouse gas
A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
es 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
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
.
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
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' bec ...
, which was more
felsic
In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...
in composition, formed by the partial melting of this mafic crust. The presence of grains of the
mineral zircon of Hadean age in
Eoarchean
The Eoarchean (; also spelled Eoarchaean) is the first era of the Archean Eon of the geologic record. It spans 400 million years, from the end of the Hadean Eon 4 billion years ago (4000 Mya) to the start of the Paleoarchean Era 3600 Mya. The ...
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
The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic.
The Earth during the Archea ...
, forming the bulk of the continental crust that now exists,
which is supported by isotopic evidence from
hafnium
Hafnium is a chemical element with the Symbol (chemistry), symbol Hf and atomic number 72. A lustre (mineralogy), lustrous, silvery gray, tetravalence, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirco ...
in
zircon
Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the ...
s 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
Rodinia (from the Russian родина, ''rodina'', meaning "motherland, birthplace") was a Mesoproterozoic and Neoproterozoic supercontinent that assembled 1.26–0.90 billion years ago and broke up 750–633 million years ago.
were probabl ...
, began to break apart. The continents later recombined to form
Pannotia
Pannotia (from Greek: '' pan-'', "all", '' -nótos'', "south"; meaning "all southern land"), also known as the Vendian supercontinent, Greater Gondwana, and the Pan-African supercontinent, was a relatively short-lived Neoproterozoic supercontinent ...
at , then finally
Pangaea
Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
, 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 reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breakin ...
s 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
The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the atmosphere, although still small in relat ...
() 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
In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state'' ...
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
A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, or bacteria alone. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in desert ...
fossils found in 3.48 billion-year-old
sandstone in
Western Australia,
biogenic
A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...
graphite found in 3.7 billion-year-old
metasediment
In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and ...
ary rocks in
Western Greenland
Kitaa, originally Vestgrønland ("West Greenland"), is a former administrative division of Greenland. It was by far the most populated of the divisions, being home to almost 90% of the total population. The divisions were de facto replaced by st ...
, and remains of
biotic material
Biotic material or biological derived material is any material that originates from living organisms. Most such materials contain carbon and are capable of decay.
The earliest life on Earth arose at least 3.5 billion years ago.Schopf, JW, Kudrya ...
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
The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago.
It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is pre ...
, , 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
The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil recor ...
, when multicellular life forms significantly increased in complexity. Following the Cambrian explosion, , there have been at least five major
mass extinctions
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It ...
and many minor ones.
Apart from the proposed current
Holocene extinction event, the
most recent was , when
an asteroid impact triggered the extinction of the non-avian
dinosaurs and other large reptiles, but largely spared small animals such as
insects,
mammals,
lizard
Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia althou ...
s and
birds. Mammalian life has diversified over the past , and several million years ago an African
ape
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
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
A civilization (or civilisation) is any complex society characterized by the development of a state, social stratification, urbanization, and symbolic systems of communication beyond natural spoken language (namely, a writing system).
Ci ...
, 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, reducing concentration to levels lethally low for plants ( for
C4 photosynthesis
carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960's discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants, when supp ...
) in approximately .
The lack of vegetation will result in the loss of oxygen in the atmosphere, making animal life impossible.
Due to the increased luminosity, Earth's mean temperature may reach in 1.5 billion years, and all ocean water will evaporate and be lost to space, which may trigger a
runaway greenhouse effect
A runaway greenhouse effect occurs when a planet's atmosphere contains greenhouse gas in an amount sufficient to block thermal radiation from leaving the planet, preventing the planet from cooling and from having liquid water on its surface. A ...
, within an estimated 1.6 to 3 billion years.
Even if the Sun were stable, a fraction of the water in the modern oceans will descend to the
mantle
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
, due to reduced steam venting from mid-ocean ridges.
The Sun will
evolve to become a
red giant
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
in about . Models predict that the Sun will expand to roughly , 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 from the Sun when the star reaches its maximum radius, otherwise, with tidal effects, it may enter the Sun's atmosphere and be vaporized.
