Ordinary Meeting, 2006 March 22

 

The March Sky

Mr Mobberley recalled that the Association's Sky Notes had a long history, running back to the 1950s, and that Patrick Moore had presented it at one time. He himself had given his first presentation in November 1990, at the suggestion of Mrs Hazel McGee, then Meetings Secretary. He could hardly have imagined back then that he would still be giving regular presentations sixteen years later. He had retired from the job in 1999, but Guy Hurst had persuaded him to make a return in 2002 for the duration of his Presidency. Having now served through Tom Boles' Presidency also, he felt it time to pass the buck on, and to see what hidden talents lay in the audience.

This month, he opened with a report of recurrent nova RS Ophiuchi's recent outburst, discovered visually at mag 4.5 by Japanese observers Kiyotaka Kanai and Hiroaki Narumi on February 12. This was an unusual object, one of only seven novae which had ever been observed to recur. RS Oph's previous outbursts had been in 1898, 1933, 1958, 1967 and 1985, and so, after 21-years' wait, another had been widely anticipated, even though its timing had not been predictable in advance. The first known BAA observation of the new outburst had been by Gary Poyner on February 15, through cloud breaks and a bedroom window. As an indication of what the latest outburst might do in coming weeks, Mr Mobberley added that the 1985 event had faded at a rate of 0.1 mag/day for its first 38 days, then returning more slowly to its normal brightness over a period of 3-4 months.

Looking further back through recent events, January 25 had seen the occultation by Saturn of mag 7.4 star BV Cancri; a few observers had imaged the merger of the two objects, though the speaker was not aware of any animations or videos. Generally the huge difference in brightness of the two objects had reduced this event's visual appeal.

Turning to the planets, Venus' present apparition was shortly to climax with its reaching dichotomy – i.e. half phase – in the morning sky on March 26. The circumstances of this apparition were not ideal, however: at this time of year the ecliptic was so orientated that the maximum solar elongation of Venus would be a little under three hours of RA, placing it rather low on the horizon in morning twilight. The speaker paused briefly to show a rather pleasing gallery of images by David Arditti of Venus' thin crescent from late January and early February.

Saturn's present apparition had passed opposition on January 27, and the speaker showed a selection of this year's images. Many observers had noted a bluish tint to the northern polar region; this had been widely anticipated, having been seen also on past occasions when one or other of Saturn's poles had emerged from the shadow of the rings. Dave Tyler had captured an image on the exact night of opposition, January 27, and had observed the rings to brighten quite dramatically for a brief time. This phenomenon, caused by the Opposition Effect, had been discussed around the time of the previous opposition1 – it was essentially a result of the near alignment of our line of sight with the direction of the Sun's rays. Tyler's image of the effect this year showed exceptionally brilliant rings, with the planet appearing rather dull behind; the speaker compared its grey appearance to that of a potato. Saturn's surface had generally been fairly uneventful of late, and so Mr Mobberley moved on, but he closed with an animated series of frames by Damian Peach from the night of March 14-15. The passage of white spots across the surface in the Southern Tropical Zone showed clearly the rotation of the planet.

Mars' disk continued to shrink ever smaller following its close approach of October 2005; in April its diameter would pass below 5". For those whose interest was not deterred by this, it could be found in Taurus and would pass eastward into Gemini on April 14 before passing about one twin-separation to the south of Castor and Pollux in late May. Damian Peach had continued to produce some impressive images in recent weeks, of which the speaker showed a selection. He closed with a fine animation of Mars' rotation by Dutchman Richard Bosman, constructed from a compilation of Bosman's best images. All of these frames had been taken with a Celestron C11, although the motion of the moons with respect to the planet had been superposed by software.

Jupiter was now observable again after passing through solar conjunction in 2005 October, but this year's would be a very southerly apparition at around δ=-15°. For comparison, Spica was at δ=-11°, and so it was clear that Jupiter would be very low indeed in the southern sky. Even Dave Tyler had struggled to get much detail of late, though he would shortly be following in Damian Peach's footsteps, setting out on an observing expedition to Barbados, and so fine images were to be expected in a future Sky Notes.

