Earth

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Earth is the third planet from the Sun and is the largest of the terrestrial planets in the Solar System, in both diameter and mass. It is also referred to as "the Earth", "Planet Earth", "Gaia", "Terra",Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. [1] Gazetteer of Planetary Nomenclature, USGS, and "the World". Home to millions of species [2] including humans.


For lessons on the topic of Earth, follow this link.


About 71% of Earth's surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water. As of 2007, water vapor has been detected in the atmosphere of extrasolar planets. See: Water vapour in the atmosphere of a transiting extrasolar planet, Nature, [3] The Earth formed about 4.57 billion years ago, and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere that exists today. This change led to the proliferation of aerobic organisms as well as to the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land.

Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.<ref>The number of solar days is one less than the number of sidereal days because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis. The Earth's axis of rotation is tilted 23.5° Ahrens, Global Earth Physics: A Handbook of Physical Constants, p. 8.</ref> away from the perpendicular to its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year. Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans. Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the orbit of the planet are believed to have caused the ice ages that have covered significant portions of the surface in glacial sheets.

The planet's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

History

Scientists have been able to reconstruct detailed information about the planet's past. Earth and the other planets in the Solar System formed 4.57 billion years ago<ref name="age_earth" /> out of the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object (sometimes called Theia with about 10% of the Earth's mass, An impact origin of the Earth-Moon system, American Geophysical Union, [4] impacting the Earth in a glancing blow. Origin of the Moon in a giant impact near the end of the Earth's formation [5]. Some of this object's mass merged with the Earth and a portion was ejected into space, but enough material survived to form an orbiting moon.

Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, produced the oceans. Source regions and time scales for the delivery of water to Earth, Meteoritics & Planetary Science, [6] The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life existed. Uprooting the tree of life, Scientific American.

The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and resulted in a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[7] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.[8] Astrobiologists Find Evidence of Early Life on Land, NASA

As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), the earliest known supercontinent, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which broke apart 180 mya. How do supercontinents assemble?, American Scientist, [9]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate. The Proterozoic Biosphere: A Multidisciplinary Study, Cambridge University Press, ISBN 0521366151

Following the Cambrian explosion, about 535 mya, there have been five mass extinctions. Mass Extinctions in the Marine Fossil Record, Science [10] The last extinction event occurred 65 mya, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright. The Evolution of Life on Earth, Scientific American[11] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had, The impact of humans on continental erosion and sedimentation, Bulletin of the Geological Society of America [12] affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 mya, then intensified during the Pleistocene about 3 mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.[13] Paleoclimatology - The Study of Ancient Climates, Paleontology Science Center

Composition and structure

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant such as Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and total mass. Of these four planets, Earth also has the highest density, the highest surface gravity and the strongest magnetic field.[14] Planetary Magnetism

Shape

The Earth's shape is very close to an oblate spheroid—a rounded shape with a bulge around the equator—although the precise shape (the geoid) varies from this by up to 100 meters (327 ft).[15] Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model, National Geodetic Survey, NOAA, The average diameter of the reference spheroid is about 12,742 km (7,913 mi). More approximately the distance is 40,000 km/π because the meter was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France.[16] NIST Reference on Constants, Units, and Uncertainty, NIST Physics Laboratory

The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km (27 mi) larger than the pole to pole diameter. [17] Exploring the Ocean Basins with Satellite Altimeter Data The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m [29,028 ft] above local sea level) and the Mariana Trench (10,911 m [35,798 ft] below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in billiard balls. [18] WPA Tournament Table & Equipment Specifications, World Pool-Billiards Association, Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador. Did Edmund Hillary Climb the Wrong Mountain, Professional Surveyor, [19]

The mass of the Earth is approximately 5.98 kg. It is composed mostly of iron (32.1%), oxygen (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 believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements. Chemical composition of Earth, Venus, and Mercury, Proceedings of the National Academy of Science, [20]

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.

Internal structure

The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents.

The internal heat of the planet is most likely produced by the radioactive decay of potassium-40, uranium-238 and thorium-232 isotopes. All three have half-life decay periods of more than a billion years. Radioactive potassium may be major heat source in Earth's core, UC Berkeley News, [21] At the center of the planet, the temperature may be up to 7,000 degrees and the pressure could reach 360 GPa. The ab initio simulation of the Earth's core, Philosophical Transaction of the Royal Society of London, [22] A portion of the core's thermal energy is transported toward the crust by Mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts. Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails, Science, [23]

Tectonic plates

According to plate tectonics theory, which is currently accepted by nearly all of the scientists working in this area, the outermost part of the Earth's interior is made up of two layers: the lithosphere, comprising the crust, and the solidified uppermost part of the mantle. Below the lithosphere lies the asthenosphere, which forms the inner part of the mantle. The asthenosphere behaves like a superheated and extremely viscous liquid.[24]