Geophysical characteristics
Size and shape
The shape of Earth is nearly spherical, with an average diameter of , making it the
fifth largest of the Solar System's planetary sized objects and largest among its
terrestrial ones. Due to
Earth's rotation
Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star Polari ...
its shape is
bulged around the Equator and slightly flattened at the
poles,
resulting in a larger diameter at the equator than at the poles.
Earth's shape therefore is more accurately described as an
oblate spheroid.
Earth's shape furthermore has local
topographic variations. Though the largest variations, like the
Mariana Trench ( below local sea level), only shortens Earth's average radius by 0.17% and
Mount Everest ( above local sea level) lengthens it by only 0.14%. Earth's surface is farthest out from Earth's
center of mass
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
at its equatorial bulge, making the summit of the
Chimborazo
Chimborazo () is a currently inactive stratovolcano in the Cordillera Occidental range of the Andes. Its last known eruption is believed to have occurred around 550 A.D.
Chimborazo's summit is the farthest point on the Earth's surface from the ...
volcano in
Ecuador () the farthest point.
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 shape called a
geoid
The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extende ...
. Such a geoid 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 gravitational irregular geoid surface, providing a mean
sea level (MSL) as a reference level for topographic measurements.
Surface
The total
surface area of Earth is about .
Earth's surface can be divided into two
hemispheres, such as into the
Northern
Northern may refer to the following:
Geography
* North, a point in direction
* Northern Europe, the northern part or region of Europe
* Northern Highland, a region of Wisconsin, United States
* Northern Province, Sri Lanka
* Northern Range, a r ...
and
Southern Hemisphere, or the
Western
Western may refer to:
Places
*Western, Nebraska, a village in the US
*Western, New York, a town in the US
*Western Creek, Tasmania, a locality in Australia
*Western Junction, Tasmania, a locality in Australia
*Western world, countries that id ...
and
Eastern Hemisphere.
Most of the
surface is made of water, in
liquid form or in smaller amounts as
ice
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...
. 70.8% () of the Earth's surface consists of the interconnected
ocean,
making it Earth's global ocean or ''world ocean''.
This makes Earth, along with its vibrant
hydrosphere
The hydrosphere () is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to change in shape. This ...
a water world
or
ocean world
An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the surfa ...
,
particularly in Earth's early history when the ocean is thought to have possibly covered Earth completely.
The world ocean is commonly divided into the
Pacific Ocean,
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth#Surface, Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Afr ...
,
Indian Ocean,
Southern Ocean and
Arctic Ocean
The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
, from largest to smallest. Below the ocean's surface are the
continental shelf, mountains, volcanoes,
oceanic trenches,
submarine canyons,
oceanic plateau
An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides.
There are 184 oceanic plateaus in the world, covering an area of or about 5.11% of the ...
s, abyssal plains, and a globe-spanning mid-ocean ridge system.
In contrast, Earth's
land
Land, also known as dry land, ground, or earth, is the solid terrestrial surface of the planet Earth that is not submerged by the ocean or other bodies of water. It makes up 29% of Earth's surface and includes the continents and various islan ...
makes 29.2%, or of Earth's surface area. Earth's land consists of many
islands around the globe, but mainly of four continental
landmass
A landmass, or land mass, is a large region or area of land. The term is often used to refer to lands surrounded by an ocean or sea, such as a continent or a large island. In the field of geology, a landmass is a defined section of contine ...
es, which are from largest to smallest:
Afroeurasia
Afro-Eurasia (also Afroeurasia, Eurafrasia or the Old World) is a landmass comprising the continents of Africa, Asia, and Europe. The terms are compound words of the names of its constituent parts. Its mainland is the largest and most populou ...
,
America
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
,
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
and
Australia.
