Earth
Etomology
The Modern English word Earth developed, via Middle English, from an Old English noun most often spelled eorðe.It has cognates in every Germanic language, and their ancestral root has been reconstructed as *erþō. In its earliest attestation, the word eorðe was used to translate the many senses of Latin terra and Greek γῆ 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/Tellūs and Greek Gaia, 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. Beginning with the use of Early Middle English, its definite sense as “the globe” was expressed as “the earth”. By the era of Early Modern English, capitalization of nouns began to prevail, and the earth was also written the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets, though “earth” and forms with “the earth” remain common.[24] House styles now vary: Oxford spelling recognizes the lowercase form as the more common, with the capitalized form an acceptable variant. Another convention capitalizes “Earth” when appearing as a name, such as a description of the “Earth’s atmosphere”, but employs the lowercase when it is preceded by “the”, such as “the atmosphere of the earth”. It almost always appears in lowercase in colloquial expressions such as “what on earth are you doing?
The name Terra /ˈtɛrə/ occasionally is used in scientific writing and especially in science fiction to distinguish humanity’s inhabited planet from others,while in poetry Tellus /ˈtɛləs/ has been used to denote personification of the Earth. Terra is also the name of the planet in some Romance languages, languages that evolved from Latin, like Italian and Portuguese, while in other Romance languages the word gave rise to names with slightly altered spellings, like the Spanish Tierra and the French Terre. The Latinate form Gæa or Gaea (English: /ˈdʒiː.ə/) of the Greek poetic name Gaia (Γαῖα; Ancient Greek: [ɡâi̯.a] or [ɡâj.ja]) is rare, though the alternative spelling Gaia has become common due to the Gaia hypothesis, in which case its pronunciation is /ˈɡaɪ.ə/ rather than the more classical English /ˈɡeɪ.ə/.
There are a number of adjectives for the planet Earth. The word “earthly” is derived from “Earth”. From the Latin Terra comes terran /ˈtɛrən/,[30] terrestrial /təˈrɛstriəl/,[31] and (via French) terrene /təˈriːn/,[32] and from the Latin Tellus comes tellurian /tɛˈlʊəriən/[33] and telluric.
From the sun earth is third planet . Earth being an ocean world. lakes, rivers, and atmospheric water combined. Earth’s crust consists of slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes.Almost all of Earth’s water is contained in its global ocean, covering 70.8% of Earth’s crust. The remaining 29.2% of Earth’s crust is land, most of which is located in the form of continental landmasses within Earth’s land hemisphere.
Earth has a dynamic atmosphere, which sustains Earth’s surface conditions and protects it from most meteoroids and UV-light at entry. It has a composition of primarily nitrogen and oxygen. Water vapor is widely present in the atmosphere, forming clouds that cover most of the planet. The water vapor acts as a greenhouse gas and, together with other greenhouse gases in the atmosphere, particularly carbon dioxide (CO2), creates the conditions for both liquid surface water and water vapor to persist via the capturing of energy from the Sun’s light. This process maintains the current average surface temperature of 14.76 °C (58.57 °F), at which water is liquid under normal atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with the equatorial region receiving more sunlight than the polar regions) drive atmospheric and ocean currents, producing a global climate system with different climate regions, and a range of weather phenomena such as precipitation, allowing components such as nitrogen to cycle.
Earth is rounded into an ellipsoid with a circumference of about 40,000 km. It is the densest planet in the Solar System. Of the four rocky planets, it is the largest and most massive. Earth is about eight light-minutes away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes). Earth’s axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons. Earth is orbited by one permanent natural satellite, the Moon, which orbits Earth at 384,400 km (1.28 light seconds) and is roughly a quarter as wide as Earth. The Moon’s gravity helps stabilize Earth’s axis, causes tides and gradually slows Earth’s rotation. Tidal locking has made the Moon always face Earth with the same side.
