Mercury's transit in front of the Sun on 8 November 2006

On Wednesday 8 November 2006, Mercury will pass directly between the Sun and the Earth. The innermost planet will be seen not as a bright point in the sky but as a tiny black dot, silhouetted against the brilliant surface of the Sun. Although this spectacle is not visible from Europe, the ESA-NASA solar satellite SOHO will be watching.

Such a crossing is known as a transit. From Earth’s vantage point, only Mercury and Venus transit the Sun, because these are the only planets inside Earth’s orbit. In the case of Venus, the planet is large enough to be seen against the Sun without optical aid, providing that a pair of solar filters is worn to cut down the damaging glare of the Sun.

Mercury, however, is much smaller. At only 1/194 of the Sun's apparent diameter, a small telescope is needed. The telescope must be fitted with a solar filter, otherwise the bright Sun will permanently damage the eyes of anyone who looks through it. Once filtered, an eyepiece that provides a magnification of 50-100 times will give the best view of this event.

This transit is the second of 14 Mercury transits that will take place during the 21st century. The previous Mercury transit was on 7 May 2003 and the next will not happen until 9 May 2016.

However, if you are in Europe, the timing of today's event is wrong. The transit begins at 20:12 CET today - the Sun will have set – and it will last until 01:10 CET tomorrow (9 November 2006). European sky watchers need not despair, however. The ESA-NASA solar watchdog SOHO (SOlar and Heliospheric Observatory) will be in the position favourable to observe the event from 21:09 CET on 8 November until 02:57 CET on 9 November, and will beam those pictures back to Earth, where they can be seen live on the internet.

The images will be visible by following this link. As the event takes place, the images will be compiled into a movie of the transit.

In previous centuries, transits – especially of Venus – were used to measure the absolute distance between the Sun and the Earth. Nowadays, they are mostly observed for enjoyment. However, the scientists and engineers working with SOHO have found a use for them. "These transits provide unique opportunities for characterising the imagers and spectrometers," says Bernhard Fleck, ESA's SOHO Project Scientist. By checking the instruments in this way, the SOHO team can be sure of producing the best science data possible.

Beyond the Solar System, transits of extrasolar planets around other stars are now being observed. The drop in light registered as the distant planet moves in front of the star can tell astronomers the diameter of the planet. In December 2006, CoRoT, the international space telescope with ESA participation, will launch from Kazakhstan. It will search hundreds of thousands of stars for transits. In this way, it is expected to detect hundreds of previously unknown planets.