In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location; the celestial sphere is a practical tool for spherical astronomy, allowing astronomers to specify the apparent positions of objects in the sky if their distances are unknown or irrelevant. In the equatorial coordinate system, the celestial equator divides the celestial sphere into two halves: the northern and southern celestial hemispheres; because astronomical objects are at such remote distances, casual observation of the sky offers no information on their actual distances. All celestial objects seem far away, as if fixed onto the inside of a sphere with a large but unknown radius, which appears to rotate westward overhead.
For purposes of spherical astronomy, concerned only with the directions to celestial objects, it makes no difference if this is the case or if it is Earth, rotating while the celestial sphere is stationary. The celestial sphere can be considered to be infinite in radius; this means any point within it, including that occupied by the observer, can be considered the center. It means that all parallel lines, be they millimetres apart or across the Solar System from each other, will seem to intersect the sphere at a single point, analogous to the vanishing point of graphical perspective. All parallel planes will seem to intersect the sphere in a coincident great circle. Conversely, observers looking toward the same point on an infinite-radius celestial sphere will be looking along parallel lines, observers looking toward the same great circle, along parallel planes. On an infinite-radius celestial sphere, all observers see the same things in the same direction. For some objects, this is over-simplified.
Objects which are near to the observer will seem to change position against the distant celestial sphere if the observer moves far enough, from one side of planet Earth to the other. This effect, known as parallax, can be represented as a small offset from a mean position; the celestial sphere can be considered to be centered at the Earth's center, the Sun's center, or any other convenient location, offsets from positions referred to these centers can be calculated. In this way, astronomers can predict geocentric or heliocentric positions of objects on the celestial sphere, without the need to calculate the individual geometry of any particular observer, the utility of the celestial sphere is maintained. Individual observers can work out their own small offsets from the mean positions. In many cases in astronomy, the offsets are insignificant; the celestial sphere can thus be thought of as a kind of astronomical shorthand, is applied frequently by astronomers. For instance, the Astronomical Almanac for 2010 lists the apparent geocentric position of the Moon on January 1 at 00:00:00.00 Terrestrial Time, in equatorial coordinates, as right ascension 6h 57m 48.86s, declination +23° 30' 05.5".
Implied in this position is. For many rough uses, this position, as seen from the Earth's center, is adequate. For applications requiring precision, the Almanac gives formulae and methods for calculating the topocentric coordinates, that is, as seen from a particular place on the Earth's surface, based on the geocentric position; this abbreviates the amount of detail necessary in such almanacs, as each observer can handle their own specific circumstances. These concepts are important for understanding celestial coordinate systems, frameworks for measuring the positions of objects in the sky. Certain reference lines and planes on Earth, when projected onto the celestial sphere, form the bases of the reference systems; these include the Earth's equator and orbit. At their intersections with the celestial sphere, these form the celestial equator, the north and south celestial poles, the ecliptic, respectively; as the celestial sphere is considered arbitrary or infinite in radius, all observers see the celestial equator, celestial poles, ecliptic at the same place against the background stars.
From these bases, directions toward objects in the sky can be quantified by constructing celestial coordinate systems. Similar to geographic longitude and latitude, the equatorial coordinate system specifies positions relative to the celestial equator and celestial poles, using right ascension and declination; the ecliptic coordinate system specifies positions relative to the ecliptic, using ecliptic longitude and latitude. Besides the equatorial and ecliptic systems, some other celestial coordinate systems, like the galactic coordinate system, are more appropriate for particular purposes; the ancients assumed the literal truth of stars attached to a celestial sphere, revolving about the Earth in one day, a fixed Earth. The Eudoxan planetary model, on which the Aristotelian and Ptolemaic models were based, was the first geometric explanation for the "wandering" of the classical planets; the outer most of these "crystal spheres" was thought to carry the fixed stars. Eudoxus used 27 concentric spherical solids to answer Plato's challenge: "By the assumption of what uniform and orderly motions can the appa
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P. Sloan Foundation. Data collection began in 2000; the main galaxy sample has a median redshift of z = 0.1. Data release 8, released in January 2011, includes all photometric observations taken with the SDSS imaging camera, covering 14,555 square degrees on the sky. Data release 9, released to the public on 31 July 2012, includes the first results from the Baryon Oscillation Spectroscopic Survey spectrograph, including over 800,000 new spectra. Over 500,000 of the new spectra are of objects in the Universe 7 billion years ago. Data release 10, released to the public on 31 July 2013, includes all data from previous releases, plus the first results from the APO Galactic Evolution Experiment spectrograph, including over 57,000 high-resolution infrared spectra of stars in the Milky Way.