Earth
Earth, like most other bodies in the Solar System, formed 4.5 billion years ago from gas and dust in the early Solar System. During the first billion years of Earth’s history, the ocean formed and then life developed within it. Life spread globally and has been altering Earth’s atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth. Humans depend on Earth’s biosphere and natural resources for their survival, but have increasingly impacted the planet’s environment. Humanity’s current impact on Earth’s climate and biosphere is unsustainable, threatening the livelihood of humans and many other forms of life, and causing widespread extinctions.
After formation
Main article: Geological history of Earth
Earth’s atmosphere and oceans were formed by volcanic activity and outgassing.Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets.
Origin of life and evolution
Main articles: Abiogenesis, Earliest known life forms, and History of life
Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose. The evolution of photosynthesis allowed the Sun’s energy to be harvested directly by life forms. The resultant molecular oxygen (O2) accumulated in the atmosphere and due to interaction with ultraviolet solar radiation, formed a protective ozone layer (O3) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.
Future
Main article: Future of Earth
See also: Global catastrophic risk
A dark gray and red sphere representing the Earth lies against a black background to the right of an orange circular object representing the Sun
Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, about 5–7 billion years in the future
Earth’s expected long-term future is tied to that of the Sun. Over the next 1.1 billion years, solar luminosity will increase by 10%, and over the next 3.5 billion years by 40%.[78] Earth’s increasing surface temperature will accelerate the inorganic carbon cycle, possibly reducing CO2 concentration to levels lethally low for current plants (10 ppm for C4 photosynthesis) in approximately 100–900 million year.
Surface
Further information: Planetary surface, Land cover, Land, Pedosphere, Ocean, Sea, Cryosphere, and Peplosphere
A composite image of Earth, with its different types of surface discernible: Earth’s surface dominating Ocean (blue), Africa with lush (green) to dry (brown) land and Earth’s polar ice in the form of Antarctic sea ice (grey) covering the Antarctic or Southern Ocean and the Antarctic ice sheet (white) covering Antarctica.
Earth’s land covers 29.2%, or 149 million km2 (58 million sq mi) of Earth’s surface. The land surface includes many islands around the globe, but most of the land surface is taken by the four continental landmasses, which are (in descending order): Africa-Eurasia, America (landmass), Antarctica, and Australia (landmass).[105][106][107] These landmasses are further broken down and grouped into the continents. The terrain of the land surface varies greatly and consists of mountains, deserts, plains, plateaus, and other landforms. The elevation of the land surface varies from a low point of −418 m (−1,371 ft) at the Dead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top of Mount Everest. The mean height of land above sea level is about 797 m (2,615 ft).[108]
Land can be covered by surface water, snow, ice, artificial structures or vegetation. Most of Earth’s land hosts vegetation,[109] but considerable amounts of land are ice sheets (10%,[110] not including the equally large area of land under permafrost)[111] or deserts (33%).[112]
The pedosphere is the outermost layer of Earth’s land surface and is composed of soil and subject to soil formation processes. Soil is crucial for land to be arable. Earth’s total arable land is 10.7% of the land surface, with 1.3% being permanent cropland.[113][114] Earth has an estimated 16.7 million km2 (6.4 million sq mi) of cropland and 33.5 million km2 (12.9 million sq mi) of pastureland.[115]
The land surface and the ocean floor form the top of Earth’s crust, which together with parts of the upper mantle form Earth’s lithosphere. Earth’s crust may be divided into oceanic and continental crust. Beneath the ocean-floor sediments, the oceanic crust is predominantly basaltic, while the continental crust may include lower density materials such as granite, sediments and metamorphic rocks.[116] Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the mass of the crust.[117]
Earth’s surface topography comprises both the topography of the ocean surface, and the shape of Earth’s land surface. The submarine terrain of the ocean floor has an average bathymetric depth of 4 km, and is as varied as the terrain above sea level. Earth’s surface is continually being shaped by internal plate tectonic processes including earthquakes and volcanism; by weathering and erosion driven by ice, water, wind and temperature; and by biological processes including the growth and decomposition of biomass into soil.
Gravitational field
Main article: Gravity of Earth
The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within Earth. Near Earth’s surface, gravitational acceleration is approximately 9.8 m/s2 (32 ft/s2). Local differences in topography, geology, and deeper tectonic structure cause local and broad regional differences in Earth’s gravitational field, known as gravity anomalies.
