Thirteen times each century the inner planet Mercury appears, from Earth, to cross the disc of the Sun. This is known as a transit. It last happened on 9 May 2016 and it will happen again on 11 November 2019 for the last time until 2032.
A transit is nowhere near as spectacular as a total solar eclipse, and can’t be seen with the naked eye, so why should anyone make the effort to see it?
Here’s everything you need to know about one of astronomy’s rarest predictable events.
What is a transit of Mercury?
A transit across the Sun takes place when an inner planet passes directly between the Sun and Earth.
The Moon totally eclipses the Sun every 18 months or so, but transits of the inner planets Mercury and Venus are much rarer.
Transits of Mercury are much more common, happening 13 times each century, always in May or November, but there’s a 13-year wait after this one.
Global Visibility of the Transit of Mercury on 2019 Nov 11. Credit: Fred Espenek & www.eclipsewise.com
What can be seen during a Transit of Mercury?
The tiny black disc of Mercury, just 10 arc seconds across, will take about five and a half hours to pass across the Sun. It will cross from east to west.
Why is a transit of Mercury interesting?
The transit of Venus is famous for helping scientists figure out the Astronomical Unit – the average distance from Earth to the Sun – by measuring where Venus appeared to be on the Sun’s disc at the same time from different places on Earth.
Above: the 2016 Mercury transit in 4K video. Credit: NASA Goddard
How to observe the transit of Mercury safely
It’s extremely dangerous to look at the Sun, and even more dangerous to do so through an unprotected telescope.
“Mercury is small, so this is not going to be like a total solar eclipse where you can watch the central phase, totality, without any special equipment,” says Massey.
“A pair of eclipse glasses won’t help because Mercury has too small an angular size, but if you project its image through a tripod-mounted telescope or binoculars onto a piece of white card you will see the disc of the Sun and Mercury crossing it as a speck.”
Another option is to put a solar filter on the front of a telescope and observe as normal through the eyepiece, which will get you greater detail.
Composite image of Mercury transiting across the sun on 9 May, 2016, as seen by HMI on NASA’s Solar Dynamics Obeservatory. Credit: NASA/SDO
Where’s the best place to watch the transit of Mercury?
Those who travel to see a transit of Mercury do so primarily for a higher chance of clear weather, but also to watch the entire event from start to finish, as shown by this interactive Google Map.
“You can see that all of the transit is visible from the Atacama desert in northern Chile and western Africa where the weather prospects are the best in November,” says Xavier Jubier, a member of the IAU Working Group on Solar Eclipses.
“This is why I will likely observe from the Canary Islands or else from the Atacama desert in northern Chile.”
However, we’re talking independent travel; don’t expect special ‘transit trips’ to be organised.
“Mercury transits don’t have the same kind of pull as eclipses, so as far as I know there aren’t special trips to see them, not least because they are visible over a large area,” says Massey.
Wherever you watch from, the transit of Mercury – the last until 2032 – promises to be a special astronomical event.
How to see the Mercury transit from the UK
Although it’s not a prime viewing spot, the transit of Mercury will be viewable from the UK during sunset.
“It’s happening at noon going forward until sunset, so the prospects are not terrible, but the Sun is rather low in the sky during November and, of course, the weather prospects are not as good as during May,” says Massey.
Local Mercury transit timings for the UK. Credit: Xavier Jubier /xjubier.free.fr/Google Maps
In the UK on 11 November 2019, the event will be begin at lunchtime and still be ongoing as the Sun sets, but the Sun will never be higher than 20° above the horizon.
However, if you want only a glimpse of the event, it will be possible … clear skies allowing.
The best places around the world to watch the transit of Mercury
The entire transit is viewable from South America, with eastern North America and West Africa also well placed.
However, likely cloud cover make some places better than others.
Mercury eclipse timings for San Pedro de Atacama. Credit: Xavier Jubier /xjubier.free.fr/Google Maps
Atacama Desert, Chile
Entire transit visible from 09:35-15:03
As well as having a view of the entire event and a high chance of clear skies, there are many boutique observatories around the small town of San Pedro de Atacama that will almost certainly be setting-up sun-scopes.