How to observe the transit
A rare transit of Mercury across the face of the Sun is visible from the UK on Monday 11 November 2019. Mercury starts to move across the face of the Sun just after 12:35 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 has been edited from the one 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 ÂŁ20. Do not be tempted to use other materials unless they are specifically designed for solar observing, as although they may appear dark they can transmit harmful infrared radiation which can cause serious eye damage without your being aware until it’s too late. So forget the shiny wrapping paper or coloured glass.
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 10 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.
The event begins at just after 12:35 pm GMT, when the tiny silhouette of Mercury moves onto the solar disc. It then moves pretty centrally across the face of the Sun, reaching its mid point at 15:19. At this time the Sun is just a few degrees above the horizon as seen from the UK, so observations after this will require a very low south-western horizon. Although the transit continues until after 6 pm, from the UK the sun sets some time after 4 pm (depending on your location – the sun sets at 4 pm from John o’ Groats and 4:44 pm from Land’s End, with other locations somewhere in between).
Choose your observing site with these times in mind. The Sun will be in roughly the same place in the sky in the afternoon several days previously, so you don’t need to wait for the day itself to find your best location. But don’t give it a miss and wait until the next transit comes around – it won’t be until 13 November 2032, when it’s a morning event, ending just after 11 am.
If you want to know more about Mercury itself and how to observe the transit, European Space Agency’s mission to the planet itself, previous missions and much more, visit the Open University’s excellent Mercury pages:
http://www.open.edu/openlearn/exploremercury
Transit of Mercury, 11 November 2019 from Society for Popular Astronomy on Vimeo.
Update added on 11 November
The Sun has now set in the UK, though the transit continues to be visible from points west. Weather conditions were mixed across the UK. Rain clouds moved away in southeast England, allowing Paul Sutherland to take a number of shots during the early part of the transit, through milky skies and with a low Sun.
Meanwhile, Robin Scagell travelled to warmer climes to catch the transit high in the sky from Trinidad and Tobago.