Magnetic field
Main article: Earth’s magnetic field
Diagram showing the magnetic field lines of Earth’s magnetosphere. The lines are swept back in the anti-solar direction under the influence of the solar wind.
A schematic view of Earth’s magnetosphere with solar wind flowing from left to right
The main part of Earth’s magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth’s surface, where it is, approximately, a dipole. The poles of the dipole are located close to Earth’s geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is 3.05×10−5 T, with a magnetic dipole moment of 7.79×1022 Am2 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.
Rotation
Main article: Earth’s rotation
Satellite time lapse imagery of Earth’s rotation showing axis tilt
Earth’s rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).[157] Because Earth’s solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 ms longer than the mean solar day.[158][159]
Earth’s rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23h 56m 4.0989s.[2][n 10] Earth’s rotation period relative to the precessing or moving mean March equinox (when the Sun is at 90° on the equator), is 86,164.0905 seconds of mean solar time (UT1) (23h 56m 4.0905s).[2] 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.
Orbit
Main articles: Earth’s orbit and Earth’s location
Exaggerated illustration of Earth’s elliptical orbit around the Sun, marking that the orbital extreme points (apoapsis and periapsis) are not the same as the four seasonal extreme points, the equinox and solstice
Earth orbits the Sun, making Earth the third-closest planet to the Sun and part of the inner Solar System. Earth’s average orbital distance is about 150 million km (93 million mi), which is the basis for the astronomical unit (AU) and is equal to roughly 8.3 light minutes or 380 times Earth’s distance to the Moon. Earth orbits the Sun every 365.2564 mean solar days, or one sidereal year. With an apparent movement of the Sun in Earth’s sky at a rate of about 1°/day eastward, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian.
The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth’s diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance from Earth to the Moon, 384,400 km (238,900 mi), in about 3.5 hours.
The Moon and Earth orbit a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system’s common orbit around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon, and their axial rotations are all counterclockwise. Viewed from a vantage point above the Sun and Earth’s north poles, Earth orbits in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth’s axis is tilted some 23.44 degrees from the perpendicular to the Earth–Sun plane (the ecliptic), and the Earth-Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.
The Hill sphere, or the sphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius.[164][n 11] This is the maximum distance at which Earth’s gravitational influence is stronger than that of the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.[164] 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
Main article: Axial tilt § Earth
Earth’s axial tilt causing different angles of seasonal illumination at different orbital positions around the Sun
The axial tilt of Earth is approximately 23.439281°[2] 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.[166] 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.[167][168]
By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when Earth’s rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.
A view of Earth with different layers of its atmosphere visible: the troposphere with its clouds casting shadows, a band of stratospheric blue sky at the horizon, and a line of green airglow of the lower thermosphere around an altitude of 100 km, at the edge of space
The atmospheric pressure at Earth’s sea level averages 101.325 kPa (14.696 psi),[211] with a scale height of about 8.5 km (5.3 mi).[3] A dry atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecule.
Upper atmosphere
Earth’s night-side upper atmosphere appearing from the bottom as bands of afterglow illuminating the troposphere in orange with silhouettes of clouds, and the stratosphere in white and blue. Next the mesosphere (pink area) extends to the orange and faintly green line of the lowest airglow, at about one hundred kilometers at the edge of space and the lower edge of the thermosphere (invisible). Continuing with green and red bands of aurorae stretching over several hundred kilometers.
Life on Earth
Main articles: Biosphere and History of life
An animation of the changing density of productive vegetation on land (low in brown; heavy in dark green) and phytoplankton at the ocean surface (low in purple; high in yellow)
Earth is the only known place that has ever been habitable for life. Earth’s life developed in Earth’s early bodies of water some hundred million years after Earth formed. Earth’s life has been shaping and inhabiting many particular ecosystems on Earth and has eventually expanded globally forming an overarching biosphere.
