PARIS: Astronomers in the Americas, East Asia and Australasia will get a rare daylight treat this week when Mercury passes in a direct line between the Earth and the Sun, a “transit” that will next occur on May 9, 2016.

Skygazers will be able to see Mercury, the closest planet to the Sun, creep across the solar disk as a tiny black dot.

The whole five-hour transit, from 1912 GMT on Wednesday to 0010 GMT on Thursday, can be visible from the western coast of the United States, Canada, New Zealand, south-eastern Australia, and the archipelagoes of the South Pacific.

Part of the transit will be visible before sunset on Wednesday in the rest of the Americas and after sunrise on Thursday in East Asia and the rest of Australia.

It will not be visible in Europe, Africa, the Middle East or in Asia west of Burma.

The only other planet that makes a transit from Earth's perspective is Venus, the second planet from the Sun. The last Venusian crossing was in June 2004.

Mercury takes only 88 days to race around the Sun, but it is rarely in direct alignment between the Earth and the Sun because its orbit is tilted.

Transits occur a dozen times a century; the last was in 2003.

Mercury is so tiny that transits could not be seen before the invention of the telescope. The first person to witness one was a French astronomer, Pierre Gassendi, in 1631.

His discovery sparked the realisation that transits could be used to establish a way of measuring distances in the Solar System.

The method is that of simple geometry -- observing an object from two points that are a known distance apart, which provides the base line for a triangle.

Anyone observing the transit should use safe solar filters on their telescope or project the image onto a screen to avoid damaging their eyesight.—AFP