1999 August 14-16
New Hall, Cambridge
After months of planning and much hard work the participants for the second International Workshop on Cometary Astronomy began to assemble at New Hall, Cambridge on the afternoon and evening of Friday, August 13th. New Hall is one of the more recent Cambridge colleges and includes a centre built for Japanese students as well as accommodation for the graduate and undergraduate students. It is a women’s college and a few participants were later disturbed by the night porter doing his rounds and making sure that all ground floor windows were closed. A hearty dinner was provided, but afterwards I had to leave to continue last minute preparations for the morning.
On Saturday morning, Dan Green and Jon Shanklin made a few opening announcements. We had nine comet discoverers present and five continents were represented. The next meeting would take place in 4 – 5 years time, possibly in America. For most of the day the British Astronomical Association had a sales desk in the entrance foyer to New Hall, with a range of eclipse memorabilia on offer, as well as copies of cometary publications.
Don Machholz gave the opening talk about comet hunting. He used to live in a light polluted site and drove out to Lomo Prieta for comet searching. In 1990 he moved 180 miles to the small town of Colfax (pop 1000) and has since discovered five comets. He had searched 1000 hours since 1994 without a discovery. If the Edgar Wilson award had been in operation he would have netted an average of $4000 a year, though some years would be more rewarding and others less. His search technique is to scan east/west and move down in the morning sky. There are three conditions for success – you must look, the comet must be bright enough and you must find it first. His first three comets were closer to the sun than those previously discovered by amateurs in the previous 25 years. Type 1 comets are 30 – 60 deg from the Sun in the morning sky, bright, few in number and have small q. Type 2 lie in the evening sky 60 – 120 deg from the Sun, are dim, common and have large q. Most discoveries were from Japan, USA and Australia. Southern Hemisphere observers only discover southern declination comets, however northern hemisphere observers find them in both hemispheres. There is no significant trend in discovery declination. The average elongation is 70 deg. Most are 20 to 60 altitude and average 9.4 mag. They are slightly brighter closer to the sun, but not much. Some bright comets (6th mag) have been found far from the Sun. Amateurs average 3.3 per year, with an average of 368 hours per comet and a median of 177 [NH 433/228, SH 165/113]. Average q is 0.9.
Kesao Takimizawa addressed the meeting on Japanese comet discoverers. He had become interested in astronomy in 1966 aged 14, and had observed Ikeya-Seki. He had searched for 33 years and discovered 5 comets. The first Japanese comet discovery occurred in 1928. Honda had been very successful with 12 comets, followed by Ikeya and Seki. Several Japanese discoveries were made simultaneously by three or more observers. 57 visual Japanese discoverers have found comets, with 72 in total (14 photographic and one CCD). Most used reflectors or binoculars, and 113 different instruments had been used. He has used large binoculars for 15 years. A plot of discoveries showed that they were mostly morning, followed by evening and opposition, with gaps between opposition and quadrature, particularly in the evening sky. He showed prints of Japanese discoverers and mentioned a medal for Japanese discoverers.
After a break for refreshments came a panel discussion on hunting for comets, however I missed most of these whilst carrying out administrative duties. LINEAR doesn’t search within 90 deg of the sun, and there is no move to set up a Southern Hemisphere equivalent. The Edgar Wilson award was discussed, most panellists didn’t think it would make much difference, but it was nice to have the money. One negative aspect is the feeling of financial loss when someone else discovers a comet.
Michael Jager had made accidental discoveries as a result of photographing other comets. He had found a fragment of Machholz 2 (what should this be called if it turned out to be the main fragment?) and P/1998 U3 whilst photographing Harrington-Abel. He uses Schmidt cameras, the smaller reaching 14m and the larger 15m and has photographed over 150 comets. He thanked the discoverers for providing him with opportunities for photographing new comets. He began photographic work in 1982 and had failed to see comet Kohoutek.
