The dark circle in this picture is the planet Mercury. Mercury is in front of the Sun. This picture was taken during a transit on May 7, 2003. The transit is almost over and Mercury is near the edge of the Sun. Click on image for full size (34K JPEG) Image courtesy of The Royal Swedish Academy of Sciences.

A "transit" is the name of a type of astronomical event. A transit is like a solar eclipse, when the moon blocks out the Sun. During a transit a planet, not the Moon, moves between Earth and the Sun. There are only two planets that ever have transits. Those two are Mercury and Venus. They are the only planets that orbit closer to the Sun than Earth does.

When a transit happens the planet doesn't cover the whole Sun like the Moon does during a total solar eclipse. The planets are so far away that they look like tiny dots moving across the Sun.

Most of the time when a planet gets between us and the Sun a transit doesn't happen. Usually Mercury or Venus is a little bit above or below the Sun from our point of view.

There are about 13 or 14 transits of Mercury every 100 years. Transits of Venus are very rare. The last Venus transit was in 1882! There will be another transit of Venus in June 2004.

In the last few years astronomers have started studying another type of transit. Some planets orbiting other stars have been found because of transits. The planets are much too far away to be seen crossing in front of a star. However, when the planet is in front of the star the dark planet blocks out a tiny amount of the star's light. Sometimes astronomers can detect the change and know that they have found a new planet!