Glossary term: Transit
Description: A transit occurs when a celestial body passes between another celestial body of larger angular size and the observer. From the point of view of the observer, the obscuring celestial body moves in front of the obscured body, blocking the view of a fraction of the surface of the obscured object. If the obscuring body that passes between the observer and the obscured object has a larger angular size than the latter, the event is referred to as an occultation rather than a transit. A transit viewed by one observer may not be seen by another observer looking at the same system from a different angle.
In the Solar System both Mercury and Venus can sometimes transit across the face of the Sun when viewed from Earth. Moons orbiting Solar System planets are commonly seen to transit across the face of their host planet when viewed from Earth.
Planets orbiting other stars (exoplanets) are often discovered when they transit their host star, blocking out a little of the star's light and making it appear to dim slightly when viewed from the observer's location. A planet orbiting a star will only transit when viewed from Earth if the plane of its orbit intersects with the line of sight from the Earth. As such searching for planets using the transit method can only ever detect a fraction of the planets in the Galaxy. Transits can also be used to estimate the size of an exoplanet.
Related Terms:
See this term in other languages
Term and definition status: This term and its definition have been approved by a research astronomer and a teacher
The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".
If you notice a factual error in this glossary definition then please get in touch.
Related Media
Jupiter, Io and its shadow, by Ralf Burkart, Germany
Caption: First place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons.
This time-lapse of Jupiter taken in 2017 from Germany beautifully illustrates the transit of one of the Galilean moons, Io, in front of Jupiter. As this is simply a moon casting a shadow on a planet it is equivalent to a lunar eclipse on Earth observed from further away. While the shadow of the moon is clearly visible from the beginning, it might be difficult to spot the moon itself against the background of the beautiful atmospheric bands of Jupiter the first time the video is seen. Watching it repeatedly allows appreciating the rapid motion and rotation in this fantastic observation.
Credit: Ralf Burkart/IAU OAE
License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
Venus Transit from Tempe, AZ, USA , June 5, 2012
Caption: While the planet Venus regularly passes between the Sun and the Earth, most of the time it passes above or below the Sun when observed from Earth. On rare occasions, the planet Venus passes directly in front of the Sun for observers on Earth. The last such Venus transits were in 2004 and 2012, respectively. The next such transit will be in 2117. The picture was taken at 18:54 PDT on June 5, 2012, from Tempe, Arizona, US. The telescope used was an 8" Schmidt-Cassegrain with ND5 filter; the camera was a Sony NEX5N with the APS sensor placed at prime focus. Clearly visible are the disk of the Sun, Venus as a prominent round spot. Several sun spots also appear on the solar disk.
Credit: Stephen Rector
credit link
License: CC-BY-SA-3.0 Creative Commons Attribution-ShareAlike 3.0 Unported icons
Transit of Phobos from Mars
Caption: This video shows the Sun viewed from Mars as Mars’ moon Phobos transits across the solar disk. This video is composed of images taken from the Martian surface on the 30th of September 2024 by the Mastcam-Z camera of NASA’s Perseverance Mars rover. While our Moon is perfectly sized to eclipse the Sun when seen from the Earth’s surface, Phobos’s size and orbit mean that it cannot cover the solar disk completely. This means that this event is a transit and not an eclipse.
This video is in real time, with the entire transit lasting just 32 seconds. This compares with the several hours a solar eclipse on Earth takes from the Moon first obscuring part of the Sun to the Sun being completely visible again. This is due to Phobos’ much shorter orbital period of just over seven and a half hours compared to the Moon’s orbital period of 27 days as well as the solar disk appearing slightly smaller from Mars than it does from Earth. Note the non-spherical shape of Phobos as it crosses the solar disk.
Credit: NASA/JPL-Caltech/ASU/MSSS
credit link
License: PD Public Domain icons