Human geography
Main article: Human geography
See also: World
A composite image of artificial light emissions at night on a map of Earth
Originating from earlier primates in Eastern Africa 300,000 years ago humans have since been migrating and with the advent of agriculture in the 10th millennium BC increasingly settling Earth’s land. In the 20th century Antarctica 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,[254] and is projected to peak at around ten billion in the second half of the 21st century.[255] Most of the growth is expected to take place in sub-Saharan Africa.
Distribution and density of human population varies greatly around the world with the majority living in south to eastern Asia and 90% inhabiting only the Northern Hemisphere of Earth,[256] 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 a few special-purpose deep underground and underwater presences and a few space stations. The human population virtually completely remains on Earth’s surface, fully depending on Earth and the environment it sustains. Since the second half of the 20th century, some hundreds of humans have temporarily stayed beyond Earth, a tiny fraction of whom have reached another celestial body, the Moon.
Earth has been subject to extensive human settlement, and humans have developed diverse societies and cultures. Most of Earth’s land has been territorially claimed since the 19th century by sovereign states (countries) separated by political borders, and 205 such states exist today, with only parts of Antarctica and a few small regions remaining unclaimed. Most of these states together form the United Nations, the leading worldwide intergovernmental organization,which extends human governance over the ocean and Antarctica, and therefore all of Earth.
Humans and the environment
Main articles: Human impact on the environment and Climate change
The graph from 1880 to 2020 shows natural drivers exhibiting fluctuations of about 0.3 degrees Celsius. Human drivers steadily increase by 0.3 degrees over 100 years to 1980, then steeply by 0.8 degrees more over the past 40 years.
Change in average surface air temperature and drivers for that change. Human activity has caused increased temperatures, with natural forces adding some variability.
Human activities have impacted Earth’s environments. Through activities such as the burning of fossil fuels, humans have been increasing the amount of greenhouse gases in the atmosphere, altering Earth’s energy budget and climate.It is estimated that global temperatures in the year 2020 were 1.2 °C (2.2 °F) warmer than the preindustrial baseline.This increase in temperature, known as global warming, has contributed to the melting of glaciers, rising sea levels, increased risk of drought and wildfires, and migration of species to colder areas.
The concept of planetary boundaries was introduced to quantify humanity’s impact on Earth. Of the nine identified boundaries, five have been crossed: Biosphere integrity, climate change, chemical pollution, destruction of wild habitats and the nitrogen cycle are thought to have passed the safe threshold. As of 2018, no country meets the basic needs of its population without transgressing planetary boundaries. It is thought possible to provide all basic physical needs globally within sustainable levels of resource use.
Cultural and historical viewpoint
Main articles: Earth in culture and Earth in science fiction
Woman seeing the Earth from space through a window
Tracy Caldwell Dyson, a NASA astronaut, observing Earth from the Cupola module at the International Space Station on 11 September 2010
Human cultures have developed many views of the planet.[280] The standard astronomical symbols of Earth are a quartered circle, 🜨, representing the four corners of the world, and a globus cruciger, ♁. Earth is sometimes personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity.Creation myths in many religions involve the creation of Earth by a supernatural deity or deities. The Gaia hypothesis, developed in the mid-20th century, compared Earth’s environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability.
Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, called the overview effect, emphasizing its beauty, uniqueness and apparent fragility.In particular, this caused a realization of the scope of effects from human activity on Earth’s environment. Enabled by science, particularly Earth observation,humans have started to take action on environmental issues globally, acknowledging the impact of humans and the interconnectedness of Earth’s environments.
Scientific investigation has resulted in several culturally transformative shifts in people’s view of the planet. Initial belief in a flat Earth was gradually displaced in Ancient Greece by the idea of a spherical Earth, which was attributed to both the philosophers Pythagoras and Parmenides. Earth was generally believed to be the center of the universe until the 16th century, when scientists first concluded that it was a moving object, one of the planets of the Solar System.
