Earth is the third
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
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 us ...
known to harbor
life
Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energ ...
. While large
volumes of water can be found throughout the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
, only
Earth sustains liquid surface water. About 71% of Earth's surface is made up of the
ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
, 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
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates are ...
, 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 dynam ...
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 sola ...
s.
The atmosphere of the Earth consists mostly of
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
.
Greenhouse gases
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 ...
in the atmosphere like
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
(CO
2) trap a part of the
energy from the Sun close to the surface.
Water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
is widely present in the atmosphere and
forms clouds that cover most of the planet. More
solar energy
Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essenti ...
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
An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...
. 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 planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the S ...
s, 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 for ...
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
Permanent may refer to:
Art and entertainment
* ''Permanent'' (film), a 2017 American film
* ''Permanent'' (Joy Division album)
* "Permanent" (song), by David Cook
Other uses
* Permanent (mathematics), a concept in linear algebra
* Permanent (cy ...
natural satellite
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'' ...
, the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, 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
tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another.
Tide t ...
s, 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 geologic ...
, 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
The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust, was a time interval during the Paleoproterozoic era when the Earth's atmosphere ...
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
Specific definitions of sustainability are difficult to agree on and have varied in the literature and over time. The concept of sustainability can be used to guide decisions at the global, national, and individual levels (e.g. sustainable livin ...
, threatening people's lives and
causing widespread extinctions of other life.
Etymology
The
Modern English word ''Earth'' developed, via
Middle English
Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old English p ...
, from an
Old English
Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...
noun most often spelled '.
It has cognates in every
Germanic language
The Germanic languages are a branch of the Indo-European language family spoken natively by a population of about 515 million people mainly in Europe, North America, Oceania and Southern Africa. The most widely spoken Germanic language, Engli ...
, 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
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
' and
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
''gē'': the ground, its
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
, 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 parthenog ...
, Earth may have been a
personified goddess in
Germanic paganism
Germanic paganism or Germanic religion refers to the traditional, culturally significant religion of the Germanic peoples. With a chronological range of at least one thousand years in an area covering Scandinavia, the British Isles, modern Germ ...
: late
Norse mythology
Norse, Nordic, or Scandinavian mythology is the body of myths belonging to the North Germanic peoples, stemming from Old Norse religion and continuing after the Christianization of Scandinavia, and into the Nordic folklore of the modern period ...
included
Jörð
Jörð ( non, Jǫrð, lit=earth) is the personification of earth and a goddess in Norse mythology. She is the mother of the thunder god Thor and a sexual partner of Odin. Jörð is attested in Danish historian ''Gesta Danorum'', composed in the ...
('Earth'), a giantess often given as the mother of
Thor
Thor (; from non, Þórr ) is a prominent god in Germanic paganism. In Norse mythology, he is a hammer-wielding æsir, god associated with lightning, thunder, storms, sacred trees and groves in Germanic paganism and mythology, sacred groves ...
.
Historically, ''earth'' has been written in lowercase. From
early Middle English
Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old English p ...
, 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 E ...
,
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 House style may refer to:
* Standards for writing as specified in the internal style guide of a particular institution, for example, a book publishing company, newspaper, professional organization, or university
* Standards for illustration or grap ...
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
The Romance languages, sometimes referred to as Latin languages or Neo-Latin languages, are the various modern languages that evolved from Vulgar Latin. They are the only extant subgroup of the Italic languages in the Indo-European language fam ...
(languages that evolved from
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
) 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 Ita ...
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 ''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 parthenog ...
'' (; or ) is rare, though the alternative spelling ''Gaia'' has become common due to the
Gaia hypothesis
The Gaia hypothesis (), also known as the Gaia theory, Gaia paradigm, or the Gaia principle, proposes that living organisms interact with their inorganic surroundings on Earth to form a synergistic and self-regulating, complex system that help ...
, 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
The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a ...
partitions a volume out of a
molecular cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen ...
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 ...
, and then the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, and
dust
Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...
(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 ...
,
planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
s 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
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
-sized object with about 10% of Earth's mass, named
Theia
In Greek mythology, Theia (; grc, Θεία, Theía, divine, also rendered Thea or Thia), also called Euryphaessa ( grc, Εὐρυφάεσσα) "wide-shining", is one of the twelve Titans, the children of the earth goddess Gaia and the sky god ...
, 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 hypoth ...
caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth.
After formation
Earth's atmosphere
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
and
ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
s 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
Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which a ...
. 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 ar ...
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 Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
es kept the oceans from freezing when the newly forming Sun
had only 70% of its
current luminosity.
By ,
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...
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 sola ...
.
