Transit of Mercury, 11 November 2019: what it is and how to see it

The last transit of Mercury until 2032 occurs on 11 November 2019, but the best views will be from outside the UK. Here’s how to watch safely from wherever you are.

Mercury passing in front of the Sun captured in 2006 by the Solar Optical Telescope. Credit: Hinode JAXA/NASA/PPARC

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.

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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.

The last transit of Venus was 6 June 2012 and the next will take place on 11 December 2117. That’s one of the rarest astronomical events going.

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
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.

These days, transits are useful for astronomers hunting for other worlds around distant stars.

“You can use transits as exoplanet analogues,” says Dr Robert Massey, Deputy Executive Director at the Royal Astronomical Society and co-author of Moon: Art, Science, Culture.

“If you want to detect extrasolar planets using transit measurements, then seeing something in-situ in our own solar system provides a model.”

On 3 June 2014, NASA’s Mars rover Curiosity observed a transit of Mercury across the Sun, marking the first time a planetary transit has been observed from a celestial body besides 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.

Baader AstroSolar filter material costs around £20 for an A4 sheet. There are more tips on observing a Transit of Mercury safely here.

Find a public event and share the experience

The best and safest option is to attend a public event at an observatory, museum or science centre, or find an observing event organised by a local amateur astronomy group.

“They’ll be set up properly, they’ll have better equipment than you, and you’ll also get the camaraderie of being with a group of people who are interested in the same thing as you,” says Massey.

Lookout for details in your area or contact your local astronomy society nearer the time.

Composite image of Mercury transiting across the sun on 9 May, 2016, as seen by HMI on NASA's Solar Dynamics Obersvatory. Credit: NASA/SDO
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
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.

Here’s the timetable:

UK Mercury transit times:

  • 12:35 – Mercury touches the Sun’s edge
  • 15:19 – Mercury is nearest the Sun’s centre
  • 16:15 – Sun sets with transit ongoing

(All times from www.timeanddate.com)

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
Mercury eclipse timings for San Pedro de Atacama. Credit: Xavier Jubier /xjubier.free.fr/Google Maps
1

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.

Try Observatorio Alarkapin and Explora Atacama, both of which host visitors.

Mercury eclipse timings for Salar de Uyuni, Bolivia. Credit: Xavier Jubier /xjubier.free.fr/Google Maps
Mercury eclipse timings for Salar de Uyuni, Bolivia. Credit: Xavier Jubier /xjubier.free.fr/Google Maps
2

Salar de Uyuni, Bolivia

Entire transit visible from 08:35-14:03

An extension of the Atacama desert, southern Bolivia has super-dark and clear skies (the rainy season starts in January).

If you’ve ever wanted to explore the mesmerising Uyuni salt flats, the world’s largest, November would be a prime time.

Chimu Adventures organise car-driver-guide tours and are well used to the demands of astronomers and astrophotographers. Take your own telescope.

Mercury transit timings for the Canary Islands. Credit: Xavier Jubier /xjubier.free.fr/Google Maps
Mercury transit timings for the Canary Islands. Credit: Xavier Jubier /xjubier.free.fr/Google Maps
3

Canary Islands, Spain

Entire transit visible from 12:35-18:03

Slightly closer to home and easy to get cheap flights to, any of the Canary Islands – Tenerife, Fuerteventura, Gran Canaria, Lanzarote and La Palma – would be fine.

However, Tenerife has a huge solar observatory that does tours and, nearby in Teide National Park 2,152 m above sea level, the Parador de Cañadas del Teide hotel, which has its own telescopes.

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Jamie Carter is an astronomy and travel writer, and the author of A Stargazing Programme for Beginners