It was only during the 19th century that geologists realized Earth’s age was at least many millions of years.] Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864, sparking a vigorous debate on the subject; it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth’s age was established, proving the planet to be billions of years old.
earth
Etomology
The Modern English word Earth developed, via Middle English, from an Old English noun most often spelled eorðe.It has cognates in every Germanic language, and their ancestral root has been reconstructed as *erþō. In its earliest attestation, the word eorðe was used to translate the many senses of Latin terra and Greek γῆ 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/Tellūs and Greek Gaia, 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. Beginning with the use of Early Middle English, its definite sense as “the globe” was expressed as “the earth”. By the era of Early Modern English, capitalization of nouns began to prevail, and the earth was also written the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets, though “earth” and forms with “the earth” remain common.[24] House styles now vary: Oxford spelling recognizes the lowercase form as the more common, with the capitalized form an acceptable variant. Another convention capitalizes “Earth” when appearing as a name, such as a description of the “Earth’s atmosphere”, but employs the lowercase when it is preceded by “the”, such as “the atmosphere of the earth”. It almost always appears in lowercase in colloquial expressions such as “what on earth are you doing?
The name Terra /ˈtɛrə/ occasionally is used in scientific writing and especially in science fiction to distinguish humanity’s inhabited planet from others,while in poetry Tellus /ˈtɛləs/ has been used to denote personification of the Earth. Terra is also the name of the planet in some Romance languages, languages that evolved from Latin, like Italian and Portuguese, while in other Romance languages the word gave rise to names with slightly altered spellings, like the Spanish Tierra and the French Terre. The Latinate form Gæa or Gaea (English: /ˈdʒiː.ə/) of the Greek poetic name Gaia (Γαῖα; Ancient Greek: [ɡâi̯.a] or [ɡâj.ja]) is rare, though the alternative spelling Gaia has become common due to the Gaia hypothesis, in which case its pronunciation is /ˈɡaɪ.ə/ rather than the more classical English /ˈɡeɪ.ə/.
There are a number of adjectives for the planet Earth. The word “earthly” is derived from “Earth”. From the Latin Terra comes terran /ˈtɛrən/,[30] terrestrial /təˈrɛstriəl/,[31] and (via French) terrene /təˈriːn/,[32] and from the Latin Tellus comes tellurian /tɛˈlʊəriən/[33] and telluric.
From the sun earth is third planet . Earth being an ocean world. lakes, rivers, and atmospheric water combined. Earth’s crust consists of slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes.Almost all of Earth’s water is contained in its global ocean, covering 70.8% of Earth’s crust. The remaining 29.2% of Earth’s crust is land, most of which is located in the form of continental landmasses within Earth’s land hemisphere.
Earth has a dynamic atmosphere, which sustains Earth’s surface conditions and protects it from most meteoroids and UV-light at entry. It has a composition of primarily nitrogen and oxygen. Water vapor is widely present in the atmosphere, forming clouds that cover most of the planet. The water vapor acts as a greenhouse gas and, together with other greenhouse gases in the atmosphere, particularly carbon dioxide (CO2), creates the conditions for both liquid surface water and water vapor to persist via the capturing of energy from the Sun’s light. This process maintains the current average surface temperature of 14.76 °C (58.57 °F), at which water is liquid under normal atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with the equatorial region receiving more sunlight than the polar regions) drive atmospheric and ocean currents, producing a global climate system with different climate regions, and a range of weather phenomena such as precipitation, allowing components such as nitrogen to cycle.
Earth is rounded into an ellipsoid with a circumference of about 40,000 km. It is the densest planet in the Solar System. Of the four rocky planets, it is the largest and most massive. Earth is about eight light-minutes away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes). Earth’s axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons. Earth is orbited by one permanent natural satellite, the Moon, which orbits Earth at 384,400 km (1.28 light seconds) and is roughly a quarter as wide as Earth. The Moon’s gravity helps stabilize Earth’s axis, causes tides and gradually slows Earth’s rotation. Tidal locking has made the Moon always face Earth with the same side.
