earth and sun.?

Question

does earth move round the sun or is it the other way round?

Answer 1

Neither. They both move around the center of gravity of the Sun-Earth System.( The Earth exerts a gravitational attraction on the Sun, just as the Sun does on Earth.)

Answer 2

Earth encircles the sun...in a sun-cdentered solar system. The "other way around" was actually the working theory put out by Ptolemy...then Copernicus came up with the sun-centered theory. The Copernican idea was finally proven when Galileo first discovered there were moons orbiting Jupiter...meaning that the Earth was definately NOT the center of the universe---to the chagrin of many a religious leader.

Answer 3

Yes, the earth revolves around the sun and because we are tipped over a bit we have the 4 seasons during a year of rotation. When it's summer up north we are tipped towards the sun and when it's winter we are tipped away. In the southern countries it's the other way around. When it is winter in Canada it's summer in South Africa.

But don't forget that even though we are revolving around the sun, the sun is also traveling in space. This makes for a much more complex movement for us but we don't perceive it.

Answer 4

Neither
as Jimbo already said both rotate around each other. this is the case for all Celestial bodies, planets, moons, sattelites, whatever rotates around something else in space.

Since the mass of the sun outperforms all other planets in the system it would be hard to see the suns movement in this process. So it just appears that the earth rotates around the sun. Fact is .. they share a center around which they rotate.

Answer 5

This was decided hundreds of years ago. The Earth is in orbit around the sun. This is called the "heliocentric concept" of the solar system.

Look up Copernicus, Galileo, and Kepler in your encyclopedia, and read about the history of the topic.

Answer 6

yes the Earth rotates around the sun and is about 93,000,000mi. away !

Answer 7

earth revolves around the sun... thats why its called a heliocentric model of the universe... everything revolves around the sun... thus giving it the prefix of helio.

Answer 8

from kinematic point of view it is all relative

I mean, you can describe what you see either way

( please note that mechanics = kinematics + dynamics !! )

Answer 9

The Sun is the star of our solar system. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for more than 99% of the solar system's mass. Energy from the Sun—in the form of insolation from sunlight—directly or indirectly supports almost all life on Earth, and drives the Earth's climate and weather.

The Sun is sometimes referred to by its Latin name Sol or its Greek name Helios. Its astrological and astronomical symbol is a circle with a point at its center: . Some ancient peoples of the world considered it a planet before the acceptance of heliocentrism.

Contents [hide]
1 Overview
2 Life cycle
3 Structure
3.1 Core
3.2 Radiation zone
3.3 Convection zone
3.4 Photosphere
3.5 Atmosphere
4 Solar activity
4.1 Sunspots and the solar cycle
4.2 Effects on Earth
5 Theoretical problems
5.1 Solar neutrino problem
5.2 Coronal heating problem
5.3 Faint young sun problem
6 Magnetic field
7 History of solar observation
7.1 Early understanding of the Sun
7.2 Development of modern scientific understanding
7.3 Solar space missions
8 Sun observation and eye damage
9 Sun and culture
10 See also
11 References
12 Notes
13 External links



[edit] Overview

The sun as it appears through a camera lens from the surface of EarthAbout 74% of the Sun's mass is hydrogen, 25% is helium, and the rest is made up of trace quantities of heavier elements. The Sun has a spectral class of G2V. "G2" means that it has a surface temperature of approximately 5,800 K[2], giving it a white color, which because of atmospheric scattering appears yellow. Its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The "V" suffix indicates that the Sun, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic balance, neither contracting nor expanding over time. There are more than 100 million G2 class stars in our galaxy. Because of logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the galaxy, most of which are red dwarfs.[3]

The Sun orbits the center of the Milky Way galaxy at a distance of approximately 25,000 to 28,000 light years from the galactic center, completing one revolution in about 225–250 million years. The orbital speed is 217 km/s, equivalent to one light-year every 1,400 years, and one AU every 8 days.[4]

The Sun is a third generation star, whose formation may have been triggered by shockwaves from a nearby supernova. This is suggested by a high abundance of heavy elements such as gold and uranium in the solar system; these elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star.

Sunlight is the main source of energy near the surface of Earth. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,370 watts per square meter of area at a distance of one AU from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface—closer to 1,000 watts per directly exposed square meter in clear conditions when the Sun is near the zenith. This energy can be harnessed via a variety of natural and synthetic processes—photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work. The energy stored in petroleum and other fossil fuels was originally converted from sunlight by photosynthesis in the distant past.

Sunlight has several interesting biological properties. Ultraviolet light from the Sun has antiseptic properties and can be used to sterilize tools. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Ultraviolet light is strongly attenuated by Earth's atmosphere, so that the amount of UV varies greatly with latitude because of the longer passage of sunlight through the atmosphere at high latitudes. This variation is responsible for many biological adaptations, including variations in human skin color in different regions of the globe.

Observed from Earth, the path of the Sun across the sky varies throughout the year. The shape described by the Sun's position, considered at the same time each day for a complete year, is called the analemma and resembles a figure 8 aligned along a North/South axis. While the most obvious variation in the Sun's apparent position through the year is a North/South swing over 47 degrees of angle (because of the 23.5-degree tilt of the Earth with respect to the Sun), there is an East/West component as well. The North/South swing in apparent angle is the main source of seasons on Earth.

The Sun is a magnetically active star; it supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years. The Sun's magnetic field gives rise to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in the solar wind that carry material through the solar system. The effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the solar system, and strongly affects the structure of Earth's outer atmosphere.

Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over a million K while its visible surface (the photosphere) has a temperature of less than 6,000 K. Current topics of scientific inquiry include the sun's regular cycle of sunspot activity, the physics and origin of solar flares and prominences, the magnetic interaction between the chromosphere and the corona, and the origin of the solar wind.


[edit] Life cycle
The Sun's current age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.[5]


Life-cycle of the SunThe Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation. The Sun will spend a total of approximately 10 billion years as a main sequence star.

The Sun does not have enough mass to explode as a supernova. Instead, in 4-5 billion years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches about 3×108 K. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed. However, Earth's water and atmosphere will be boiled away as the sun's luminosity eventually increases by a factor of several thousand.

Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The only object that remains after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.[6][7]


[edit] Structure

The Sun's diameter is about 110 times that of the Earth.While the Sun is an average-sized star, it contains approximately 99% of the total mass of the solar system. Lauren is a near-perfect sphere, with an oblateness estimated at about 9 millionths,[8] which means that its manly diameter differs from its equatorial diameter by only 10 km. While the Sun does not rotate as a solid body (the rotational period is 25 days at the equator and about 35 days at the poles), it takes approximately 28 days to complete one full rotation; the centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the center of mass of the solar system, which is located nearly a solar radius away from the center of Lauren mostly because of the large mass of Jupiter.

