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
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The term is used in astronomy, but can refer to any situation in which an object in the foreground blocks from view an object in the background. In this general sense, occultation applies to the visual scene observed from low-flying aircraft when foreground objects obscure distant objects dynamically, as the scene changes over time; the term occultation is most used to describe those frequent occasions when the Moon passes in front of a star during the course of its orbital motion around the Earth. Since the Moon, with an angular speed with respect to the stars of 0.55 arcsec/s or 2.7 µrad/s, has a thin atmosphere and stars have an angular diameter of at most 0.057 arcseconds or 0.28 µrad, a star, occulted by the Moon will disappear or reappear in 0.1 seconds or less on the Moon's edge, or limb. Events that take place on the Moon's dark limb are of particular interest to observers, because the lack of glare allows these occultations to more be observed and timed.
The Moon's orbit is inclined to the ecliptic, any stars with an ecliptic latitude of less than about 6.5 degrees may be occulted by it. There are three first magnitude stars that are sufficiently close to the ecliptic that they may be occulted by the Moon and by planets – Regulus and Antares. Occultations of Aldebaran are presently only possible by the Moon, because the planets pass Aldebaran to the north. Neither planetary nor lunar occultations of Pollux are possible. However, in the far future, occultations of Pollux will be possible; some deep-sky objects, such as the Pleiades, can be occulted by the Moon. Within a few kilometres of the edge of an occultation's predicted path, referred to as its northern or southern limit, an observer may see the star intermittently disappearing and reappearing as the irregular limb of the Moon moves past the star, creating what is known as a grazing lunar occultation. From an observational and scientific standpoint, these "grazes" are the most dynamic and interesting of lunar occultations.
The accurate timing of lunar occultations is performed by astronomers. Lunar occultations timed to an accuracy of a few tenths of a second have various scientific uses in refining our knowledge of lunar topography. Photoelectric analysis of lunar occultations have discovered some stars to be close visual or spectroscopic binaries; some angular diameters of stars have been measured by timing of lunar occultations, useful for determining effective temperatures of those stars. Early radio astronomers found occultations of radio sources by the Moon valuable for determining their exact positions, because the long wavelength of radio waves limited the resolution available through direct observation; this was crucial for the unambiguous identification of the radio source 3C 273 with the optical quasar and its jet, a fundamental prerequisite for Maarten Schmidt's discovery of the cosmological nature of quasars. Several times during the year, someone on Earth can observe the Moon occulting a planet. Since planets, unlike stars, have significant angular sizes, lunar occultations of planets will create a narrow zone on Earth from which a partial occultation of the planet will occur.
An observer located within that narrow zone could observe the planet's disk blocked by the moving moon. The same mechanic can be seen with the Sun, where observers on Earth will view it as a solar eclipse. Therefore, a total solar eclipse is the same event as the Moon occulting the Sun. Stars may be occulted by planets. Occultations of bright stars are rare. In 1959, Venus occulted Regulus, the next occultation of a bright star will be in 2044. Uranus's rings were first discovered when that planet occulted a star in 1977. On 3 July 1989, Saturn passed in front of the 5th magnitude star 28 Sagittarii. Pluto occulted stars in 1988, 2002, 2006, allowing its tenuous atmosphere to be studied via atmospheric limb sounding. In rare cases, one planet can pass in front of another. If the nearer planet appears larger than the more distant one, the event is called a mutual planetary occultation. An occultation occurs; these occultations are useful for measuring the size and position of minor planets much more than can be done by any other means.
A cross-sectional profile of the shape of an asteroid can be determined if a number of observers at different, locations observe the occultation. Occultations have been used to estimate the diameter of trans-Neptunian objects such as 2002 TX300, Varuna. In addition, mutual occultation and eclipsing events can occur between a minor planet and its satellite. A large number of these minor-planet moons have been discovered analyzing the photometric light curves of rotating minor planets and detecting a second, superimposed brightness variation, from which an orbital period for the satellite, a secondary-to-primary diameter-ratio can be derived. On 29 May 1983, 2 Pallas occulted the naked-eye bright spectroscopic binary star 1 Vulpeculae along a track across the southern United States, northern Mexico, north parts of the Caribbean. Observations from 130 different locations defined the shape of about two-thirds of the asteroid, detected the secondary companion of the bright binary star.
