California, US (BBN)-Skywatchers across the globe will see Mercury transit the Sun on Monday.
The Solar System’s smallest planet will pass directly between Earth and our star from around 11:12 GMT to 18:42, reports BBC.
Such a transit only rolls around 13 or 14 times each century; the next occasions will be in 2019 and 2032.
The event is impossible – and dangerous – to view with the naked eye or binoculars, but astronomy groups worldwide will offer a chance to view it through filtered telescopes.
Views from space and ground telescopes will also be live-streamed online.
They will show Mercury as a tiny black disc, smaller but darker than many sunspots, slowly traversing the Sun’s giant yellow disc.
Mercury spins around the Sun every 88 days, but its orbit is tilted relative to the Earth’s. It is that discrepancy which makes it relatively rare for the three bodies to line up in space.
From western Europe, north-western Africa and much of the Americas, Mercury’s seven-and-a-half-hour glide across the Sun will be visible in its entirety. A further swathe of the planet (see map below) will be able to see part of the transit, depending on local sunrise and sunset times.
The only land masses to miss out completely will be Australasia, far eastern Asia and Antarctica.
Because Mercury is so small – just one-third as big as Earth and, from our perspective, 1/150th of the Sun’s diameter – its transit can only be glimpsed under serious magnification; the “eclipse glasses” used by thousands of people to view last year’s solar eclipse will be useless.
And to avoid permanent eye damage, any telescope must be fitted with a solar filter before being trained on the Sun. The British Astronomical Association explains on its website how amateur stargazers can enjoy the spectacle safely.
Open University’s Prof David Rothery said the celestial event would not present any novel scientific opportunities – but was special nonetheless.
“From this transit, we’re unlikely to learn anything we don’t already know,” he told BBC Inside Science. “But what a wonderful event for showing people Mercury. It’s a hard planet to see.
“Historically, transits were of immense importance.”
In the 1700s, for example, it was observations of Mercury and Venus slipping across the Sun that allowed astronomers, led by Edmund Halley, to pin down the dimensions of the known Solar System.
Prof Rothery is a Mercury expert and a leading scientist on the European Space Agency’s BepiColombo mission to the diminutive planet, which will launch in 2017 or 2018.
Mercury has already been visited by two Nasa probes: Mariner 10 flew past in 1974 and 1975 and Messenger spent four years in orbit until its planned crash landing in 2015.
“[Messenger] told us an awful lot. It really told us we don’t understand Mercury – because there’s a lot of things which just don’t stack up,” Prof Rothery said.
“It’s an airless body, with lots of craters… But there’s been a long history of volcanic activity, fault activity – and the composition, that began to be revealed by Messenger, is weird.
“There’s very little iron at the surface but it must have a ginormous iron core, because it generates a magnetic field – which Venus, Mars and the Moon don’t.”