A very special few weeks to catch elusive Mercury

Many people, including astronomers, go through their entire lives without ever seeing Mercury, the closest planet to the Sun. But this week offers a great chance to glimpse it in the evening sky and there is a chance to see it in a more unusual way in May when it passes directly in front of the Sun.

Mercury on a previous appearance in the evening sky, from Sandwich, Kent. Image credit: Paul Sutherland

The reason Mercury is hard to see is because its proximity to the Sun means it never shines in a very dark sky, and only comes out when relatively low over the horizon. The best time to look is when it is at its furthest from the Sun – a point known as an elongation. At its greatest elongations east, Mercury follows the Sun down to the horizon and so appears in the evening sky. At greatest elongations west, it rises before the Sun in the morning.

In the northern hemisphere, the plane of the Earth’s orbit, along which all the planets roughly travel, is steeply inclined to the horizon during spring evenings. This has the effect of allowing Mercury to appear higher in the sky than it would at other times of the year. (For pre-dawn appearances, fall, or autumn, is the best time).

Because Mercury has a very elliptical orbit around the Sun, its apparent distance from it at an elongation can vary from 18° to 28°. From mid-northern latitudes, on this occasion, Mercury will be about 17° above the western horizon when the Sun sets on 18 May, so keep an eye out after that to catch it glinting at about magnitude -2 after that as soon as the sky darkens enough. You should be able to find it for a few nights afterwards too, as long as you have a clear, unobstructed horizon.

Mercury in the evening sky in April. Dates are for 20h UT. Chart by Paul Sutherland, using Skychart.

The writer spotted Mercury on the evening of 16 April from the village of Guston, near Dover, Kent. Once found it was unmistakable, but of course magnitude -2 looks nothing like as bright in the twilight as it would in the darkness of full night!

After we lose Mercury back into the Sun’s glare we will have to wait until 9 May when it makes a relatively rare transit across the face of the Sun. Although much more common than transits of Venus, Mercury only performs this passage about 13 times a century. The last was in 2006. At other times, because the orbits of Mercury and the Earth are inclined to each other, Mercury passes either above or below the Sun.

A transit of Mercury in 2003 imaged by the Soho solar observatory in space. Image credit: SOHO (ESA & NASA)

This transit begins shortly after 11.12 UT as Mercury begins to come onto the solar disc. It then spends seven and a half hours crossing the disc before leaving it completely just after 18.42 UT. Unlike a solar eclipse (which is really a transit of the Moon) the transit of Mercury can be seen from anywhere that has the Sun above the horizon at the time. Mercury is so far away that its position will appear almost exactly the same from any part of the Earth.

Viewing the Transit of Mercury will not be easy if you don’t have the right equipment. But the dangers of looking at the Sun will be just as real, so do not try. Unfortunately, those eclipse glasses with their shiny foil will not help because Mercury will be too small to see with the unaided eye.

The traditional way is to project an image of the Sun’s disc through a telescope and onto a sheet of white paper or card, making sure not to look through the instrument. Cover the telescope’s finder too, so that it does not focus the Sun’s light and start a fire. The growing use of plastics in telescope construction mean that the heat from the Sun within the optical assembly could melt the plastic somewhere. It is better therefore to use an approved filter which must fit completely over the end of the telescope tube where the Sun’s rays enter. It is possible to buy specialist glass filters to fit particular telescopes, but you can also make one by purchasing a sheet of Baader AstroSolar Safety Film. Find out more here: http://astrosolar.com/en/

Homemade alternatives such as sweet foil or CDs will NOT do. They could let through invisible radiation that could permanently damage your sight, so stick to the authorised products. Don’t risk blindness!

An excellent guide to the Transit of Mercury, presented by solar scientist Professor Lucie Green, has been produced by the Society for Popular Astronomy, a friendly UK-based organisation that is specially geared to helping beginners and non-experts in astronomy. It is an organisation well worth joining for the very reasonable price of an annual subscription. See the SPA’s transit page here.

The Transit of Mercury – 9 May 2016 from Society for Popular Astronomy on Vimeo.


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