Earth
Earth, like most other bodies in the Solar System, formed 4.5 billion years ago from gas and dust in the early Solar System. During the first billion years of Earth’s history, the ocean formed and then life developed within it. Life spread globally and has been altering Earth’s atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth. Humans depend on Earth’s biosphere and natural resources for their survival, but have increasingly impacted the planet’s environment. Humanity’s current impact on Earth’s climate and biosphere is unsustainable, threatening the livelihood of humans and many other forms of life, and causing widespread extinctions.
After formation
Main article: Geological history of Earth
Earth’s atmosphere and oceans were formed by volcanic activity and outgassing.Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets.
Origin of life and evolution
Main articles: Abiogenesis, Earliest known life forms, and History of life
Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose. The evolution of photosynthesis allowed the Sun’s energy to be harvested directly by life forms. The resultant molecular oxygen (O2) accumulated in the atmosphere and due to interaction with ultraviolet solar radiation, formed a protective ozone layer (O3) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.
Future
Main article: Future of Earth
See also: Global catastrophic risk
A dark gray and red sphere representing the Earth lies against a black background to the right of an orange circular object representing the Sun
Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, about 5–7 billion years in the future
Earth’s expected long-term future is tied to that of the Sun. Over the next 1.1 billion years, solar luminosity will increase by 10%, and over the next 3.5 billion years by 40%.[78] Earth’s increasing surface temperature will accelerate the inorganic carbon cycle, possibly reducing CO2 concentration to levels lethally low for current plants (10 ppm for C4 photosynthesis) in approximately 100–900 million year.
Surface
Further information: Planetary surface, Land cover, Land, Pedosphere, Ocean, Sea, Cryosphere, and Peplosphere
A composite image of Earth, with its different types of surface discernible: Earth’s surface dominating Ocean (blue), Africa with lush (green) to dry (brown) land and Earth’s polar ice in the form of Antarctic sea ice (grey) covering the Antarctic or Southern Ocean and the Antarctic ice sheet (white) covering Antarctica.
Earth’s land covers 29.2%, or 149 million km2 (58 million sq mi) of Earth’s surface. The land surface includes many islands around the globe, but most of the land surface is taken by the four continental landmasses, which are (in descending order): Africa-Eurasia, America (landmass), Antarctica, and Australia (landmass).[105][106][107] These landmasses are further broken down and grouped into the continents. The terrain of the land surface varies greatly and consists of mountains, deserts, plains, plateaus, and other landforms. The elevation of the land surface varies from a low point of −418 m (−1,371 ft) at the Dead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top of Mount Everest. The mean height of land above sea level is about 797 m (2,615 ft).[108]
Land can be covered by surface water, snow, ice, artificial structures or vegetation. Most of Earth’s land hosts vegetation,[109] but considerable amounts of land are ice sheets (10%,[110] not including the equally large area of land under permafrost)[111] or deserts (33%).[112]
The pedosphere is the outermost layer of Earth’s land surface and is composed of soil and subject to soil formation processes. Soil is crucial for land to be arable. Earth’s total arable land is 10.7% of the land surface, with 1.3% being permanent cropland.[113][114] Earth has an estimated 16.7 million km2 (6.4 million sq mi) of cropland and 33.5 million km2 (12.9 million sq mi) of pastureland.[115]
The land surface and the ocean floor form the top of Earth’s crust, which together with parts of the upper mantle form Earth’s lithosphere. Earth’s crust may be divided into oceanic and continental crust. Beneath the ocean-floor sediments, the oceanic crust is predominantly basaltic, while the continental crust may include lower density materials such as granite, sediments and metamorphic rocks.[116] Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the mass of the crust.[117]
Earth’s surface topography comprises both the topography of the ocean surface, and the shape of Earth’s land surface. The submarine terrain of the ocean floor has an average bathymetric depth of 4 km, and is as varied as the terrain above sea level. Earth’s surface is continually being shaped by internal plate tectonic processes including earthquakes and volcanism; by weathering and erosion driven by ice, water, wind and temperature; and by biological processes including the growth and decomposition of biomass into soil.
Gravitational field
Main article: Gravity of Earth
The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within Earth. Near Earth’s surface, gravitational acceleration is approximately 9.8 m/s2 (32 ft/s2). Local differences in topography, geology, and deeper tectonic structure cause local and broad regional differences in Earth’s gravitational field, known as gravity anomalies.