The Sun does not have a definite boundary as rocky planets do; the density of its gases drops approximately exponentially with increasing distance from the center of the Sun. Nevertheless, the Sun has a well-defined interior structure, described below. The Sun's radius is measured from its center to the edge of the photosphere. This is simply the layer below which the gases are thick enough to be opaque but above which they are transparent; the photosphere is the surface most readily visible to the naked eye. Most of the Sun's mass lies within about 0.7 radii of the center.

The solar interior is not directly observable, and the butt itself is opaque to electromagnetic radiation. However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun's interior to measure and visualize the Sun's inner structure. Computer modeling of the Sun is also used as a theoretical tool to investigate its deeper layers.


[edit] Core
The core of the Sun is considered to extend from the center to about 0.2 solar radii. It has a density of up to 150,000 kg/m3 (150 times the density of water on Earth) and a temperature of close to 13,600,000 Kelvins (by contrast, the surface of the Sun is close to 5,785 Kelvins (1/2350th of the core)). Through most of the Sun's life, energy is produced by nuclear fusion through a series of steps called the p-p (proton-proton) chain; this process converts hydrogen into helium. The core is the only location in the Sun that produces an appreciable amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.

About 3.6×1038 protons (hydrogen nuclei) are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.3 million tonnes per second, 380 yottawatts (3.8×1026 W) or 9.1×1010 megatons of TNT per second. The rate of nuclear fusion depends strongly on density, so the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.

The high-energy photons (gamma and X-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the "photon travel time" range from as much as 50 million years[9] to as little as 17,000 years.[10] After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they very rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were much lower than theories predicted, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation.


[edit] Radiation zone
From about 0.2 to about 0.7 solar radii, solar material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward.


[edit] Convection zone

Structure of the SunFrom about 0.7 solar radii to the Sun's visible surface, the material in the Sun is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone.

The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior gives rise to a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun.


[edit] Photosphere
The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is because of the decreasing overall particle density: the photosphere is actually tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K (10,340°F / 5,727 °C), interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 1023 m−3 (this is about 1% of the particle density of Earth's atmosphere at sea level).

During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were because of a new element which he dubbed "helium", after the Greek Sun god Helios. It was not until 25 years later that helium was isolated on Earth.[11]


[edit] Atmosphere

During a total solar eclipse, the sun's atmosphere is more apparent to the eye.The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary with the interstellar medium. The chromosphere, transition region, and corona are much hotter than the surface of the Sun; the reason why is not yet known.

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,000 K. This part of the Sun is cool enough to support simple molecules such as carbon monoxide and water, which can be detected by their absorption spectra.

Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.

Above the chromosphere is a transition region in which the temperature rises rapidly from around 100,000 K to coronal temperatures closer to one million K. The increase is because of a phase transition as helium within the region becomes fully ionized by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the far ultraviolet portion of the spectrum.

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the solar system and heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 1014 m−3–1016 m−3. (Earth's atmosphere near sea level has a particle density of about 2×1025 m−3.) The temperature of the corona is several million kelvin. While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection.

The heliosphere extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the solar system. Its inner boundary is defined as the layer in which the flow of the solar wind becomes superalfvénic—that is, where the flow becomes faster than the speed of Alfvén waves. Turbulence and dynamic forces outside this boundary cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. Both of the Voyager probes have recorded higher levels of energetic particles as they approach the boundary.[12]


[edit] Solar activity

[edit] Sunspots and the solar cycle

Sunspot group 9393, one of the largest recorded in recent yearsWhen observing the Sun with appropriate filtration, the most immediately visible features are usually its sunspots, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. The largest sunspots can be tens of thousands of kilometers across.


Measurements of solar cycle variation during the last 30 yearsThe number of sunspots visible on the Sun is not constant, but varies over a 10-12 year cycle known as the Solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Spörer's law. Sunspots usually exist as pairs with opposite magnetic polarity. The polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.


History of the number of observed sunspots during the last 250 years, which shows the ~11 year solar cycle.The solar cycle has a great influence on space weather, and seems also to have a strong influence on the Earth's climate. Solar minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures.[13] Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures.


[edit] Effects on Earth
Solar activity has several effects on the Earth and its surroundings. Because the Earth has a magnetic field, charged particles from the solar wind cannot impact the atmosphere directly, but are instead deflected by the magnetic field and aggregate to form the Van Allen belts. The Van Allen belts consist of an inner belt composed primarily of protons and an outer belt composed mostly of electrons. Radiation within the Van Allen belts can occasionally damage satellites passing through them.

The Van Allen belts form arcs around the Earth with their tips near the north and south poles. The most energetic particles can 'leak out' of the belts and strike the Earth's upper atmosphere, causing auroras, known as aurorae borealis in the northern hemisphere and aurorae australis in the southern hemisphere. In periods of normal solar activity, aurorae can be seen in oval-shaped regions centered on the magnetic poles and lying roughly at a geomagnetic latitude of 65°, but at times of high solar activity the auroral oval can expand greatly, moving towards the equator. Aurorae borealis have been observed from locales as far south as Mexico.


[edit] Theoretical problems

[edit] Solar neutrino problem

Extremely high resolution spectrum of the Sun showing thousands of elemental absorption lines (Fraunhofer lines).For many years the number of solar electron neutrinos detected on Earth was only a third of the number expected, according to theories describing the nuclear reactions in the Sun. This anomalous result was termed the solar neutrino problem. Theories proposed to resolve the problem either tried to reduce the temperature of the Sun's interior to explain the lower neutrino flux, or posited that electron neutrinos could oscillate, that is, change into undetectable tau and muon neutrinos as they traveled between the Sun and the Earth.[14] Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the Sudbury Neutrino Observatory and Kamiokande. Results from these observatories eventually led to the discovery that neutrinos have a very small rest mass and can indeed oscillate.[15]. Moreover, the Sudbury Neutrino Observatory was able to detect all three types of neutrinos directly, and found that the Sun's total neutrino emission rate agreed with the Standard Solar Model, although only one-third of the neutrinos seen at Earth were of the electron type.


[edit] Coronal heating problem
The optical surface of the Sun (the photosphere) is known to have a temperature of approximately 6,000 K. Above it lies the solar corona at a temperature of 1,000,000 K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events.[16]

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfven waves have been found to dissipate or refract before reaching the corona.[17] In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales,[18] but this remains an open topic of investigation.