These landmasses are further broken down and grouped into the
continent
A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...
s. The
terrain
Terrain or relief (also topography, topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The ...
varies greatly and consists of mountains, deserts, plains, plateaus, and other
landform
A landform is a natural or anthropogenic land feature on the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Landforms include hills, m ...
s. The elevation of the land surface varies from the low point of at the
Dead Sea
The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Bank t ...
, to a maximum altitude of at the top of Mount Everest. The mean height of land above sea level is about .
The
continental crust
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' bec ...
consists of lower density material such as the igneous rocks
granite and
andesite. Less common is
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
, a denser volcanic rock that is the primary constituent of the
ocean floor
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
s.
Sedimentary rock is formed from the accumulation of sediment that becomes buried and
compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the crust.
The third form of rock material found on Earth is
metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant
silicate mineral
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, silica (silicon dioxide, ) is usually cons ...
s on Earth's surface include
quartz,
feldspars,
amphibole
Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is Am ...
,
mica,
pyroxene and
olivine.
Common
carbonate mineral
Carbonate minerals are those minerals containing the carbonate ion, .
Carbonate divisions Anhydrous carbonates
*Calcite group: trigonal
**Calcite CaCO3
** Gaspéite (Ni,Mg,Fe2+)CO3
**Magnesite MgCO3
** Otavite CdCO3
**Rhodochrosite MnCO3
**Sid ...
s include
calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
(found in
limestone) and
dolomite Dolomite may refer to:
*Dolomite (mineral), a carbonate mineral
*Dolomite (rock), also known as dolostone, a sedimentary carbonate rock
*Dolomite, Alabama, United States, an unincorporated community
*Dolomite, California, United States, an unincor ...
.
Erosion and tectonics
The interaction between erosion and tectonics has been a topic of debate since the early 1990s. While the tectonic effects on surface processes such as erosion have long been recognized (for example, river formation as a result of tectonic uplift ...
,
volcanic eruptions
Several types of volcanic eruptions—during which lava, tephra ( ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are of ...
,
flooding,
weathering,
glaciation
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
, the growth of
coral reefs, and meteorite impacts are among the processes that constantly reshape Earth's surface over
geological time
The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochrono ...
.
The
pedosphere
The pedosphere (from Greek ''pedon'' "ground" or "earth" and ''sphaira'' "sphere") 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, h ...
is the outermost layer of Earth's continental surface and is composed of soil and subject to
soil formation processes. The total
arable land
Arable land (from the la, arabilis, "able to be ploughed") is any land capable of being ploughed and used to grow crops.'' Oxford English Dictionary'', "arable, ''adj''. and ''n.''" Oxford University Press (Oxford), 2013. Alternatively, for th ...
is 10.7% of the land surface, with 1.3% being permanent cropland. Earth has an estimated of cropland and of pastureland.
Tectonic plates
Earth's mechanically rigid outer layer, 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
A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a ...
, 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
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
,
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
Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic c ...
is
subducted
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
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
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' bec ...
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
Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipela ...
,
African,
Antarctic
The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...
,
Indo-Australian, and
South American
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
. Other notable plates include the
Arabian Plate
The Arabian Plate is a minor tectonic plate in the Northern and Eastern Hemispheres.
It is one of the three continental plates (along with the African and the Indian Plates) that have been moving northward in geological history and collidi ...
, the
Caribbean Plate
The Caribbean Plate is a mostly Oceanic crust, oceanic tectonic plate underlying Central America and the Caribbean Sea off the north coast of South America.
Roughly 3.2 million square kilometers (1.2 million square miles) in area, the Caribbea ...
, the
Nazca Plate off the west coast of South America and the
Scotia Plate
The Scotia Plate () is a tectonic plate on the edge of the South Atlantic and Southern oceans. Thought to have formed during the early Eocene with the opening of the Drake Passage that separates South America from Antarctica, it is a minor plat ...
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
and the Pacific Plate moving . At the other extreme, the slowest-moving plate is the South American Plate, progressing at a typical rate of .
