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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}}
[[File:Phobos transit in real color.webm|upright=1.5|thumb|[[Phobos (moon)|Phobos]] transits the [[Sun]], as viewed by the [[Perseverance (rover)|''Perseverance'' rover]] on 2 April 2022]]
{{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 [[lunar distance (astronomy)|Earth–Moon distance]].]]
 
In [[astronomy]], a '''transit''' (or '''astronomical transit''') is athe [[celestialpassage event|phenomenon]] whenof a [[astronomical object|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, [[eclipse|covering]] a small portion of it.<ref>{{Cite web|url=https://www.merriam-webster.com/dictionary/transit|title=Definition of TRANSIT|website=www.merriam-webster.com|language=en|access-date=2018-12-16}}</ref>
 
The word "transit" refers to cases where the nearer object [[apparent size|appears]] smaller than the more distant object. Cases where the nearer object appears larger and completely hides the more distant object are known as [[occultation|''occultations'']].
 
However, the probability of seeing a transiting planet is low because it is dependent on the alignment of the three objects in a nearly perfectly straight line.<ref>{{Cite web|url=https://lco.global/spacebook/transit-method/|title=Transit Method {{!}} Las Cumbres Observatory|website=lco.global|language=en|access-date=2018-11-27}}</ref> Many parameters of a planet and its parent star can be determined 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 sunSun and Earth not being in the same line.]]
One exampletype of a transit involves the motion of a [[planet]] between a [[Earth|terrestrial]] observer and the [[Sun]]. This can happen only with [[inferior and superior planets|inferior planets]], namely [[Mercury (planet)|Mercury]] and [[Venus]] (see [[transit of Mercury]] and [[transit of Venus]]). However, because a transit is dependent on the point of observation, the [[transit of Earth from Mars|Earth itself transits the Sun]] if observed from Mars. In the solar transit ofby 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 [[lunar distance (astronomy)|Earth–Moon distance]].
 
The term can also be used to describe the motion of a [[natural satellite|satellite]] across its parent planet, for instance one of the Galilean satellites ([[Io (moon)|Io]], [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], [[Callisto (moon)|Callisto]]) across [[Jupiter]], as seen from [[Earth]].
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== Outside the Solar System ==
'''{{further|Exoplanet Detection'''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 [[exoplanet]]s. 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 [[Charge-coupled device|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 measured to determine the characteristics which tend to occur at regular intervals. Multiple planets orbiting the same host star can cause [[Transit-timing variation|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.▼
 
▲The transit method can be used to discover [[exoplanet]]s. 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 [[Charge-coupled device|chargedcharge-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 measured to determine the characteristics which tend to occur at regular intervals. Multiple planets orbiting the same host star can cause [[Transit-timing variation|Transittransit-timing Timevariations Variations(TTV).]] TTV is causecaused 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.
<math>P_\text{transit}= (R_\text{star} + R_\text{planet})/a</math><ref name="Asher">{{Cite book|title=How do you find an exoplanet?|last=Asher|first=Johnson, John|isbn=9780691156811|location=Princeton, New Jersey|oclc=908083548}}</ref>▼
 
▲:<math>P_\text{transit}= (R_\text{star} + R_\text{planet})/a,</math><ref name="Asher">{{Cite book|title=How do you find an exoplanet?|last=Asher|first=Johnson, John|date=29 December 2015|isbn=9780691156811|location=Princeton, New Jersey|oclc=908083548}}</ref>
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 Jupiter]]s 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.▼
 
▲Wherewhere Rstar''R''<sub>star</sub> and Rplanet''R''<sub>planet</sub> isare the radius of the star and planet, respectfully.respectively, and ''a'' Theis semithe semi-major axis length represented by a. Because of the low probability of a transit in any specific system, large selections of the sky must be regularly observed in order to see a transit. [[Hot Jupiter]]s are more likely to be seen because of their larger radius and short semi -major axis. In order to find earth sizeEarth-sized 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 infor discovering exoplanets.
In recent years, the discovery of [[extrasolar planet]]s has excited interest in the possibility of detecting their transits across their own [[star|stellar]] primaries. [[HD 209458b]] was the first such transiting planet to be detected.▼
 
