Skygazers are in for a treat on Monday

Irish sky watchers will have an opportunity to witness one of the rarest astronomical events: a transit of Mercury across the face of the Sun this Monday.

Because this transit lasts more than seven hours, the Earth will turn enough that parts of the transit will be visible nearly everywhere except for Australia, New Zealand, Indonesia, the Philippines and a small area of eastern Asia.

Because Mercury and Venus are closer to the sun than us, the two planets occasionally pass between Earth and the Sun in what is called a solar transit. This happens less than twice a century with Venus, and 13 or 14 times a century with Mercury.

The longest transit of Mercury this century begins at 12.12pm BST, with the planet exiting the Sun’s disk seven and a half hours later at 7.40pm on Monday. The next such transit visible from Ireland will be in November 2019 and then not until 2032.

Sky watchers will need a good telescope equipped with a proper solar filter to see Mercury silhouetted against the sun. That is because Mercury is very small compared to the Sun — only 1/160 the diameter of our star. (Warning: NEVER look directly at the sun without proper protection. Serious and permanent eye damage can result).

If you don’t own a good telescope equipped with a solar filter, don’t despair. Many astronomy clubs will be hosting public viewing events. Contact your local club or the Irish Federation of Astronomical Societies (IFAS) at www.irishastronomy.org or https://www.facebook.com/irishastronomy for further details.