The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: convergent, divergent and transform. The last occurs where two plates move laterally relative to each other, creating a strike-slip fault. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.[25], Understanding plate motions, USGS

The main plates are:[26] SFT and the Earth's Tectonic Plates, Los Alamos National Laboratory

Notable minor plates include the Indian Plate, 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 actually fused with Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr[27] Plate Tectonic Evolution of the Cocos-Nazca Spreading Center, Proceedings of the Ocean Drilling Program, Texas A&M University (3.0 in/yr) and the Pacific Plate moving 52–69 mm/yr (2.1–2.7 in/yr). At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr (0.8 in/yr).[28]

Surface

Present day Earth altimetry and bathymetry. Data from the National Geophysical Data Center's TerrainBase Digital Terrain Model.]] The Earth's terrain varies greatly from place to place. About 70.8%[29] Fundamentals of Physical Geography of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,<ref name="ngdc2006" /> oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts. [30]Terrestrial Impact Cratering and Its Environmental Effects, Lunar and Planetary Laboratory, also act to reshape the landscape.

As the continental plates migrate across the planet, the ocean floor is subducted under the leading edges. At the same time, upwellings of mantle material create a divergent boundary along mid-ocean ridges. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.[31] Pacific Plate Motion, University of Hawaii, [32] Age of the Ocean Floor Poster, NOAA

The continental plates consist 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.[33] Layers of the Earth, Volcano World, Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust. [34] Weathering and Sedimentary Rocks, Cal Poly Pomona, 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 the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine. [35] Minerals, Museum of Natural History, Oregon, Common carbonate minerals include calcite (found in limestone), aragonite and dolomite.[36] Carbonate sediments

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. [37], The World Factbook, U.S. C.I.A. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3 km² (3.3 acres) of cropland and 3.4 km² (8.4 acres) of pastureland. Production Yearbook 1994, Food and Agriculture Organization of the United Nations,Rome, Italy ISBN 9250038445

The elevation of the land surface of the Earth varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m (29,028 ft) at the top of Mount Everest. The mean height of land above sea level is 686(2,250 ft). The Permanence of Ocean Basins, The Geographical Journal,[38]

Hydrosphere

The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the solar system. The 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 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911 m (35,798 ft or 6.78 mi). [39] "Deep Ocean Studies", Ocean Studies, RAIN National Public Internet and Community Technology Center, Takuyo measurement; see Mariana Trench for details. The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents.

The mass of the oceans is approximately 1.35 e|18 ;metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386 e|9 km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km (approximately 1.7 mi). The total volume of the Earth's oceans is: 1.4 km³. The total surface area of the Earth is 5.1 km². So, to first approximation, the average depth would be the ratio of the two, or 2.7km. About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.Igor A. Shiklomanov, [40] World Water Resources and their use Beginning of the 21st century" Prepared in the Framework of IHP UNESCO, State Hydrological Institute, St. Petersburg.

About 3.5% of the total mass of the oceans consists of salt. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[41],Salt of the Early Earth,NASA Astrobiology Magazine. The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms. [42] NASA Astrobiology Magazine, Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[43]Earth's Big heat Bucket, NASA Earth Observatory. Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation. [44]

Atmosphere

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.<ref name="earth_fact_sheet"/> It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The atmosphere protects the Earth's life forms by absorbing ultraviolet solar radiation, moderating temperature, transporting water vapor, and providing useful gases.[45] Earth's Atmosphere,NASA. It also serves as a shield that causes small meteors to burn up before they strike the surface.

In a phenomenon known as the greenhouse effect, trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the net temperature. Carbon dioxide, water vapor, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C and life would likely not exist.

Weather and climate

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km (about 4 mi) of the planet's 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 heat energy.[46] World Book Online Reference Center,NASA/World Book, Inc.

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°. [47] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.[48]

When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.<ref name="moran2005" /> Most of the water is then transported back to lower elevations by river systems, usually returning to the oceans or being 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, topological features and temperature differences determine the average precipitation that falls in each region.[49]

The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly-used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes<ref name="berger2002" />

Upper atmosphere

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.<ref name="atmosphere" /> Each of these layers 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 Earth's magnetic fields interact with the solar wind).[50] Stratosphere and Weather; Discovery of the Stratosphere, Science WeekAn important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. The Kármán line, defined as 100 km (62 mi) above the Earth's surface, is a working definition for the boundary between atmosphere and space.

Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can escape from the planet's gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate.Liu, S. C.; Donahue, T. M., The Aeronomy of Hydrogen in the Atmosphere of the Earth | journal=Journal of Atmospheric Sciences [51]For this reason, the Earth's current environment is oxidizing, rather than reducing, with consequences for the chemical nature of life which developed on the planet. The oxygen-rich atmosphere also preserves much of the surviving hydrogen by locking it up in water molecules. [52]

Magnetic field

The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[53] = MHD dynamo theory

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.[54] Exploration of the Earth's Magnetosphere.


