Transits of Mercury

A transit (or passage) of the planet Mercury, pictured left, across the face of the Sun is quite a rare occurence: there are, on average, only 13 or 14 transits of Mercury per century. Due to the particular orientation of Mercury's orbit, these all fall within a few days of 8th May and 10th November: those in May are relatively less common (by a factor of two) but the planet's disc is 44% larger in area as its elliptical orbit takes it farther from the Sun (and therefore nearer the Earth) at that time. I didn't really get interested in observing a transit until that of May 2003, so I was lucky to have had the optimum conditions for observing the event as the previous May transit was back in 1970.

The relationships between the orbital periods of Mercury and the Earth mean that successive transits are separated by periods of 3.5, 7, 9.5, 10 or 13 years (though not in a regular sequence). The next opportunity to see Mercury in transit after 2003 was in fact at the minimal interval afterwards, on 8th November 2006, but it was unobservable from the UK as it happened after the Sun had set. That on 9th May 2016 (i.e. after another 9.5yrs) was much better placed though, as it happened in the afternoon and early evening. I had clear skies for the first part of the transit but high cloud drifted in thereafter, reducing my opportunity for observation. However, despite some problems, I did get a good number of useful images. The next one, on 11th November 2019 (another 3.5yr interval), was of unusually long duration as Mercury passed very close to the centre of the Sun. Unfortunately, from the UK sunset occured well before the end of the transit and weather conditions were not good. I was therefore only able to get a few images, but just enough to make it worthwhile.

Mercury's disc is so small (only ¹/160 of the apparent diameter of the Sun in May and just ¹/192 in November) that optical aid is required to observe it during a transit. Naturally, this must not be done by direct vision as even the slightest glimpse of the Sun through a telescope or binoculars can permanently damage eyesight. Although a dark filter can be used, the best and safest method is projection. In this technique, the image of the Sun is directed out of the telescope eyepiece onto a card held some distance away. Not only does this avoid any possibility of eye damage, it also provides an image of sufficient size for easy observation by several people. The card must, of course, be shaded as the image is not very bright: this can be achieved by pointing the telescope barrel out through heavy curtains, for example. A further advantage of projection is that a small telescope will give an excellent result - my old 60cm refractor is perfectly adequate and much easier to handle than a larger instrument.

Safety aside, projection was in any case really the only practical method to obtain images in 2003, as the large lenses necessary to achieve adequate magnification for direct photography were very expensive at that time. However, the advent of digital cameras, and in particular "super-zoom" bridge models, made things much easier in 2016 and 2019. A filter is still needed of course, but the ability to use a 1365mm lens only a few inches long (as I did) was a great asset.

Just after the 2003 transit described above, the first transit of Venus since 1882 occurred on 8th June 2004. This was much easier to observe, as Venus is considerably larger than Mercury and much nearer the Earth and in fact it was visible to the naked eye (using suitable protection, of course) so direct photography was possible. I used two telescopes however: a refractor to project the image (as per the Mercury transit) and a reflector fitted with a digital webcam. To see how I got on, click here.