The elongations of Mercury 2007−2016, and the 2016 solar transit

2017 July 21

Richard McKim

Introduction
In early 2008 we summarised the best BAA Mercury observations for the 1978-2007 period, with collages of the highest resolution images and drawings available from our files. This paper acknowledges more recent efforts and summarises the period from late 2007 to the end of 2016. It would not have been worthwhile to analyse a shorter time period. The improvement in resolution since 2007 is sometimes noticeable in the submitted data. Ironically, with the completion of the mapping process by the Messenger spacecraft, we now receive fewer observations than in the past. Nevertheless, it is worth putting on record what the amateur astronomer can now resolve upon the little disk of Mercury.
In this paper we also discuss and illustrate the solar transit of 2016. Even better amateur images of Mercury’s albedo markings have been seen online from time to time, but in this paper we use only those contributed to the Section.

Observations of Mercury’s sunlit disk, 2007-2016
Observational circumstances
The difficulties in observing Mercury hardly need restating here: its small angular separation from the Sun coupled with its tiny angular diameter frustrate most observers’ attempts even to locate it, while low altitude and consequent poor seeing often prevent any useful observation being made. Furthermore, the observable part of any elongation is brief. But for many observers a nice wide-angle photograph, perhaps of some conjunction involving Mercury, represents an achievable goal: see Figures 1A and 1B.
Only a few elongations of Mercury per year are relatively favourable for observation: the BAA Handbook always gives full details. It is suggested that the writer’s previous Section note should be read for some details about the planet’s observational history. Since its publication, a number of other Section notes and relevant papers about telescopic observations of Mercury have appeared in the Journal.

Observers
Many of the observers (Table 1) will be recognisable for their regular work for other BAA planetary Sections. Most appearances in this list represent the result of chance opportunities for an occasional view, for few observers concentrate upon the planet. All observers provided images except those marked V (visual data only). Drawings and images were supplied by Adamoli, Bailey and Niechoy. An image by Boudreau was received via Melillo. Johnson contributed wide-angle sky photos and the other observers contributed telescopic observations at east or west elongations. We list the 2016 solar transit observers separately (Table 3).

Elongation data
In Table 2 we list the greatest elongation (GE) dates from 2007 Nov to 2016 Oct. In his Section Reports, the late J. Hedley Robinson found it suitable to summarise Mercury observations every two calendar years. (F. J. Melillo, who is the current Mercury Recorder for the ALPO, reports upon the work of his contributors annually.) The sheer number of these elongations is immediately apparent from the table. (The BAA Handbook also gives the times of superior conjunction.) We also indicate the number of contributors.
We have shown the availability of observational material in bold type. Each bold entry is accompanied in brackets first by the number of visual observers, followed by the number of imaging observers (e.g., (2/1)). A number of elongations, particularly the morning ones, went unobserved.

Observational results
It can readily be seen that most eastern elongations received observational coverage, but that an increasing number of western elongations were not observed. The majority of BAA work has always taken place at the more favourable Spring eastern elongation each year, although some observers such as Gray are obliged to concentrate upon western elongations due to local obstructions. The best-observed elongations in terms of volume of data were the eastern ones of 2008 May (bringing drawings from Adamoli, Frassati, Niechoy and Phelps, and images from the Ackermanns, Arditti, Kivits, Lomeli, Niechoy and Walker) and 2010 Apr (drawings by Adamoli, Bailey, Frassati, Grego and Niechoy, and images by the Ackermanns, Edwards, Ikemura, Kivits and Meredith). Understandably, some of the best quality images and drawings were obtained in the still morning air at western elongations. (continued…)

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