We broke for a buffet lunch, which turned out to be another filling meal, with more than just bites on offer. After lunch Charles Morris spoke to the title "Why you don't get your papers published in the ICQ and other rants". He began by defining a rant as a heated one sided discussion. His topic was on amateur research. Many amateurs observe at a professional level. The ICQ would like to publish amateur papers, but their quality is often below the acceptable standard.
To demonstrate the problem that occurs with some amateur publications, he cited a situation at the last IWCA where informal exchange about his use of averted vision was then used as the basis of a paper to (incorrectly) discredit the Morris magnitude estimation method. This paper was published in an amateur comet publication and neither the author nor editor bothered to check the validity of the reference to the informal conversation. The ICQ tries to avoid such problems by using a referee review system.
As a side note, he pointed out that the Morris method actually integrates the Sidgwick and Bobrovnikoff methods. That is, when properly used these two methods are actually subsets of the Morris method. Morris then asserted (Charles Morris personal communication!) that using the Sidgwick technique for DC3+ or Bobrovnikoff method for DC7- will give biases brighter and fainter, respectively. In discussion Nick James pointed out that this assertion about the Morris method was not proven; Charles was not allowed to forget it for the rest of the meeting, though he commented that the underestimate of the Bobrovnikoff method was well documented. Joe Marcus later commented that the extensive work by the Dutch comet section did demonstrate the delta effect.
Morris noted that several groups want additional observational information to be published in the ICQ tabulations. However, prior to adding parameters, it must be shown that the information would be useful. This is the responsibility of the person/group proposing the parameters, not the ICQ staff. Adding additional parameters to the database, particularly for past observations, is non-trivial. It will be done (for instance, the changes made in the DC parameter after the last IWCA) when the change clearly improves the database.
Most amateur research requires statistics. Statistics are very important and you can't just assert a correlation. There is a difference between precision and accuracy and you need to quote errors. The delta effect may exist, but as r and delta are correlated, any delta effect study must not blindly use multiple regression analyses. (Morris has yet to be convinced that the delta effect has been proven.) Extrapolation also presents problems when observations only cover a limited magnitude range. Adding extra parameters, eg coma diameter, doesn't necessarily improve estimates or analyses of M1, particularly if the parameter is poorly defined. Morris' advice was to listen to reviewers comments, they should help to improve the paper, though reasoned argument can convince an editor that the reviewer is incorrect.
The next item was a panel discussion between Charles Morris, Jonathan Shanklin, Guy Hurst, Dan Green and Andreas Kammerer on the World Wide Web, the Internet and the influence on comet observing. Although there had been some feeling that the new media acted to bias observers, there was little demonstrable evidence. Guy Hurst made the point that the scatter in variable star estimates was typically no more than ±0.8 whilst the scatter in comet observations was often 2 magnitudes. When the extreme observations were queried the observers sometimes admitted that they were guestimates rather than actual observations. There might be a case for always including the actual magnitude estimate in reported observations, as is done with variable stars. Magnitude estimates of comet Hyakutake were quite discordant, with experienced and inexperienced observers making systematically different estimates. It was pointed out that the telephone had existed before the Internet and that it had always been possible to exchange information. Charles Morris said that he regarded the Internet as an educational tool, and the beginner observers would eventually become experienced.
Andreas Kammerer commented that the long tails reported for comet Hyakutake were physically impossible. The waxing moon coincided with the publication of IAUC 6360, which first cast doubts on these estimates, thus preventing further observations, which might have settled the question of influence on observers. The tail had shrunk significantly by the time the moon waned after closest approach. In his poster Andreas showed results of his investigations on this matter: the contentious observations can only be explained by the somewhat dubious assumption of a tail that must have deviated in step with the changing position of the earth.
After the discussion, tea was a little delayed and we took the opportunity for the first group photograph. Following the break Herman Mikuz explained his careful procedures for CCD photometry [I missed this talk whilst finding new supplies of poster pins and mains adaptors.] Nicolas Biver spoke on his work on the outgassing of carbon monoxide from distant comets. He concluded that there was a good correlation between visual magnitude and CO outgassing. He suggested that any comet brighter than 14th magnitude should be observable if CO drives the activity, even out to 30 AU.