Internal structure
Earth's interior, like that of the other terrestrial planets, is divided into layers by their
chemical or physical (
rheological
Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applie ...
) properties. The outer layer is a chemically distinct
silicate solid crust, which is underlain by a highly
viscous
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the in ...
solid mantle. The crust is separated from the mantle by the
Mohorovičić discontinuity.
The thickness of the crust varies from about under the oceans to for the continents. The crust and the cold, rigid, top of the
upper mantle
The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appro ...
are collectively known as the lithosphere, which is divided into independently moving tectonic plates.
Beneath the lithosphere is the
asthenosphere
The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...
, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at below the surface, spanning a
transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid
outer core
Earth's outer core is a fluid layer about thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. The outer core begins approximately beneath Earth's surface at the core-mantle boundary and e ...
lies above a solid
inner core.
Earth's inner core may be rotating at a slightly higher
angular velocity
In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an objec ...
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.
Density increases with depth, as described in the table on the right.
Among the Solar System's planetary sized objects Earth is the
object with the highest density.
Chemical composition
Earth's mass is approximately (5,970
Yg). It is composed mostly of
iron (32.1%),
oxygen (30.1%),
silicon (15.1%),
magnesium (13.9%),
sulfur (2.9%),
nickel (1.8%),
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
(1.5%), and
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It h ...
(1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to
mass segregation
In astronomy, dynamical mass segregation is the process by which heavier members of a gravitationally bound system, such as a star cluster, tend to move toward the center, while lighter members tend to move farther away from the center.
Equipar ...
, the core region is estimated to be primarily composed of 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 nearly all
oxides: chlorine, sulfur, and fluorine are the important exceptions to this and their total amount in any rock is usually much less than 1%. Over 99% of the crust is composed of 11 oxides, principally silica, alumina, iron oxides, lime, magnesia,
potash, and soda.
Heat
The major heat-producing
isotopes within Earth are
potassium-40,
uranium-238, and
thorium-232.
At the center, the temperature may be up to , and the pressure could reach .
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
Mantle convection is the very slow creeping motion of Earth's solid silicate mantle as convection currents carrying heat from the interior to the planet's surface.
The Earth's surface lithosphere rides atop the asthenosphere and the two form ...
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
Hotspot, Hot Spot or Hot spot may refer to:
Places
* Hot Spot, Kentucky, a community in the United States
Arts, entertainment, and media Fictional entities
* Hot Spot (comics), a name for the DC Comics character Isaiah Crockett
* Hot Spot (Tra ...
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 because the crust there is much thinner than that of the continents.
Gravitational field
The
gravity of Earth is the
acceleration
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...
that is imparted to objects due to the distribution of mass within Earth. Near Earth's surface,
gravitational acceleration is approximately . Local differences in topography,
geology, and deeper tectonic structure cause local and broad regional differences in Earth's gravitational field, known as
gravity anomalies
The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity meas ...
.
Magnetic field
The main part of Earth's magnetic field is generated in the core, the site of a
dynamo
"Dynamo Electric Machine" (end view, partly section, )
A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundat ...
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
In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...
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
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dyna ...
. Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the dayside of the magnetosphere, to about 10 Earth radii, and extends the nightside 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
In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of th ...
precedes the dayside magnetosphere within the solar wind.
Charged particle
In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can also be an electron or a proton, or another elementary particle, ...
s 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 storm
A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field.
The disturbance that d ...
s and
substorm
A substorm, sometimes referred to as a magnetospheric substorm or an auroral substorm, is a brief disturbance in the Earth's magnetosphere that causes energy to be released from the "tail" of the magnetosphere and injected into the high latitud ...
s, 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 the
aurora
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
.
Orbit and rotation
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 star
In astronomy, fixed stars ( la, stellae fixae) is a term to name the full set of glowing points, astronomical objects actually and mainly stars, that appear not to move relative to one another against the darkness of the night sky in the backgro ...
s, called its ''stellar day'' by the
International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Pa ...
