How to observe the transit
A rare transit of Mercury across the face of the Sun is visible from the UK on 11 November 2019. Mercury starts to move across the face of the Sun around 12:30 pm and continues throughout the afternoon, leaving the disc after sunset as seen from the UK. So how should you go about observing it? Watch the video recorded by Lucie Green, which was made for the 2016 transit, for more info.
Your quick guide to observing the transit
The Sun is so bright that even a glance at it through a telescope can blind you. So you need either a full-aperture filter of suitable material such as Baader AstroSolar on the front of your telescope, or to project the image onto a piece of white card. Baader AstroSolar filter material is available from many suppliers in A4 sheets for about £20.
If your telescope has a lens cap that has a smaller removable cap, all you need to do is to cover the inside of the small aperture with filter material, making sure that no chinks of light are visible around the edge and that it is well-secured. Otherwise you will have to make a cap that fits snugly, with no risk of it falling off.
With this in place, you can observe the Sun directly through the telescope, ideally using a magnification of about 50 or more. You’ll need such a magnification because Mercury is quite small – only 12 arc seconds across – so it will be only a tiny dot. This rules out naked-eye observation using eclipse glasses, and using ordinary binoculars with solar filters. In theory, those with good eyesight could just see Mercury with filtered 10 x binoculars, but it will be hard to spot. If you viewed the Transit of Venus in 2004, you will remember that Venus was quite noticeable against the face of the Sun. But, as the simulation at right shows, Mercury (shown above Venus) is much smaller.
The other method is to project the Sun’s image through your telescope onto a piece of white card or similar material. Binoculars might also give a large enough image in this case. But while the method is fairly safe, there is a risk that if you let the Sun’s image drift out of the field of view its image and heat will fall on the inner parts of the eyepiece. Many eyepieces have plastic interiors which can easily be damaged! The Solar Section site also has more information about observing the Sun in general.
If you want to know more about Mercury itself and how to observe the transit, European Space Agency plans to send a mission to the planet itself, previous missions and much more, visit the Open University’s excellent Mercury pages: