Observers in Ireland and Great Britain will have an opportunity, weather permitting, to observe a rare astronomical event on Monday 11th November, when the planet Mercury will transit across the Sun. Transits occur when a planet lies directly between the Earth and the Sun. With suitable observing equipment, Mercury may be viewed as a small black dot crossing the face of the Sun over a 4-hr period. 

Viewing the Sun through AOP’s historical Grubb refractor during the May 2016 transit of Mercury. Rok Nezic is in attendance. Image Credit: Apostolos Christou

Transits of Mercury occur on average thirteen times a century. Because of the relative orientation of the orbits of Mercury and the Earth at the present era [see diagram here], transits can only occur in May or November. May transits offer better weather prospects and have the advantage that Mercury is near the farthest point of its orbit to the Sun. As a result, it appears slightly larger, by some 20%, against the solar disk than during a November transit. 

The last transit visible from Northern Ireland was in May 2016. The 2019 event, while visible in its entirety from the Americas, will only be partly visible from Europe with the transit still in progress at sunset. Nonetheless, missing this transit will mean that one has to wait 13 years, until 2032, to see another one, also in November. The next May transit will, in fact, not occur until 7 May 2049! 

Image of the Sun obtained at AOP during the 2016 transit of Mercury. The planet is the small black dot just below centre. The dark smudge slightly above centre is a sunspot. Image Credit: Apostolos Christou

The 2019 transit begins at 1235 GMT, as Mercury appears to touch the edge of the Sun, and ends at 1804 GMT with Mercury exiting the Sun’s disc. This is, however, interrupted by sunset which occurs at 1633 GMT in Armagh and at 1629 GMT in Belfast. Mercury is closest to the Sun’s centre (greatest transit) at 1520 GMT, about an hour before sunset and at a distance of 76 seconds of arc. This is equal to the apparent size of a lunar crater as viewed from the Earth and the closest for any future transit until that of 12 November 2190, in 171 years’ time! Mercury, a cratered world with a diameter of 3,025 miles, will be only about 1/194th the apparent diameter of the Sun and will appear similar in size to a small sunspot.

Interestingly,  this small planet will come under close scrutiny in 2025 when BepiColombo, a joint mission between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) launched in October 2018, will be inserted into orbit around the planet and make scientific measurements during a one-year nominal mission. 

View this spectacle by projecting an image of the Sun through a telescope onto a white card held about one foot beyond the eyepiece end of the telescope. Never look directly at the sun, with or without a telescope or even binoculars, as irreparable eye damage can result

Mercury, which makes one revolution about the Sun every 88 days, is currently close to a much brighter Venus but practically unobservable, setting only 20 min after the Sun. 

AOP In-House Public Event: On Monday, 11th November 2019, the planet Mercury will transit the Sun for the first time since 2016. The transit or passage of a planet across the face of the Sun is a relatively rare occurrence. As seen from Earth, only transits of Mercury and Venus are possible. There are approximately 13 transits of Mercury each century!

In 2016 we opened our doors to view the transit, and this year we would like to do the same! Come in and see the Observatory Boardroom, our historic clocks and a rare opportunity to see the telescope used by King George III to observe the 1769 transit of Venus.  Then head out to the Human Orrery and use our telescopes to view the face of the Sun. Also grab the chance to use our 19th Century, Grubb telescope, that was used to compile the famous New General Catalogue of Nebulae and Clusters of Stars in 1888, to see the transit in action. 

The transit starts at 12.30pm and will be at its greatest at at 3pm and continues until sunset. 

Links:

2019 Transit of Mercury info by Fred Espenak

Transit of Mercury FAQs at the Royal Museums Greenwich

Transit of Mercury Viewing and Exhibition at AOP


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