(IERS), is of mean solar time (
UT1
Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with ...
), or
Earth's rotation period relative to the
precessing or moving mean
March equinox
The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the verna ...
(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
The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projecti ...
, 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 , which is the basis for the
Astronomical Unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
and is equal to roughly 8.3
light minute
The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly .
Just as the second forms the basis fo ...
s or 380 times
Earth's distance to the Moon.
Earth orbits the Sun every 365.2564 mean
solar day
A synodic day (or synodic rotation period or solar day) is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time.
The synodic day is distinguished from the sidereal day, which is ...
s, 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
A synodic day (or synodic rotation period or solar day) is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time.
The synodic day is distinguished from the sidereal day, which is ...
—for Earth to complete a full rotation about its axis so that the Sun returns to the
meridian
Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to
Science
* Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon
* ...
.
The orbital speed of Earth averages about , which is fast enough to travel a distance equal to Earth's diameter, about , in seven minutes, and the distance to the Moon, , in about 3.5 hours.
The Moon and Earth orbit a common
barycenter
In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...
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
In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.
Variations
In Shona, Middle Eastern, and Euro ...
, 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
Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite s ...
. 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
The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sph ...
, or the sphere of
gravitational influence, of Earth is about in radius.
This is the maximum distance at which Earth's gravitational influence is stronger than 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 its orbit plane, always pointing towards the
Celestial Poles
The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at ...
. 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
Winter is the coldest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultures ...
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
The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth. Its southern equivalent is the Antarctic Circle.
The Arctic Circle marks the southernmost latitude at wh ...
and below the
Antarctic Circle
The Antarctic Circle is the most southerly of the five major circles of latitude that mark maps of Earth. The region south of this circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone. So ...
there is no daylight at all for part of the year, causing a
polar night
The polar night is a phenomenon where the nighttime lasts for more than 24 hours that occurs in the northernmost and southernmost regions of Earth. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnig ...
, 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
The summer solstice, also called the estival solstice or midsummer, occurs when one of Earth's poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere ( Northern and Southern). For that hemisphere, the summer s ...
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
Nutation () is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference frame ...
; 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
A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time fro ...
. 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 mathematics and theoretical physics, quasiperiodic motion is in rough terms the type of motion executed by a dynamical system containing a finite number (two or more) of incommensurable frequencies.
That is, if we imagine that the phase spac ...
. In addition to an annual component to this motion, there is a 14-month cycle called the
Chandler wobble
The Chandler wobble or Chandler variation of latitude is a small deviation in the Earth's axis of rotation relative to the solid earth, which was discovered by and named after American astronomer Seth Carlo Chandler in 1891. It amounts to change o ...
. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.
In modern times, Earth's
perihelion occurs around 3 January, and its
aphelion
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
around 4 July. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as
Milankovitch cycles. The changing 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.
Earth–Moon system
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
In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the wor ...
is larger relative to the
dwarf planet Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest k ...
. 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 (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 Earth's crust.
The gravitational attraction between Earth and the Moon causes
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 . 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
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of foss ...
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
The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it ...
(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
Trojan or Trojans may refer to:
* Of or from the ancient city of Troy
* Trojan language, the language of the historical Trojans
Arts and entertainment Music
* ''Les Troyens'' ('The Trojans'), an opera by Berlioz, premiered part 1863, part 189 ...
. There are at least five quasi-satellites, including
469219 Kamoʻoalewa
469219 Kamoʻoalewa (), provisionally designated , is a very small asteroid, fast rotator and near-Earth object of the Apollo group, approximately in diameter. At present it is a quasi-satellite of Earth, and currently the smallest, closest, ...
.
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
Vanguard 1 (Harvard designation: 1958-Beta 2, COSPAR ID: 1958-005B ) is an American satellite that was the fourth artificial Earth-orbiting satellite to be successfully launched, following Sputnik 1, Sputnik 2, and Explorer 1. It was launch ...
