Astronomers and amateur star gazers will gather today to watch Mercury transit the Sun's disc as it is projected on a giant screen outside Marina Mall in Abu Dhabi.

Mercury will start crossing the Sun's disc at 9.13am local time and will continue for about five hours and 17 minutes.

The Astronomy Amateur Group at the Emirates Heritage Club will install a telescope outside the mall and project the planet's transit on a screen to give the public a live celestial spectacle.

"This is an exciting event for all astronomers, and for the first time we decided to share it with people and give them a chance to see it," said Ibrahim Obeidallah, Director of the Group.

Obeidallah said the transit of Mercury, the planet closest to the Sun, will be difficult to notice because of the vast differences in relative sizes.

"Mercury will only appear like a spot, because it is 158 times smaller than the sun," he explained.

Even the smallest sunspots on the Sun's surface are larger than Mercury, whose diameter is only 4,800 kilometres. Its tiny silhouette against the Sun will barely be bigger than the point of a pencil.

Obeidallah said that Mercury's transit was last seen in the UAE in 1999.

Such solar transits by Mercury happen about 13 times each century, at irregular intervals of approximately 13, 7, 10, and three years.

This year the Earth passes through Mercury's orbital plane in May and November. The next Mercury transit will be on November 8 2006.

Transits by Mercury happen when the planet is between the Earth and the Sun, on an orbital plane that intersects ours. Obeidallah said that while the best viewing conditions are in Europe, Africa and Asia, it can be viewed by telescopes wherever the Sun is above the horizon and the sky is clear.

He stressed that one should not try to look at the Sun with the naked eye or by telescope as this can cause blindness.

"As looking at the Sun directly can be dangerous, people are advised to watch through telescopes only if provided with Sun filters or a projection on a screen. Otherwise, human sight can be irremediably damaged," he said.

Astronomers and photographers are now busy dusting off their telescopes to watch the occurrence, despite Mercury's transit not being a major astronomical event, said Obeidallah.

"Mercury's transit is not as exciting as a lunar eclipse, but to astronomers it is still an important event."