CAPE CANAVERAL (Florida) • Astronomers were preparing yesterday for one of the highlights of the skywatchers' year, when the Sun, Mercury and Earth all line up - a phenomenon that happens just a dozen or so times in a century.

Mercury will be seen through telescopes as a black dot inching over the face of our star, providing a celestial spectacle that will last 71/2 hours, where even amateur astronomers will be able to see the tiny planet tens of millions of kilometres away.

"At the start, Mercury will look as if it is nibbling at the edge of the Sun, and then it will very slowly cross its surface and leave the other side," said the Paris Observatory. "It is something rare because it requires the Sun, Mercury and Earth to be in almost perfect alignment."

Britain's Royal Astronomical Society said observers in East and South-east Asia and Australasia will miss out on the transit.

Mercury spins around the Sun every 88 days, but its orbit is tilted relative to the Earth's. It is that discrepancy which makes it relatively rare for the three bodies to line up in space. The last Mercury line-up was 10 years ago, and the next will be in 2019, followed by 2032 and 2049.

Several websites are providing "live" coverage of the event.

AGENCE FRANCE-PRESSE