Earth

The Modern English word Earth developed, via Middle English, from an Old English noun most often spelled '. It has cognates in every Germanic language, from which has been reconstructed. In its earliest attestation, the word ' was used to translate the many senses of Latin '''' and Greek : the ground, its soil, dry land, the human world, the surface of the world (including the sea), and the globe itself. As with Roman (or ) and Greek , Earth may have been a personified goddess in Germanic paganism: late Norse mythology included ('Earth'), a giantess often given as the mother of Thor. Historically, Earth has been written in lowercase. During the Early Middle English period, its definite sense as "the globe" began being expressed using the phrase the earth. By the period of Early Modern English, capitalization of nouns began to prevail, and the earth was also written the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets, though earth and forms with the earth remain common. The name Terra is occasionally used in scientific writing; it also sees use in science fiction to distinguish humanity's inhabited planet from others, while in poetry Tellus has been used to denote personification of the Earth. Terra is also the name of the planet in some Romance languages, languages that evolved from Latin, like Italian and Portuguese, while in other Romance languages the word gave rise to names with slightly altered spellings, like the Spanish and the French . The Latinate form ( ) of the Greek poetic name ( or ) is rare, though the alternative spelling Gaia has become common due to the Gaia hypothesis, in which case its pronunciation is rather than the more traditional English . There are a number of adjectives for the planet Earth. The word earthly is derived from Earth. From the Latin comes terran , terrestrial , and (via French) terrene , and from the Latin comes tellurian and telluric. == Natural history ==

Formation 's protoplanetary disk from which Earth and other Solar System bodies were formed The oldest material found in the Solar System is dated to Ga (billion years) ago. Estimates of the age of the Moon range from 4.5 Ga to significantly younger. A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object with about 10% of Earth's mass, named Theia, collided with Earth. Between approximately 4.0 and , numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth. After formation Earth's atmosphere and oceans were formed by volcanic activity and outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets. In this model, atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. As the molten outer layer of Earth cooled it formed the first solid crust, which is thought to have been mafic in composition. The first continental crust, which was more felsic in composition, formed by the partial melting of this mafic crust. Hadean-aged zircon grains found in Western Australia, dating back as far as , provide the oldest direct evidence of Earth's surface, indicating that felsic continental crust and liquid water existed within 140–160 million years of the planet's formation. and then intensified during the Pleistocene about . High- and middle-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating about every 21,000, 41,000, and 100,000 years. Future 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 , solar luminosity will increase by 10%, and over the next by 40%. Earth's increasing surface temperature will accelerate the inorganic carbon cycle, possibly reducing concentration to levels lethally low for current plants ( for C4 photosynthesis) in approximately . A lack of vegetation would result in the loss of oxygen in the atmosphere, making current animal life impossible. Due to the increased luminosity, Earth's mean temperature may reach in 1.5 billion years, and all ocean water will evaporate and be lost to space, which may trigger a runaway greenhouse effect, within an estimated 1.6 to 3 billion years.{{Cite journal The Sun will evolve to become a red giant 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, with the heavier elements sinking to the core of the dying sun. == Bulk properties ==

Size and shape showing topography relative to Earth's center instead of to mean sea level, as in common topographic maps Earth has a rounded shape, through hydrostatic equilibrium, with an average diameter of , making it the fifth largest planetary sized and largest terrestrial object of the Solar System. Due to Earth's rotation it has the shape of an ellipsoid, bulging at its equator; its diameter is longer there than at its poles. shortens Earth's average radius by 0.17% and Mount Everest ( above local sea level) lengthens it by 0.14%. Since Earth's surface is farthest out from its center of mass at its equatorial bulge, the summit of the volcano Chimborazo in Ecuador () is its farthest point out. Parallel to the rigid land topography the ocean exhibits a more dynamic topography. To measure the local variation of Earth's topography, geodesy employs an idealized Earth producing a geoid shape. Such a shape is gained if the ocean is idealized, covering Earth completely and without any perturbations such as tides and winds. The result is a smooth but irregular geoid surface, providing a mean sea level as a reference level for topographic measurements. Internal structure Earth's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. The outer layer is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity. 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 that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core. The major heat-producing isotopes within Earth are potassium-40, uranium-238, and thorium-232. and the pressure could reach . 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. Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonic bow shock precedes the day-side magnetosphere within the solar wind. Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates. The ring current is defined by medium-energy particles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field, and the Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere. During magnetic storms and substorms, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's ionosphere, where atmospheric atoms can be excited and ionized, causing an aurora. == Surface environment ==