A panel discussion between Charles Morris, Dan Green, Herman Mikuz, Nicolas Biver and for the last few minutes Jonathan Shanklin followed. One conclusion was that a group should be set up to discuss the issues of CCD photometry and set up standard procedures. Jon Shanklin commented that current ICQ coding didn’t include a code for the type of CCD chip being used in the photometry.
As the weather looked a bit threatening (we had heard thunder rolling around and a gust front had thrown up dust outside the college), a fleet of taxis took the participants the short drive to the Cambridge University Press bookshop in the centre of Cambridge. It should have been a short drive, but at least one taxi was sufficiently unfamiliar with Cambridge that they went to the Press Building on the other side of town. Here we were treated to a generous reception from the Press, and were able to purchase books at 20% of list price. Most participants managed to walk back to New Hall for dinner. By the end of dinner the storm clouds were retreating and Jon Shanklin took all those that were interested over to the University Observatory, a 20 minute walk from New Hall. Here we were able to use 20x80 binoculars to observe comet Lynn, the Thorrowgood refractor to observe comet 10P/Tempel 2 and the Northumberland refractor for a variety of deep sky objects. The two refractors are historic instruments, with the Northumberland first being used to observe comets over 150 years ago. Skies were very transparent and most observers spotted fragments of 109P/Swift-Tuttle blazing through our atmosphere. Observing finished around midnight, though we managed to loose at least a couple of observers on the walk back to New Hall. They were eventually retrieved and I stayed up till dawn at a dark sky site observing Perseid meteors and the other two comets visible in the morning sky.
On Sunday morning the participants were free to explore Cambridge and discussions resumed after lunch. Kay Williams introduced the legendary British observer, George Alcock. By way of background she revealed that her son Gareth had wanted to be an astronomer from the age of seven and had eventually gone to Cambridge, MA to work with Brian Marsden. At a dinner party Nancy Marsden had suggested that her next work should be a biography of George Alcock. She was a bit daunted by this as all her previous subjects had been dead! George’s work speaks for itself and includes history, architecture, ornithology, meteorology and astronomy as well as a lifetime in teaching. George is perhaps most famous for his comet and novae discoveries, but some of his comet drawings were also on display. George said a few words in response and sat down to a standing ovation.
George was followed by another comet discoverer, Kesao Takamizawa, who had been observing comets, variable stars, novae, supernovae etc since the 1960s. He now uses a 10-cm f4 astrograph, with limiting magnitude 15.5 (B) on T-Max 400 and a 25-cm f2.8 Baker-Schmidt with limiting magnitude 17. He sometimes visits a 1500-m altitude site in the mountains. He searches in 720 areas and checks for variable stars and minor planets using a PC. Over 5 years he has observed on 367 occasions, taken 16530 shots and discovered two comets, two novae, one supernova and 502 new variables. He changed from visual to photographic search techniques in 1994.
Jean-Claude Merlin spoke about his work at Le Creusot, IAU station 504, which has a 40-cm f5 reflector and CCD, with more than 120 clear nights a year. He averages 6 runs per month, taking two hours per run while observing two - five comets, with up to 30 second exposures. He has measured 900 positions since 1997, with an average accuracy of about 0.5". As a guide he suggested that Exposure time = pixel size/object speed. As an example if pixel size was 2" and the object was moving 30" per hour it would require a total of 4 minutes exposure. Looking at combined observations he had found systematic trends in the position residuals, for example 104P/Kowal 2 showed about a 160 day period. 73P/Schwassmann-Wachmann 3 showed larger residuals than average and he thought there might be a 75 day period to them. Questions suggested that these might be related to poor orbital determinations, or that the non-gravitational parameters didn't quite match reality. However the possibility of a true physical phenomenon is not completely ruled out (for example precession effect on a small nucleus, such as in the case of comet 104P/Kowal 2).