, 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.
Hydrosphere
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 down to a depth of . The mass of the oceans is approximately 1.35
metric ton
The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States ...
s or about 1/4400 of Earth's total mass. The oceans cover an area of with a mean depth of , resulting in an estimated volume of .
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 . 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 cap
In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets.
Description
Ice caps are not constrained by topographical feature ...
s and
glaciers.
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
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
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
El Niño–Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the Tropics, tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of t ...
.
The abundance of
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
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
showing signs of large reservoirs of
extraterrestrial liquid water
Extraterrestrial liquid water () is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and thus surmised as essential ...
, 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
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
at Earth's sea level averages ,
with a
scale height of about .
A dry atmosphere is composed of 78.084%
nitrogen, 20.946% oxygen, 0.934%
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
, 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 at the poles to at the equator, with some variation resulting from weather and seasonal factors.
Earth's
biosphere
The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be ...
has significantly altered its
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
.
Oxygenic photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
evolved ,
forming the primarily nitrogen–oxygen atmosphere of today.
This change enabled the proliferation of
aerobic organisms
Aerobic means "requiring air," in which "air" usually means oxygen.
Aerobic may also refer to
* Aerobic exercise, prolonged exercise of moderate intensity
* Aerobics, a form of aerobic exercise
* Aerobic respiration, the aerobic process of cell ...
and, indirectly, the formation of the ozone layer due to the subsequent
conversion of atmospheric into . The ozone layer blocks
ultraviolet solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/ ...
, 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 , in contrast to the current ,
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 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
Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from year to year, but t ...
that drives the weather and climate through redistribution of thermal energy.
The primary atmospheric circulation bands consist of the
trade winds
The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...
in the equatorial region below 30° latitude and the
westerlies in the mid-latitudes between 30° and 60°.
Ocean heat content and
currents
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (str ...
are also important factors in determining climate, particularly the
thermohaline circulation
Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to tempe ...
that distributes thermal energy from the equatorial oceans to the polar regions.
Earth receives 1361 W/m
2 of
solar irradiance. The amount of solar energy that reaches the 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 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
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referred to ...
(or equatorial),
subtropical,
temperate and
polar
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
*Polar climate, the cli ...
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
A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most a ...
,
humid middle latitudes,
continental
Continental may refer to:
Places
* Continent, the major landmasses of Earth
* Continental, Arizona, a small community in Pima County, Arizona, US
* Continental, Ohio, a small town in Putnam County, US
Arts and entertainment
* ''Continental'' ( ...
and cold
polar
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
*Polar climate, the cli ...
), 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 in
hot desert
A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one ...
s, such as
Death Valley
Death Valley is a desert valley in Eastern California, in the northern Mojave Desert, bordering the Great Basin Desert. During summer, it is the hottest place on Earth.
Death Valley's Badwater Basin is the point of lowest elevation in Nort ...
, and
can fall as low as in
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
.
Upper atmosphere
The upper atmosphere, the atmosphere above the troposphere, is usually divided into the
stratosphere
The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
,
mesosphere
The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...
, 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 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
The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...
, it can achieve
escape velocity
In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non- propelled object to escape from the gravitational influence of a primary body, thus reaching an infinite distance from it. It is typically s ...
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 is
habitable and has hosted 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
The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be ...
. 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 over time greatly diversified, allowing the biosphere to have different
biome
A biome () is a biogeographical unit consisting of a biological community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. Biome is a broader ...
s, which are inhabited by comparatively similar plants and animals. The different biomes develope at distinct
elevations or
water depths, planetary temperature
latitude
In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pole ...
s 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.
Earth provides liquid water—an environment where complex
organic molecules
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
can assemble and interact, and sufficient energy to sustain a
metabolism.
Plants and other organisms take up
nutrients from water, soils and the atmosphere. These nutrients are constantly recycled between different species.
