Mercury only passes in front of the Sun a few times every century, and the last time was a decade ago. While the celestial event is visible from most of Western Europe and North America on Monday 9 May between 12.12pm BST and 7.42pm BST, catching a glimpse of it is no easy task. The first thing to know is that you should not look directly at the Sun, because it'll injure your eyes. Special glasses used to view solar eclipses are also pretty useless - Mercury is so small that magnification is needed to see it.

A pair of binoculars or a telescope is likely to suffice, but the eye risk remains. That means you'll need a special solar filter, or to project an image of the Sun on to a piece of paper. If that sounds like too much fuss, there is a far easier way to see it: by watching one of the numerous online live streams. Both the European Space Agency and NASA are broadcasting the event. At one time the passing of Mercury in front of the Sun was a useful scientific opportunity because it allowed astronomers to measure distances in the solar system.

The Open University's David Rothery told the New Scientist: "When I was a lad in Birmingham I saw the 1973 transit. "It's going to be especially important for me this time, because I’m now involved in the European Space Agency’s mission to Mercury." The agency's spacecraft is due to blast off in two years and arrive at Mercury in 2024. Its mission is to study Mercury's surface and its magnetic field. Last year NASA's Messenger spacecraft slammed into the surface of Mercury, ending a successful 11-year mission.