Bob Neville gave a very enthusiastic talk about the need to use CCD equipment to make positional measurements. His own set-up at 967 Greens Norton was entirely homemade. He used a 30-cm guide scope to a 22-cm reflector with a Starlight Xpress SX CCD. The system allowed for offset guidance to about 1 degree. The telescope had a roller drive in RA, which was very smooth and gave symmetrical star images. For reduction he used Astrometrica and the USNO catalogue which gives dense coverage, with Guide 6 or Megastar for finder charts. ACLOCK (share/freeware) provided LST. Observers need to use short exposures to avoid saturation. Sometimes poor seeing can actually help as it fuzzes out images; it is possible to make worthwhile observations even in poor environments. Nicolas Biver commented that speedy astrometry to provide good orbits is essential to help professionals target radio telescopes.
During the break for tea we had a group photograph and managed to capture 11 discoverers on film, namely: George Alcock, Doug Biesecker (SOHO), Kazimieris Cernis, Alan Hale, Michael Jager, Bill Liller, Don Machholz, David Seargent, Patrick Stonehouse, Kesao Takamizawa and Keith Tritton who between them had discovered 28 comets and many SOHO comets. Research in Sky & Tel showed that seven discoverers who at that time had discovered 15 comets had been present at an RTM meeting in 1990 [David Levy (6), Jean Mueller (1), Don Machholz (4), Clyde Tombaugh (1?), William Sorrels (1), Doug Berger (1) and Jeff Phinney (1)].
Before we resumed the formal sessions Charles Morris took centre stage and confessed that it was time to honour his bet with Alan Hale that his comet wouldn’t become brighter than 0 mag. It did, so Alan received 10 one dollar bills. Our first speaker after tea was Doug Biesecker, a member of the SOHO LASCO team, which has discovered a large number of sungrazing comet fragments. They are all remains of a single progenitor, which had a period of around 800 years, with a highly inclined orbit (and therefore not affected much by Jupiter). The date of the original progenitor is not known and multiple fragmentation has occurred. The most famous member of the familly is Ikeya-Seki.
SOHO orbits at the L1 Lagrangian point between the earth and sun. The LASCO C2 camera has an orange filter with a bandwith of 100nm and views the region from 2.5 to 6 solar radii with a resolution of 13". The C3 camera has an orange/clear filter with a bandwith of 300nm and views from 4 to 30 solar radii with a resolution of 56". They offer 360 deg coverage round the sun, with C3 taking about 1 frame an hour and C2 2 frames an hour. The camera support pylon hides the track of typical sungrazers between March and April. Before LASCO about 10 sungrazers had been discovered from the ground between –371 and 1970. Six were discovered by Solwind between 1979 and 1984 and 10 by SMM between 1980 and 1989. SOHO has now discovered 78 comets (updated to 79 that evening); it finds about 1.9 comets a month, when corrected for the duty cycle. Of a subset of 53 Kreutz group comets, 33 were seen in C2 and 52 in C3. It has not been possible to compute an orbit for SOHO-45, though an mpeg loop shown later clearly showed the object. There was some speculation as to whether the object was a sungrazer or earth approacher. The team are getting better at visual detection, but are still running the automated search program. This will only detect potential Kreutz group members, so there could be other faint comets being missed. The comets show fairly slow motion, and ‘disc’ like ones are difficult to spot, and may only be seen in a few frames.
Most of the comets brighten as 10 log r, but after a certain point fade quickly. Magnitude scales are not well calibrated on SOHO, partly because solar physicists require less accuracy than comet observers do. There is a problem with vignetting and this makes reduction of the C2 and C3 magnitudes uncertain. At T-20 hours the median magnitude is around 8, with the brightest 1st magnitude and the faintest 10th. Most stop brightening 6 – 12 hours before perihelion, which implies a fairly narrow range of sizes. No comets have shown tail features, and none have been observed closer than 3 solar radii.
The SOHO spectrometer had observed two comets. Lyman alpha emissions give an upper limit to the solar wind velocity of 640 km/sec. The comets suffer a 20 kg/sec mass loss. A body 6.7-m in diameter would weigh 120,000 kg and evaporate in about 5 hours.