As the molten outer layer of Earth cooled it
formed the first solid
crust, which is thought to have been
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
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'' be ...
, 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. Th ...
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s 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
Earth ...
, 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 Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...
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 r ...
s and
neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes i ...
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
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 ...
, 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
supercontinent
In geology, a supercontinent is the assembly of most or all of Earth's continent, continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", ...
s 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 probably ...
, 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 age
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gree ...
s began about , and then intensified during the
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
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 IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
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
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 i ...
allowed the Sun's energy to be harvested directly by life forms. The resultant
molecular oxygen
There are several known allotropes of oxygen. The most familiar is molecular oxygen (O2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O3). Others are:
*A ...
() 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 rela ...
() in the upper atmosphere.
The incorporation of smaller cells within larger ones resulted in the
development of complex cells called
eukaryote
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
s.
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
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
by the ozone layer, life colonized Earth's surface.
Among the earliest
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
evidence for
life
Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energ ...
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 deserts. ...
fossils found in 3.48 billion-year-old
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
in
Western Australia
Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...
,
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
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
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 er ...
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
Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
n rocks showing fossils of
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s.
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 ...
, , much of Earth might have been covered in ice. This hypothesis has been termed "
Snowball Earth
The Snowball Earth hypothesis proposes that, during one or more of Earth's Greenhouse and icehouse Earth, icehouse Climate, climates, the Earth's surface, planet's surface became entirely or nearly entirely Freezing, frozen. It is believed that ...
", 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
The Holocene extinction, or Anthropocene extinction, is the ongoing extinction event during the Holocene epoch. The extinctions span numerous families of bacteria, fungi, plants, and animals, including mammals, birds, reptiles, amphibians, f ...
event, the
most recent was , when
an asteroid impact triggered the extinction of the non-avian
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s and other large reptiles, but largely spared small animals such as
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s,
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s,
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 alt ...
s and
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s. 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 sup ...
) 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, 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 Planet#Planetary-mass object, planetary sized objects and largest among its terrestrial planet, terrestrial ones. Due to Earth's rotation its shape is equatorial bulge, bulged around the Equator and slightly flattened at the Geographical pole, 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 topography, 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 at its equatorial bulge, making the summit of the Chimborazo (volcano), Chimborazo volcano in Ecuador () the farthest point.
Parallel to the rigid land topography Ocean surface 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. 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 Spheroid#Area, surface area of Earth is about .
Earth's surface can be divided into two Hemispheres of Earth, hemispheres, such as into the Northern Hemisphere, Northern and Southern Hemisphere, or the Western Hemisphere, Western and Eastern Hemisphere.
Most of the Water distribution on Earth, surface is made of water, in liquid water, liquid form or in smaller amounts as ice. 70.8% () of the Earth's surface consists of the interconnected
ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
,
making it Earth's global ocean or ''world ocean''.
This makes Earth, along with its vibrant hydrosphere a water world
or ocean world,
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, Indian Ocean, Southern Ocean and Arctic Ocean, from largest to smallest. Below the ocean's surface are the continental shelf, mountains, volcanoes,
oceanic trenches, submarine canyons, oceanic plateaus, 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 landmasses, which are from largest to smallest: Afroeurasia,Americas, America, Antarctica and Mainland Australia, Australia.
These landmasses are further broken down and grouped into the continents. The terrain varies greatly and consists of mountains, deserts, plains, plateaus, and other landforms. The elevation of the land surface varies from the low point of at the Dead Sea, 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'' be ...
consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.
Sedimentary rock is formed from the accumulation of sediment that becomes buried and Diagenesis, 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 minerals on Earth's surface include quartz, feldspars, amphibole, mica, pyroxene and olivine.
Common carbonate minerals include calcite (found in limestone) and Dolomite (mineral), dolomite.
Erosion and tectonics, Types of volcanic eruptions, volcanic eruptions, flooding, weathering, glaciation, the growth of coral reefs, and meteorite impacts are among the processes that constantly reshape Earth's surface over geological time.
The pedosphere is the outermost layer of Earth's continental surface and is composed of soil and subject to pedogenesis, soil formation processes. The total arable land 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 boundary, convergent boundaries, two plates come together; at Divergent boundary, divergent boundaries, two plates are pulled apart; and at Transform boundary, transform boundaries, two plates slide past one another laterally. Along these plate boundaries, earthquakes, Volcanism, volcanic activity, Orogeny, mountain-building, and oceanic trench formation can occur.
The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates.
As the tectonic plates migrate, oceanic crust is Subduction, subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes recycles the oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than old. The oldest oceanic crust is located in the Western Pacific and is estimated to be old.