Orbit and rotation

Relative to the background stars, it takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds (one sidereal day) to rotate around the axis that connects the north and the south poles.[55]. From Earth, the main apparent motion of celestial bodies in the sky (except that of meteors within the atmosphere and low-orbiting satellites) is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes. (The apparent sizes of the Sun and the Moon are approximately the same.)

Earth orbits the Sun at an average distance of about 150 million kilometers (93.2 million miles) every 365.2564 mean solar days (1 sidereal year). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1°/day (or a Sun or Moon diameter every 12 hours) eastward. Because of 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 the Earth averages about 30 km/s (108,000 km/h or 67,000 mi/h), which is fast enough to cover the planet's diameter (about 12,600 km [7,800 mi]) in seven minutes, and the distance to the Moon (384,000 km or 238,000 mi) in four hours.[56] Earth Fact Sheet

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common revolution 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 counter-clockwise. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.5 degrees from the perpendicular to the Earth–Sun plane (which causes the seasons); and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane (without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses).<ref name="moon_fact_sheet"[57] Moon Fact Sheet

Because of the axial tilt of the Earth, the position of the Sun in the sky (as seen by an observer on the surface) varies over the course of the year. For an observer at a northern latitude, when the northern pole is tilted toward the Sun the day lasts longer and the Sun climbs higher in the sky. This results in warmer average temperatures from the increase in solar radiation reaching the surface. When the northern pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the arctic circle, an extreme case is reached where there is no daylight at all for part of the year. (This is called a polar night.)

This variation in the climate (because of the direction of the Earth's axial tilt) results in the seasons. By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo a slight, irregular motion (known as nutation) with a main period of 18.6 years. The orientation (rather than the angle) of the Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the 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. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.[58]

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4 (for other eras, see precession and Milankovitch cycles). The changing Earth-Sun distance results in an increase of about 6.9%<ref>Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% the energy at aphelion.</ref> in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the 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. However, 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.[59] Earth's tilt creates seasons

Observation

Earth was first photographed from space by Explorer 6 in 1959.[60] Explorers: Searching the Universe Forty Years Later | publisher = NASA/Goddard | accessdate = 2007-03-05 }}</ref> Yuri Gagarin became the first human to view Earth from space in 1961. The crew of the Apollo 8 was the first to view an Earth-rise from lunar orbit in 1968. In 1972 the crew of the Apollo 17 produced the famous "Blue Marble" photograph of the planet Earth (see top of page). NASA archivist Mike Gentry has speculated that "The Blue Marble" is the most widely distributed image in human history.


From space, the Earth can be seen to go through phases similar to the phases of the Moon and Venus. This appearance is caused by light that reflects off the Earth as it moves around the Sun. The phases seen depend upon the observer's location in space, and the rate is determined by their orbital period, and by the orbital period of the Earth itself. The phases of the Earth can be simulated by shining light on a globe of the Earth.

An observer on Mars would be able to see the Earth go through phases similar to those that an Earth-bound observer sees the phases of Venus (as discovered by Galileo). However, a fictional observer on the Sun would not see the Earth going through phases. The Sun observer would only be able to see the lit side of the Earth.

Moon

The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's. It is the largest moon in the solar system relative to the size of its planet. (Charon is larger relative to the dwarf planet Pluto.) The natural satellites orbiting other planets are called "moons", after Earth's Moon.

The gravitational attraction between the Earth and Moon cause tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the 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: The dark part of the face is separated from the light part by the solar terminator.

Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm (1.5 in) a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs a year—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. Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, as it appears to be for Mars. If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life.However, this is a controversial subject, and further studies of Mars—which has a similar rotation period and axial tilt as Earth, but not its large Moon or liquid core—may settle the matter.

Viewed from Earth, the Moon is just far enough away to have very nearly 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 eclipses to occur on Earth.

The most widely accepted theory of the Moon's origin, the giant impact theory, 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 the Earth's crust.

Earth has at least two co-orbital satellites, the asteroids 3753 Cruithne and [[2002 AA29|2002 AA

Habitability

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble, and sufficient energy to sustain metabolism. [61] Astrobiology Roadmap, NASA, Lockheed Martin The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.Habitable Planets for Man, American Elsevier Publishing, Co.[62]ISBN 0-444-00092-5]

Biosphere

The planet's life forms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion years ago. Earth is the only place in the universe (officially recognized by the communities of Earth) where life is absolutely known to exist. Some scientists believe that Earth-like biospheres might be rare.

The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land primarily latitude and height above the sea level separates biomes. Terrestrial biomes lying within the Arctic, Antarctic Circle or in high altitudes are relatively barren of plant and animal life, while the greatest latitudinal diversity of species is found at the Equator.