Brian G. Marsden, (Harvard-Smithonian Center for Astrophysics) provided a synopsis of his talk on 'Discoveries, astrometry, catalogues and awards'.
This month we are celebrating the fortieth anniversary of George Alcock's discovery of what were announced on the IAU Circulars at the time as "Comet Alcock (1959e)" and "Comet Alcock (1959f)". Following the tradition, the year/letter designations supplied in order of discovery announcement were later changed to 1959 IV and 1959 VI, showing the order of passage of the year's comets through perihelion. In terms of the new system introduced in 1994 the announcements would have involved the single appellations "Comet C/1959 Q1 (Alcock)" and "Comet C/1959 Q2 (Alcock)", the 1 and 2 indicating the order of announcement of discoveries in half-month "Q" of the year, i.e., the second half of August. Although we intended no disrespect, some astronomers have been condemning the IAU Circulars for this "new" procedure of placing the name of the discoverer, rather than the designation, in parentheses. As it happens, this procedure is not new at all--early IAU Circulars speak of "Comet 1922c (Baade)", for example--and since "Comet Alcock" is not by itself a unique form of address, it is surely more logical to state the unique designator for the comet first, backing it up with the additional information identifying the discoverer. The parenthetical use of the discoverer's name was for many decades also standard use in the Astronomische Nachrichten, the principal international source for information about discoveries, astrometry and orbits of comets prior to the first IAU Circular. The discoverer's name, and sometimes also the date of discovery, were specified in this way, even in cases when the year/letter designations were not used and the Roman numeral designations had not yet been supplied. "Comet 1889 ... (Barnard 1888 Sept. 2)", already recognizing the year in which the object would pass perihelion, uniquely defined the comet that later became 1889 I, that was from the start defined in some publications as 1888e, and that we now know as C/1888 R1 (Barnard).
Since two earlier speakers have provided admirable accounts of their astrometric activities, there is little I need add, except perhaps to point out that it was not always this way. Modern CCD astrometry has turned out to be a much more automatic, accurate, reliable, rapid and straightforward process than the older astrometric processes involving photography and micrometry.
Although I am happy to announce that the thirteenth edition of the Central Bureau for Astronomical Telegrams/Minor Planet Center "Catalogue of Cometary Orbits" has just now become available, I again want to stress that by far the best and most detailed such catalogue ever published is that by Galle of 1894, which has the sole disadvantage that it is just very much out of date! One feature of the 1894 catalogue is that it defined the 15 multiple-apparition comets as (H) = Halley, (E) = Encke, ..., (Wo) = Wolf, (Fi) = Finlay. Actually, these periodic comets are the only named objects in the catalogue. Since there are now 140 such comets, it might have been preferable if Galle had instead given them consecutive numbers. The 1994 designation system has taken care of this by calling them 1P, 2P, etc. As many as 55 comets were discovered during the twelve months preceding the end of July 1999. Although 22 and 14 of these were discovered by the very automated LINEAR and SOHO projects, respectively, even the remaining activity was prodigious in comparison with a year as recent as 1971, which yielded only a single discovery. Cataloguing new discoveries has been complicated by the fact that, more often than not, LINEAR does not recognize its discoveries as cometary. The same is also true of other CCD discovery programs, which often involve exposures sufficient only to detect moving objects. Cometary status is often established only by careful scrutiny of objects that have orbits suggesting cometary nature. Although some of these objects, like P/1999 DN3 (Korlevic-Juric), had already received designations as minor planets, the beauty of the new system is that such designations can be retained and combined with genuine cometary designations in a transparent manner. Again to complicate matters, June 1999 saw for the first time the discovery of an object (two objects, in fact) having a retrograde orbit but no trace of cometary activity!
