A transit occurs when a celestial body passes directly between a larger body and a viewer. On Earth, planetary transits occur when Mercury or Venus pass between the Sun and the Earth, appearing as tiny black dots traveling across the face of the Sun. Only Mercury and Venus can transit the Sun for a viewer on Earth because those are the only two planets that orbit the Sun inside of the Earth's own orbit. In a way, a transit can be thought of as a "mini solar eclipse," although the sunlight blocked by the distant planet equals just a tiny fraction of that which is blocked when the Moon obscures the Sun during an actual solar eclipse.
Transits of Mercury and Venus are much rarer than solar eclipses. Transits of Mercury only occur about 13 or 14 times per century in either May or November. Transits of Venus are even rarer, with two transits occurring in June or December within eight years of each other, but with gaps of over 100 years in between pairs.
Transits of Mercury and Venus can be seen by almost everyone on the daytime side of the Earth for up to several hours. As with solar eclipses, transits require that safe solar viewing procedures are followed. Because Mercury and Venus appear so small against the solar disc, binoculous or telescopes with special front-mounted (the Sun side) solar filters are used for viewing. (See our Safety page for more information on safe solar viewing.)
Transits are used to discover new planets outside of our Solar System. Since a transiting planet blocks a small amount of light from its host star, atronomers look for this faint dimming of starlight to detect transits and, therefore, new planets.