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. of Earth's crust Earth's surface is the boundary between the atmosphere and the solid Earth and oceans. Defined in this way, it has an area of about . Most of Earth's surface is ocean water: 70.8% or . and makes Earth with its dynamic hydrosphere a water world or ocean world. Indeed, in Earth's early history the ocean may have covered Earth completely. The world ocean is commonly divided into the Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern Ocean, and Arctic Ocean, from largest to smallest. The ocean covers Earth's oceanic crust, with the shelf seas covering the shelves of the continental crust to a lesser extent. The oceanic crust forms large oceanic basins with features like abyssal plains, seamounts, submarine volcanoes, At Earth's polar regions, the ocean surface is covered by seasonally variable amounts of sea ice that often connects with polar land, permafrost and ice sheets, forming polar ice caps. Earth's land covers 29.2%, or 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, Antarctica, and Australia. These landmasses are further broken down and grouped into the continents. Land can be covered by surface water, snow, ice, artificial structures or vegetation. Most of Earth's land hosts vegetation, but considerable amounts of land are ice sheets (10%, not including the equally large area of land under permafrost) or deserts (33%). The pedosphere is the outermost layer of Earth's land surface and is composed of soil and subject to soil formation processes. Soil is crucial for land to be arable. Earth's total arable land is 10.7% of the land surface, with 1.3% being permanent cropland. Earth has an estimated of cropland and of pastureland. The land surface and the ocean floor form the top of Earth's crust, which together with parts of the upper mantle form Earth's lithosphere. Earth's crust may be divided into oceanic and continental crust. Beneath the ocean-floor sediments, the oceanic crust is predominantly basaltic, while the continental crust may include lower density materials such as granite, sediments and metamorphic rocks. 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. Tectonic plates , which are: Hydrosphere and cloud cover, which dominate Earth's surface and hydrosphere; at Earth's polar regions, its hydrosphere forms larger areas of ice cover. Earth's hydrosphere is the sum of Earth's water and its distribution. Most of Earth's hydrosphere consists of Earth's global ocean. Earth's hydrosphere also consists of water in the atmosphere and on land, including clouds, inland seas, lakes, rivers, and underground waters. The mass of the oceans is approximately 1.35 metric tons or about 1/4400 of Earth's total mass. The oceans cover an area of with a mean depth of , resulting in an estimated volume of . About 97.5% of the water is saline; the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is present as ice in ice caps and glaciers. The remaining 30% is ground water, 1% surface water (covering only 2.8% of Earth's land) and other small forms of fresh water deposits such as permafrost, water vapor in the atmosphere, biological binding, etc. In Earth's coldest regions, snow survives over the summer and changes into ice. This accumulated snow and ice eventually forms into glaciers, bodies of ice that flow under the influence of their own gravity. Alpine glaciers form in mountainous areas, whereas vast ice sheets form over land in polar regions. The flow of glaciers erodes the surface, changing it dramatically, with the formation of U-shaped valleys and other landforms. Sea ice in the Arctic covers an area about as big as the United States, although it is quickly retreating as a consequence of climate change. The average salinity of Earth's oceans is about 35 grams of salt per kilogram of seawater (3.5% salt). Despite some moons showing signs of large reservoirs of extraterrestrial liquid water, with possibly even more volume than Earth's ocean, all of them are large bodies of water under a kilometers-thick frozen surface layer. Atmosphere 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 , with a scale height of about . 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. is usually divided into the stratosphere, mesosphere, and thermosphere. 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 amount of solar energy that reaches Earth's surface decreases with increasing latitude. At higher latitudes, the sunlight reaches the surface at lower angles, and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about per degree of latitude from the equator. Places close to oceans typically have colder summers and warmer winters, due to the fact that oceans can store large amounts of heat. The wind transports the cold or the heat of the ocean to the land. Atmospheric circulation also plays an important role: San Francisco and Washington DC are both coastal cities at about the same latitude. San Francisco's climate is significantly more moderate as the prevailing wind direction is from sea to land. Finally, temperatures decrease with height causing mountainous areas to be colder than low-lying areas. Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and falls to the surface as precipitation. Surface air temperature can rise to around in hot deserts, such as Death Valley, and can fall as low as in Antarctica. == Orbit and rotation ==

Rotation of Earth's rotation showing axis tilt Earth's rotation period relative to the Sun—its mean solar day—is of mean solar time (). 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 Above the Arctic Circle and below the Antarctic Circle there is no daylight at all for part of the year, causing a polar night, and this night extends for several months at the poles themselves. These same latitudes also experience a midnight sun, where the sun remains visible all day. By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when Earth's rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped. == Gravitational domain and influence ==