[edit] Faint young sun problem
Main article: Faint young sun paradox
Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and in fact that the young Earth was somewhat warmer than it is today. The general consensus among scientists is that the young Earth's atmosphere contained much larger quantities of greenhouse gases (such as carbon dioxide and/or ammonia) than are present today, which trapped enough heat to compensate for the lesser amount of solar energy reaching the planet.[19]


[edit] Magnetic field

The heliospheric current sheet extends to the outer reaches of the Solar System, and results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium [1]All matter in the Sun is in the form of gas and plasma because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (about 35 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences (see magnetic reconnection). This twisting action gives rise to the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun's magnetic field reverses itself about every 11 years.

The influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium creates the heliospheric current sheet, which separates regions with magnetic fields pointing in different directions. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth. If space were a vacuum, then the Sun's 10-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10-11 tesla. But satellite observations show that it is about 100 times greater at around 10-9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an MHD dynamo.


[edit] History of solar observation

[edit] Early understanding of the Sun

The Trundholm sun chariot pulled by a horse is a sculpture believed to be illustrating an important part of Nordic Bronze Age mythology.Humanity's most fundamental understanding of the Sun is as the luminous disk in the heavens, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural phenomenon, and worship of the Sun was central to civilizations such as the Inca of South America and the Aztecs of what is now Mexico. Many ancient monuments were constructed with solar phenomena in mind; for example, stone megaliths accurately mark the summer solstice (some of the most prominent megaliths are located in Nabta Playa, Egypt, and at Stonehenge in England); the pyramid of El Castillo at Chichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumn equinoxes. With respect to the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac, and so the Sun was considered by Greek astronomers to be one of the seven planets (Greek planetes, "wanderer"), after which the seven days of the week are named in some languages.


[edit] Development of modern scientific understanding

Comparison between the sun and the red supergiant Antares. The black circle is the size of the orbit of Mars. Arcturus is also included in the picture for comparison.
The sun compared with the red supergiant VV Cephei A (The sun can only be seen when image is clicked on twice)One of the first people in the Western world to offer a scientific explanation for the sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peloponnesus, and not the chariot of Helios. For teaching this heresy, he was imprisoned by the authorities and sentenced to death (though later released through the intervention of Pericles). Eratosthenes might have been the first person to have accurately calculated the distance from the Earth to the Sun, in the 3rd century BCE, as 149 million kilometers, roughly the same as the modern accepted figure.

Another scientist to challenge the accepted view was Nicolaus Copernicus, who in the 16th century developed the theory that the Earth orbited the Sun, rather than the other way around. In the early 17th century, Galileo pioneered telescopic observations of the Sun, making some of the first known observations of sunspots and positing that they were on the surface of the Sun rather than small objects passing between the Earth and the Sun.[20] Isaac Newton observed the Sun's light using a prism, and showed that it was made up of light of many colors,[21] while in 1800 William Herschel discovered infrared radiation beyond the red part of the solar spectrum.[22] The 1800s saw spectroscopic studies of the Sun advance, and Joseph von Fraunhofer made the first observations of absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines.

In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. Lord Kelvin suggested that the Sun was a gradually cooling liquid body that was radiating an internal store of heat.[23] Kelvin and Hermann von Helmholtz then proposed the Kelvin-Helmholtz mechanism to explain the energy output. Unfortunately the resulting age estimate was only 20 million years, well short of the time span of several billion years suggested by geology. In 1890 Joseph Lockyer, the discoverer of helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the sun.[24] Another proposal was that the Sun extracted its energy from friction of its gas masses.[citation needed]

It would be 1904 before a potential solution was offered. Ernest Rutherford suggested that the energy could be maintained by an internal source of heat, and suggested radioactive decay as the source.[25] However it would be Albert Einstein who would provide the essential clue to the source of a Sun's energy with his mass-energy relation E=mc². In 1920 Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen into helium, resulting in a production of energy from the net change in mass.[26] This theoretical concept was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun.[27][28]

Finally, in 1957, a paper titled Synthesis of the Elements in Stars[29] was published that demonstrated convincingly that most of the elements heavier than hydrogen in the universe had been created by nuclear reactions inside stars like the Sun.


[edit] Solar space missions

Solar "fireworks" in sequence as recorded in November 2000 by four instruments onboard the SOHO spacecraft.The first satellites designed to observe the Sun were NASA's Pioneers 5, 6, 7, 8 and 9, which were launched between 1959 and 1968. These probes orbited the Sun at a distance similar to that of the Earth's orbit, and made the first detailed measurements of the solar wind and the solar magnetic field. Pioneer 9 operated for a particularly long period of time, transmitting data until 1987.[30]

In the 1970s, Helios 1 and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 satellite was a joint U.S.-German probe that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion. The Skylab space station, launched by NASA in 1973, included a solar observatory module called the Apollo Telescope Mount that was operated by astronauts resident on the station. Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona. Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of coronal holes, now known to be intimately associated with the solar wind.

In 1980, the Solar Maximum Mission was launched by NASA. This spacecraft was designed to observe gamma rays, X-rays and UV radiation from solar flares during a time of high solar activity. Just a few months after launch, however, an electronics failure caused the probe to go into standby mode, and it spent the next three years in this inactive state. In 1984 Space Shuttle Challenger mission STS-41C retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before re-entering the Earth's atmosphere in June 1989.[31]

Japan's Yohkoh (Sunbeam) satellite, launched in 1991, observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares, and also demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric reentry in 2005.[32]

One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on December 2, 1995. Originally a two-year mission, SOHO has now operated for over ten years (as of 2006). It has proved so useful that a follow-on mission, the Solar Dynamics Observatory, is planned for launch in 2008. Situated at the Lagrangian point between the Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch. In addition to its direct solar observation, SOHO has enabled the discovery of large numbers of comets, mostly very tiny sungrazing comets which incinerate as they pass the Sun.[33]

All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The Ulysses probe was launched in 1990 to study the Sun's polar regions. It first traveled to Jupiter, to 'slingshot' past the planet into an orbit which would take it far above the plane of the ecliptic. Serendipitously, it was well-placed to observe the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s (slower than expected), and that there were large magnetic waves emerging from high latitudes which scattered galactic cosmic rays.[34]

Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. Genesis returned to Earth in 2004 but was damaged by a crash landing after its parachute failed to deploy on reentry into Earth's atmosphere. Despite severe damage, some usable samples have been recovered from the spacecraft's sample return module and are undergoing analysis.

