Ordinary Meeting, 2004 May 26


The May Sky

The speaker opened with the lunar eclipse of May 4, for which the UK had not been an ideal observing site, the eclipse taking place at Moonrise. The eclipse also had a relatively short 1h16m period of totality because the Moon passed only just within the umbra. These factors combined, totality had ended when the Moon had risen to an altitude of only 11° above the horizon. In addition, there had been patchy cloud on the night, though all of these factors had not deterred Association members, and the speaker proceeded to show a fine crop of images from Maurice Gavin, Nick James, as well as his own. Perhaps the finest of all was an evocative image of the partially eclipsed Moon through thin cloud, taken by Damian Peach, normally a name associated with rather higher resolution images.

Mr Mobberley congratulated the Association's resident supernova hunters on an exceptionally fruitful period: Mark Armstrong had discovered five events since the previous meeting. His latest and 62nd discovery, 2004bs, had been classified as a type Ib supernova on the grounds of its spectrum, as obtained by the ESO Calar Alto Observatory in Spain. It was estimated to have been three weeks past maximum at discovery. In addition, the President had made two further supernova discoveries. After the discovery of 2004bd, Ron Arbour had found several pre-discovery images from two weeks earlier amongst his observations of host galaxy NGC 3786. He had failed to claim the discovery himself as it was so close to the nucleus that it had been missed. Reviewing the progress, Tom Boles now had 65 discoveries, with Mark Armstrong coming up fast behind with 62.

Presently an easy variable star to catch was χ-Cygni, variable between mag 3.3 and 14.2 on a 407-day period, and presently close to maximum. Moving onto the comet scene, the big news was that Comet C/2001 Q4 (NEAT) had arrived in the northern hemisphere in early May, peaking at around mag 3 at closest approach to the Earth on May 7, slightly fainter than had been hoped. Loke Tan had obtained exceptionally clear images from the Atacama Desert on April 18-19, around a month before its May 15 perihelion. Association members James Weightman, Maurice Gavin, and the speaker himself, had all obtained their first images on May 10, though not with favourable seeing. The speaker showed a number of images by these members, and Nick James, on the proceeding nights. Michael Jäger, an expert cometary photography had caught a fine image of the comet and tail.

At the previous meeting, Mr Mobberley had mentioned the close-pass of C/2001 Q4 past M44. The show had been not quite as exciting as hoped as the comet appeared much smaller than the cluster, however many Association members had caught images of the two objects in the same frame. Moving onto perihelion, the speaker showed mosaic images from May 15 by himself and Mike Holloway in Van Buren, Arkansas. In all cases, it was clear that the ion tail was slightly spoiled as it was superimposed on a wide swathe of dust, reducing the contrast. In the coming months, C/2001 Q4 would remain in the northern hemisphere, fading to mag 10 by the end of October, and remaining a circumpolar object from the UK until 2008.

The other comet-of-the-moment was C/2002 T7 (LINEAR), and was performing rather better, having peaked at mag 3 in mid-May. However, despite being tantalisingly close to northern-hemisphere visible, its path presently took it a few degrees south of Orion, perching it on the horizon in civil twilight. Southern observers, by contrast, were returning fine images. A 10-minute exposure by Vello Tabur, in Australia, showed the comet to have an anti-tail feature on April 17. This same feature was also apparent in an image by Masi and Mallia from a 14" reflector in Chile on the same night.

Bill Bradfield, an Australian comet hunter, had on April 12 made his 18th discovery, C/2004 F4 (Bradfield). Southern hemisphere comet hunters did, of course, enjoy somewhat of an advantage over their northern counterparts, as the southern skies were not scanned by robotised surveys, and the speaker noted that of all Bradfield's 18 discoveries, he did not share any with a co-discoverer. All were named simply 'Bradfield'. The speaker proceeded to show SOHO LASCO images of C/2004 F4 from April 16-20, around perihelion. It was curious that the comet should have been discovered only four days earlier – its pre-discovery orbit had wound a zig-zag path through Aries for some months previously, brightening from around mag 18 in early 2003, to mag 8 at discovery. Now a fine northern hemisphere comet, the speaker showed superb images by Michael Jäger in Austria, reporting a tail of >9°, and another by Endo in Japan of the comet streaking up above sunset. In early June, C/2004 F4 would be around mag 10, before passing through Cassiopeia and fading to around mag 12 by early August.

Comet C/2003 T3 (Tabur) was one to watch at around mag 9, but was skirting along the north-eastern pre-dawn horizon, hiding from easy view. C/2003 K4 was passing from Vulpecula into Cygnus at the time of the meeting, now approaching mag 10. By mid-June, it would reach mag 8, then in Hercules, perhaps reaching mag 6 by August, but plunging south before reaching perihelion in mid-September.

Meteor observers might be treated to a return of the Bootid/Pons-Winneckeds shower on June 26, the day of the Exhibition Meeting. After a 70-year dormancy, this shower had returned in 1998 to yield a rate of 50-100 per hour. A ten-day-old Moon would set at 0h27 UT that night, but summer twilight would likely be a greater bother.

The Cassini probe was now drawing very close to its target, and on 2004 March 24 took its final image of Saturn such that the whole planet could fit within a single frame. Close-up images of the rings on May 10 showed a complex spiders-web-like network of shadows of the rings cast down onto the surface below. Insertion into Saturnian orbit was scheduled for July 1. The speaker then displayed the fine results being obtained by the Association's own planetary observers in tracing surface details on Jupiter. Having contributions from observers with wide geographic spread, the Jupiter Section was often able to obtain images of the surface on every rotation, allowing features to be monitored with precision previously only available to costly space missions. The speaker remarked that this work was ideally suited for amateurs, since good images could be obtained from a standard 8" or 12" instrument, with no benefit gained by using giant apertures.

Finally, the speaker closed by continuing the theme of the previous talk, giving the details of the forthcoming transit of Venus on the morning of June 8. This would be the first transit of the planet for 122 years – its rarity was a result of the 3° inclination of Venus' orbit with respect to that of the Earth. Thus a transit was only observed from Earth on the rare occasions when inferior conjunction coincided with the passing of Venus within 0.25° of the ecliptic – the radius of the Sun. To good approximation, the orbits returned to their start-point and repeated themselves every 243 years, and hence transits were observed in cycles of four, with 122, 8, 105, 8 years (total 243 years) between successive events. The appearance of eight-year interludes was because in this time the Earth completed eight orbits in almost exactly the same length of time that Venus took to complete five. Because of the non-exactness of this match, however, the accuracy of alignment required for a transit was only retained for one eight-year interval; after sixteen years a near miss would result.

Mr Mobberley showed several computer-generated images to simulate the apparent angular size of Venus during the transit – around a 30th that of the solar disk. He noted that the ratio of Venus' angular size to that of the Sun was three times the ratio of physical sizes, as Venus was three times closer. Finally, the speaker showed an animated video sequence of the 1882 transit, which had been compiled by Dr William Sheehan from photographic plates taken by the Lick Observatory.

Following the applause for Mr Mobberley's lively and complete account, Mr Maurice Gavin asked whether there were any plans to observe the transit with the space-based SOHO solar observatory. The speaker replied that SOHO was sufficiently far from the Earth that the geometry was different, and SOHO would not see Venus pass in front of the Sun at all. The President then invited Dr Richard McKim, director of the Association's Mars Section, to deliver the evening's final talk.




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