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 How many species are there on earth? [2] including humans,

Earth is the only place in the universe known to harbor life. About 71% of Earth's surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for life as we know it, is not known to exist on any other planet's surface. Other planets in the solar system are either too hot or too cold to support liquid water. However, it is believed to have existed on the surface of Mars in the past, and may still appear today. See: Simulations Show Liquid Water Could Exist on Mars, University of Arkansas, [3] As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: Water vapour in the atmosphere of a transiting extrasolar planet, Nature, [4] The Earth formed about 4.57 billion years The Age of the Earth, Stanford University Press, California, ISBN 0-8047-1569-6 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.[1] 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[2] 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, [5] impacting the Earth in a glancing blow. Origin of the Moon in a giant impact near the end of the Earth's formation [6]. 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, [7] 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.Cite error: Closing </ref> missing for <ref> tag 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.[8] Earth's tilt creates seasons

Observation

Earth was first photographed from space by Explorer 6 in 1959.[9] 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. [10] 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.[11]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.<ref>{{cite journal [12]

The estimated amount of irrigated land in 1993 was 2,481,250 km².[3]

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,[13]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 [14] 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.[15], 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.

External links


for more see:[16]

  1. 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.
  2. Cite error: Invalid <ref> tag; no text was provided for refs named age_earth
  3. Cite error: Invalid <ref> tag; no text was provided for refs named cia