DR10 includes over 670,000 new BOSS spectra of galaxies and quasars in the distant universe. The publicly available images from the survey were made between 1998 and 2009. SDSS uses a dedicated 2.5 m wide-angle optical telescope. The imaging camera was retired in late 2009, since the telescope has observed in spectroscopic mode. Images were taken using a photometric system of five filters; these images are processed to produce lists of objects observed and various parameters, such as whether they seem pointlike or extended and how the brightness on the CCDs relates to various kinds of astronomical magnitude. For imaging observations, the SDSS telescope used the drift scanning technique, which tracks the telescope along a great circle on the sky and continuously records small strips of the sky; the image of the stars in the focal plane drifts along the CCD chip, the charge is electronically shifted along the detectors at the same rate, instead of staying fixed as in tracked telescopes.. This method allows consistent astrometry over the widest possible field, minimises overheads from reading out the detectors.
The disadvantage is minor distortion effects. The telescope's imaging camera is made up of 30 CCD chips, each with a resolution of 2048×2048 pixels, totaling 120 megapixels; the chips are arranged in 5 rows of 6 chips. Each row has a different optical filter with average wavelengths of 355.1, 468.6, 616.5, 748.1 and 893.1 nm, with 95% completeness in typical seeing to magnitudes of 22.0, 22.2, 22.2, 21.3, 20.5, for u, g, r, i, z respectively. The filters are placed on the camera in the order r, i, u, z, g. To reduce noise, the camera is cooled to 190 kelvins by liquid nitrogen. Using these photometric data, stars and quasars are selected for spectroscopy; the spectrograph operates by feeding an individual optical fibre for each target through a hole drilled in an aluminum plate. Each hole is positioned for a selected target, so every field in which spectra are to be acquired requires a unique plate; the original spectrograph attached to the telescope was capable of recording 640 spectra while the updated spectrograph for SDSS III can record 1000 spectra at once.
Over the course of each night, between six and nine plates are used for recording spectra. In spectroscopic mode, the telescope tracks the sky in the standard way, keeping the objects focused on their corresponding fibre tips; every night the telescope produces about 200 GB of data. During its first phase of operations, 2000–2005, the SDSS imaged more than 8,000 square degrees of the sky in five optical bandpasses, it obtained spectra of galaxies and quasars selected from 5,700 square degrees of that imaging, it obtained repeated imaging of a 300 square degree stripe in the southern Galactic cap. In 2005 the survey entered a new phase, the SDSS-II, by extending the observations to explore the structure and stellar makeup of the Milky Way, the SEGUE and the Sloan Supernova Survey, which watches after supernova Ia events to measure the distances to far objects; the survey covers over 7,500 square degrees of the Northern Galactic Cap with data from nearly 2 million objects and spectra from over 800,000 galaxies and 100,000 quasars.
The information on the position and distance of the objects has allowed the large-scale structure of the Universe, with its voids and filaments, to be investigated for the first time. All of these data were obtained in SDSS-I, but a small part of the footprint was finished in SDSS-II; the Sloan Extension for Galactic Understanding and Exploration obtained spectra of 240,000 stars in order to create a detailed three-dimensional map of the Milky Way. SEGUE data provide evidence for the age and phase space distribution of stars within the various Galactic components, providing crucial clues for understanding the structure, formation a
An altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two perpendicular axes – one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth of the pointing direction of the instrument. Rotation about the horizontal axis varies the altitude of the pointing direction; these mounts are used, for example, with telescopes, radio antennas, heliostat mirrors, solar panels, guns and similar weapons. Several names are given to this kind of mount, including altitude-azimuth, azimuth-elevation and various abbreviations thereof. A gun turret is an alt-azimuth mount for a gun, a standard camera tripod is an alt-azimuth mount as well; when used as an astronomical telescope mount, the biggest advantage of an alt-azimuth mount is the simplicity of its mechanical design. The primary disadvantage is its inability to follow astronomical objects in the night sky as the Earth spins on its axis. On the other hand, an equatorial mount only needs to be rotated about a single axis, at a constant rate, to follow the rotation of the night sky.
