Ordinary Meeting and Exhibition Meeting, 2004 June 26


The Summer Sky

Opening with the supernova scene, Mr Mobberley explained that preparing this month's Sky Notes had been a somewhat frustrating business, as every time he thought he had finished, another discovery would come in. Since the previous meeting, Mark Armstrong had contributed four discoveries, his latest confirmed on the evening before the meeting. One further discovery by Tom Boles landed the two rivals neck and neck with 66 discoveries apiece. Having surpassed the record of 100 UK supernova discoveries last year, it now seemed very likely that both the Association's leading hunters would have 100 each by early next year.

The speaker then showed the latest images of comet C/2001 Q4 (NEAT), including a mosaic of eight two-minute exposures taken on May 16, close to perihelion, by Mike Holloway in Van Buren, Arkansas. Mr Mobberley went on to show one of his own mosaics from a week later. Throughout July and August, C/2001 Q4 would be passing through Ursa Major at mag 7-8, skirting close by a series of galaxies in the first half of August: NGCs 3613, 3610, 3642, 3945 and 4036. Most of these close approaches would be within around 30', and so they would be fine photographic opportunities. In September it would cut across Draconis, heading on into Ursa Minor in October, fading to around mag 10 by November.

Comet C/2004 F4 (Bradfield) was now passing through Cassiopeia, and fading from mag 10 at the start of June to mag 12 by the end of July. An intrinsically compact comet, a CCD would soon be required to observe it, though it would remain a northern object in coming months. Similarly, Comet Tabur was fading at around mag 11, and passing through Auriga and Lynx in July and August respectively. This too was essentially confined to the realm of CCD observation now.

The next good northern comet would be C/2003 K4, brightening through the summer, having passed from Cygnus into Lyra at around mag 9 in late May, and passing on into Hercules in mid-June at mag 8. By mid-August, it might reach mag 6, passing 8° north-west of Arcturus. But then it would plunge west into evening twilight, being lost around the first week of September. Another comet of interest was C/2004 H6 (SWAN), the discovery of which had been announced on May 27. This had come an unusually long time after the first reported observations, which had come to the attention of the Central Bureau for Astronomical Telegrams some two weeks earlier on May 13, based on images taken by the SWAN instrument on the SOHO satellite on April 29. The delay seemed to have been caused by dubious astrometry in some of the follow-up observations, which made it impossible to fit an orbit to them. The speaker also noted that the comet had been called 'SWAN', not 'SOHO' as had previous discoveries with the same instrument.

Starting with an image by Michael Mazziato, one of the comet's co-discoverers on May 13, the speaker discussed amateur observing prospects. It would pass through Cetus between mid-June and mid-July, at around mag 8, appearing in the pre-dawn UK sky around the second week of July. It would be easiest to observe in late July and early August, when it would be passing through Aquarius, before spending much of the autumn in Aquila, fading out of sight.

The most easily observable planets – Mars, Jupiter and Saturn – would be absent from the night's sky over the summer, but Mr Mobberley urged members to take advantage of the summer oppositions of Uranus and Neptune on August 27 at mag 5.7, and on August 6 at mag 7.8 respectively. He wondered, given the ever-increasing level of detail that amateurs were able to resolve on the nearer planets, whether it might soon be possible to resolve detail on the disks of these also, having diameters of 3"9 and 2"5 respectively. The speaker also noted that the Association's own asteroid, 4522 Britastra, would pass within 6' of Neptune on August 1 in Capricornus at mag 14.5.

The coming months would bring two occultations of binocular stars by minor planets. Firstly, on September 16, mag 8.8 star TYC1286-00191-1 would be occulted by mag 15.6 minor planet 638 Moira for a maximum duration of 6.2 seconds at 04h32 UT. This event would be visible on a track passing through southern Britain. Then, on October 15, mag 9.7 star TYC1921-02606-1 would be occulted by mag 12.5 minor planet 63 Ausonia for a maximum duration of 8.6 seconds at 02h01 UT, this time visible in more northerly parts of England. On both occasions, the Moon would be helpfully near-new.

Mr Mobberley briefly reviewed the progress of the Cassini mission to Saturn, already mentioned by the preceding speaker. One of his favourite images thus far was of the ring system, taken on May 10. The filamentary shadows of each ring on the surface of the planet could be seen through the inter-ring spaces, having an appearance similar to a spider's web. Though these images had been taken while the probe was still a considerable distance from Saturn, the speaker noted that each pixel represented a distance of only 100 miles on the surface. On May 23, Cassini had turned its cameras upon Titan for the first time, one of the most curious moons in the system, and to be the ultimate destination of the European Huygens probe, scheduled to be released from mothership Cassini on Christmas Day, and to descend into the Moon's atmosphere on 2005 January 14. Perhaps the most stunning images of all were those from the close fly-by of Phoebe on June 14, already mentioned by Mr Graham.

