On Monday, May 9, for the first time since 1927, the Great Equatorial Telescope at the Royal Observatory, Greenwich, will be turned to the Sun to view a Transit of Mercury.

This transit will last upwards of seven hours and the public has been invited to view the phenomena through the telescope , with 99% of sunlight filtered out to make the viewing safe.

Astronomer Dr Edward Bloomer explained what would be happening from about midday to about 6.30pm.

Dr Edward Bloomer will be watching the Transit of Mercury for the first time

He said: “The planet Mercury is going to cut across the surface of the Sun. Because the orbits of the planets are elliptical and slightly inclined it means that these are fairly rare events and each transit is not quite the same, not all of them are very easy to see or in the right place at the right time. This is a really good one for us because it’s going to be visible across western Europe for quite a long time.”

Although the event has very little wider scientific significance in the modern era, transits have helped astronomers through the ages piece together their understanding of how the universe works.

Dr Bloomer said: “ [German astronomer] Johannes Kepler predicted that these transits should occur in the 17th century so they were important to observe as it would prove, in some way, the mechanics of the solar system.

“We know these things now, of course, but it’s great to witness it yourself. There are lots of things in astronomy that are very long-lived, or very short-lived, but this is something where you can see it change on a very human time-scale.”

Mercury is a small rocky planet, with a radius a third of that of the Earth’s. It turns slowly and has no insulating atmosphere meaning that the side facing the Sun is more than 400C while the side away from the Sun drops to –173C.

The Great Equatorial Telescope at the Royal Observatory, Greenwich

“It’s a dead surface, covered in impact craters. Superficially it’s quite like the Moon because it doesn’t have the erosion of Earth where we have natural processes like the water cycle. If you make a mark on Mercury it remains there. That’s very useful in astronomy because they help you understand the history of the planet.”

Astronomers are hoping for a clear day on May 9, unlike the weather that spoiled the Transit of Venus in 2012.

Dr Bloomer said: “Ideally we would love people to look through the telescope directly but we want to be absolutely safe with the filter cap on.

“The back-up would be to attach a camera to the eyepiece so people are looking at a screen. That means we can also simultaneously broadcast around the site, capturing that data live.”

Dr Bloomer warned that, even an apparent “filter” of a cloud covering, it was dangerous to look at the Sun.

“There’ll be lots of people in the astronomical community looking at this in different ways and different wavelengths and different magnifications to capture all the data we can.”