By comparison, the oldest dated
continental crust
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'' be ...
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 Plate, Pacific, North American Plate, North American, Eurasian Plate, Eurasian, African Plate, African, Antarctic Plate, Antarctic, Indo-Australian Plate, Indo-Australian, and South American Plate, South American. Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between . The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of
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 (Rheology, rheological) properties. The outer layer is a chemically distinct Silicate minerals, silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity.
The thickness of the crust varies from about under the oceans to for the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, which is divided into independently moving tectonic plates.
Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at below the surface, spanning a Transition zone (Earth), 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 Earth's inner core, inner core.
Earth's inner core may be rotating at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year, although both somewhat higher and much lower rates have also been proposed.
The radius of the inner core is about one-fifth of that of Earth.
Density increases with depth, as described in the table on the right.
Among the Solar System's planetary sized objects Earth is the List of solar system objects by size, object with the highest density.
Chemical composition
Earth mass, Earth's mass is approximately (5,970 yottagram, Yg). It is composed mostly of iron (32.1%),
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
(30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminum (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, 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 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 Hotspot (geology), hotspots and flood basalts.
More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs under the oceans because the crust there is much thinner than that of the continents.
Gravitational field
The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within Earth. Near Earth's surface, gravitational acceleration is approximately . Local differences in topography, geology, and deeper tectonic structure cause local and broad regional differences in Earth's gravitational field, known as Gravity anomaly, gravity anomalies.
Magnetic field
The main part of Earth's magnetic field is generated in the core, the site of a Dynamo theory, dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth's surface, where it is, approximately, a dipole. The poles of the dipole are located close to Earth's geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is , with a magnetic dipole moment of at epoch 2000, decreasing nearly 6% per century (although it still remains stronger than its long time average).
The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes Geomagnetic secular variation, secular variation of the main field and geomagnetic reversal, 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 dynam ...
. 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 precedes the dayside magnetosphere within the solar wind. Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates. The ring current is defined by medium-energy particles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field,
and the Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.
During magnetic storms and substorms, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's ionosphere, where atmospheric atoms can be excited and ionized, causing the Aurora (astronomy), aurora.
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 acceleration, tidal deceleration, each day varies between longer than the mean solar day.
Earth's rotation period relative to the fixed stars, called its ''stellar day'' by the International Earth Rotation and Reference Systems Service (IERS), is of mean solar time (UT1), or
Earth's rotation period relative to the precession (astronomy), precessing or moving mean March equinox (when the Sun is at 90° on the equator), is of mean solar time (UT1) .
Thus the sidereal day is shorter than the stellar day by about 8.4 ms.
Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.
Orbit
Earth orbits the Sun, making Earth the third-closest planet to the Sun and part of the inner Solar System. Earth's average orbital distance is about , which is the basis for the Astronomical Unit 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 for ...
s or 380 times Lunar distance (astronomy), Earth's distance to the Moon.
Earth orbits the Sun every 365.2564 mean solar days, or one sidereal year. With an apparent movement of the Sun in Earth's sky at a rate of about 1°/day eastward, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a Solar time, solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the Meridian (astronomy), meridian.
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 every 27.32 days relative to the background stars. When combined with the Earth-Moon system's common orbit around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial pole, celestial north pole, the motion of Earth, the Moon, and their axial rotations are all counterclockwise. Viewed from a vantage point above the Sun and Earth's north poles, Earth orbits in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axial tilt, axis is tilted some 23.44 degrees from the perpendicular to the Earth-Sun plane (the ecliptic), and the Earth-Moon plane is tilted up to ±5.1 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.
The Hill sphere, or the sphere of Gravity, 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. Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes the seasonal change in climate, with summer in the Northern Hemisphere occurring when the Tropic of Cancer is facing the Sun, and in the Southern Hemisphere when the Tropic of Capricorn faces the Sun. In each instance, winter occurs simultaneously in the opposite hemisphere. During the summer, the day lasts longer, and the Sun climbs higher in the sky. In winter, the climate becomes cooler and the days shorter. Above the Arctic Circle and below the Antarctic Circle there is no daylight at all for part of the year, causing a polar night, and this night extends for several months at the poles themselves. These same latitudes also experience a midnight sun, where the sun remains visible all day.
By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when Earth's rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and September equinox, autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.
The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years.
The orientation (rather than the angle) of Earth's axis also changes over time, axial precession, precessing around in a complete circle over each 25,800-year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth's equatorial bulge. The poles also migrate a few meters across Earth's surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.
