Planet Mercury has always been one of the least-observed of the major planets, ranked with the outer Ice-Giants, Uranus and Neptune when it comes to lack of observations. In many ways this is understandable; Mercury’s orbit is much closer to the Sun than Earth’s so it spends most of its time in the daylight sky, only becoming visible in twilight skies at certain times near sunrise or sunset.
When, from our point of view on Earth, Mercury sits as far to the west of the Sun as it can we call it at greatest western elongation, and this always occurs in the morning sky with Mercury rising before the Sun. Conversely greatest eastern elongation occurs in the evening with Mercury trailing behind, to the east of the Sun and setting after it in evening twilight. At these times Mercury never appears much above the horizon, especially from the extremes of the northern or southern hemisphere, so the UK is never well placed to observe it, however modern equipment and observing techniques in the hands of talented amateurs have started to reveal some excellent detail on this small and illusive planet.
For a brief period in late February and early March Mercury was at its greatest eastern, evening, elongation and could be observed shortly after sunset at up to around 16 degrees above the western horizon; the actual altitude depends on latitude with those further south getting a slightly improved view. At this low altitude Mercury has to be observed through a great depth of our own atmosphere and therefore requires very steady air (good ‘seeing’ conditions) to be observed in detail. In addition the low altitude leads to atmospheric dispersion effects with red and blue light being bent at differing angles by its passage through our atmosphere. This makes it difficult to get a sharp focus and a clean view of the target.
Amateurs can use a number of techniques to improve the view, particularly when making photographic observations. The most common of these is to use planetary video cameras to capture a long movie of the target rather than individual snapshots. Software can then be used to sort through the thousands of frames of video in order to pick out those taken when atmospheric conditions were at their best. These can be averaged together in a process known as ‘stacking’ to produce a final clean image of much higher quality than any individual frame in the captured video.
Red or infra red light is much less dispersed on passing through our atmosphere than blue light, so by using a video camera that is sensitive to the deep-red or infra red part of the spectrum and appropriate filters it is possible to cut through some of the effects of poor seeing conditions and improve the quality of the final image. Finally use of a device known as an Atmospheric Dispersion Corrector (ADC) can noticeably improve the image still further. An ADC is about the size of an average telescope eyepiece, sits between the telescope and the camera and contains rotatable prisms that can be turned to tune out the effects of atmospheric dispersion.
Which brings us to the results obtained by members of the planetary section of the SPA during the recent greatest eastern, evening, elongation of Mercury. Simon Kidd, Martin Lewis and Dave Finnigan captured Mercury on the 23rd, 24th, 25th and 27th of February using exactly the techniques described above. Software simulations of Mercury for the dates and times of the captured images can be placed alongside the actual observations and striking similarities are immediately obvious. Major Mercurian features can be identified proving the capture and processing techniques are valid. I hope that this work done by SPA members will encourage others to attempt observations of Mercury during its next elongation, a morning elongation on the 23rd of June this year.