In the context of spaceflight, a satellite is an artificial object, intentionally placed into orbit. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as Earth's Moon. On 4 October 1957 the Soviet Union launched the world's first artificial satellite, Sputnik 1. Since about 8,100 satellites from more than 40 countries have been launched. According to a 2018 estimate, some 4,900 remain in orbit, of those about 1,900. 500 operational satellites are in low-Earth orbit, 50 are in medium-Earth orbit, the rest are in geostationary orbit. A few large satellites have been assembled in orbit. Over a dozen space probes have been placed into orbit around other bodies and become artificial satellites to the Moon, Venus, Jupiter, Saturn, a few asteroids, a comet and the Sun. Satellites are used for many purposes. Among several other applications, they can be used to make star maps and maps of planetary surfaces, take pictures of planets they are launched into.
Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, space telescopes. Space stations and human spacecraft in orbit are satellites. Satellite orbits vary depending on the purpose of the satellite, are classified in a number of ways. Well-known classes include low Earth orbit, polar orbit, geostationary orbit. A launch vehicle is a rocket, it lifts off from a launch pad on land. Some are launched at sea aboard a plane. Satellites are semi-independent computer-controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, attitude control and orbit control. "Newton's cannonball", presented as a "thought experiment" in A Treatise of the System of the World, by Isaac Newton was the first published mathematical study of the possibility of an artificial satellite. The first fictional depiction of a satellite being launched into orbit was a short story by Edward Everett Hale, The Brick Moon.
The idea surfaced again in Jules Verne's The Begum's Fortune. In 1903, Konstantin Tsiolkovsky published Exploring Space Using Jet Propulsion Devices, the first academic treatise on the use of rocketry to launch spacecraft, he calculated the orbital speed required for a minimal orbit, that a multi-stage rocket fuelled by liquid propellants could achieve this. In 1928, Herman Potočnik published The Problem of Space Travel -- The Rocket Motor, he described the use of orbiting spacecraft for observation of the ground and described how the special conditions of space could be useful for scientific experiments. In a 1945 Wireless World article, the English science fiction writer Arthur C. Clarke described in detail the possible use of communications satellites for mass communications, he suggested. The US military studied the idea of what was referred to as the "earth satellite vehicle" when Secretary of Defense James Forrestal made a public announcement on 29 December 1948, that his office was coordinating that project between the various services.
The first artificial satellite was Sputnik 1, launched by the Soviet Union on 4 October 1957, initiating the Soviet Sputnik program, with Sergei Korolev as chief designer. This in turn triggered the Space Race between the United States. Sputnik 1 helped to identify the density of high atmospheric layers through measurement of its orbital change and provided data on radio-signal distribution in the ionosphere; the unanticipated announcement of Sputnik 1's success precipitated the Sputnik crisis in the United States and ignited the so-called Space Race within the Cold War. Sputnik 2 was launched on 3 November 1957 and carried the first living passenger into orbit, a dog named Laika. In May, 1946, Project RAND had released the Preliminary Design of an Experimental World-Circling Spaceship, which stated, "A satellite vehicle with appropriate instrumentation can be expected to be one of the most potent scientific tools of the Twentieth Century." The United States had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics of the United States Navy.