▲In recent years, the discovery of [[extrasolar planet]]s has excitedprompted interest in the possibility of detecting their transits across their own [[star|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 [[exoplanet]]ary systems. Today, [[Methods of detecting exoplanets#Transit photometry|transit photometry]] is the leading form of [[Discoveries of exoplanets|exoplanet discovery]].<ref name="Asher"/> As exoplanets move in front of its host stars there is a dimming in the luminosity of its host star that can be measured.<ref>{{Cite web|url=http://www.planetary.org/explore/space-topics/exoplanets/transit-photometry.html|title=Transit Photometry|website=www.planetary.org|language=en|access-date=2018-11-27}}</ref> 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]].▼
 
▲The transit of celestial objects is one of the few key phenomena used today for the study of [[exoplanet]]ary systems. Today, [[Methods of detecting exoplanets#Transit photometry|transit photometry]] is the leading form of [[Discoveries of exoplanets |exoplanet discovery]].<ref name="Asher"/> As exoplanetsan exoplanet movemoves in front of its host starsstar there is a dimming in the luminosity of itsthe host star that can be measured.<ref>{{Cite web|date = February 2020|url=http://www.planetary.org/explore/space-topics/exoplanets/transit-photometry.html |title=Down in Front!: The Transit Photometry Method|website=www.planetary.orgThe Planetary Society|language=en|access-date=2018-11-27}}</ref> Larger planets make the dip in luminosity more noticeable and easier to detect. Followup observations using other [[methods of detecting exoplanets|methods]] are often donecarried out 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.<ref>{{Cite web|url=https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html|title=Exoplanet Archive Planet Counts|website=exoplanetarchive.ipac.caltech.edu|access-date=2018-12-17}}</ref>
<!-- removed: [[File:Screen Shot 2018-12-17 at 12.28.55 AM.png|left|thumb|305x305px|The number of exoplanets discover by method. ]] -->
[[File:Exoplanets discovery methods chart.png|thumb|553x553px|Exoplanets found by different search methods each year through 2018, transit method in purple.|alt=|center]]
 
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*'''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")<ref name="UCLsafety">{{cite web|url=http://www.transit-of-venus.org.uk/safety.htm|title=Transit of Venus&nbsp;– Safety|publisher=University of Central Lancashire|accessdateaccess-date=21 September 2006|url-status=dead|archiveurlarchive-url=https://web.archive.org/web/20060925140910/http://www.transit-of-venus.org.uk/safety.htm|archivedatearchive-date=25 September 2006}}</ref>
 
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.<ref name="UCLsafety" />
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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 [[Transiting Exoplanet Survey Satellite|TESS]], HATPI, and others which can be found among the [[List of exoplanet search projects|List of Exoplanet Search Projects]].
 
==== HATNet ====
[[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.<ref>{{Cite web|url=https://hatnet.org/|title=The HATNet Exoplanet Survey|website=hatnet.org|access-date = 2018-12-16|publisher = Princeton University}}</ref> 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. IIn 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.<ref>{{Cite web|url=https://hatsurveys.org/|title=The HAT Exoplanet Surveys|website=hatsurveys.org|access-date=2018-12-16|archive-date=25 September 2021|archive-url=https://web.archive.org/web/20210925064147/https://hatsurveys.org/|url-status=dead}}</ref>
 
A third sub-project, HATPI, is currently under construction and will survey most of the night sky seen from its location in Chile.<ref>{{Cite web|url=https://hatpi.org/|title=The HATPI Project|website=hatpi.org|access-date=2018-12-16}}</ref>
 
