The term transit or astronomical transit has three meanings in astronomy:
- A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point. If the first celestial body hides a major part, or all of, the second celestial body, then it is an occultation rather than a transit.
- A transit occurs when a celestial body crosses the meridian due to the Earth's rotation, about halfway between rising and setting. For instance, the Sun transits the meridian at solar noon. Observation of meridian transits was once very important for timekeeping purposes (see transit instrument).
- The term star transit is used for the passage of a star through the eyepiece of a telescope. Precise observations of elevation or time are carried out to determine star positions or the local vertical (geographic latitude/longitude).
The rest of this article refers to the first kind of transit.
On 3 June 2014, the Curiosity rover on the planet Mars observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides Earth.
The word "transit" refers to cases where the nearer object appears considerably smaller than the more distant object. Cases where the nearer object appears larger and completely hides the more distant object are known as occultations.
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, as seen from outer planets such as Mars, the Earth itself transits the Sun on occasion.
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.
A transit requires three bodies to be lined up in a single line. More rare are cases where four bodies are lined up. The one closest to the present occurred on 27 April 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.
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 is the first such transiting planet to be discovered.
Mutual planetary transits and occultations
In rare cases, one planet can transit in front of another. The next time this will happen (as seen from Earth) will be on 22 November 2065 at about 12:43 UTC, when Venus near superior conjunction (with an angular diameter of 10.6") will transit in front of Jupiter (with an angular diameter of 30.9"); however, this will take place only 8° west of the Sun, and will therefore not be visible to the unaided/unprotected eye. When the nearer object has a larger angular diameter than the farther object, thus covering it completely, the event is not a transit but an occultation. Before transiting Jupiter, Venus will occult Jupiter's moon Ganymede at around 11:24 UTC as seen from some southernmost parts of Earth. Parallax will cause actual observed times to vary by a few minutes, depending on the precise location of the observer.
There are only 18 mutual planetary transits and occultations as seen from Earth between 1700 and 2200. Note the long break of events between 1818 and 2065.
- 19 Sep 1702 – Jupiter occults Neptune
- 20 Jul 1705 – Mercury transits Jupiter
- 14 Jul 1708 – Mercury occults Uranus
- 4 Oct 1708 – Mercury transits Jupiter
- 28 May 1737 – Venus occults Mercury
- 29 Aug 1771 – Venus transits Saturn
- 21 Jul 1793 – Mercury occults Uranus
- 9 Dec 1808 – Mercury transits Saturn
- 3 Jan 1818 – Venus transits Jupiter
- 22 Nov 2065 – Venus transits Jupiter
- 15 Jul 2067 – Mercury occults Neptune
- 11 Aug 2079 – Mercury occults Mars
- 27 Oct 2088 – Mercury transits Jupiter
- 7 Apr 2094 – Mercury transits Jupiter
- 21 Aug 2104 – Venus occults Neptune
- 14 Sep 2123 – Venus transits Jupiter
- 29 Jul 2126 – Mercury occults Mars
- 3 Dec 2133 – Venus occults Mercury
Occultations after 2200 include:
The 1737 event was observed by John Bevis at Greenwich Observatory – it is the only detailed account of a mutual planetary occultation. A transit of Mars across Jupiter on 12 Sep 1170 was observed by the monk Gervase at Canterbury, and by Chinese astronomers. In addition, an occultation of Mars by Venus was observed by Michael Maestlin at Heidelberg on 3 October 1590.
Future transits that can be seen from planets other than Earth include:
- 23 Jun 2021 – Earth occults Pluto as seen from Venus 
- 29 Nov 2022 – Earth occults Mercury as seen from Mars 
- 12 Jan 2032 – Earth transits Saturn as seen from Venus (south pole)
- 11 Sep 2032 – Mercury transits Jupiter as seen from Mars
- 2 Sep 2064 – Venus occults Uranus as seen from Mercury
- 22 Nov 2065 – Venus occults Earth as seen from Jupiter
- 11 Aug 2079 – Mercury transits Earth as seen from Mars
- 4 Nov 2079 – Jupiter occults Uranus as seen from Mars
- 27 Oct 2088 – Mercury transits Earth as seen from Jupiter
- 7 Apr 2094 – Mercury transits Earth as seen from Jupiter
- 20 Apr 2100 – Venus transits Saturn as seen from Mercury
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")
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.
- Kepler Mission
- Occultations, transits, and eclipses
- Transit of asteroids
- Transit of Deimos from Mars
- Transit of Phobos from Mars
- Transit of Vulcan
For transit of planets in front of others, see also occultation.
- 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: Geodätische Astronomie, Vol.2a of Handbuch der Vermessungskunde, 900 p., J.B.Metzler, Stuttgart 1969.