Altazimuth mounts need to be rotated about both axes at variable rates, achieved via microprocessor based two-axis drive systems, to track equatorial motion. This imparts an uneven rotation to the field of view that has to be corrected via a microprocessor based counter rotation system. On smaller telescopes an equatorial platform is sometimes used to add a third "polar axis" to overcome these problems, providing an hour or more of motion in the direction of right ascension to allow for astronomical tracking; the design does not allow for the use of mechanical setting circles to locate astronomical objects although modern digital setting circles have removed this shortcoming. Another limitation is the problem of gimbal lock at zenith pointing; when tracking at elevations close to 90°, the azimuth axis must rotate quickly. Thus, altazimuth telescopes, although they can point in any direction, cannot track smoothly within a "zenith blind spot" 0.5 or 0.75 degrees from the zenith. Typical current applications of altazimuth mounts include the following.
Research telescopesIn the largest telescopes, the mass and cost of an equatorial mount is prohibitive and they have been superseded by computer-controlled altazimuth mounts. The simple structure of an altazimuth mount allows significant cost reductions, in spite of the additional cost associated with the more complex tracking and image-orienting mechanisms. An altazimuth mount reduces the cost in the dome structure covering the telescope since the simplified motion of the telescope means the structure can be more compact. Amateur telescopesBeginner telescopes: Altazimuth mounts are cheap and simple to use. Dobsonian telescopes: John Dobson popularized a simplified altazimuth mount design for Newtonian reflectors because of its ease of construction. "GoTo" telescopes: It has proved more convenient to build a mechanically simpler altazimuth mount and use a motion controller to manipulate both axes to track an object, when compared with a more mechanically complex equatorial mount that requires minimally complex control of a single motor.
Dobsonian mount Equatorial mount Heliostat Horizontal coordinate system - a system to locate objects on the celestial sphere via Alt-azimuth coordinates Parallactic angle Solar tracker Tripod Images of the Unitron altazimuth mount
Earth's rotation is the rotation of Planet Earth around its own axis. Earth rotates eastward, in prograde motion; as viewed from the north pole star Polaris, Earth turns counter clockwise. The North Pole known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where Earth's axis of rotation meets its surface; this point is distinct from Earth's North Magnetic Pole. The South Pole is the other point where Earth's axis of rotation intersects its surface, in Antarctica. Earth rotates once in about 24 hours with respect to the Sun, but once every 23 hours, 56 minutes, 4 seconds with respect to other, stars. Earth's rotation is slowing with time; this is due to the tidal effects. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago increasing the rate at which UTC is adjusted by leap seconds. Analysis of historical astronomical records shows a slowing trend of about 2.3 milliseconds per century since the 8th century BCE.
Among the ancient Greeks, several of the Pythagorean school believed in the rotation of Earth rather than the apparent diurnal rotation of the heavens. The first was Philolaus, though his system was complicated, including a counter-earth rotating daily about a central fire. A more conventional picture was that supported by Hicetas and Ecphantus in the fourth century BCE who assumed that Earth rotated but did not suggest that Earth revolved about the Sun. In the third century BCE, Aristarchus of Samos suggested the Sun's central place. However, Aristotle in the fourth century BCE criticized the ideas of Philolaus as being based on theory rather than observation, he established the idea of a sphere of fixed stars. This was accepted by most of those who came after, in particular Claudius Ptolemy, who thought Earth would be devastated by gales if it rotated. In 499 CE, the Indian astronomer Aryabhata wrote that the spherical Earth rotates about its axis daily, that the apparent movement of the stars is a relative motion caused by the rotation of Earth.
He provided the following analogy: "Just as a man in a boat going in one direction sees the stationary things on the bank as moving in the opposite direction, in the same way to a man at Lanka the fixed stars appear to be going westward."In the 10th century, some Muslim astronomers accepted that Earth rotates around its axis. According to al-Biruni, Abu Sa'id al-Sijzi invented an astrolabe called al-zūraqī based on the idea believed by some of his contemporaries "that the motion we see is due to the Earth's movement and not to that of the sky." The prevalence of this view is further confirmed by a reference from the 13th century which states: "According to the geometers, the Earth is in constant circular motion, what appears to be the motion of the heavens is due to the motion of the Earth and not the stars." Treatises were written to discuss its possibility, either as refutations or expressing doubts about Ptolemy's arguments against it. At the Maragha and Samarkand observatories, Earth's rotation was discussed by Qushji.
In medieval Europe, Thomas Aquinas accepted Aristotle's view and so, did John Buridan and Nicole Oresme in the fourteenth century. Not until Nicolaus Copernicus in 1543 adopted a heliocentric world system did the contemporary understanding of Earth's rotation begin to be established. Copernicus pointed out that if the movement of Earth is violent the movement of the stars must be much more so, he pointed to examples of relative motion. For Copernicus this was the first step in establishing the simpler pattern of planets circling a central Sun. Tycho Brahe, who produced accurate observations on which Kepler based his laws, used Copernicus's work as the basis of a system assuming a stationary Earth. In 1600, William Gilbert supported Earth's rotation in his treatise on Earth's magnetism and thereby influenced many of his contemporaries; those like Gilbert who did not support or reject the motion of Earth about the Sun are called "semi-Copernicans". A century after Copernicus, Riccioli disputed the model of a rotating Earth due to the lack of then-observable eastward deflections in falling bodies.