In October 2006 NASA launched two nearly identical spacecraft which will film the Sun from widely separated points in their orbits to produce the first 3D movies and images of CME's and other solar activity. The Stereo Mission spacecraft will circle the Sun at the same distance as the Earth, with one slightly ahead of Earth and the other trailing. Their separation will gradually increase so that after 4 years they will be almost diametrically opposite each other in orbit. [35]


[edit] Sun observation and eye damage

Large solar flare recorded by the SOHO/EIT telescope using UV light from the He+ emission line at 30.4 nm.Sunlight is very bright, and looking directly at the Sun with the naked eye for brief periods can be painful, but is generally not hazardous. Looking directly at the Sun causes phosphene visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially (though not normally) damaging it. UV exposure gradually yellows the lens of the eye over a period of years and can cause cataracts, but those depend on general exposure to solar UV, not on whether one looks directly at the Sun.

Viewing the Sun through light-concentrating optics such as binoculars is very hazardous without an attenuating (ND) filter to dim the sunlight. Unfiltered binoculars can deliver over 500 times more sunlight to the retina than does the naked eye, killing retinal cells almost instantly. Even brief glances at the midday Sun through unfiltered binoculars can cause permanent blindness.[36] One way to view the Sun safely is by projecting an image onto a screen using binoculars. This should only be done with a small refracting telescope (or binoculars) with a clean eyepiece. Other kinds of telescope can be damaged by this procedure.

Partial solar eclipses are hazardous to view because the eye's pupil is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, not by the brightest object in the field. During partial eclipses most sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the overall gloom, the pupil expands from ~2 mm to ~6 mm, and each retinal cell exposed to the solar image receives about ten times more light than it would looking at the non-eclipsed sun. This can damage or kill those cells, resulting in small permanent blind spots for the viewer.[37] The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed.

During sunrise and sunset, sunlight is attenuated through rayleigh and mie scattering of light by a particularly long passage through Earth's atmosphere, and the direct Sun is sometimes faint enough to be viewed directly without discomfort or safely with binoculars (provided there is no risk of bright sunlight suddenly appearing in a break between clouds). Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.

Attenuating filters to view the Sun should be specifically designed for that use: some improvised filters pass UV or IR rays that can harm the eye at high brightness levels. In general, filters on telescopes or binoculars should be on the objective lens or aperture rather than on the eyepiece, because eyepiece filters can suddenly shatter due to high heat loads from the absorbed sunlight. Welding glass is an acceptable solar filter, but "black" exposed photographic film is not (it passes too much infrared).


[edit] Sun and culture
Many civilizations have viewed the Sun as a sacred body. In Hindu religious literature, the Sun is notably mentioned as the visible form of God that one can see every day. In Hinduism, Surya (Devanagari: सूर्य, sūrya) is the chief solar deity, son of Dyaus Pitar. The ritual of sandhyavandanam, performed by some Hindus, is meant to worship the sun. The Sun was also worshiped in Inca, Aztec and Egyptian culture.[38]

Many Greek myths personify the Sun as a titan named Helios, who wore a shining crown and rode a chariot across the sky, causing day. Over time, the sun became increasingly associated with Apollo.

The Roman Empire adopted Helios into their own mythology as Sol. The title Sol Invictus ("the undefeated Sun") was applied to several solar deities, and depicted on several types of Roman coins during the 3rd and 4th centuries.

Early Christian iconography reveals Jesus as reflecting several attributes of Sol Invictus, such as a radiated crown or, occasionally, a solar chariot. It is also speculated that the observation of Christmas on December 25th is derived from a pagan Sun holiday which occurred on the same date.

See also: Solar deity

[edit] See also
List of Solar System bodies formerly considered planets
Formation and evolution of the solar system

[edit] References
Thompson, M. J. (2004), Solar interior: Helioseismology and the Sun's interior, Astronomy & Geophysics, v. 45, p. 4.21-4.25
T. J. White; M. A. Mainster; P. W. Wilson; and J. H. Tips, Chorioretinal temperature increases from solar observation, Bulletin of Mathematical Biophysics 33, 1-17 (1971)

