Armagh Observatory, 29th April 2016: Observers in Northern Ireland will have an opportunity, weather permitting, to observe a rare astronomical event on Monday 9th May 2016, when a transit of Mercury will take place. Transits of Mercury occur when the planet Mercury, the closest planet to the Sun, passes in its orbit directly between the Earth and the Sun. With suitable observing equipment Mercury can then be viewed as a small black dot crossing the face of the Sun over a period of several hours. All observers are advised that it is NEVER safe to look at the Sun without proper eye protection. Failure to use proper observing methods can result in permanent eye damage and severe retinal loss.

Transits of Mercury can only occur in May or November, and occur on average approximately 13 times per century. Apart from offering better weather prospects (the Sun is more likely to be visible in May than November!), May transits also have the advantage that Mercury lies close to the farthest point of its elliptical orbit around the Sun. As a result, Mercury is slightly closer to Earth during a May transit and so appears slightly larger, by some 20%, against the solar disc than during a November transit.

There was a transit of Mercury visible from North America in November 2006, but the last transit visible from Northern Ireland was on 7th May 2003. This year´s event, which will be visible in its entirety from Northern Ireland, will also be visible in part throughout Europe, Africa the Americas and most of Asia. The next transit of Mercury will take place on 11th November 2019, but this will only be briefly visible from Northern Ireland for a short time before sunset. The next May transit visible from Northern Ireland will not occur until 7th May 2049, that is, in 33 years´ time.

This year's transit begins at approximately 12:12 (BST), when the edge of Mercury appears to touch the edge of the eastern quadrant of the Sun. The transit ends at 19:42 (BST) as Mercury emerges from the southwest section of the Sun´s disc, with both objects sitting at a comfortable 10 degrees above the horizon. Mid-transit occurs around 15:57 (BST), at which point the planet will lie at about one-sixth the solar diameter from the Sun´s centre. Mercury, a cratered world with a diameter slightly less than 5,000 kilometres, will be 1/157th the apparent diameter of the Sun and will appear similar in size to a small sunspot.

Interestingly, our knowledge of this small planet has increased manifold since the 2003 transit owing to its coming under close scrutiny by NASA's "Messenger" probe, the first spacecraft to orbit Mercury, between 2008 and 2015. Mercury is also due to receive its next visitor in 2024 when "BepiColombo", a joint mission between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA), now scheduled for launch in 2017, will be inserted into orbit around the planet and make scientific measurements during a one-year nominal mission.

Astronomers at Armagh Observatory will be observing the transit of Mercury by the method of projection, weather permitting, and there will be a small exhibition open to visitors from 14:00 to 17:00.

Those who may wish to visit the Observatory to view the exhibition and participate in a free tour are advised to write, telephone or e-mail: Mrs Aileen McKee, Armagh Observatory, College Hill, Armagh, BT61 9DG; Tel: 028-3752-2928; e-mail:

FOR FURTHER INFORMATION PLEASE CONTACT: Mark Bailey or Apostolos Christou at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; E-mail:; URL:

Live Webcast Links

Web Links
Amazing transit of Mercury, latest images from Armagh Observatory
The 2016 Transit of Mercury in detail by Fred Espenak (EclipseWise.Com)
2016 Transit page by the British Astronomical Association
The 2016 transit at Paris Observatory (in french)
Article on the 2016 transit in Sky and Telescope
2003 Mercury Transit at Armagh Observatory
Mercury Transits on Wikipedia
Transit of Mercury resources at the National Space Centre in Leicester
ToM resources incl images and animations from past transits by Jay Pasachoff

Last Revised: 2016 May 12th