BAA / RMetS Joint Meeting, 2004 November 27


The November Sky

The last few weeks had not been at all good for observing: even by British standards it had been an exceptionally cloudy month. Indeed Mr Mobberley doubted if anyone had managed to glimpse the stars in recent days. However, he started his round-up by discussing lunar observation: first of all, the total eclipse of October 28. Though most observers had been clouded out, a few resourceful Association members had caught glimpses of it through cloud gaps, including Peter Lawrence in Selsey, Sussex, and Jamie Cooper.

Continuing on a lunar theme, the SMART-1 probe, launched by ESA on September 27, had arrived in its initial polar orbit of the Moon on November 15. Primarily a mission to evaluate new technologies, SMART-1's main scientific aim with regard to the Moon was to provide detailed surface maps to help understand how features had developed and evolved. The speaker remarked on the incredibly long travel time, due to the use of an ion-propulsion engine, being tested in flight for only the second time, and which provided no more force than the weight of a postcard, an acceleration of 0.2mm/s2 and an average speed towards the Moon of 20mph.

A key question, useful if humans were ever to establish colonies, was whether surface water deposits existed. This focussed particular attention on the craters of the south-polar region, the bottoms of which were believed to be in perpetual darkness, the Sun being at such a low altitude in the sky as to never rise above the sides, creating cold conditions under which ice might survive for millions of years without sublimation. For this reason, considerable time would be invested in looking for the spectral signature of water-ice deposits in such places, particularly the Aitkin basin, the largest known impact crater in the Solar System.

The UK's supernova hunters had, as always, been hard at work, finding 20 events between them since June, leaving the UK total now standing at 162 discoveries. Of these, some of which were shared, 15 were Ron Arbour's discoveries, 70 by Mark Armstrong, and 82 by Tom Boles. Notable among recent non-UK discoveries was 2004et in NGC 6946, which had already hosted seven previous supernovae starting in 1917. The spectra of the most recent event revealed it to be of Type II. On November 20, Tago and Sakurai had discovered a new mag 7.6 nova in Puppis, the eighth in the constellation since 2001. At declination –27°, a further 10° south than Sirius, it was close to the southern limit of the UK observable sky, but despite this, Nick James claimed to have obtained an image the previous night at 3:17 UT, when it had been at altitude 10°.

The next big comet looked set to be C/2004 Q2, an amateur discovery by Dan Machholz on August 28. Undoubtedly one of the 20th Century's leading comet hunters, Machholz had accumulated nine discoveries between 1978 and 1994, when his success had come to an abrupt halt with the switching on of the automated searches of LINEAR and NEAT. However, not deterred, he had continued to search, though this discovery had been the first fruit of his labours since that time. The speaker noted that the discovery location, in Eridanus, had been exceptionally close to the Sun at the time. In December and January, C/2004 Q2 would be in a familiar part of the sky, passing from Eridanus into Taurus on December 27, possibly peaking around mag 4 shortly thereafter, and passing into Perseus on January 11. It would be around mag 5 as it passed into Cassiopeia on January 27. Mike Holloway, in Van Buren, had already produced a number of fine images, revealing dust and ion tails. The speaker noted that its orbit was orientated such that it would remain a northern-visible object for many years to come.

Moving onto fainter comets, the speaker apologised to those in the audience who found it difficult to get excited about mag 13 comets, only visible, after all, with CCDs, but he explained that they were all that comet-enthusiasts had to observe much of the time. Comet 2004 Q1 (Tucker) would spend December and January passing northwards through Andromeda at around mag 13, ~10° north of the Square of Pegasus. Tongue-twister 29P/Schwassmann-Wachmann presently lay on the Pisces-Pegasus border, a couple of degrees south of the Square, in almost permanent outburst at mag 12. 78P/Gehrels, presently in Aries, would pass into Taurus around January 15, moving through into Orion in the latter part of April, gradually fading from its present mag 11. C/2003 T4, presently in Draconis, would pass through Taurus and move into Lyra around New Year, edging around five degrees south-east of Vega on January 6, and hopefully brightening to around mag 9 by the end of that month. 32P/Comas Sola would spend the next couple of months taking a circuit around Aries, ~10° west of Hamal, remaining close to mag 12. 62P/Tsuchinshan, presently a little over 5° west of Regulus, would pass into Virgo in early January, before spending the remainder of the spring in Coma Berenices. The speaker remarked that it would pass close by several of the Virgo cluster of galaxies. Finally, C/2001 Q4, the big northern comet of 2004, remained a northern object, presently in Draco, moving into Cepheus on December 12, and into Cassiopeia in late January, whilst fading at around mag 11. Last of all, for asteroid enthusiasts, 2004 RZ164, a rock of around 700m diameter, would pass within two million miles of the Earth on December 9, perhaps reaching mag 11.8 in the Perseus region.

Moving onto the planetary scene, the speaker remarked that here transparency was often of secondary importance to seeing: planetary imagers wanted the sky to be as steady as possible, but their targets were so bright that a little cloud didn't worry them. It was found that high-pressure systems often provided the best seeing, because they deflected the 40,000ft jet stream, and turbulence brought with it, away from the UK. For this reason, the misty, foggy conditions often associated with high pressure was the favoured weather for planetary work. Saturn was now up for most of the night, transiting at 3am. Its north polar region was slowly emerging from beneath the rings. The speaker showed some fine images by Dave Tyler, using Damian Peach's old Celestron-11, having taken some lessons from that Great Master. Briefly commenting on the Cassini mission, the speaker looked forward eagerly to the separation of the Huygens probe at 02h00 UT on Christmas Day, later to plunge into the atmosphere of Titan on January 14. Larger than Mercury, Titan was the only moon in the solar system to have its own atmosphere, although it was not known whether its observed cloud-like structures were composed of liquid or gas. Mr Mobberley noted with some amazement that the atmosphere had been discovered in 1908 by the prolific Spanish planetary observer Josep Comas i Sola, on account of his observations of limb-darkening – how this observation had been possible by visual means on such a small body baffled him.

Jupiter now transited at 8am, and so was still largely lost in dawn twilight, but would soon be rising earlier in the night. Observers were urged to watch out for the Geminid meteor shower between December 7-16, predicted to peak December 13/14. As this coincided with New Moon, conditions were particularly favourable. By coincidence, the progenitor of the shower, 3200 Phaethon, was visible at around mag 15 in December, making closest approach to the Earth at 0.22 AU on 22nd.

Finally, Mr Mobberley urged anyone who might be in the southern half of France the following day to observe the occultation of mag 8.48 star HIP 30327 by mag 12.43 minor planet 238 Hypatia. By combining timings of such occultations from widely spread geographic locations, including negative observations, it would be possible to constrain the size of the minor planet. In this instance, the occultation would take place around 22:46.5UT (or 2-3 minutes earlier over Poland and Germany), for a maximum duration of up to 16.5 seconds. A previous occultation by the same minor planet of TYC 0158-01520-1 on November 11 had provided an estimated size of 159.8km by 148km. Another imminent occultation was that of mag 11.5 star TYC 1163-00239-1u by 1712 Angola at 18:08 – 18:21UT on November 29, passing over northern England at 18:12.5UT, around 2° from Markab in Pegasus. To close, the speaker wished to record his warmest congratulations to Sir Patrick Moore upon the 70th anniversary of his joining the BAA, on 1934 November 28.

Following the applause for Mr Mobberley's lively instalment, the meeting broke for tea, after which Mr Tom Boles invited Dr Brian Marsden, who had delivered the first annual BAA George Alcock memorial lecture last year, to introduce this year's speaker.




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