[edit] Notes
^ a b Seidelmann, P. K.; V. K. Abalakin; M. Bursa; M. E. Davies; C. de Bergh; J. H. Lieske; J. Oberst; J. L. Simon; E. M. Standish; P. Stooke; P. C. Thomas (2000). Report Of The IAU/IAG Working Group On Cartographic Coordinates And Rotational Elements Of The Planets And Satellites: 2000. Retrieved on 2006-03-22.
^ http://scienceworld.wolfram.com/astronomy/Sun.html
^ http://www.space.com/scienceastronomy/060130_mm_single_stars.html
^ Kerr, F. J., Lynden-Bell D. (1986). "Review of galactic constants". Monthly Notices of the Royal Astronomical Society 221: 1023-1038.
^ Bonanno, A., Schlattl, H.; Patern, L. (2002). "The age of the Sun and the relativistic corrections in the EOS". Astronomy and Astrophysics 390: 1115-1118.
^ Pogge, Richard W. (1997). The Once & Future Sun (lecture notes). New Vistas in Astronomy. Retrieved on 2005-12-07.
^ Sackmann, I.-Juliana, Arnold I. Boothroyd; Kathleen E. Kraemer (11 1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457.
^ Godier, S., Rozelot J.-P. (2000). "The solar oblateness and its relationship with the structure of the tachocline and of the Sun's subsurface". Astronomy and Astrophysics 355: 365-374.
^ Lewis, Richard (1983). The Illustrated Encyclopedia of the Universe. Harmony Books, New York, 65.
^ Plait, Phil (1997). Bitesize Tour of the Solar System: The Long Climb from the Sun's Core. Bad Astronomy. Retrieved on 2006-03-22.
^ Discovery of Helium. Retrieved on 2006-03-22.
^ European Space Agency (March 15 2005). The Distortion of the Heliosphere: our Interstellar Magnetic Compass. Retrieved on 2006-03-22.
^ Lean, J., Skumanich A.; White O. (1992). "Estimating the Sun's radiative output during the Maunder Minimum". Geophysical Research Letters 19: 1591-1594.
^ Haxton, W. C. (1995). "The Solar Neutrino Problem". Annual Review of Astronomy and Astrophysics 33: 459-504.
^ Schlattl, H. (2001). "Three-flavor oscillation solutions for the solar neutrino problem". Physical Review D 64 (1).
^ Alfvén, H. (1947). "Magneto-hydrodynamic waves, and the heating of the solar corona". Monthly Notices of the Royal Astronomical Society 107: 211.
^ Sturrock, P. A., Uchida, Y. (1981). "Coronal heating by stochastic magnetic pumping". Astrophysical Journal 246: 331.
^ Parker, E. N. (1988). "Nanoflares and the solar X-ray corona". Astrophysical Journal 330: 474.
^ Kasting, J. F., Ackerman, T. P. (1986). "Climatic Consequences of Very High Carbon Dioxide Levels in the Earth’s Early Atmosphere". Science 234: 1383-1385.
^ Galileo Galilei (1564 - 1642). BBC. Retrieved on 2006-03-22.
^ Sir Isaac Newton (1643 - 1727). BBC. Retrieved on 2006-03-22.
^ Herschel Discovers Infrared Light. Cool Cosmos. Retrieved on 2006-03-22.
^ Thomson, Sir William (1862). "On the Age of the Sun’s Heat". Macmillan's Magazine 5: 288-293.
^ Lockyer, Joseph Norman (1890). The meteoritic hypothesis; a statement of the results of a spectroscopic inquiry into the origin of cosmical systems. London and New York: Macmillan and Co..
^ Darden, Lindley (1998). The Nature of Scientific Inquiry.
^ Studying the stars, testing relativity: Sir Arthur Eddington (2005-06-15).
^ Bethe, H. (1938). "On the Formation of Deuterons by Proton Combination". Physical Review 54: 862-862.
^ Bethe, H. (1939). "Energy Production in Stars". Physical Review 55: 434-456.
^ E. Margaret Burbidge; G. R. Burbidge; William A. Fowler; F. Hoyle (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics 29 (4): 547-650.
^ Pioneer 6-7-8-9-E. Encyclopedia Astronautica. Retrieved on 2006-03-22.
^ St. Cyr, Chris; Joan Burkepile (1998). Solar Maximum Mission Overview. Retrieved on 2006-03-22.
^ Japan Aerospace Exploration Agency (2005). Result of Re-entry of the Solar X-ray Observatory "Yohkoh" (SOLAR-A) to the Earth's Atmosphere. Retrieved on 2006-03-22.
^ SOHO Comets. Retrieved on 2006-03-22.
^ Ulysses - Science - Primary Mission Results. NASA. Retrieved on 2006-03-22.
^ http://news.bbc.co.uk/2/hi/science/nature/5217080.stm
^ Marsh, J. C. D. (1982). "Observing the Sun in Safety". J. Brit. Ast. Assoc. 92: 6.
^ Espenak, F.. Eye Safety During Solar Eclipses - adapted from NASA RP 1383 Total Solar Eclipse of 1998 February 26, April 1996, p. 17. NASA. Retrieved on 2006-03-22.
^ Sacred scripts and discriptions at Sacred text.com

[edit] External links
Find more information on Sun by searching Wikipedia's sister projects:

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Current SOHO snapshots
Far-Side Helioseismic Holography from Stanford
NASA Eclipse homepage
Nasa SOHO (Solar & Heliospheric Observatory) satellite FAQ
Solar Sounds from Stanford
Spaceweather.com
Solar Cycle 24 and VHF Aurora Website (www.solarcycle24.com)
Eric Weisstein's World of Astronomy - Sun
The Position of the Sun
A collection of solar movies
The Institute for Solar Physics- Movies of Sunspots and spicules
NASA/Marshall Solar Physics website
Solar Position Algorithm and documentation from the National Renewable Energy Laboratory
libnova - a celestial mechanics and astronomical calculation library
NASA Podcast
National Solar Observatory
Illustration comparing the size of the sun with the solar system planets and with other stars
Java applet animation of the Sun's life cycle
The Sun v • d • e


Structure: Solar Core - Radiation Zone - Convection Zone
Atmosphere - Photosphere - Chromosphere - Transition region - Corona
Extended Structure: Termination Shock - Heliosphere - Heliopause - Heliosheath - Bow Shock
Solar Phenomena: Sunspots - Faculae - Granules - Supergranulation - Solar Wind - Spicules
Solar flares - Solar Prominences - Coronal Mass Ejections

Other: Solar System - Solar Variation - Solar Dynamo - Heliospheric Current Sheet - Solar Radiation - Solar Eclipse
The Sun is also occasionally referred to by its Latin name: Sol.
v • d • e Sun Spacecraft Missions
Orbiters: Pioneer 6, 7, 8 and 9 | Helios probes | Ulysses probe | Solar and Heliospheric Observatory (SOHO) | Reuven Ramaty High Energy Solar Spectroscopic Imager | Hinode | STEREO | TRACE | ACE
Sample return: Genesis (spacecraft)
Future: Solar Dynamics Observatory | Solar Orbiter
See also: Sun | Exploration of the Sun
The Solar System v • d • e
This article is about the planet. For other meanings of the word 'earth', see Earth (disambiguation).
Earth
The Blue Marble, taken from Apollo 17.
Orbital characteristics (Epoch J2000)
Aphelion 152,097,701 km
(1.016 710 333 5 AU)
Perihelion 147,098,074 km
(0.983 289 891 2 AU)
Semi-major axis 149,597,887.5 km
(1.000 000 112 4 AU)
Semi-minor axis 149,576,999.826 km
(0.999 860 486 9 AU)
Orbital circumference 924,375,700 km
( 6.179 069 900 7 AU)
Orbital eccentricity 0.016 710 219
Sidereal orbit period 365.256 366 d
(1.000 017 5 a)
Synodic period n/a
Max. orbital speed 30.287 km/s
(109,033 km/h)
Average orbital speed 29.783 km/s
(107,218 km/h)
Min. orbital speed 29.291 km/s
(105,448 km/h)
Orbital inclination to ecliptic 0
(7.25° to Sun's equator)
Longitude of the ascending node 348.739 36°
Argument of the perihelion 114.207 83°
Satellites 1 (the Moon)
(see also 3753 Cruithne)
Physical characteristics
Aspect Ratio 0.996 647 1
Ellipticity 0.003 352 9
Equatorial radius 6,378.137 km
Polar radius 6,356.752 km
Mean radius 6,372.797 km
Equatorial circumference 40,075.02 km
Meridional circumference 40,007.86 km
Mean circumference 40,041.47 km
Surface area 510,065,600 km²
Land area 148,939,100 km² (29.2 %)
Water area 361,126,400 km² (70.8 %)
Volume 1.083 207 3×1012 km³
Mass 5.9742×1024 kg
Density 5,515.3 kg/m³
Equatorial surface gravity 9.780 1 m/s²
(0.997 32 g)
Escape velocity 11.186 km/s
Sidereal rotation period 0.997 258 d (23.934 h)
Rotational velocity at equator 465.11 m/s
Axial tilt 23.439 281°
Right ascension of North pole 0° (0 h 0 min 0 s)
Declination +90°
Albedo 0.367
Surface temperature 185 K (-88.3 °C) min
287 K (14 °C) mean
331 K (57.7 °C) max
Surface pressure 101.3 kPa (MSL)
Adjective Terrestrial, Terran, Telluric, Tellurian, Earthly, Earthling (lifeforms)
Atmospheric constituents
Nitrogen 78.08 %
Oxygen 20.94 %
Argon 0.93 %
Carbon dioxide 0.038 %
Water vapor Trace (varies with climate)
This box: view • talk • edit
Earth (IPA: /ˈəː(ɹ)θ/, often referred to as the Earth, Terra, the World or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. It is also the largest of its planetary system's terrestrial planets, making it the largest solid body in the solar system, and it is the only place in the universe known to humans to support life. It is also the densest planet in the solar system. Widely accepted scientific evidence indicates that the Earth was formed around 4.57 billion years ago[1] and its natural satellite, the Moon, was orbiting it shortly thereafter, around 4.53 billion years ago.

The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convecting yet solid mantle and an iron core that generates a magnetic field. Its atmospheric conditions have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered in salt-water oceans, and the remainder consists of continents and islands.

There is significant interaction between the Earth and its space environment. The relatively large moon provides ocean tides and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet is believed to have played a role in the formation of the oceans. Later, asteroid impacts are understood to have caused significant changes to the surface environment. Long term periodic changes in the orbit of the planet may also be responsible for the ice ages that have covered significant portions of the surface in glacial sheets.

Contents [hide]
1 Lexicography
2 History
3 Shape
4 Composition
5 Internal structure
6 Tectonic plates
7 Surface
7.1 Extremes
8 Hydrosphere
9 Atmosphere
10 Climate
11 Pedosphere
12 Biosphere
13 Natural resources
14 Land use
15 Natural and environmental hazards
16 Human geography
17 Solar system
18 Phases
18.1 Magnetic field
18.2 Moon
19 Descriptions
20 Future
21 See also
22 References
23 Notes
24 External links



Lexicography
In American English usage, the name can be capitalized or spelled in lowercase interchangeably, either when used absolutely or prefixed with "the" (i.e. Earth, the Earth, earth or the earth). Many deliberately spell the name of the planet with a capital, both as "Earth" or "the Earth". This is to distinguish it as a proper noun, distinct from the senses of the term as a count noun or verb (e.g. referring to soil, the ground, earthing in the electrical sense, etc.). Oxford Spelling recognizes the lowercase form as the most common, with the capitalized form as a variant of it. Another convention that is very common is to spell the name with a capital when occurring absolutely (e.g. Earth's atmosphere) and lowercase when preceded by "the" (e.g. the atmosphere of the earth). The term almost exclusively exists in lowercase when appearing in common phrases, even without "the" preceding it (e.g. it doesn't cost the earth; what on earth are you doing?).[2]

Terms that refer to the Earth can use the Latin root terr-, as in terraform and terrestrial. An alternative Latin root is tellur-, which is used in words such as tellurian and tellurium. Such terms derive from Latin terra and tellus, which refer variously to the world, the element earth, the earth goddess and so forth[3]. Scientific terms such as geography, geocentric and geothermal use the Greek prefix geo- (γαιο-, gaio-), from gē (again meaning "earth").[4] In many science fiction books and video games, Earth is referred to as Terra or Gaia[citation needed]. Astronauts refer to the Earth as "Terra Firma"[citation needed].

The English word "earth" has cognates in many modern and ancient languages. Examples in modern tongues include aarde in Afrikaans and Dutch, and Erde in German. The root has cognates in extinct languages such as ertha in Old Saxon and ert (meaning "ground") in Middle Irish, derived from the Old English eorðe. All of these words derive from the Proto-Indo-European base *er-.

Several Semitic languages have words for "earth" similar to those in Indo-European languages. Arabic has ard; Akkadian, irtsitu; Aramaic, araa; Phoenician, erets (which appears in the Mesha Stele); and Hebrew, ארץ (arets, or erets when not preceded by a definite article, or when followed by a noun modifier). The etymological connection between the words in Indo-European and Semitic languages are uncertain, though, and may simply be coincidence.

The standard name for people from Earth is Earthling, although Terran, Gaian, and Earther are alternate names that have been used in Science fiction.

Words for Earth in other languages include: Terre (French), पृथ्वी pr̥thvī (Sanskrit), Maa (Finnish and Estonian), Pamînt (Romanian), Föld (Hungarian), Ziemia (Polish), Zemlja (Russian and Serbian), Tierra (Spanish), Terra (Italian), Diqiu (Mandarin), Deiqao (Cantonese), Jigu (Korean), Bumi (Malay), Chikyuu (Japanese), Jorden (Danish, Norwegian, Swedish), Gi, Choma (Greek), Dunia (Swahili), Âlem, Dünya الْمَسْكُونَة (Arabic), Dinê (Kurdish), Ergir երկիր (Armenian), Jehun, Zamin (Persian), and Acun, Yeryüzü, Yerküre (Turkish).[5], כדור הארץ (Hebrew), Bhoomi (Telugu)


History
Main article: History of Earth
Based on the available evidence, current scientists have been able to reconstruct detailed information about the planet's past. Earth is believed to have formed around 4.57 billion years ago out of the solar nebula, along with the Sun and the other planets. Initially molten, the outer layer of the planet cooled when water began accumulating in the atmosphere when the planet was about half its current radius, resulting in the solid crust. The moon formed soon afterwards, possibly as the result of the impact with a Mars-sized object known as Theia. Outgassing and volcanic activity produced the primordial atmosphere; condensing water vapor, augmented by ice delivered by comets, produced the oceans.[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 lived.[7]

The development of photosynthesis allowed the sun's energy to be harvested directly; the resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[8] Cells within colonies became increasingly specialized, resulting in true multicellular organisms. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.

Over hundreds of millions of years, continents formed and broke up as the surface of Earth continually reshaped itself. The continents have migrated across the surface of the Earth, 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.[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 lifeforms began to proliferate.[10]

Since the Cambrian explosion, about 535 mya, there have been five mass extinctions.[11] The last 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, a small African ape gained the ability to stand upright. 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 timespan as no other life form had, affecting both the nature and quantity of other life forms, and the global climate.