The United States Air Force's Project RAND released the report, but considered the satellite to be a tool for science and propaganda, rather than a potential military weapon. In 1954, the Secretary of Defense stated, "I know of no American satellite program." In February 1954 Project RAND released "Scientific Uses for a Satellite Vehicle," written by R. R. Carhart; this expanded on potential scientific uses for satellite vehicles and was followed in June 1955 with "The Scientific Use of an Artificial Satellite," by H. K. Kallmann and W. W. Kellogg. In the context of activities planned for the International Geophysical Year, the White House announced on 29 July 1955 that the U. S. intended to launch satellites by the spring of 1958. This became known as Project Vanguard. On 31 July, the Soviets announced that they intended to launch a satellite by the fall of 1957. Following pressure by the American Rocket Society, the National Science Foundation, the International Geophysical Year, military interest picked up and in early 1955 the Army and Navy were worki
La Silla Observatory
La Silla Observatory is an astronomical observatory in Chile with three telescopes built and operated by the European Southern Observatory. Several other telescopes are located at the site and are maintained by ESO; the observatory is one of the largest in the Southern Hemisphere and was the first in Chile to be used by ESO. The La Silla telescopes and instruments are located 150 km northeast of La Serena at the outskirts of the Chilean Atacama Desert, one of the driest and most remote areas of the world. Like other observatories in this geographical area, La Silla is located far from sources of light pollution and, like the Paranal Observatory, home to the Very Large Telescope, it has one of the darkest night skies on the Earth. Following the decision in 1963 to approve Chile as the site for the ESO observatory, scouting parties were sent to various locations to assess their suitability; the site, decided upon was La Silla in the southern part of the Atacama desert, 600 km north of Santiago de Chile and at an altitude of 2400 metres.
Besides being government property, it had the added benefits of being in a dry and accessible area, yet isolated and remote from any artificial light and dust sources. Named the Cinchado, it was renamed La Silla after its saddle-like shape. On October 30, 1964, the contracts were signed and an area of 245 square miles was purchased the following year. During 1965, temporary facilities were erected with a workshop and storage area; the dedication ceremony of the road to the summit took place in March 1966, two months after completion of the road. On 25 March 1969, the ESO site at La Silla was formally inaugurated by President Eduardo Frei Montalva. With a permanent base of dormitories, workshops and several functioning telescopes, the observatory was operational; the ESO 1.5-metre and ESO 1-metre telescopes had been erected in the late 1960s, were joined in 1968 by the Gran Prismo Objectif telescope, been used in South Africa. These three telescopes can be seen in this order from right to left in the background of the image on the left from June 1968.
By 1976, the largest telescope planned, the § ESO 3.6 m Telescope, started operations. It was subsequently to have a 1.4m CAT attached. In 1984, the 2.2m telescope began operations, while in March 1989, the 3.5 m New Technology Telescope saw first light. The program reached its apex with the installation of the SEST in 1987, the only large submillimetre telescope in the southern hemisphere, a combined project between ESO and the Swedish Natural Science Research Council. Around the end of the century some of the original telescopes were closed: the 1m Schmidt closed in 1998 and the 1.5m in 2002, while new equipment owned by various foreign observatories was introduced. A 1-metre telescope owned by Marseille Observatory opened in 1998, followed by a 1.2-metre telescope from Geneva Observatory in 2000. ESO operates three major optical and near infrared telescopes at the La Silla site: the New Technology Telescope, the 3.6-m ESO Telescope, the 2.2-m Max-Planck-ESO Telescope. In addition La Silla hosts several other national and project telescopes such as the ESO 1-metre Schmidt Telescope, the 1.54-m Danish Telescope, the 1.2-m Leonhard Euler Telescope, the Rapid Eye Mount telescope, TRAPPIST and TAROT.
These telescopes are not operated by ESO and hence do not fall under the responsibility of La Silla Science Operations. This 3.6 m Cassegrain telescope started operations in 1976 and has been upgraded since, including the installation of a new secondary mirror that has kept the telescope in its place as one of the most efficient and productive engines of astronomical research. The telescope hosts HARPS, the High Accuracy Radial velocity Planet Searcher, the world's foremost exoplanet hunter. HARPS is a spectrograph with unrivalled precision and is the most successful finder of low-mass exoplanets to date. Since April 2008, HARPS has been the only instrument available at the 3.6 m telescope. The ESO New Technology Telescope is an Alt-Az, 3.58-metre Richey-Chretien telescope which pioneered the use of active optics. The telescope and its enclosure had a revolutionary design for optimal image quality. NTT saw first light in March 1989; the telescope chamber is ventilated by a system of flaps which optimize the air flow across the NTT optimizing the dome and mirror seeing.
