PARIS-Astronomers on Monday were preparing 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 per century. Mercury will be seen through telescopes as a black dot inching over the face of our star, providing a celestial spectacle - weather permitting - that will last seven and a half hours.

“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 Pascal Descamps of the Paris Observatory. “It’s something rare, because it requires the Sun, Mercury and Earth to be in almost perfect alignment.”

The smallest recognised planet in the Solar System, Mercury completes an orbit every 88 days, and passes between the Earth and the Sun every 116 days. But its orbit is tilted in relation to Earth’s, which means it usually appears - from our perspective - to pass above or below the Sun.

Thirteen times each century, however, the two orbits align such that even amateur astronomers can see the tiny planet tens of millions of kilometres (miles) away.

According to Britain’s Royal Astronomical Society (RAS), most of Western Europe, the western parts of North and West Africa, eastern North America, and most of South America will be able to view the entire transit, which will last from 1112 GMT to 1842 GMT.

The rest of north and south America, the eastern Pacific, the remainder of Africa and most of Asia, will see parts of the event. Observers in east and southeast Asia and Australasia, however, will miss out entirely.

The closest planet to the Sun and a third the size of Earth, Mercury is one of the Solar System’s curiosities. It is one of the four rocky planets of the inner Solar System but has no atmosphere and its metallic body is scarred by collisions from space rocks.