Magnetic field
Main article: Earth’s magnetic field
Diagram showing the magnetic field lines of Earth’s magnetosphere. The lines are swept back in the anti-solar direction under the influence of the solar wind.
A schematic view of Earth’s magnetosphere with solar wind flowing from left to right
The main part of Earth’s magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth’s surface, where it is, approximately, a dipole. The poles of the dipole are located close to Earth’s geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is 3.05×10−5 T, with a magnetic dipole moment of 7.79×1022 Am2 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.
Rotation
Main article: Earth’s rotation
Satellite time lapse imagery of Earth’s rotation showing axis tilt
Earth’s rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).[157] Because Earth’s solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 ms longer than the mean solar day.[158][159]
Earth’s rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23h 56m 4.0989s.[2][n 10] Earth’s rotation period relative to the precessing or moving mean March equinox (when the Sun is at 90° on the equator), is 86,164.0905 seconds of mean solar time (UT1) (23h 56m 4.0905s).[2] 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.
Orbit
Main articles: Earth’s orbit and Earth’s location
Exaggerated illustration of Earth’s elliptical orbit around the Sun, marking that the orbital extreme points (apoapsis and periapsis) are not the same as the four seasonal extreme points, the equinox and solstice
Earth orbits the Sun, making Earth the third-closest planet to the Sun and part of the inner Solar System. Earth’s average orbital distance is about 150 million km (93 million mi), which is the basis for the astronomical unit (AU) and is equal to roughly 8.3 light minutes or 380 times Earth’s distance to the Moon. Earth orbits the Sun every 365.2564 mean solar days, or one sidereal year. With an apparent movement of the Sun in Earth’s sky at a rate of about 1°/day eastward, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian.
The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth’s diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance from Earth to the Moon, 384,400 km (238,900 mi), in about 3.5 hours.
The Moon and Earth orbit a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system’s common orbit around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon, and their axial rotations are all counterclockwise. Viewed from a vantage point above the Sun and Earth’s north poles, Earth orbits in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth’s axis is tilted some 23.44 degrees from the perpendicular to the Earth–Sun plane (the ecliptic), and the Earth-Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.
The Hill sphere, or the sphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius.[164][n 11] This is the maximum distance at which Earth’s gravitational influence is stronger than that of the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.[164] 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
Main article: Axial tilt § Earth
Earth’s axial tilt causing different angles of seasonal illumination at different orbital positions around the Sun
The axial tilt of Earth is approximately 23.439281°[2] 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.[166] 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.[167][168]
By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when Earth’s rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.
A view of Earth with different layers of its atmosphere visible: the troposphere with its clouds casting shadows, a band of stratospheric blue sky at the horizon, and a line of green airglow of the lower thermosphere around an altitude of 100 km, at the edge of space
The atmospheric pressure at Earth’s sea level averages 101.325 kPa (14.696 psi),[211] with a scale height of about 8.5 km (5.3 mi).[3] A dry atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecule.
Upper atmosphere
Earth’s night-side upper atmosphere appearing from the bottom as bands of afterglow illuminating the troposphere in orange with silhouettes of clouds, and the stratosphere in white and blue. Next the mesosphere (pink area) extends to the orange and faintly green line of the lowest airglow, at about one hundred kilometers at the edge of space and the lower edge of the thermosphere (invisible). Continuing with green and red bands of aurorae stretching over several hundred kilometers.
Life on Earth
Main articles: Biosphere and History of life
An animation of the changing density of productive vegetation on land (low in brown; heavy in dark green) and phytoplankton at the ocean surface (low in purple; high in yellow)
Earth is the only known place that has ever been habitable for life. Earth’s life developed in Earth’s early bodies of water some hundred million years after Earth formed. Earth’s life has been shaping and inhabiting many particular ecosystems on Earth and has eventually expanded globally forming an overarching biosphere.