To prevent heat input to the building, all motors in the telescope are water cooled and all the electronics boxes are insulated and cooled. The primary mirror of the NTT is controlled to preserve its figure at all telescope positions; the secondary mirror position is actively controlled in three directions. The optimized airflow, the thermal controls, the active optics give the excellent image quality of the NTT. Note that the NTT has active instead of adaptive optics: it corrects the defects and deformation of the telescope and mirror, but does not correct the turbulence. Together with the thermal control, it allows the NTT to reach the ambient seeing, but it does not improve it; the 2.2-metre telescope has been in operation at La Silla since early 1984, is on indefinite loan to ESO from the Max Planck Society. Telescope time is shared between MPG and ESO observing programmes, while the operation and maintenance of the telescope are ESO's responsibility. However, due to a new agreement between the Max Planck Institute for Astronomy and ESO, the instrument is operated by MPG until the end of September 2016.
The telescope hosts three instruments: the 67-million pixel
The full moon is the lunar phase when the Moon appears illuminated from Earth's perspective. This occurs when Earth is located between the Moon; this means that the lunar hemisphere facing Earth – the near side – is sunlit and appears as a circular disk, while the far side is dark. The full moon occurs once every month; when the Moon moves into Earth's shadow, a lunar eclipse occurs, during which all or part of the Moon's face may appear reddish due to the Rayleigh scattering of blue wavelengths and the refraction of sunlight through Earth's atmosphere. Lunar eclipses happen only during full moon and around points on its orbit where the satellite may pass through the planet's shadow. A lunar eclipse does not occur every month because the Moon's orbit is inclined 5.14° with respect to the ecliptic plane of Earth. Lunar eclipses happen only. Therefore, a lunar eclipse occurs every 6 months and 2 weeks before or after a solar eclipse, which occurs during new moon around the opposite node; the interval period between a new or full moon and the next same phase, a synodic month, averages about 29.53 days.
Therefore, in those lunar calendars in which each month begins on the day of the new moon, the full moon falls on either the 14th or 15th day of the lunar month. Because a calendar month consists of a whole number of days, a lunar month may be either 29 or 30 days long. A full moon is thought of as an event of a full night's duration; this is somewhat misleading because its phase seen from Earth continuously wanes. Its maximum illumination occurs at the moment waxing. For any given location, about half of these maximum full moons may be visible, while the other half occurs during the day, when the full moon is below the horizon. Many almanacs list full moons not only by date, but by their exact time in Coordinated Universal Time. Typical monthly calendars that include lunar phases may be offset by one day when used in a different time zone. Full moon is a suboptimal time for astronomical observation of the Moon because shadows vanish, it is a poor time for other observations because the bright sunlight reflected by the Moon, amplified by the opposition surge outshines many stars.
On 12 December 2008, the full moon occurred closer to the Earth than it had been at any time for the previous 15 years, called a supermoon. On 19 March 2011, another full supermoon occurred, closer to the Earth than at any time for the previous 18 years. On 14 November 2016, a full supermoon occurred closer to the Earth than at any time for the previous 68 years; the date and approximate time of a specific full moon can be calculated from the following equation: d = 20.362000 + 29.530588861 × N + 102.026 × 10 − 12 × N 2 where d is the number of days since 1 January 2000 00:00:00 in the Terrestrial Time scale used in astronomical ephemerides. The true time of a full moon may differ from this approximation by up to about 14.5 hours as a result of the non-circularity of the moon's orbit. See New moon for an explanation of the formula and its parameters; the age and apparent size of the full moon vary in a cycle of just under 14 synodic months, referred to as a full moon cycle. Full moons are traditionally associated with temporal insomnia and various "magical phenomena" such as lycanthropy.
Psychologists, have found that there is no strong evidence for effects on human behavior around the time of a full moon. They find that studies are not consistent, with some showing a positive effect and others showing a negative effect. In one instance, the 23 December 2000 issue of the British Medical Journal published two studies on dog bite admission to hospitals in England and Australia; the study of the Bradford Royal Infirmary found that dog bites were twice as common during a full moon, whereas the study conducted by the public hospitals in Australia found that they were less likely. Month names are names of moons in lunisolar calendars. Since the introduction of the solar Julian calendar in the Roman Empire, the Gregorian calendar worldwide, people no longer perceive month names as "moon" names; the traditional Old English month names were equated with the names of the Julian calendar from an early time. Some full moons have developed new names in modern times, e.g. the blue moon, the names "harvest moon" and "hunter's moon" for the full moons of autumn.