Extreme weather, such as
tropical cyclones (including
hurricane
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
s and
typhoon
A typhoon is a mature tropical cyclone that develops between 180° and 100°E in the Northern Hemisphere. This region is referred to as the Northwestern Pacific Basin, and is the most active tropical cyclone basin on Earth, accounting for a ...
s), 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,
blizzard
A blizzard is a severe snowstorm characterized by strong sustained winds and low visibility, lasting for a prolonged period of time—typically at least three or four hours. A ground blizzard is a weather condition where snow is not falling b ...
s, 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
Acid rain is rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions (low pH). Most water, including drinking water, has a neutral pH that exists between 6.5 and 8.5, but acid ...
, loss of vegetation (
overgrazing
Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, game reserves, or nature res ...
,
deforestation,
desertification
Desertification is a type of land degradation in drylands in which biological productivity is lost due to natural processes or induced by human activities whereby fertile areas become increasingly arid. It is the spread of arid areas caused b ...
), loss of wildlife, species
extinction,
soil degradation
Soil retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a stable soil. Retrogression is primarily due to soil erosion and corresponds to a phenomenon where succession reverts the land to ...
,
soil depletion and
erosion. Human activities release greenhouse gases into the atmosphere which cause
global warming.
This is driving
changes
Changes may refer to:
Books
* ''Changes'', the 12th novel in Jim Butcher's ''The Dresden Files'' Series
* ''Changes'', a novel by Danielle Steel
* ''Changes'', a trilogy of novels on which the BBC TV series was based, written by Peter Dickinson ...
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
Originating from earlier
primates in
eastern Africa 300,000 years ago
humans have since been migrating and with the advent of
agriculture
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...
in the 10th millennium BC increasingly
settling
Settling is the process by which particulates move towards the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction ...
Earth's land. In the 20th century
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
had been the last continent to see a first and until today limited human presence.
Human population has since the 19th century grown exponentially to seven billion in the early 2010s, 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
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
and 90% inhabiting only 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 on a temporary basis, with only special purpose deep
underground and
underwater presence, and a few
space stations. Human population virtually completely remains on Earth's surface, fully depending on Earth and the environment it sustains. Humans have gone and temporarily stayed beyond Earth with some hundreds of people, since the latter half of the 20th century, and only a fraction of them reaching another celestial body, the Moon.
Humans have developed diverse
societies
A society is a group of individuals involved in persistent social interaction, or a large social group sharing the same spatial or social territory, typically subject to the same political authority and dominant cultural expectations. Societ ...
and
cultures, which have marked Earth significantly. Earth has been the claim of extensive human sedetary, extractive and political activity. Earth's land has been mostly territorially claimed since the 19th century by
states, of which today
more than 200 exist, with only Antarctica and few
areas remaining unclaimed. Most of these states together form the
United Nations
The United Nations (UN) is an intergovernmental organization whose stated purposes are to maintain international peace and security, develop friendly relations among nations, achieve international cooperation, and be a centre for harmonizi ...
, the leading worldwide
intergovernmental organization, with international governance having provided legal regimes extraterritorially, extanding human governance
over the ocean and
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
, and therefore all of Earth.
Natural resources and land use
Earth has resources that have been exploited by humans. Those termed
non-renewable resource
A non-renewable resource (also called a finite resource) is a natural resource that cannot be readily replaced by natural means at a pace quick enough to keep up with consumption. An example is carbon-based fossil fuels. The original organic mat ...
s, such as
fossil fuels, are only replenished over geological timescales. Large deposits of fossil fuels are obtained from Earth's crust, consisting of
coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when dead ...
,
petroleum, and
natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon di ...
. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral
ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...
bodies have also been formed within the crust through a process of
ore genesis
Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.
Ore-genesis theories generally involve three components: sou ...
, resulting from actions of
magmatism
Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of ...