In modern times, Earth's perihelion occurs around 3 January, and its aphelion 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, terrestrial, Planetary-mass moon, 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 (moon), Charon is larger relative to the dwarf planet Pluto. The natural satellites of other planets are also referred to as "moons", after Earth's. The most widely accepted theory of the Moon's origin, the giant-impact hypothesis, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (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
tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another.
Tide t ...
s 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 Microsecond, 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. Paleontology, Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.
Some theorists think that without this stabilization against the torques applied by the Sun and planets to Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting large changes over millions of years, as is the case for Mars, though this is disputed.
Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.
This allows total and annular solar eclipses to occur on Earth.
Asteroids and artificial satellites
Earth's Co-orbital configuration, co-orbital asteroids population consists of quasi-satellites, objects with a horseshoe orbit and Trojan (celestial body), trojans. There are at least five quasi-satellites, including 469219 Kamoʻoalewa.
A Earth trojan, trojan asteroid companion, , is Libration, librating around the leading Lagrangian point, Lagrange triangular point, L4, in Earth's orbit around the Sun.
The tiny near-Earth asteroid makes close approaches to the Earth–Moon system roughly every twenty years. During these approaches, it can orbit Earth for brief periods of time.
, there are 4,550 operational, human-made satellites orbiting Earth.
There are also inoperative satellites, including Vanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of tracked space debris.
Earth's largest artificial satellite is the International Space Station.
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 tons 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 water, saline; the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.
In Earth's coldest regions, snow survives over the summer and Ice formation, changes into ice. This accumulated snow and ice eventually forms into glaciers, bodies of ice that flow under the influence of their own gravity. Alpine glaciers form in mountainous areas, whereas vast ice sheets form over land in polar regions. The flow of glaciers erodes the surface changing it dramatically, with the formation of U-shaped valleys and other landforms. Sea ice in the Arctic covers an area about as big as the United States, although it is quickly retreating as a consequence of climate change.
The average salinity of Earth's oceans is about 35 grams of salt per kilogram of seawater (3.5% salt).
Most of this salt was released from volcanic activity or extracted from cool igneous rocks.
The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.
Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.
Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño–Southern Oscillation.
The abundance of water on Earth's surface is a unique feature that distinguishes it from other planets in the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. Solar System planets with considerable atmospheres do partly host atmospheric water vapor, but they lack surface conditions for stable surface water.
Despite some Natural satellite, moons showing signs of large reservoirs of extraterrestrial liquid water, with possibly even more volume than Earth's ocean, all of them are List of largest lakes and seas in the Solar System, large bodies of water under a kilometers thick frozen surface layer.
Atmosphere
The atmospheric pressure at Earth's sea level averages ,
with a scale height of about .
A dry atmosphere is composed of 78.084%
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecules.
Water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
content varies between 0.01% and 4%
but averages about 1%.
Cloud cover, 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 of Earth, atmosphere. Oxygen evolution#Oxygen evolution in nature, Oxygenic photosynthesis evolved , oxygen catastrophe, forming the primarily nitrogen–oxygen atmosphere of today.
This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent Ozone–oxygen cycle, conversion of atmospheric into . The ozone layer blocks ultraviolet solar radiation, permitting life on land.
Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.
This last phenomenon is the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the surface, thereby raising the average temperature. Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be , 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 that drives the weather and climate through redistribution of thermal energy.
The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.
Ocean heat content and Ocean current, currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.
Earth receives 1361 W/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 Tropics, tropical (or equatorial), Subtropics, subtropical, temperate and Polar region, polar climates.
Further factors that affect a location's climates are its Continentality, 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 Lapse rate, 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 (tropical climate, humid tropics, arid, humid subtropical climate, humid middle latitudes, Continental climate, continental and cold polar climate, polar), which are further divided into more specific subtypes.
The Köppen system rates regions based on observed temperature and precipitation. Surface Highest temperature recorded on Earth, air temperature can rise to around in hot deserts, such as Death Valley National Park, Death Valley, and Lowest temperature recorded on Earth, can fall as low as in Antarctica.
Upper atmosphere
The upper atmosphere, the atmosphere above the troposphere, is usually divided into the stratosphere, mesosphere, and thermosphere.
Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the geomagnetic fields interact with the solar wind.
Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 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 Atmospheric escape, loss of the atmosphere into space. Because unfixed hydrogen has a low molecular mass, it can achieve escape velocity more readily, and it leaks into outer space at a greater rate than other gases.
The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface from an initially redox, 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 Planetary habitability, 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 biomes, which are inhabited by comparatively similar plants and animals. The different biomes develope at distinct elevations or Ocean depths, water depths, planetary temperature latitudes and on land also with different humidity. Latitudinal gradients in species diversity, Earth's species diversity and Biomass (ecology), biomass reaches a peak in shallow waters and with tropical rainforest, forests, particularly in equatorial, warm and humid conditions. While freezing Polar regions of Earth, polar regions and Alpine tundra, high altitudes, or desert, extremely arid areas are relatively barren of plant and animal life.