Lunar eclipses only happen during a full moon and cast a reddish tint over the face of the moon. This has been called a blood moon in popular culture; the "harvest moon" and "hunter's moon" are traditional terms for the full moons occurri
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three celestial objects is known as a syzygy. Apart from syzygy, the term eclipse is used when a spacecraft reaches a position where it can observe two celestial bodies so aligned. An eclipse is the result of either a transit; the term eclipse is most used to describe either a solar eclipse, when the Moon's shadow crosses the Earth's surface, or a lunar eclipse, when the Moon moves into the Earth's shadow. However, it can refer to such events beyond the Earth–Moon system: for example, a planet moving into the shadow cast by one of its moons, a moon passing into the shadow cast by its host planet, or a moon passing into the shadow of another moon. A binary star system can produce eclipses if the plane of the orbit of its constituent stars intersects the observer's position.
For the special cases of solar and lunar eclipses, these only happen during an "eclipse season", the two times of each year when the plane of the Earth's orbit around the Sun crosses with the plane of the Moon's orbit around the Earth. The type of solar eclipse that happens during each season depends on apparent sizes of the Sun and Moon. If the orbit of the Earth around the Sun, the Moon's orbit around the Earth were both in the same plane with each other eclipses would happen each and every month. There would be a lunar eclipse at every full moon, a solar eclipse at every new moon, and if both orbits were circular each solar eclipse would be the same type every month. It is because of non-circular differences that eclipses are not a common event. Lunar eclipses can be viewed from the entire nightside half of the Earth, but solar eclipses total eclipses occurring at any one particular point on the Earth's surface, are rare events that can be many decades apart. The term is derived from the ancient Greek noun ἔκλειψις, which means "the abandonment", "the downfall", or "the darkening of a heavenly body", derived from the verb ἐκλείπω which means "to abandon", "to darken", or "to cease to exist," a combination of prefix ἐκ-, from preposition ἐκ, "out," and of verb λείπω, "to be absent".
For any two objects in space, a line can be extended from the first through the second. The latter object will block some amount of light being emitted by the former, creating a region of shadow around the axis of the line; these objects are moving with respect to each other and their surroundings, so the resulting shadow will sweep through a region of space, only passing through any particular location in the region for a fixed interval of time. As viewed from such a location, this shadowing event is known as an eclipse; the cross-section of the objects involved in an astronomical eclipse are disk shaped. The region of an object's shadow during an eclipse is divided into three parts: The umbra, within which the object covers the light source. For the Sun, this light source is the photosphere; the antumbra, extending beyond the tip of the umbra, within which the object is in front of the light source but too small to cover it. The penumbra, within which the object is only in front of the light source.
A total eclipse occurs when the observer is within the umbra, an annular eclipse when the observer is within the antumbra, a partial eclipse when the observer is within the penumbra. During a lunar eclipse only the umbra and penumbra are applicable; this is because Earth's apparent diameter from the viewpoint of the Moon is nearly four times that of the Sun. The same terms may be used analogously in describing other eclipses, e.g. the antumbra of Deimos crossing Mars, or Phobos entering Mars's penumbra. The first contact occurs when the eclipsing object's disc first starts to impinge on the light source. For spherical bodies, when the occulting object is smaller than the star, the length of the umbra's cone-shaped shadow is given by: L = r ⋅ R o R s − R o where Rs is the radius of the star, Ro is the occulting object's radius, r is the distance from the star to the occulting object. For Earth, on average L is equal to 1.384×106 km, much larger than the Moon's semimajor axis of 3.844×105 km. Hence the umbral cone of the Earth can envelop the Moon during a lunar eclipse.