However, the contributions of Kepler and Newton gathered support for the theory of the rotation of Earth. Earth's rotation implies that the geographical poles are flattened. In his Principia, Newton predicted this flattening would occur in the ratio of 1:230, pointed to the pendulum measurements taken by Richer in 1673 as corroboration of the change in gravity, but initial measurements of meridian lengths by Picard and Cassini at the end of the 17th century suggested the opposite. However, measurements by Maupertuis and the French Geodesic Mission in the 1730s established the oblateness of Earth, thus confirming the positions of both Newton and Copernicus. In Earth's rotating frame of reference, a moving body follows an apparent path that deviates from the one it would follow in a fixed frame of reference; because of the Coriolis effect, falling bodies veer eastward from the vertical plumb line below their point of release, projectiles veer right in the Northern Hemisphere from the direction in which they are shot.
The Coriolis effect is observable at a meteorological scale, where it is responsible for the opposite directions of cyclone rotation in th
European Southern Observatory
The European Southern Observatory, formally the European Organisation for Astronomical Research in the Southern Hemisphere, is a 16-nation intergovernmental research organization for ground-based astronomy. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities and access to the southern sky; the organisation employs about 730 staff members and receives annual member state contributions of €162 million. Its observatories are located in northern Chile. ESO has operated some of the largest and most technologically advanced telescopes; these include the 3.6 m New Technology Telescope, an early pioneer in the use of active optics, the Very Large Telescope, which consists of four individual 8.2 m telescopes and four smaller auxiliary telescopes which can all work together or separately. The Atacama Large Millimeter Array observes the universe in the millimetre and submillimetre wavelength ranges, is the world's largest ground-based astronomy project to date, it was completed in March 2013 in an international collaboration by Europe, North America, East Asia and Chile.
Under construction is the Extremely Large Telescope. It will use a 39.3-metre-diameter segmented mirror, become the world's largest optical reflecting telescope when operational in 2024. Its light-gathering power will allow detailed studies of planets around other stars, the first objects in the universe, supermassive black holes, the nature and distribution of the dark matter and dark energy which dominate the universe. ESO's observing facilities have made astronomical discoveries and produced several astronomical catalogues, its findings include the discovery of the most distant gamma-ray burst and evidence for a black hole at the centre of the Milky Way. In 2004, the VLT allowed astronomers to obtain the first picture of an extrasolar planet orbiting a brown dwarf 173 light-years away; the High Accuracy Radial Velocity Planet Searcher instrument installed on the older ESO 3.6 m telescope led to the discovery of extrasolar planets, including Gliese 581c—one of the smallest planets seen outside the solar system.
The idea that European astronomers should establish a common large observatory was broached by Walter Baade and Jan Oort at the Leiden Observatory in the Netherlands in spring 1953. It was pursued by Oort, who gathered a group of astronomers in Leiden to consider it on June 21 that year. Thereafter, the subject was further discussed at the Groningen conference in the Netherlands. On January 26, 1954, an ESO declaration was signed by astronomers from six European countries expressing the wish that a joint European observatory be established in the southern hemisphere. At the time, all reflector telescopes with an aperture of 2 metres or more were located in the northern hemisphere; the decision to build the observatory in the southern hemisphere resulted from the necessity of observing the southern sky. Although it was planned to set up telescopes in South Africa, tests from 1955 to 1963 demonstrated that a site in the Andes was preferable. On November 15, 1963 Chile was chosen as the site for ESO's observatory.
The decision was preceded by the ESO Convention, signed 5 October 1962 by Belgium, France, the Netherlands and Sweden. Otto Heckmann was nominated as the organisation's first director general on 1 November 1962. A preliminary proposal for a convention of astronomy organisations in these five countries was drafted in 1954. Although some amendments were made in the initial document, the convention proceeded until 1960 when it was discussed during that year's committee meeting; the new draft was examined in detail, a council member of CERN highlighted the need for a convention between governments. The convention and government involvement became pressing due to rising costs of site-testing expeditions; the final 1962 version was adopted from the CERN convention, due to similarities between the organisations and the dual membership of some members. In 1966, the first ESO telescope at the La Silla site in Chile began operating; because CERN had sophisticated instrumentation, the astronomy organisation turned to the nuclear-research body for advice and a collaborative agreement between ESO and CERN was signed in 1970.