==== KELT ====
[[Kilodegree Extremely Little Telescope|KELT]] is a terrestrial telescope mission designed to search for transiting systems of planets of magnitude 8<M<10. It began operation in October 2004 in Winer Observatory and has a southern companion telescope added in 2009.<ref>{{Cite journal|lastlast1=Pepper|firstfirst1=J.|last2=Pogge|first2=R.|last3=Depoy|first3=D. L.|last4=Marshall|first4=J. L.|last5=Stanek|first5=K.|last6=Stutz|first6=A.|last7=Trueblood|first7=M.|last8=Trueblood|first8=P.|date=1 July 2007|title=Early Results from the KELT Transit Survey|journal=Transiting Extrapolar Planets Workshop|location=eprint: arXiv:astro-ph/0611947|volume=366|pages=27|bibcode=2007ASPC..366...27P|arxiv=astro-ph/0611947}}</ref> KELT North observes "26-degree wide strip of sky that is overhead from North America during the year", while KELT South observes single target areas of the size 26 by 26 degrees. Both telescopes dancan detect and identify transit events as small as a 1% flux dip, which allows for detection of planetary systems similar to those in our solarplanetary system.<ref>{{Cite web|url=http://www.astronomy.ohio-state.edu/keltnorth/Method.html|title=KELT-North: Method|website=www.astronomy.ohio-state.edu|access-date=2018-12-16|archive-date=24 January 2019|archive-url=https://web.archive.org/web/20190124180526/http://www.astronomy.ohio-state.edu/keltnorth/Method.html|url-status=dead}}</ref><ref>{{Cite journal|lastlast1=Stassun|firstfirst1=Keivan|last2=James|first2=David|last3=Siverd|first3=Robert|last4=Kuhn|first4=Rudolf B.|last5=Pepper|first5=Joshua|date=7 March 2012|title=The KELT-South Telescope*|journal=Publications of the Astronomical Society of the Pacific|language=en|volume=124|issue=913|pages=230|doi=10.1086/665044|issn=1538-3873|arxiv=1202.1826|bibcode=2012PASP..124..230P|s2cid=119207060 }}</ref>
 
==== Kepler / K2 ====
 
The [[Kepler (spacecraft)|Keplerspace telescope]] satellite served the Kepler mission between 7 March 2009 and 11 May 2013, where it observed one part of the sky in search of transiting planets within a 115 square degrees of the sky around the [[Cygnus (constellation)|Cygnus]], [[Lyra]], and [[Draco (constellation)|Draco]] constellations.<ref>{{Cite web|url=http://www.nasa.gov/mission_pages/kepler/overview/index.html|title=Mission overview|last=Johnson|first=Michele|date=13 April 2015|website=NASA|access-date=2018-12-16}}</ref> After that, the satellite continued operating until 15 November 2018, this time changing its field along the ecliptic to a new area roughly every 75 days due to reaction wheel failure.<ref>{{Cite journal|lastlast1=Fortney|firstfirst1=Jonathan J.|last2=Twicken|first2=J. D.|last3=Smith|first3=Marcie|last4=Najita|first4=Joan R.|last5=Miglio|first5=Andrea|last6=Marcy|first6=Geoffrey W.|last7=Huber|first7=Daniel|last8=Cochran|first8=William D.|last9=Chaplin|first9=William J.|date=1 April 2014|title=The K2 Mission: Characterization and Early Results|journal=Publications of the Astronomical Society of the Pacific|language=en|volume=126|issue=938|pages=398|doi=10.1086/676406|issn=1538-3873|arxiv=1402.5163|bibcode=2014PASP..126..398H|s2cid=119206652 }}</ref>
 
==== TESS ====
 
[[Transiting Exoplanet Survey Satellite|TESS]] was launched on 18 April 2018, and is planned to survey most of the sky by observing it strips defined along the [[right ascension]] lines for 27 days each. Each area surveyed is 27 by 90 degrees. Because of the positioning of sections, the area near TESS's [[Rotation around a fixed axis|rotational axis]] will be surveyed for up to 1 year, allowing for the identification of planetary systems with longer orbital periods.
 
==See also==
{{Wikipedia books|Transits of Planets}}
* [[Eclipse]]
* [[Kepler Mission|''Kepler'' Mission]]
* [[Occultation]]
* [[Syzygy (astronomy)]]
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* [[Transit of Phobos from Mars]]
* [[Transit of Vulcan]]
* [[Transit of Mercury from Mars]]
* [[Transit of Earth from Mars]]
 
== References==
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