If you are in the Americas or the Pacific region and you have a suitably-equipped telescope, you will be able to observe a rare event on Monday.

Mercury, the innermost planet, will transit, or move across, the face of the Sun.

So-called transits of Mercury only occur on average 13 times a century and a grazing transit like this one is extremely rare. The next one will not occur until 2314.

Unless you are in the right part of the world and have a telescope rigged for projection, the only way to observe the transit is on the web. Remember, you should never look at the Sun directly with the unaided eye or through binoculars.

Curiously, the effect is exactly the same as that recently used to detect the shadow of a planet moving across the face of another star.

North and South America will be in the best position to see Mercury since it will pass in front of the solar disk at sunset. The transit will begin at 21:15:01 GMT and last only 52 minutes.

Distance scale

Observers in north-eastern Australia, Papua New Guinea, and Hawaii should be able to see at least part of the transit. Mercury will appear similar to a small sunspot. It will pass across the northernmost edge of the Sun.

In general, such a transit is of only historical interest. Sir Edmund Halley realised in the 17th Century that transits, especially of Venus, could be used to calibrate the distance scale of the Solar System and allow the distances to the Sun and planets to be calculated.

One satellite will be using the transit to calibrate its solar detectors. For the first time since the Soho solar monitoring satellite was launched in late 1995, scientists will be able to take advantage of a Mercury transit to improve the quality of data its gathers.

From Soho's perspective 1.5 million km closer to the Sun than the Earth, the closest Mercury will get to the Sun will be at an angle of about 92 arc seconds. This means that Soho will not see Mercury crossing the Sun's disk but rather see it pass just above the disk. This will be right in front of the Sun's hot outer atmosphere, the corona.

Bernhard Fleck, Soho's Project said: "We need to distinguish between light from the solar disk and that from the corona. That will allow us to get more refined measurements of the corona."

A mission to Mercury, called Bepi Colombo, is one of the European Space Agency's "cornerstone" missions, and it will be launched in 2009. According to preliminary studies completed in April 1999, it would include two small orbiters and a probe to land on the surface.