A transit is the passage of a smaller astronomical object across the face of a larger one, as in a transit of Venus or Mercury across the Sun, or the transit of a moon across its primary. In the latter case, if the transit involves the moons shadow rather than the moon itself, it is known as a shadow transit. Transits of Venus and Mercury can only take place when the planets are close to the nodes of their orbits at inferior conjunction: in early December or early June for Venus, and in early November or early May for Mercury. Only five transits of Venus have been observed, those of 1639, 1761, 1769, 1874, and 1882; the next is due in 2004.
A transit is also the instant at which a star or other celestial object crosses the observers meridian; also known as culimination or meridian passage.