Shape
The Earth's shape is very close to an oblate spheroid, although the precise shape (the geoid) varies from this by up to 100 meters (327 ft). The average diameter of the reference spheroid is approximately 12,742 km (more roughly, 40,000 km/π). The rotation of the Earth causes the equator to bulge out slightly so that the equatorial diameter is 43 km larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,850 m above local sea level) and the Mariana Trench (10,924 m 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%. For comparison, this is less than the 0.22% tolerance allowed in billiard balls. Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.


Composition
See also: Abundance of the chemical elements#Abundance of elements on Earth
The mass of the Earth is approximately 5980 yottagrams (5.98 ×1024 kg). It is composed mostly of iron (35.0%), oxygen (28.0%), silicon (17.0%), magnesium (15.7%), nickel (1.5%), calcium (1.4%) and aluminium (1.4%)[12].


Internal structure
Main article: Structure of the Earth

Earth cutaway from core to exosphere. Partially to scaleThe 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 geologic component layers of the Earth[13] are at the following depths below the surface:

Depth Layer
Kilometers Miles
0–60 0–37 Lithosphere (locally varies between 5 and 200 km)
0–35 0–22 ... Crust (locally varies between 5 and 70 km)
35–60 22–37 ... Uppermost part of mantle
35–2890 22–1790 Mantle
100–700 62–435 ... Asthenosphere
2890–5100 1790–3160 Outer core
5100–6378 3160–3954 Inner core


Tectonic plates
Main article: Plate tectonics

A map pointing out the Earth's major plates.According to plate tectonics theory currently accepted by the vast majority of 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 comprises the inner, viscous part of the mantle. The mantle behaves like a superheated and extremely viscous liquid.

The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates move in relation to one another at one of three types of plate boundaries: convergent, divergent, and transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries.

The main plates are

African Plate, covering Africa - Continental plate
Antarctic Plate, covering Antarctica - Continental plate
Australian Plate, covering Australia (fused with Indian Plate between 50 and 55 million years ago) - Continental plate
Eurasian Plate covering Asia and Europe - Continental plate
North American Plate covering North America and north-east Siberia - Continental plate
South American Plate covering South America - Continental plate
Pacific Plate, covering the Pacific Ocean - Oceanic plate
Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate and the Scotia Plate.


Surface
Main article: Landforms

Surface of the Earth, colors reflect changes in elevationThe Earth's terrain varies greatly from place to place. About 70% of the surface is covered by water, with much of the continental shelf below sea level. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2500 metres (approximately 8000 ft.). The remaining 30% not covered by water consists of mountains, deserts, plains, plateaus, etc.

Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.[14] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 × 109 acres of cropland and 8.4 × 109 acres of pastureland.[15]


Extremes
Main article: Extreme points of the world
Elevation extremes: (measured relative to sea level)

Lowest point on land: Dead Sea −417 m
Lowest point overall: Challenger Deep of the Mariana Trench in the Pacific Ocean −10,924 m [16]
Highest point: Mount Everest 8,844 m (2005 est.)

Hydrosphere
Main article: Hydrosphere
The abundance of water on Earth is a unique feature that distinguishes the "Blue Planet" from others in the solar system. Approximately 70.8 percent of the Earth is covered by water and only 29.2 percent is terra firma.

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. 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 × 10^18 tonnes, or about 1/4400 of the total mass of the Earth.


Atmosphere
Main article: Earth's atmosphere
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 of the planet's surface. This lowest layer is called the troposphere. Further up, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Beyond these, the exosphere thins out into the magnetosphere (where the Earth's magnetic fields interact with the solar wind). An important part of the atmosphere for life on Earth is the ozone layer.

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 6 km. It is 78% nitrogen and 21% oxygen, with trace amounts of other gaseous molecules such as water vapor. The atmosphere protects the Earth's life forms by absorbing ultraviolet solar radiation, moderating temperature, transporting water vapor, and providing useful gases. The atmosphere is one of the principal components in determining weather and climate.

Because hydrogen gas is light and based on Earth's mean temperature, achieves escape velocity, unfixed hydrogen leaves the Earth. For this reason, the Earth's environment is oxidizing, with consequences for the chemical nature of life which developed on the planet.


Climate
Main article: Climate

A part of the earth as it looks from high orbit.The most prominent features of the Earth's climate are its two large polar regions, two narrow temperate zones, and a wide equatorial tropical region. Precipitation patterns vary widely, ranging from several metres of water per year to less than a millimetre.

Ocean currents are important factors in determining climate, particularly the spectacular thermohaline circulation which distributes heat energy from the equatorial oceans to the polar regions.


Pedosphere
Main article: Pedosphere
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.


Biosphere
Main article: Biosphere
The planet's lifeforms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion (3.5×109) years ago. Earth is the only place in the universe officially recognized by the communities of Earth where life is absolutely known to exist, and some scientists believe that biospheres might be rare.

The biosphere is divided into a number of biomes, inhabited by broadly similar flora and fauna. 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 most of the more populous biomes lie near the Equator.


Natural resources
Main article: Natural resource
Earth's crust contains large deposits of fossil fuels: (coal, petroleum, natural gas, methane clathrate). These deposits are used by humans both for energy production and as feedstock for chemical production.
Mineral ore bodies have been formed in Earth's crust by the action of erosion and plate tectonics. These bodies form concentrated sources for many metals and other useful elements.
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.
Some of these resources, such as mineral fuels, are difficult to replenish on a short time scale, called non-renewable resources. The exploitation of non-renewable resources near the surface by human civilization has become a subject of significant controversy in modern environmentalism movements.


Land use
Humans use the Earth's land to support themselves through the production of food, energy, and building material. They also live on the land by building shelters. Human use of land is approximately:

Arable land: 13.13%[14]
Permanent crops: 4.71%[14]
Permanent pastures: 26%
Forests and woodland: 32%
Urban areas: 1.5%
Other: 30% (1993 est.)
Irrigated land: 2,481,250 km² (1993 est.)


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 localize 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.

Long-term climate alteration from enhancement of the greenhouse effect caused by the earth itself and human industrial carbon dioxide emissions is an increasing concern, the focus of intense study and debate.


