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The word "transit" refers to cases where the nearer object appears considerably smaller in apparent size than the more distant object. Cases where the nearer object appears larger and completely hides the more distant object are known as occultations. Cases where one object moves into the shadow of another are known as eclipses. Each of these three terms are the visible effects of a syzygy.

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. The one closest to the present occurred on April 27, 1586, 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.

(from wikipedia)