, erosion, and plate tectonics. These
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...
s 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, of Earth's land surface consisted of forest and woodlands, was shrub and grassland, were used for animal feed production and grazing, and 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 material
Building material is material used for construction. Many naturally occurring substances, such as clay, rocks, sand, wood, and even twigs and leaves, have been used to construct buildings. Apart from naturally occurring materials, many man-ma ...
s 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 gas
A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
es in the atmosphere, altering
Earth's energy budget
Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but ma ...
and climate.
[ It is estimated that global temperatures in the year 2020 were warmer than the preindustrial baseline. This increase in temperature, known as global warming, has contributed to the melting of glaciers, ]rising sea levels
Rising may refer to:
* Rising, a stage in baking - see Proofing (baking technique)
*Elevation
* Short for Uprising, a rebellion
Film and TV
* "Rising" (''Stargate Atlantis''), the series premiere of the science fiction television program ''Starg ...
, increased risk of drought and wildfires, and migration of species to colder areas.
The concept of planetary boundaries
Planetary boundaries is a concept highlighting human-caused perturbations of Earth systems making them relevant in a way not accommodated by the environmental boundaries separating the three ages within the Holocene epoch. Crossing a planetar ...
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 symbol
Astronomical symbols are abstract pictorial symbols used to represent astronomical objects, theoretical constructs and observational events in European astronomy. The earliest forms of these symbols appear in Greek papyrus texts of late antiq ...
s of Earth are a quartered circle, , representing the four corners of the world, and a globus cruciger, . Earth is sometimes personified
Personification occurs when a thing or abstraction is represented as a person, in literature or art, as a type of anthropomorphic metaphor. The type of personification discussed here excludes passing literary effects such as "Shadows hold their b ...
as a deity. In many cultures it is a mother goddess
A mother goddess is a goddess who represents a personified deification of motherhood, fertility goddess, fertility, creation, destruction, or the earth goddess who embodies the bounty of the earth or nature. When equated with the earth or t ...
that is also the primary fertility deity
A fertility deity is a god or goddess associated with fertility, sex, pregnancy, childbirth, and crops. In some cases these deities are directly associated with these experiences; in others they are more abstract symbols. Fertility rites may ac ...
. Creation myth
A creation myth (or cosmogonic myth) is a symbolic narrative of how the world began and how people first came to inhabit it., "Creation myths are symbolic stories describing how the universe and its inhabitants came to be. Creation myths develop ...
s 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
The overview effect is a cognitive shift reported by some astronauts while viewing the Earth from space. Researchers have characterized the effect as "a state of awe with self-transcendent qualities, precipitated by a particularly striking vis ...
, 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
Earth observation (EO) is the gathering of information about the physical, chemical, and biological systems of the planet Earth. It can be performed via remote-sensing technologies (Earth observation satellites) or through direct-contact sensors ...
, 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
The flat-Earth model is an archaic and scientifically disproven conception of Earth's shape as a plane or disk. Many ancient cultures subscribed to a flat-Earth cosmography, including Greece until the classical period (5th century BC), th ...
was gradually displaced in Ancient Greece
Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cult ...
by the idea of a spherical Earth
Spherical Earth or Earth's curvature refers to the approximation of figure of the Earth as a sphere.
The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of Greek philosophers. ...
, 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
The age of Earth is estimated to be 4.54 ± 0.05 billion years This age may represent the age of Earth's accretion, or core formation, or of the material from which Earth formed. This dating is based on evidence from radiometric age-dating of m ...
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.
See also
Notes
References
External links
Earth – Profile
– Solar System Exploration – NASA
Earth Observatory
– NASA
* Earth – Videos – International Space Station:
*
Video (01:02)
– Earth (time-lapse)
*
Video (00:27)
– Earth and aurora
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
s (time-lapse)
Google Earth 3D
interactive map
Interactive 3D visualization of the Sun, Earth and Moon system
GPlates Portal
(University of Sydney)
{{Authority control
Astronomical objects known since antiquity
Global natural environment
Planets in the circumstellar habitable zone
Nature
Planets of the Solar System
Terrestrial planets