This event, visible from Europe, Africa and Asia and parts of North America, will provide a close-up example of the "transit method," one of the techniques scientists are using to detect or confirm the existence of planets around other stars. The transit method involves measuring the slight decrease in light from a star when a planet passes across its face.

The disk of Mercury is very small and will be difficult to see. Observation of the event requires being in the right location (see map) and using a telescope with special protection. It is dangerous to observe the Sun directly; the only completely safe approach is to project the image onto a sheet of paper.

The Mercury transit, which occurs once every seven years, provides a prelude to an even more important celestial event next year. On June 8, 2004, the larger planet Venus will pass in front of the Sun, and will be easily observable from the same continents

If you aren't in the right place to observe the Mercury event directly, you can watch it virtually via one of the the webcams that will transmit live images on the Internet. See "Find out more" (upper right) for links to webcams, and a list of sites that provide comprehensive information on observing the Mercury transit.

Webcams:

• ESO
• Paris Observatory
• 2003 Transit of Mercury page
• ESO Mercury Transit page