Several months ESO's telescope division moved into a CERN building in Geneva and ESO's Sky Atlas Laboratory was established on CERN property. ESO's European departments moved into the new ESO headquarters in Garching, Germany in 1980. Although ESO is headquartered in Germany, its telescopes and observatories are in northern Chile, where the organisation operates advanced ground-based astronomical facilities: La Silla, which hosts the New Technology Telescope Paranal, where the Very Large Telescope is located Llano de Chajnantor, which hosts the APEX submillimetre telescope and where ALMA, the Atacama Large Millimeter/submillimeter Array, is locatedThese are among the best locations for astronomical observations in the southern hemisphere. An ESO project is the Extremely Large Telescope, a 40-metre-class telescope based on a five-mirror design and the planned Overwhelmingly Large Telescope; the ELT will be the near-infrared telescope in the world. ESO began its design in early 2006, aimed to begin construction in 2012.
Construction work at the ELT site started in June 2014. As decided by the ESO council on 26 April 2010, a fou
A planet is an astronomical body orbiting a star or stellar remnant, massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, has cleared its neighbouring region of planetesimals. The term planet is ancient, with ties to history, science and religion. Five planets in the Solar System are visible to the naked eye; these were regarded by many early cultures as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union adopted a resolution defining planets within the Solar System; this definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas and Vesta, Pluto, that were once considered planets by the scientific community, are no longer viewed as such.
The planets were thought by Ptolemy to orbit Earth in epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular; as observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes and hydrology. Planets are divided into two main types: large low-density giant planets, smaller rocky terrestrials. There are eight planets in the Solar System.
In order of increasing distance from the Sun, they are the four terrestrials, Venus and Mars the four giant planets, Saturn and Neptune. Six of the planets are orbited by one or more natural satellites. Several thousands of planets around other stars have been discovered in the Milky Way; as of 1 April 2019, 4,023 known extrasolar planets in 3,005 planetary systems, ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar habitable zone. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age; the concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy; the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες or πλανῆται, from which today's word "planet" was derived. In ancient Greece, China and indeed all pre-modern civilizations, it was universally believed that Earth was the center of the Universe and that all the "planets" circled Earth.
The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC; the oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that dates as early as the second millennium BC. The MUL. APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun and planets over the course of the year; the Babylonian astrologers laid the foundations of what would become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.
Venus and the outer planets Mars and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times; the ancient Greeks did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5t
The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process, it is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Three quarters of the Sun's mass consists of hydrogen; the Sun is a G-type main-sequence star based on its spectral class. As such, it is informally and not accurately referred to as a yellow dwarf, it formed 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System; the central mass became so hot and dense that it initiated nuclear fusion in its core. It is thought that all stars form by this process.
The Sun is middle-aged. It fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result; this energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand to become a red giant, it is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, render Earth uninhabitable. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, no longer produce energy by fusion, but still glow and give off heat from its previous fusion; the enormous effect of the Sun on Earth has been recognized since prehistoric times, the Sun has been regarded by some cultures as a deity.
The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of, the predominant calendar in use today. The English proper name Sun may be related to south. Cognates to English sun appear in other Germanic languages, including Old Frisian sunne, Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, Gothic sunnō. All Germanic terms for the Sun stem from Proto-Germanic *sunnōn; the Latin name for the Sun, Sol, is not used in everyday English. Sol is used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars; the related word solar is the usual adjectival term used for the Sun, in terms such as solar day, solar eclipse, Solar System. A mean Earth solar day is 24 hours, whereas a mean Martian'sol' is 24 hours, 39 minutes, 35.244 seconds. The English weekday name Sunday stems from Old English and is a result of a Germanic interpretation of Latin dies solis, itself a translation of the Greek ἡμέρα ἡλίου.
The Sun is a G-type main-sequence star. The Sun has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. The Sun is heavy-element-rich, star; the formation of the Sun may have been triggered by shockwaves from more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars; the heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star. The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is 1 astronomical unit, though the distance varies as Earth moves from perihelion in January to aphelion in July.
At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports all life on Earth by photosynthesis, drives Earth's climate and weather; the Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere. For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometres; the tidal effect of the planets is weak and does not affect the shape of the Sun. The Sun rotates faster at its equator than at its poles; this differential rotation is caused by convective motion