While monetary prizes and other awards specifically for the discoveries of comets date back to the year 1831, there was no such international award between the 1950s and this past year. The Edgar Wilson Award, made possible by a bequest from a businessman in Kentucky, has recently been instituted for cometary discoveries by amateur astronomers (or individuals acting in an amateur capacity) for whom those comets are named and who are using for the discoveries amateur, privately-owned equipment. The amount available each year, roughly $20 000, is shared according to the number of comets with eligible discoveries during the year, which for this purpose is taken as beginning at 0 hours UT on June 11. The first year of operation has just ended, and there were six eligible comets, including the aforementioned P/1999 DN3 (with the two Croatian CCD discoverers having an equal share), a CCD discovery in Arizona, a photographic discovery in Austria and three visual discoveries in Australia.
My notes show a few further asides, which Brian mentioned in passing. The D/ designation for some periodic comets implies defunct or comets which JPL shouldn't send a mission to as they might not find it. He would like to see the numeric sequence disappearing from the named periodic comets (eg S-L 1 to 9). Orbital computations are now not quite good enough to fit all the available observations, even with the inclusion of non-gravitational forces, and an improved model is clearly needed. July and August is the rainy season in New Mexico, so there are not many LINEAR observations at this time of year. LINEAR doesn't follow objects from night to night, which gives the amateur the chance to do two night linkages. By definition tailed asteroids are comets (eg 133P/Elst-Pizzarro). The Edgar Wilson award begins on June 11th because his brother died on June 10th. The original bequest included recoveries, however this was thought to be unfair for recoveries with well-known orbits, though accidental recoveries might count towards future awards.
Responding to a question, Brian said that comets are not allocated a provisional designation until positions were available. Several recent SOHO comets have yet to have their positions measured and so do not yet have designations, and one has positions but no derivable orbit, despite clearly existing. Brian's talk continued into a panel discussion also including Doug Biesecker, Bill Liller and Alan Hale. Kuiper belt objects are cometary objects. There might be some bright Kreutz group comets to come, but he wasn't sure about different sub-groups. The IAU could decline to name a comet if this might cause aggravation. The first Solwind object had been named, and the team's intention had been to name each subsequent object with the next three team-member's names, however they were instead named after the instrument. New spacecraft missions planned for the future include stereo solar imaging and all sky imaging down to 12th magnitude every 90 minutes. The first named comet is 1760 A1 as this was the year the Messier first began deliberate comet hunting.
Guiseppe Canonaco commented that useful positional data had been found in the logbook of a Dutch ship, enabling an orbit to be computed for a comet for the first time. Jon Shanklin noted that he had been forwarded recent meteorological logbook entries relating to comets Hyakutake and Hale-Bopp, which showed that mariners were generally unaware of the comets despite widespread information being available and that they only spotted them when they reached 2nd or 3rd magnitude.
The skies again cleared after dinner, giving another very transparent night (for Cambridge), with the Milky Way clearly visible from the University Observatory. It was possible to observe comets Lynn and 10P/Tempel 2 and more fragments of 109P/Swift-Tuttle were seen.
Monday began with the final session, which was devoted to short presentations and posters. Philippe Morel of the Societe Astronomique de France described the history of the comet section of the SAF. L'Astronmie, the Journal of the SAF had published observations of comets since 1887. The comet section was formed in 1970 and has held several camps. There are about 120 members and future projects include transcribing old observations from L'Astronomie. The SAF web pages include a section on comets. Stephane Garro went on to elaborate about the SAF comet database. They had decided to use database software and set up a form input so that those not familiar with the ICQ codes could enter observations correctly. The first observations were from 1882, but there are few in the ICQ database prior to 1980 and they were concentrating on entering these observations. There were 713 observations between 1939 and 1959. Many of the early observations are rough, lacking supporting data. Sometimes the date is imprecise, for example only quoting October 1901, others gave no instrument and often only the date and magnitude were quoted.