If the occulting object has an atmosphere, some of the luminosity of the star can be refracted into the volume of the umbra. This occurs, for example, during an eclipse of the Moon by the Earth—producing a faint, ruddy illumination of the Moon at totality. On Earth, the shadow cast during an eclipse moves approximately at 1 km per sec; this depends on the angle in which it is moving. An eclipse cycle takes place; this happens. A particular instance is the saros, which results in a repetition of a solar or lunar eclipse every 6,585.3 days, or a little over 18 years. Because this is not a
In astronomy, a transit is a phenomenon when a celestial body passes directly between a larger body and the observer. As viewed from a particular vantage point, the transiting body appears to move across the face of the larger body, covering a small portion of it; the word "transit" refers to cases where the nearer object appears smaller than the more distant object. Cases where the nearer object appears larger and hides the more distant object are known as occultations. However, the probability of a seeing a transiting planet is low because it is dependent on the alignment of the three objects in a nearly straight line. Many parameters can be determined by about a planet and its host star based on the transit One example of a transit involves the motion of a planet between a terrestrial observer and the Sun; this can happen only with inferior planets, namely Venus. However, because a transit is dependent on the point of observation, the Earth itself transits the Sun if observed from Mars. In the solar transit of the Moon captured during calibration of the STEREO B spacecraft's ultraviolet imaging, the Moon appears much smaller than it does when seen from Earth, because the spacecraft–Moon separation was several times greater than the Earth–Moon distance.
The term can be used to describe the motion of a satellite across its parent planet, for instance one of the Galilean satellites across Jupiter, as seen from Earth. Although rare, cases where four bodies are lined up do happen. One of these events occurred on 27 June 1586, when Mercury transited the Sun as seen from Venus at the same time as a transit of Mercury from Saturn and a transit of Venus from Saturn. No missions were planned to coincide with the transit of Earth visible from Mars on 11 May 1984 and the Viking missions had been terminated a year previously; the next opportunity to observe such an alignment will be in 2084. On December 21, 2012, the Cassini–Huygens probe, in orbit around Saturn, observed the planet Venus transiting the Sun. On 3 June 2014, the Mars rover Curiosity observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides Earth. In rare cases, one planet can pass in front of another. If the nearer planet appears smaller than the more distant one, the event is called a mutual planetary transit.
Exoplanet Detection The transit method can be used to discover exoplanets. As a planet eclipses/transits its host star it will block a portion of the light from the star. If the planet transits in-between the star and the observer the change in light can be measured to construct a light curve. Light curves are measured with a charged-coupled device; the light curve of a star can disclose several physical characteristics of the planet and star, such as, density. Multiple transit events must be measure to determine the characteristics which tend to occur at regular intervals if the others only one planet. Multiple planets orbiting the same host star can cause Transit Time Variations. TTV is cause by the gravitational forces of all orbiting bodies acting upon each other; the probability of seeing a transit from Earth is low, however. The probability is given by the following equation. P t r a n s i t = / a Where Rplanet is the radius of the star and planet, respectfully; the semi major axis length represented by a.
Because of low probability large selections of the sky must be observed in order to see a transit. Hot Jupiters are more to be seen because of their larger radius and short semi major. In order to find earth size planets red dwarf stars are observed because of their small radius. Though transiting has a low probability it has proven itself to be a good technique in discovering exoplanets. In recent years, the discovery of extrasolar planets has excited interest in the possibility of detecting their transits across their own stellar primaries. HD 209458b was the first such transiting planet; the transit of celestial objects is one of the few key phenomena used today for the study of exoplanetary systems. Today, transit photometry is the leading form of exoplanet discovery; as exoplanets move in front of its host stars there is a dimming in the luminosity of its host star that can be measured. Larger planets make the dip in easier to detect. Followup observations are done to ensure it is a planet through other methods of detecting exoplanets.
There are 2345 planets confirmed with Kepler light curves for stellar host. During a transit there are four "contacts", when the circumference of the small circle touches the circumference of the large circle at a single point. Measuring the precise time of each point of contact was one of the most accurate ways to determine the positions of astronomical bodies; the contacts happen in the following order: First contact: the smaller body is outside the larger body, moving inward Second contact: the smaller body is inside the larger body, moving further inward Third contact: the smaller body is inside the larger body, moving outward Fourth contact: the smaller body is outside the larger body, moving outward A fifth named point is that of greatest transit, when the apparent ce