Ordinary Meeting, 2009 December 12

 

Meteors, Showers and Streams: George Alcock's Legacy

Dr Mason remarked that this was a timely opportunity to reflect on George Alcock's meteor-observing work, undertaken exclusively in the formative early years of his observing career, because the next few days promised to provide one of the best meteor displays seen in UK skies for many decades – the Geminid meteor shower – due to reach maximum over the coming three nights of December 12-14. He added that despite the ground-breaking nature of Alcock's meteor observations, they were often, perhaps understandably, remembered only as a footnote among the outstanding achievements for which he was better known: the discovery of five comets and six novae. For those later accomplishments, he had been awarded the Association's Merlin Medal on no fewer than three occasions. But the speaker explained that he would seek to shed some light on the ways in which his early meteor-observing work may have nurtured the acuity and incredible memory which were to become Alcock's hallmark in later years.

Dr Mason explained that Alcock had had the good fortune to observe two solar eclipses in his childhood – the partial eclipse of 1921 and the total eclipse of 1929 – though he had been clouded out on the latter occasion. However, if a single event had sparked his interest in astronomy, that seemed not to have been those eclipses, but rather the sight, witnessed by chance at the age of 18 whilst walking across Town Bridge in Peterborough, of a meteor fireball crossing the sky on the night of 1930 November 30. He had found that sight so striking that he had written a detailed account of it and submitted his observation to the then Director of the BAA Meteor Section, J.P. Manning Prentice, even though he had had no previous involvement with the Association.

Dr Mason explained that Prentice had been so impressed by the diligent observation that he had received that he had gone on to urge Alcock to attend the Section's annual meeting on 1931 July 4 – the very meeting at which the death of W.F. Denning was to be reported. Alcock's attendance at that meeting had marked the beginning of a regular correspondence between the two men, in which Alcock had regularly submitted his observations to Prentice in the form of handwritten letters, and Prentice had replied, often in the form of several pages of closely typed foolscap paper. Dr Mason observed that it was fortunate that Alcock had responded well to criticism, as if there was ever any suggestion that his observations had fallen short of the highest standards, Prentice could prove remarkably scathing in his response. The speaker wondered to what extent this stern mentoring hand had inspired Alcock to excel in later life.

Turning to describe the observing project into which Prentice had recruited Alcock, Dr Mason explained that one of the most interesting outstanding unknowns of meteor astronomy in the early 1930s had been the height within the atmosphere at which meteors tended to burn up. Within the BAA, the Meteor Section had made enormous contributions to meteor science thanks to the work of Denning and A. Grace Cooke, now entering her tenth decade of life, but Prentice had realised the pressing need for a new generation of younger observers. He had very quickly spotted the great potential of a partnership between Alcock and himself: the 60-mile separation between their respective homes in Peterborough and Stowmarket would allow them to triangulate the exact three-dimensional positions of any meteors which were seen simultaneously from both locations, because the meteors would appear to the two observers in differing parts of the sky. The pair had carefully arranged the times when they would both go out to observe, and had afterwards brought their observations together and trawled through them for what they called concordances – records of meteors which they had both observed at exactly the same moment.

Over the following 20 years, this work had proven so successful that, at the end of that time, around 80% of the total of 1,000 heights which had been calculated worldwide for sporadic meteor trails had been derived from observations by Prentice and Alcock. From that large body of data, it had become clear that meteor trails typically became visible at an altitude of between 80 and 110 km and that they typically burnt out at altitudes of between 50 and 80 km. By the early 1950s, Alcock and Prentice's technique of triangulation was beginning to be superseded by the use of radio-frequency radar which could detect the reflection of radio waves from the ionised material left in the wake of meteor trails; Alcock would, in fact, help to pioneer that new method by offering advice to the Jodrell Bank Observatory for a short time. In the more recent modern era of automated photographic astronomy, however, the triangulation method had seen a modest revival in the hands of Steve Evans (1953–2008) amongst others, who had had some success in triangulating the exact positions of meteors by recording images of their trails from cameras placed at widely spaced locations.

Turning to look at the relevance of these observations to Alcock's later and better known discoveries, the speaker noted one similarity in particular. He explained that Prentice had been particularly anxious about the amount of time spent with star charts after each meteor was seen, whilst the observer was busy recording his observation, and was keen that this observing downtime should be kept to a minimum. He had realised that the time spent recording the path of each meteor trail could be drastically reduced if, rather than drawing a line on a star chart, the observer committed to memory the designations of all of the brightest 3,000 naked-eye stars and recorded the path simply by naming naked-eye stars close to its start and end points in a notebook. Prentice's consequent insistence that Alcock should learn his way around the naked-eye sky to such a phenomenal degree must have stood him in good stead when Alcock had later taken it upon himself to learn the pattern made by over 30,000 stars in the binocular sky to aid his search for comets and novae.

The speaker closed with a discussion of the prospects for the meteor showers and storms of coming years. The imminent 2009 Geminids were an extremely promising prospect, due to reach maximum on December 13, two days before New Moon. The radiant, close to the star Castor, would rise to an altitude of 45° by 10pm, allowing for easy early-evening observation without the need to stay up late. A peak rate in excess of 110 ZHR, spread over more than one evening, was widely expected. In coming years, this shower looked set to outclass the Perseids and become the most spectacular of the annual meteor showers. Modelling by NASA's Meteoroid Environment Office, for example, predicted that its rate might grow to regularly exceed 200 ZHR within the next 20 years.

In 2010, however, the Geminids would be less favourably placed on account of reaching maximum with some interference from an eight-day-old Moon. Consequently, the Perseids were likely to be the year's best shower, reaching maximum in the early hours of August 13, only two days after New Moon, and reaching rates of perhaps 80 ZHR. Looking further ahead, the best prospect for a truly spectacular meteor storm in the next 50 years would perhaps be the Giacobinids of 2011 October 9, expected to reach maximum at sometime between 19h10 UT and 20h40 UT on that evening. The timing of their maximum meant that they would be best seen from eastern Europe or the Middle East, where it would coincide with local midnight. Despite its recurrence every 13 years, this was a storm which Alcock himself claimed never to have seen: the 1933 storm had not been predicted and other engagements had prevented him from observing that evening, and he had been clouded out for the 1946 storm. Other observers in 1933, however, had reported a rate in excess of 40 meteors/minute for a period of around an hour, and within that time, a peak rate of around 90 meteors/minute for around 10 minutes. In 1946, the observed rates had been only a fraction slower, peaking at around 70 meteors/minute. More recent displays, including those in 1985 and 1998, had been more modest but nonetheless spectacular, and so the speaker strongly recommended making an effort to travel to see the 2011 shower.

Following the applause, the President invited the evening's final speaker, Dr Stewart Moore, Director of the Association's Deep Sky Section, to present his sky notes.

Fairfield

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41.14°N
73.26°W
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