Gyula Szabo described his observations at Konkoly in northern Hungary. The 0.60-m Schmidt is now equipped with a CCD camera which has a 29'x18' field of view, compared to the original plates which gave a 5°x5° field, however the CCD reaches 22 magnitude compared to 19 magnitude on film. He showed images of 1998 K5, which revealed a bright tail, but little coma. Several comets showed features in the coma. A short period light curve of P/1998 U3 over 2 hours showed variation in the nuclear brightness. They had carried out surface photometry using varying apertures, calculating the magnitude in various rings. Theoretically (d ln B)/(d ln P) = -1 where P is the radius of the annulus and B its width. Some LINEAR comets, eg 1998 K5, show much greater slopes than this. Solar activity, diffusivity in the coma or activity in the comet may explain the variation. Some of the data may show a trend, with minimum values occurring some 20 days after perihelion.
Bill Liller had followed comet Hale-Bopp with his 0.20-m f1.5 Schmidt with an ST5 CCD at the Newtonian focus on 338 nights. This gave a 27'x36' field at a scale of 7" per pixel. He could get a photometric accuracy of ±0.04 magnitude using a broadband V filter (effectively minus IR). Looking at the inner coma only, there was a 20±5 day periodicity when the comet was inbound. After perihelion there were ongoing recurrent outbursts about 100 days apart, which showed an outflow of 40 - 55 ms-1.
After the tea break Bill Liller presented Jonathan Shanklin with a bottle of Chilean wine and thanked him for making all the local arrangements for what had been a very successful meeting. Bernd Brinkmann gave a short talk on his CCD observations with an SX camera on a C8 and with the Askania 0.34/0.50-m Schmidt camera and ST6 camera, which also has several smaller telescopes which are under used. He processes the images, which are mostly 60-second exposures, with bias, dark and flat field frames and uses co-added frames for fainter objects. He showed high quality images of several recent comets, which had been used for astrometry. He concluded with some spectacular slides of Hale-Bopp.
The final speaker was Simona Nikolova who spoke about the endurance lifetime of meter sized cometary fragments. Meter sized fragments were common in meteor streams [though the audience was a little sceptical that photographs had showed such objects prior to atmospheric entry] and fragmentation in comets was common. She had developed a sublimation model, in collaboration with Martin Beech, using the interaction of solar radiation with water ice and several variable parameters. She concluded that 2P/Encke looses 0.65-m per revolution and a 10-meter sized fragment would last around 50 years; 55P/Tempel-Tuttle looses 0.18-m and a fragment would last 1900 years.
During the meeting several posters were on display, though I'm afraid I didn't have time to make detailed notes. The BAA Comet Section and TA had light curves of recent comets on display, a selection of superb comet drawings by George Alcock and the discovery observations of comet 1980 Y2 by Roy Panther. Comets Hyakutake and Hale-Bopp featured prominently in several posters. Nicolas Biver presented work on the estimation of the rotation period of Hale-Bopp using visual drawings. Philippe Morel discussed possible irregularities in Hale-Bopp's rotation, illustrating his poster with 15 drawings made through his 0.41-m Newtonian. Bill Liller had illustrations from his talk on Hale-Bopp and the Belgian VVS showed many photographs. Andreas Kammerer displayed a comprehensive analysis of observations of the two comets and this will also be displayed at the Meteor Section meeting at the end of October. Simona Nikolova showed a synopsis of her talk on cometary fragment lifetimes.
After lunch we boarded an air-conditioned coach for the trip to Avebury and Stonehenge. It is quite a long journey to Avebury and I kept everyone entertained with descriptions of the passing scenery and was persuaded to recount some tales from my other hobbies, which include church bell-ringing, ice hockey, natural history and cricket. Although there was rain en route, it had stopped by the time we reached Avebury and there was time to look round the large complex and discover its history in the local museum. After a light tea we continued on to Stonehenge, where we waited until the public had left before being ushered into the stone circle. We had an hour to ourselves and the lighting conditions provided a spectacular backdrop to the ancient monument, which is Britain's earliest astronomical observatory. The trip back was much quicker than the outward journey and we arrived back before midnight.
Final goodbyes were said on Tuesday morning and the remaining participants departed from New Hall for further touring round the UK and their journeys home. During the three days of the Workshop 65 astronomers and friends participated in what was a very rewarding meeting. My thanks to all those that contributed to making it such a success and I look forward to the next one in a few years time.