Transit, in astronomy, the passage of one celestial body across the disc (face) of a larger, more distant body, or across the observer's meridian. (A meridian is an imaginary line that extends north and south through the point in the sky directly above the observer's location.) Meridian transits are caused by the earth's daily rotation and are useful for measuring time.
The transit of one celestial body across another is caused by the relative positions of the earth and the two bodies. Two planets, Mercury and Venus, transit the sun. For a planet to transit the sun, the planet must be at inferior conjunction (between the earth and the sun) while the sun is at one of the nodes of the planet's orbit. (The nodes are the two points where the planet's orbit crosses the plane of the earth's orbit. As seen from earth, the sun appears to follow a yearly course in the plane of the earth's orbit, thus crossing the nodes of each planet's orbit twice a year.) Transits of Mercury occur about 13 times per century, always in May and November. Transits of Venus, which are rare, occur in June and December. The latest transit of Venus occurred in 2004. The previous one occurred in 1882 and the next will occur in 2012.