The speaker went on to discuss the orbital insertion process that would take place on July 1. The beginning of the manoeuvre would be marked at 00h47 UT, when the probe would ascend through the plane of the rings, passing between the F-ring and the G-ring at an altitude of 60,000 miles above the planet's cloud tops. Though the probe was likely to encounter some small particles of debris between these two rings, the risk posed by them was minimal. The insertion burn itself would take place over a 96-minute period between 01h12 and 02h48, during which time Cassini would dip to a minimum altitude of 11,000 miles above the cloud tops. The speaker noted that this was the lowest altitude that Cassini would attain at any point during its forthcoming four-year mission. At 04h34, all going to plan, the probe would descend back through the ring plane, once again passing between the F-ring and the G-ring. This would mark the end of the critical orbital-insertion period. However, the light travel time between Saturn and Earth being 85 minutes, mission controllers would only be sure of the success of each step after a considerable delay.

Mr Mobberley drew members' attention to some of the most exciting highlights of the mission which would be coming up in the autumn, including the first close fly-by of Titan on July 2. A second close pass would follow on October 26, at an altitude of a mere 750-miles from Titan's cloud tops. A third flypast of Titan would come on December 13, at a distance of 1,400-miles, to be followed by the separation of the Huygens probe on December 25. In the meantime, high-resolution imaging of several other moons would also be undertaken, including Enceladus, Mimas, Phoebe and Iapetus.

Closer to home, meteor spotters were reminded of the Bootid Pons-Winneckeds storm, which might return on that very evening. Rates of 50-100 ZHR had been reported in 1998 after a 70-year dormant period, and a return this year was possible, though the speaker noted that this was likely to be hampered by a ten-day-old Moon, evening twilight, and the forecast of rain across the UK. A better-known summer meteor shower would come in the form of the Perseids, spread between July 23 and August 20, peaking on August 12 with a likely ZHR of 80, four-days prior to New Moon.

On September 29, asteroid Toutatis would pass within 1.5 million km of the Earth. Though this pass did not pose a threat, the speaker noted that no larger asteroid had passed closer to the Earth for 250 years, and in the long-term, Toutatis was deemed the most hazardous such rock yet discovered. It was believed to be several kilometres across. Mr Mobberley showed images from a previous close approach, at a distance of 5.3 million km, in 1996. Radar ranging had been used in an attempt to map the surface on that occasion, revealing it to have an unusual topography, apparently formed of two large bulges connected by a narrow neck. It remained possible that these were in fact two disconnected components. Though at a bright mag 9, the forthcoming approach would not be readily observable, taking place at declinations inaccessible to northern observers, and where even southern observers would be hampered by the Sun. Toutatis would travel towards perihelion after passing the Earth, fading dramatically immediately after its close approach due to the sudden change in its apparent phase.

Having come to the end of his Sky Notes, Mr Mobberley explained that he had been asked to summarise the observations of the June 8 transit of Venus which had been made from Sir Patrick Moore's garden in Selsey, on the southern-most tip of the Sussex coast. To celebrate the historical occasion, a great many astronomers had descended to join Sir Patrick, including Brian May, former guitarist from the band Queen, Damian Peach, the production team of the BBC's Sky at Night, as well as the speaker himself and many others. During the course of the morning, Sir Patrick had made a number of appearances on breakfast television to report the progress of Venus across the face of the Sun.

As observers began to arrive, an ever-larger collection of telescopes had filled Sir Patrick's house, though the speaker drew particular attention to a Dobsonian reflector brought by Brian May, which had been his childhood instrument, and to which an eyepiece projection unit had been added for transit observation. The speaker joked that perhaps some would have preferred him to have brought his guitar along instead. On the evening before, an expedition to a local curry house had been arranged, during which a card was signed by all to form a permanent historical record of the occasion.

As the Sun rose on transit day, the weather forecast was to prove good, the south-east of the country enjoying bright sunshine, though the skies of northern Britain were rather more overcast. The observers of Selsey, at least, were promised a good view. As the time of ingress drew near, the assembled gathering congregated in the only small corner of the garden from which the early-morning Sun was visible, peeping a little over 10° above the horizon. The seeing was to prove good, and Sir Patrick was soon able to report to BBC1 audiences that ingress had been observed. The speaker showed animations of some of his own observations, followed by superb high-resolution images by Damian Peach and Dave Tyler.

Mr Mobberley also took the opportunity of showing an image of egress taken by Paolo Lazzaroti from his Italian observatory. Stacking 1/100 second frames over a 15 second period, Lazzarotti had achieved such high resolution that the solar granulation could clearly be seen, as could the refractive effect of Venus' atmosphere on this granulation around the sharp edge of the disk of the planet. Finally, the speaker wished to dispel one popular myth: there would in fact be another chance to observe a transit of Venus from the UK in 2012. On 2012 June 6, the Sun would rise at 03h36 UT, at which time the transit would be approaching egress, which would occur an hour later at 04h37 UT. So dedicated observers with flat horizons would be sure to make some observations in eight years time.

Following the applause for another lively instalment from Mr Mobberley, the President explained that the rest of the meeting would consist of a number of members presenting their observations of the transit from other observing sites. Mr Nick James was invited to chair the discussion.






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