Transit (astronomy)

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Transit (astronomy)
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{{short description|Term in astronomy}}{{About|the passage of one celestial body in front of another|the passage of a body over a meridian|Culmination|the passage of a star across the field of view of a telescope eyepiece|Star transit}}{{Refimprove|date=December 2016}}File:Moon transit of sun large.ogv|thumb|A 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.]]In astronomy, a transit (or astronomical 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.WEB,weblink Definition of TRANSIT,, en, 2018-12-16, 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 completely 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 perfectly straight line.WEB,weblink Transit Method {{!, Las Cumbres Observatory||language=en|access-date=2018-11-27}} Many parameters can be determined by about a planet and its host star based on the transit

In the Solar System

(File:Jupiter-io-transit feb 10 2009.gif|thumb|A simulation of Io transiting Jupiter as seen from the Earth in February 2009. Io's shadow is seen on the surface of Jupiter, leading Io slightly due to the sun and Earth not being in the same line.)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 Mercury and Venus (see transit of Mercury and transit of 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 also be used to describe the motion of a satellite across its parent planet, for instance one of the Galilean satellites (Io, Europa, Ganymede, Callisto) 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. {{Citation needed|date=June 2017}}

Notable observations

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. Consequently, 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.Cassini Spacecraft Tracks Venus Transit From Saturn, Space Coast Daily. Retrieved on 2016-02-08. 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.WEB,weblink Mercury Passes in Front of the Sun, as Seen From Mars, Webster, Guy, June 10, 2014, NASA, June 10, 2014,

Mutual planetary transits

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.File:2012 Transit of Venus from SF.jpg|Transit of Venus as seen from Earth, 2012File:PIA02879 - A New Year for Jupiter and Io.jpg|Io transits across Jupiter as seen by Cassini spacecraftFile:PIA18389-MarsCuriosityRover-MercuryTransitsSun-20140603.gif|Mercury transiting the Sun, seen from Curiosity rover on Mars (June 3, 2014).File:Dark side of the Moon.png|The Moon transiting in front of Earth, seen by Deep Space Climate Observatory on 4 August 2015.

Outside the Solar System

Exoplanet Detection(File:Light curve of binary star Kepler-16.jpg|thumb|The light curve shows the change in Luminosity of star as a result of transiting. The data was collected from the Kepler mission. |alt=|333x333px)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). 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_text{transit}= (R_text{star} + R_text{planet})/aBOOK, How do you find an exoplanet?, Asher, Johnson, John, 9780691156811, Princeton, New Jersey, 908083548, Where Rstar and 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 regularly observed in order to see a transit. Hot Jupiters are more likely 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. Even 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 to be detected. 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.BOOK, How do you find an exoplanet?, Asher, Johnson, John, 9780691156811, Princeton, New Jersey, 908083548, As exoplanets move in front of its host stars there is a dimming in the luminosity of its host star that can be measured. WEB,weblink Transit Photometry,, en, 2018-11-27, Larger planets make the dip in luminosity more noticeable and easier to detect. Followup observations are often done to ensure it is a planet through other methods of detecting exoplanets. There are currently (December 2018) 2345 planets confirmed with Kepler light curves for stellar host.WEB,weblink Exoplanet Archive Planet Counts,, 2018-12-17, (File:Exoplanets discovery methods chart.png|thumb|553x553px|Exoplanets found by different search methods each year through 2018, transit method in purple.|alt=|center)


During a transit there are four "contacts", when the circumference of the small circle (small body disk) touches the circumference of the large circle (large body disk) at a single point. Historically, 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 entirely outside the larger body, moving inward ("exterior ingress")
  • Second contact: the smaller body is entirely inside the larger body, moving further inward ("interior ingress")
  • Third contact: the smaller body is entirely inside the larger body, moving outward ("interior egress")
  • Fourth contact: the smaller body is entirely outside the larger body, moving outward ("exterior egress")WEB,weblink Transit of Venus â€“ Safety, University of Central Lancashire, 21 September 2006, yes,weblink" title="">weblink 25 September 2006,
A fifth named point is that of greatest transit, when the apparent centers of the two bodies are nearest to each other, halfway through the transit.


Since transit photometry allows for scanning large celestial areas with a simple procedure, it has been the most popular and successful form of finding exoplanets in the past decade and includes many projects, some of which have already been retired, others in use today, and some in progress of being planned and created. The most successful projects include HATNet, KELT, Kepler, and WASP, and some new and developmental stage missions such as TESS, HATPI, and others which can be found among the List of Exoplanet Search Projects.


HATNet Project is a set of northern telescopes in Fred Lawrence Whipple Observatory, Arizona and Mauna Kea Observatories, HI, and southern telescopes around the globe, in Africa, Australia, and South America, under the HATSouth branch of the project.WEB,weblink The HATNet Exoplanet Survey,, 2018-12-16, These are small aperture telescopes, just like KELT, and look at a wide field which allows them to scan a large area of the sky for possible transiting planets. I addition, their multitude and spread around the world allows for 24/7 observation of the sky so that more short-period transits can be caught.WEB,weblink The HAT Exoplanet Surveys,, 2018-12-16, A third sub-project, HATPI, is currently under construction and will survey most of the night sky seen from its location in Chile.WEB,weblink The HATPI Project,, 2018-12-16,


KELT is a terrestrial telescope mission designed to search for transiting systems of planets of magnitude 8

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