In astronomy, the term transit has three related meanings, with the primary and most important definition being an event where one celestial body visibly passes across the face of a larger one and blocks a small portion of it from view as seen by an observer. This is the usage most people mean when they refer to a transit, such as a planet crossing the disk of the Sun or a moon moving across a planet, and it differs from an occultation where most or all of the background object is covered. The second meaning refers to the moment a celestial object crosses the local meridian in the sky due to Earth’s rotation, roughly halfway between rising and setting, such as the Sun reaching its highest point at solar noon, a measurement that was historically important for timekeeping and navigation. The third meaning, often called a star transit, describes the passage of a star across a telescope’s eyepiece field, where careful timing and measurement are used to determine accurate star positions and refine local latitude and longitude.
In the primary sense, a transit is a line of sight event that depends on precise alignment between the observer and the two celestial bodies involved. The foreground object must pass directly between the observer and the background object so that it appears as a small silhouette moving across the brighter disk behind it. Because of the geometry required, transits are relatively uncommon and often predictable far in advance. They are especially important in planetary astronomy, since observing transits can reveal orbital motion, size, and sometimes even atmospheric properties when studied with sensitive instruments.
This type of transit also plays a major role in modern exoplanet discovery. When a distant planet passes in front of its host star from our viewpoint, it causes a slight and measurable dip in the star’s brightness. By recording and analyzing these repeated dips, astronomers can detect planets, estimate their sizes, and determine their orbital periods. For both professional research and amateur observation.
It’s the first or primary definition of “transit” that we are highlighting here and while that often means Mercury and Venus transits across the face of the Sun, there are also observable (using a telescope) transits that happen between Jupiter and its large Galilean moons (Io, Europa, Ganymede and Callisto), and Saturn and its moons. These transits involving the the moons of Jupiter and Saturn happen much more frequently than the previously mentioned Venus and Mercury transits. If you are interested in seeing these transits on Jupiter and Saturn and require information on when to look, check out the following site: shallowsky.com/galilean/
Click on a transit link below to be taken to a page with transit images I captured of that specific event.
So when’s the next Transit?
The next Mercury Transit occurs on November 12, 2032. Note: This transit will not be visible from the U.S.
The next Venus Transit occurs on December 10, 2117.