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The word transit has two 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, as seen by an observer at some particular vantage point.
- 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.
The rest of this article refers to the first kind of transit.
The word "transit" refers to cases where the nearer object appears considerably smaller in apparent size than the more distant object. Cases where the objects are comparable sizes are known as eclipses; 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. One such case occurred on March 21 1894 at around 23:00 UTC, when Mercury transited the Sun as seen from Venus, and Mercury and Venus both simultaneously transited the Sun as seen from Saturn (see Transit of Mercury from Saturn and 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
In rare cases, one planet can transit in front of another. The next time this will happen (as seen from Earth) will be on November 22 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
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.
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) at a single point. The contacts happen in the following order:
- First contact: the smaller body is entirely outside the larger body, moving inward
- Second contact: the smaller body is entirely inside the larger body, moving further inward
- Third contact: the smaller body is entirely inside the larger body, moving outward
- Fourth contact: the smaller body is entirely outside the larger body, moving outward
See also
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