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Observers in Northern Ireland will have an opportunity, weather permitting, to observe a relatively rare astronomical event on Wednesday, 7th May, when a transit of Mercury will take place. Transits of Mercury occur when the planet Mercury, the closest planet to the Sun, 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 period of a few hours.

Transits of Mercury occur on average thirteen times a century. There was a transit of Mercury in 1999 for viewers in north America. However, the last transit visible from Northern Ireland was in 1973. This year's event will be visible throughout Europe, Africa and Asia. The next transit of Mercury after this May will take place on 8 November 2006, but it will not be visible from the longitude of Northern Ireland. We will have to wait until 9 May 2016 for another opportunity to see Mercury passing in front of the Sun's disc from Europe. However, we will have an opportunity to witness an even rarer transit of the planet Venus on 8 June 2004.

The transit begins at 6:12a.m. as the edge of Mercury appears to touch the edge of the northeast quadrant of the Sun. The transit ends at 11:31a.m. as Mercury emerges in the northwest section of the Sun's disc. Mercury passes closest to the Sun's centre at 8:52a.m. at which time it will be at a distance of about one-third the solar diameter from the Sun's centre. Mercury, a cratered world with a diameter of 3,025 miles, will be only about 1/158th the apparent diameter of the Sun and will appear similar in size to a small sunspot.

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, as irreparable eye damage can result.

Mercury, which makes one revolution about the Sun every 88 days, is currently situated low in the northwest during evening twilight.

See also:
Webcast from Armagh Observatory

John McFarland at the Armagh Observatory,
College Hill, Armagh, BT61 9DG.
Tel.: 028-3752-2928; FAX: 028-3755-7174;

Last Revised: 2003 May 2nd
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