transit

A transit is the passage of an astronomical body between an observer and another astronomical body, generally used when the nearer body blocks only a bit of the further body, i.e., the further body occupies a larger portion of the celestial sphere, appearing larger; the terms eclipse and occultation mean roughly the same thing but are more generally used if the appearance of the nearer body that is blocking the view is nearly the same size or larger than that of the body blocked. However some specific situations have conventions regarding which term or terms are used. Planets transiting in front of stars are termed transiting planets, such as Venus transiting across the Sun or an extra-solar planet crossing in front of its own star. Transits (and eclipses and occultations) offer opportunities for useful measurement and study: occultations of astronomical objects behind the Moon can be used to refine position information and transits of Jupiter and Saturn by their moons have yielded useful data about both the moon and its host planet. The transit (e.g., by an exoplanet) is sometimes referred to as the primary eclipse, if the secondary eclipse is also under discussion.

The transit method is one method of detecting exoplanets and binary stars, through detection of the magnitude variation of stars due to periodic transits. Transits are also useful for follow-up studies of exoplanets discovered by other means, particularly to determine its radius relative to its host star, to help estimate the planet's density.

The word transit is also used more generally in astronomy, for an astronomical body crossing something. One example is a body crossing the field of view of an instrument such as a telescope. Another example is an astronomical body (e.g., the Sun or Moon) crossing the celestial meridian, the meridian line of the celestial sphere that (locally) happens to be directly overhead, through the zenith. A transit telescope or a meridian circle is an astronomical instrument designed to observe and record such an occurrence, often timing the moment of transit as a means to determine the body's right ascension.