Human geography
Main article: Human geography


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The Earth at night, a composite of satellite photographs showing human made illumination on the Earth's surface. Taken between October 1994 and March 1995.
Earth has approximately 6,500,000,000 human inhabitants (February 24, 2006 estimate).[17] Projections indicate that the world's human population will reach seven billion in 2013 and 9.1 billion in 2050 (2005 UN estimates). Most of the growth is expected to take place in developing nations. Human population density varies widely around the world.

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, high mountains or other unsuitable terrain.

The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada. The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole.

There are 267 administrative divisions, including nations, dependent areas, other, and miscellaneous entries. Earth does not have a sovereign government with planet-wide authority. Independent sovereign nations claim all of the land surface except for some segments of Antarctica. There is a worldwide general international organization, the United Nations. The United Nations is primarily an international discussion forum with only limited ability to pass and enforce laws.

In total, about 400 people have been outside the Earth's atmosphere as of 2004, and of these, twelve have walked on the Moon. Most of the time the only humans in space are those on the International Space Station, currently three people who are usually replaced every 6 months. See human spaceflight.



Solar system

An animation showing the rotation of the Earth.
Earth seen as a tiny dot by the Voyager 1 spacecraft, four billion miles from EarthIt 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. 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, i.e., an apparent Sun or Moon diameter every two minutes.

Earth orbits the Sun 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, i.e., a Sun or Moon diameter every 12 hours, eastward. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is enough to cover the planet's diameter (~12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.

The Moon revolves with the Earth around a common barycenter, from fixed star to fixed star, every 27.32 days. 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. The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (930,000 miles) in radius. Viewed from Earth's north pole, the motion of Earth, its moon and their axial rotations are all counterclockwise. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.5 degrees against the Earth–Sun plane (which causes the seasons); and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane (without a tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses).

In an inertial reference frame, the Earth's axis undergoes a slow precessional motion with a period of some 25,800 years, as well as a nutation with a main period of 18.6 years. These motions are caused by the differential attraction of Sun and Moon on the Earth's equatorial bulge because of its oblateness. In a reference frame attached to the solid body of the Earth, its rotation is also slightly irregular from polar motion. The polar motion is quasi-periodic, containing an annual component and a component with a 14-month period called the Chandler wobble. In addition, the rotational velocity varies, in a phenomenon known as length of day variation.

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4 (near the solstices, which are on about December 21 and June 21). For other eras, see precession and Milankovitch cycles.


Phases

Earth and Moon from Mars, imaged by Mars Global Surveyor.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. The phases of the Earth can be simulated by shining light on a globe of the Earth.

From orbit around the Earth, one can see all of the phases of the Earth in progression from New Earth to New Earth. The speed at which one sees these phases is related to the orbit of the observer and the speed of the observer around the Earth.

A Martian observer can see the Earth go through phases similar to those that an Earth-bound observer sees the phases of Venus (as discovered be Galileo), going for the Martian's perspective from New Earth to Fat Crescent to wane to New Earth. It is can be shown that an imaginary 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.


Magnetic field
Main article: Earth's 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. The field forms the magnetosphere, which deflects particles in the solar wind. The bow shock is located about at 13.5 RE. 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.



Moon
Main articles: Moon and Earth and Moon
Name Diameter (km) Mass (kg) Semi-major axis (km) Orbital period
Moon 3,474.8 7.349×1022 384,400 27 days, 7 hours, 43.7 minutes

Earthrise as seen from lunar orbit on Apollo 8, 24 December 1968.The Moon, sometimes called 'Luna', 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 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 a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 17 µs a year—add up to significant changes. During the Devonian period, there were 400 days in a year, with each day lasting 21.8 hours.

The Moon may dramatically affect the development of life by taming the weather. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[18] 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.[19] However, this is a controversial subject, and further studies of Mars—which shares Earth's rotation period and axial tilt, 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 angular size as the Sun (the Sun is 400 times larger, and the Moon is 400 times closer). This allows total eclipses and annular eclipses to occur on Earth.



The relative sizes of and distance between Earth and Moon, to scale
The most widely accepted theory of the Moon's origin, the giant impact theory, states that it was formed from the collision of a Mars-size protoplanet 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.


Descriptions

The first time an "Earth-rise" was seen from the moon.Earth has often been personified as a deity, in particular a goddess (see Gaia and Mother Earth). The Chinese Earth goddess Hou-Tu is similar to Gaia, the deification of the Earth. As the patroness of fertility, her element is 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 (goddess).

Although commonly thought to be a sphere, the Earth is actually an oblate spheroid. It bulges slightly at the equator and is slightly flattened at the poles. In the past there were varying levels of belief in a flat Earth, but ancient Greek philosophers and, in the Middle Ages, thinkers such as Thomas Aquinas believed that it was spherical. A 19th-century organization called the Flat Earth Society advocated the even-then discredited idea that the Earth was actually disc-shaped, with the North Pole at its center and a 150 foot (50 m) high wall of ice at the outer edge. It and similar organizations continued to promote this idea, based on religious beliefs and conspiracy theories, through the 1970s. Today, the subject is more frequently treated tongue-in-cheek or with mockery.

Prior to the introduction of space flight, these inaccurate beliefs were countered with deductions 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 an 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.[20] 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."[21] Earth has also been described as a massive spaceship, with a life support system that requires maintenance, or as having a biosphere that forms one large organism. See Spaceship Earth and Gaia theory.


Future

Artist's conception of the remains of artificial structures on the Earth after the Sun enters its red giant phase and swells to roughly 100 times its current size.
Comparison between the red supergiant Antares and the Sun. The black circle is the size of the orbit of Mars. Arcturus is also included in the picture for comparisonThe future of the planet is closely tied to that of the Sun. The luminosity of the Sun will continue to steadily increase, growing from the current luminosity by 10% in 1.1 billion years (1.1 Gyr) and up to 40% in 3.5 Gyr.[22] Climate models indicate that the increase in radiation reaching the Earth is likely to have dire consequences, including possible loss of the oceans.[23]

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.[22]

The increased heat will accelerate the inorganic CO2 cycle,reducing its concentration to the lethal dose for plants (10 ppm for C4 photosynthesis) in 900 million years. But even if the Sun were eternal and stable, the continued internal cooling of the Earth would have resulted in a loss of much of its atmosphere and oceans (due to lower volcanism).[24] More specifically, for Earth's oceans, the lower temperatures in the crust will permit their water to leak more deeply than today(at certain debt the water is evaporating) resulting in their total disappearance in 1 billion years.



The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris
Planets · Dwarf planets · Moons: Terran · Martian · Asteroidal · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
Small bodies: Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects and the solar system's list of objects, sorted by radius or m