Editing Astronomical transit

{{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]]

In [[astronomy]], a '''transit''' (or '''astronomical transit''') is the passage of a [[astronomical object|celestial body]] 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 Sun and Earth not being in the same line.]]
One type of 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 by 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]].

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 21 December 2012, the ''[[Cassini–Huygens]]'' probe, in orbit around [[Saturn]], observed the planet [[Venus]] transiting the Sun.<ref>''[http://spacecoastdaily.com/2012/12/cassini-spacecraft-tracks-venus-transit-from-saturn/ Cassini Spacecraft Tracks Venus Transit From Saturn]'', Space Coast Daily. Retrieved on 8 February 2016.</ref>

On 3 June 2014, the Mars rover [[Curiosity (rover)|''Curiosity'']] observed the planet [[Mercury (planet)|Mercury]] transiting the Sun, marking the first time a [[planetary transit]] has been observed from a celestial body besides Earth.<ref name="NASA-20140610">{{cite web|url=http://www.jpl.nasa.gov/news/news.php?release=2014-183|title=Mercury Passes in Front of the Sun, as Seen From Mars|last=Webster|first=Guy|date=10 June 2014|work=[[NASA]]}}</ref>

=== Mutual planetary transits ===
{{main|Planetary transits and occultations}}
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''.<gallery>
File:2012 Transit of Venus from SF.jpg|Transit of Venus as seen from Earth, 2012
File:PIA02879 - A New Year for Jupiter and Io.jpg|[[Io (moon)|Io]] transits across [[Jupiter]] as seen by ''[[Cassini-Huygens|Cassini]]'' spacecraft
File:PIA18389-MarsCuriosityRover-MercuryTransitsSun-20140603.gif|Mercury transiting the Sun, seen from [[Curiosity (rover)|''Curiosity'']] rover on Mars (3 June 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.
</gallery>

== In Hindu astrology ==

In [[Hindu astrology]] (''Jyotisha''), the concept of an astronomical transit is referred to as '''[[Gochar]]'''. Planetary transits through the twelve zodiac signs (''Rāśis'') are believed to have significant effects on an individual’s life depending on the planetary positions at the time of birth. Hindu astrology uses the [[sidereal zodiac]], which aligns the signs with the fixed constellations, unlike the tropical zodiac used in Western astrology.

Transits are used to predict the timing of important events, such as career changes, relationships, or health matters. The impact of a transit is influenced by:

* The nature of the transiting planet (benefic or malefic),
* The house it occupies in the natal chart,
* Aspects and conjunctions formed with other planets,
* The current planetary periods (''[[Dasha (astrology)|Dasha]]'') in effect.

=== Planetary transit durations and general effects ===

{| class="wikitable"
! Planet
! Approximate transit duration per sign
! General influence during transit
|-
| Sun || 1 month || Vitality, ego, authority
|-
| Moon || 2.5 days || Emotions, mood fluctuations
|-
| Mars || ~1.5 months || Energy, aggression, courage
|-
| Mercury || ~1 month || Intellect, communication
|-
| Jupiter || ~1 year || Growth, wisdom, expansion
|-
| Venus || ~1 month || Love, relationships, beauty
|-
| Saturn || ~2.5 years || Karma, discipline, delay
|-
| Rahu || ~1.5 years || Obsession, material desires
|-
| Ketu || ~1.5 years || Detachment, spirituality
|}

''Note: Rahu and Ketu are shadow planets (lunar nodes) and always move in a retrograde direction.''

== Outside the Solar System ==
{{further|Exoplanet detection}}
[[File:Transit method variable-size planet 4K.webm|thumb|Visualization of transit method for planets of different sizes, showing different light-curves.]]
[[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]]. 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-timing variations (TTV).]] TTV is caused 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|date=29 December 2015|isbn=9780691156811|location=Princeton, New Jersey|oclc=908083548}}</ref>

where ''R''<sub>star</sub> and ''R''<sub>planet</sub> are the radius of the star and planet, respectively, and ''a'' is the semi-major axis. 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-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 for discovering exoplanets.

In recent years, the discovery of [[extrasolar planet]]s has prompted 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 an exoplanet moves in front of its host star there is a dimming in the luminosity of the 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=The Planetary Society|language=en}}</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 carried out to ensure it is a planet.

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]]

== Contacts ==
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) [[tangent|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")<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|access-date=21 September 2006|url-status=dead|archive-url=https://web.archive.org/web/20060925140910/http://www.transit-of-venus.org.uk/safety.htm|archive-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" />

== Missions ==
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|date = 2018|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. In 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|last1=Pepper|first1=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|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 can detect and identify transit events as small as a 1% flux dip, which allows for detection of planetary systems similar to those in our planetary 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|last1=Stassun|first1=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 space telescope]] 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|last1=Fortney|first1=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==
* [[Eclipse]]
* [[Kepler Mission]]
* [[Occultation]]
* [[Syzygy (astronomy)]]
** [[Conjunction (astronomy)]]
** [[Opposition (astronomy)]]
* [[Transit of asteroids]]
* [[Transit of Deimos from Mars]]
* [[Transit of Phobos from Mars]]
* [[Transit of Vulcan]]
* [[Transit of Mercury from Mars]]
* [[Transit of Earth from Mars]]

== References==
{{reflist}}

==External links==
{{Commons category|Astronomical transits}}
*''[http://www.sil.si.edu/exhibitions/chasing-venus/intro.htm Chasing Venus, Observing the Transits of Venus]'' Smithsonian Institution Libraries
*[[Jean Meeus]]: ''Transits.'' Richmond, Virginia: Willmann-Bell, Inc., 1989, {{ISBN|0-943396-25-5}}
* Jean Meeus: ''Astronomical Tables of the Sun, Moon and Planets.'' Richmond, Virginia: Willmann-Bell, Inc., 1995, {{ISBN|0-943396-45-X}}
*[[Karl Ramsayer]]: ''[[Geodetic astronomy|Geodätische Astronomie]]'', Vol.2a of ''Handbuch der Vermessungskunde'', 900 p., J.B.Metzler, Stuttgart 1969.

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