Earth provides liquid water—an environment where complex Organic compound, organic molecules 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 hurricanes and typhoons), occurs over most of Earth's surface and has a large impact on life in those areas. From 1980 to 2000, these events caused an average of 11,800 human deaths per year. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, blizzards, floods, droughts, wildfires, and other calamities and disasters. Human impact is felt in many areas due to pollution of the air and water, acid rain, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion and erosion. Human activities release greenhouse gases into the atmosphere which cause global warming.
This is driving Effects of climate change, changes such as the Retreat of glaciers since 1850, melting of glaciers and ice sheets, a Sea level rise, 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 History of human migration, humans have since been migrating and with the advent of agriculture in the 10th millennium BC increasingly Sedentism, settling Earth's land. In the 20th century Antarctica had been the last continent to see a first and until today limited human presence.
World population, 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 Population density#Human population density, density of human population varies greatly around the world with the majority living in south to eastern Asia and 90% inhabiting only the Northern Hemisphere of Earth, partly due to the Land hemisphere, 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 living, underground and underwater living, 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 Society, societies 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 State (polity), states, of which today List of sovereign states, more than 200 exist, with only Antarctica and few Terra nullius, areas remaining unclaimed. Most of these states together form the United Nations, the leading worldwide intergovernmental organization, with international governance having provided legal regimes extraterritorially, extanding human governance Law of the Sea, over the ocean and Antarctic Treaty System, Antarctica, and therefore all of Earth.
Natural resources and land use
Earth has resources that have been exploited by humans. Those termed non-renewable resources, such as fossil fuels, are only replenished over geological timescales. Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed within the crust through a process of ore genesis, resulting from actions of magmatism, erosion, and plate tectonics. These metals and other elements are extracted by mining, a process which often brings environmental and health damage.
Earth's biosphere produces many useful biological products for humans, including food, wood, pharmaceuticals, oxygen, and the recycling of organic waste. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends on dissolved nutrients washed down from the land.
In 2019, 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 materials to construct shelters.
Humans and the environment
Human activities have impacted Earth's environments. Through activities such as the burning of fossil fuels, humans have been increasing the amount of
greenhouse gas
A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
es in the atmosphere, altering Earth's energy budget 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 Retreat of glaciers since 1850, melting of glaciers, Sea level rise, rising sea levels, increased risk of drought and wildfires, and migration of species to colder areas.]
The concept of planetary boundaries was introduced to quantify humanity's impact on Earth. Of the nine identified boundaries, five have been crossed: Biodiversity loss, 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
Culture, Human cultures have developed many views of the planet. The standard Astronomical symbols, astronomical symbols of Earth are a quartered circle, , representing the four corners of the world, and a globus cruciger, . Earth is sometimes Personification, personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity. Creation myths in many religions involve the creation of Earth by a supernatural deity or deities. The Gaia hypothesis
The Gaia hypothesis (), also known as the Gaia theory, Gaia paradigm, or the Gaia principle, proposes that living organisms interact with their inorganic surroundings on Earth to form a synergistic and self-regulating, complex system that help ...
, 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.
Timeline of first images of Earth from space, Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, called the overview effect, emphasizing its beauty, uniqueness and apparent fragility. In particular, this caused a realization of the scope of effects from human activity on Earth's environment. Enabled by science, particularly Earth observation, humans have started to take Environmentalism, action on environmental issues globally, acknowledging the impact of humans and the Ecological network, interconnectedness of Earth's environments.
Scientific investigation has resulted in several culturally transformative shifts in people's view of the planet. Initial belief in a flat Earth was gradually displaced in Ancient Greece by the idea of a spherical Earth, which was attributed to both the philosophers Pythagoras and Parmenides. Earth was generally believed to be Geocentric model, the center of the universe until the 16th century, when scientists first concluded that it was heliocentrism, a moving object, one of the planets of the Solar System.
It was only during the 19th century that geologists realized Earth's age was at least many millions of years. William Thomson, 1st Baron Kelvin, 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 Radiometric dating, 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 auroras (time-lapse)
Google Earth 3D
interactive map
Interactive 3D visualization of the Sun, Earth and Moon system
GPlates Portal
(University of Sydney)
{{Authority control
Earth,
Astronomical objects known since antiquity
Global natural environment
Planets in the circumstellar habitable zone
Nature
Planets of the Solar System
Terrestrial planets