Natural resources and land use

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.

Large deposits of Fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth's crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics. These bodies form concentrated sources for many metals and other useful elements.

The Earth's biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.[63]

The estimated amount of irrigated land in 1993 was 2,481,250 km².<ref name="cia" />

Natural and environmental hazards

Large areas are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species. Human activities are also producing global warming due to industrial carbon dioxide emissions. This is expected to produce changes such as the melting of glaciers and Arctic ice, more extreme temperatures, significant changes in weather conditions and a global rise in average sea levels.

Human geography

The Earth at night, a composite of DMSP/OLS ground illumination data on a simulated night-time image of the world. This image is not photographic and many features are brighter than they would appear to a direct observer.]] Earth has approximately 6,600,000,000 human inhabitants."LiveScience"

It is estimated that only one eighth of the surface of the Earth is suitable for humans to live on—three-quarters is covered by oceans, and half of the land area is desert (14%) (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)

Independent sovereign nations claim all of the planet's land surface, with the exception of some parts of Antarctica. As of 2007 there are 201 sovereign states, including the 192 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities. Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.

The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict. It is not, however, a world government. While the U.N. provides a mechanism for international law and, when the consensus of the membership permits, armed intervention,[64]it serves primarily as a forum for international diplomacy.

In total, about 400 people have been outside the Earth's atmosphere as of 2004, and, of these, twelve have walked on the Moon. Normally the only humans in space are those on the International Space Station. The station's crew of three people is usually replaced every six months.

Cultural viewpoint

The name of the planet originated from the 8th century Anglo-Saxon word erda, which means ground or soil. In Old English the word became eorthe, then erthe in Middle English. It is the only planet whose name in English is not derived from greco-roman mythology.

Earth has often been personified as a deity, in particular a goddess. In many cultures the mother goddess, also called the Mother Earth, is also portrayed as a fertility deity.

To the Aztec, Earth was called [[Tonantzin]"our mother". The Chinese Earth goddess Hou-T'u Myths & Legends of China [65] is similar to Gaia, the Greek goddess personifying the Earth. To Hindus it is called Bhuma Devi, the Goddess of Earth. In Norse mythology, the Earth goddess Jord was the mother of Thor and the daughter of Annar. Ancient Egyptian mythology is different from that of other cultures because Earth is male, Geb, and sky is female, Nut.

In many religions, accounts of creation of the Earth exist, recalling a story involving the creation of the Earth by a supernatural deity or deities.

In the ancient past there were varying levels of belief in a flat Earth, with the Mesopotamian culture portraying the world as a flat disk afloat in an ocean. The spherical form of the Earth was suggested by early Greek philosophers; a belief espoused by Pythagoras. By the Middle Ages—as evidenced by thinkers such as Thomas Aquinas—European belief in a spherical Earth was widespread.[66], The Myth of the Flat Earth , American Scientific Affiliation, but see also Cosmas Indicopleustes. Prior to the introduction of space flight, belief in a spherical Earth was based on observations of the secondary effects of the Earth's shape and parallels drawn with the shape of other planets.

Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

The technological developments of the latter half of the 20th century are widely considered to have altered the public's perception of the Earth. Before space flight, the popular image of Earth was of a green world. Science fiction artist Frank R. Paul provided perhaps the first image of a cloudless blue planet (with sharply defined land masses) on the back cover of the July 1940 issue of Amazing Stories, a common depiction for several decades thereafter.

Apollo 17's 1972 "Blue Marble" photograph of Earth from cislunar space became the current iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. A photo taken of a distant Earth by Voyager 1 in 1990 inspired Carl Sagan to describe the planet as a "Pale Blue Dot."

Over the past two centuries a growing environmental movement has emerged that is concerned about humankind's effects on the Earth. The key issues of this socio-political movement are the conservation of natural resources, elimination of pollution, and the usage of land. Environmentalists advocate sustainable management of resources and stewardship of the natural environment through changes in public policy and individual behavior. Of particular concern is the large-scale exploitation of non-renewable resources. Changes sought by the environmental movements are often in conflict with commercial interests due to the significant additional costs associated with managing the environmental impact.

Future

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium ash at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will increase by 10 percent over the next 1.1 billion years (1.1 Gyr), and by 40% over the next 3.5 Gyr.

The Sun, as part of its solar lifespan, will expand to a red giant in 5 Gyr. Models predict that the Sun will expand out to about 99% of the distance to the Earth's present orbit (1 astronomical unit, or AU). However, by that time, the orbit of the Earth may have expanded to about 1.7 AUs because of the diminished mass of the Sun. The planet might thus escape envelopment by the expanded Sun's sparse outer atmosphere, though most (if not all) existing life will have been destroyed by the Sun's proximity to the Earth.

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