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 anomalies. The Hill sphere, or the sphere of gravitational influence, of Earth is about in radius. The natural satellites of other planets are also referred to as "moons", after Earth's. 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 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. The orbital planes are not precisely aligned: the Earth-Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane (the ecliptic). Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases. Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately . Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 μs/yr—add up to significant changes. The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon. The most widely accepted theory of the Moon's origin, the giant-impact hypothesis, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains the Moon's relative lack of iron and volatile elements and the fact that its composition is nearly identical to that of Earth's crust. Asteroids and artificial satellites s and space debris around Earth in geosynchronous and low Earth orbit Earth's co-orbital asteroids population consists of quasi-satellites, objects with a horseshoe orbit and trojans. There are at least seven quasi-satellites, including 469219 Kamoʻoalewa, ranging in diameter from 10 m to 5000 m. A trojan asteroid companion, , is librating around the leading Lagrange triangular point, L4, in Earth's orbit around the Sun. , there are 4,550 operational, human-made satellites orbiting Earth. == Life on Earth ==

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, roughly 4 billion years ago. Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain a metabolism. Earth's life has been shaping and inhabiting many particular ecosystems on Earth and has eventually expanded globally forming an overarching biosphere. Earth's life has also over time greatly diversified, allowing the biosphere to have different biomes, which are inhabited by comparatively similar plants and animals. The different biomes developed at distinct elevations or water depths, planetary temperature latitudes and on land also with different humidity. Earth's species diversity and biomass reaches a peak in shallow waters and with forests, particularly in equatorial, warm and humid conditions. While freezing polar regions and high altitudes, or extremely arid areas are relatively barren of plant and animal life. Humans furthermore have impacted Earth, its life and development. Origin of life and evolution , the eon after Earth's formation, featuring round stromatolites, which are early oxygen-producing forms of life from billions of years ago. After the Late Heavy Bombardment, Earth's crust had cooled, its water-rich barren surface is marked by continents and volcanoes, with the Moon still orbiting Earth half as far as it is today, appearing 2.8 times larger and producing strong tides. 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 incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes. biogenic graphite found in 3.7 billion-year-old metasedimentary rocks in Western Greenland, and remains of biotic material found in 4.1 billion-year-old rocks in Western Australia. The earliest direct evidence of life on Earth is contained in 3.45 billion-year-old Australian rocks showing fossils of microorganisms. During the Neoproterozoic, , much of Earth might have been covered in ice. This hypothesis has been termed "Snowball Earth", and it is of particular interest because it preceded the Cambrian explosion, when multicellular life forms significantly increased in complexity. Following the Cambrian explosion, , there have been at least five major mass extinctions and many minor ones. This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which led to the evolution of humans. The development of agriculture, and then civilization, led to humans having an influence on Earth and the nature and quantity of other life forms that continues to this day. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, blizzards, floods, droughts, wildfires, and other calamities and disasters. Human impact is felt in many areas due to pollution of the air and water, acid rain, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion and erosion. Human activities release greenhouse gases into the atmosphere which cause global warming. This is driving changes such as the melting of glaciers and ice sheets, a global rise in average sea levels, increased risk of drought and wildfires, and migration of species to colder areas. == Earth and humans ==

Human geography at night on a map of Earth Humans, who originated from earlier primates in Eastern Africa 300,000years ago, have since been migrating around Earth, and with the advent of agriculture in the 10th millennium BCE, have been increasingly settling Earth's land. In the 20th century, Antarctica became the last continent to be explored and settled by humans, although human presence there remains limited. Since the 19th century, the human population has grown exponentially to eight billion in the 2020s, and is projected to peak at around ten billion in the second half of the 21st century. Most of the growth is expected to take place in sub-Saharan Africa. 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. of the United Nations Earth has been subject to extensive human settlement, and humans have developed diverse societies and cultures. Most of Earth's land has been territorially claimed since the 19th century by sovereign states (countries) separated by political borders, and 205 such states exist today, with only parts of Antarctica and a few small regions remaining unclaimed. Together, most of these states form the United Nations, the leading worldwide intergovernmental organization, which extends human governance over the ocean and Antarctica, and therefore all of Earth. Natural resources and land use Earth has resources that have been exploited by humans. Those termed non-renewable resources, such as fossil fuels, are only replenished over geological timescales. Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed within the crust through a process of ore genesis, resulting from actions of magmatism, erosion, and plate tectonics. These metals and other elements are extracted by mining, a process which often causes 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. Humans use natural and manufactured building materials to construct shelters and infrastructure. Environmental impact 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 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. As of 2018, no country meets the basic needs of its population without transgressing planetary boundaries. It is thought possible to provide all basic physical needs globally within sustainable levels of resource use. Cultural and historical viewpoint Human cultures have developed many views of the planet. The standard astronomical symbols of Earth are a quartered circle, , Creation myths in many religions involve the creation of Earth by a supernatural deity or deities. Images of Earth taken from space, particularly Earthrise and The Blue Marble, 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. Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864, sparking a vigorous debate on the subject; it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth's age was established, proving the planet to be billions of years old. ==See also==