Human geography
Main article: Human geography
See also: World
A composite image of artificial light emissions at night on a map of Earth
Originating from earlier primates in Eastern Africa 300,000 years ago humans have since been migrating and with the advent of agriculture in the 10th millennium BC increasingly settling Earth’s land. In the 20th century Antarctica 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,[254] and is projected to peak at around ten billion in the second half of the 21st century.[255] Most of the growth is expected to take place in sub-Saharan Africa.
Distribution and density of human population varies greatly around the world with the majority living in south to eastern Asia and 90% inhabiting only the Northern Hemisphere of Earth,[256] 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 a few special-purpose deep underground and underwater presences and a few space stations. The human population virtually completely remains on Earth’s surface, fully depending on Earth and the environment it sustains. Since the second half of the 20th century, some hundreds of humans have temporarily stayed beyond Earth, a tiny fraction of whom have reached another celestial body, the Moon.
Earth has been subject to extensive human settlement, and humans have developed diverse societies and cultures. Most of Earth’s land has been territorially claimed since the 19th century by sovereign states (countries) separated by political borders, and 205 such states exist today, with only parts of Antarctica and a few small regions remaining unclaimed. Most of these states together form the United Nations, the leading worldwide intergovernmental organization,which extends human governance over the ocean and Antarctica, and therefore all of Earth.
Humans and the environment
Main articles: Human impact on the environment and Climate change
The graph from 1880 to 2020 shows natural drivers exhibiting fluctuations of about 0.3 degrees Celsius. Human drivers steadily increase by 0.3 degrees over 100 years to 1980, then steeply by 0.8 degrees more over the past 40 years.
Change in average surface air temperature and drivers for that change. Human activity has caused increased temperatures, with natural forces adding some variability.
Human activities have impacted Earth’s environments. Through activities such as the burning of fossil fuels, humans have been increasing the amount of greenhouse gases in the atmosphere, altering Earth’s energy budget and climate.It is estimated that global temperatures in the year 2020 were 1.2 °C (2.2 °F) warmer than the preindustrial baseline.This increase in temperature, known as global warming, has contributed to the melting of glaciers, rising sea levels, increased risk of drought and wildfires, and migration of species to colder areas.
The concept of planetary boundaries was introduced to quantify humanity’s impact on Earth. Of the nine identified boundaries, five have been crossed: Biosphere integrity, climate change, chemical pollution, destruction of wild habitats and the nitrogen cycle are thought to have passed the safe threshold. As of 2018, no country meets the basic needs of its population without transgressing planetary boundaries. It is thought possible to provide all basic physical needs globally within sustainable levels of resource use.
Cultural and historical viewpoint
Main articles: Earth in culture and Earth in science fiction
Woman seeing the Earth from space through a window
Tracy Caldwell Dyson, a NASA astronaut, observing Earth from the Cupola module at the International Space Station on 11 September 2010
Human cultures have developed many views of the planet.[280] The standard astronomical symbols of Earth are a quartered circle, 🜨, representing the four corners of the world, and a globus cruciger, ♁. Earth is sometimes personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity.Creation myths in many religions involve the creation of Earth by a supernatural deity or deities. The Gaia hypothesis, developed in the mid-20th century, compared Earth’s environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability.
Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, called the overview effect, emphasizing its beauty, uniqueness and apparent fragility.In particular, this caused a realization of the scope of effects from human activity on Earth’s environment. Enabled by science, particularly Earth observation,humans have started to take action on environmental issues globally, acknowledging the impact of humans and the interconnectedness of Earth’s environments.
Scientific investigation has resulted in several culturally transformative shifts in people’s view of the planet. Initial belief in a flat Earth was gradually displaced in Ancient Greece by the idea of a spherical Earth, which was attributed to both the philosophers Pythagoras and Parmenides. Earth was generally believed to be the center of the universe until the 16th century, when scientists first concluded that it was a moving object, one of the planets of the Solar System.
It was only during the 19th century that geologists realized Earth’s age was at least many millions of years.] Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864, sparking a vigorous debate on the subject; it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth’s age was established, proving the planet to be billions of years old.