Quoting from John Rogers, the speaker gave a brief account of surface activity to watch out for. In the South Equatorial Belt (SEB), an outbreak of activity discovered by Hideo Einaga in 2005 December was still vigorous. In the North Equatorial Belt (NEB), white spot Z was moving even faster than in previous years. Preceding it, a merger of two dark 'barges' was taking place, the fourth such event to have been seen preceding Z in as many years. Perhaps most exciting of all, Oval BA had been reported by several observers recently to appear unusually red in colour. It was now the only remaining great oval in the South Temperate Region, and it would be interesting to see whether this colour persisted. The oval had been around since 2000, and by Jovian standards, six years was an exceptionally long time for a feature to persist. No one knew how the Great Red Spot (GRS) had formed, and some were asking whether Oval BA might be turning into a new spot. Certain parts of media were already claiming this, as the speaker showed in a recent Science@NASA article2. If this were to be true, Oval BA might tell us something about the GRS' history.

Mr Mobberley briefly paused his tour of the sky to express the gladness with which he received the news of Sir Patrick Moore's swift return to health after having an operation earlier in the month to have a pacemaker fitted, as the media had widely reported. Sir Patrick had been taken ill only days after a lavish celebration at his Selsey home in honour of his 83rd birthday.

Turning to comets, there was rather a lack of good observing prospects at present. Over the past few months, C/2006 A1 (Pojmański) had been observed by many; it had been discovered at mag 13 on January 2 by Grzegorz Pojmański of the Warsaw University Astronomical Observatory in images from the All Sky Automated Survey (ASAS) – a robotic f/2.8 telephotal lens situated in Las Campanas, Chile, and managed by Pojmański. At perihelion on, February 22, 2006 A1 had fringed upon naked-eye visibility at mag 5, but had now faded to mag 7, and the speaker estimated that it would sink below mag 11 by the end of April. The comet's orbital plane was inclined at i=93° to the ecliptic, meaning that its orbit now carried it rapidly northwards. Presently in Lacerta, it was already verging upon becoming circumpolar; it would drift into Cassiopeia and northwards of Deneb's declination within the first week of April. It remained primarily a morning object for the time being, but was fast becoming evening-observable also.

Comet C/2005 E2 (McNaught) had also recently passed perihelion in late February, though it had been somewhat fainter at mag 9. In Aries, it was still just about observable in evening twilight given a low western horizon.

Looking ahead, the return of 73P/Schwassman-Wachmann – not to be confused with comet 29P of the same name – was an exciting prospect. 73P was known to be breaking apart, and to have already shed half a dozen fragments. This stream of debris would pass the Earth in late April and early May, and over this period, several large fragments would pass the Earth to varying degrees of closeness. The stream itself would make closest approach, passing within 7 million miles of the Earth, on May 12. When, in 1930, it had passed only fractionally closer than this, a meteor shower of fragments had been seen, and Mr Mobberley wondered whether such a show might be repeated. 73P itself would peak at mag ~5, but show a very diffuse coma of ~30'; the speaker recommended the use of a telephoto lens or binoculars rather than a telescope to observe it.

Presently ~5° south of Arcturus in Boëtes, 73P's nucleus would pass through Corona Borealis, Hercules and into Lyra over the next five weeks. A photographic challenge would come in the early hours of May 8, when fragment 'C' of the debris stream would pass within ~5' of the Ring Nebula (M57), potentially making a photogenic combination. A gibbous Moon on the western horizon and fast approaching dawn twilight would make for exceptionally difficult photographic conditions, though.

Turning to UK supernova patrolling, Tom Boles had discovered four events in as many days earlier in the month: 2006ao on March 1, 2006ap on March 2, and 2006aq and 2006ar on March 5. These brought to 99 his tally of discoveries; Mr Mobberley wondered when he would make his century. Though not a UK discovery, the speaker also gave mention to 2006X in M100, discovered independently by Shoji Suzuki of Japan and Marco Migliardi of Italy on February 4. Supernovae in Messier galaxies often seemed to be picked up by amateurs rather than professional patrollers, as had happened on this occasion; presumably this was simply because these galaxies were so well observed. 2006X was the sixth supernova to have been seen in a Messier galaxy since 2000, and the fifth in M100 since 1900. Given the brightness of its host galaxy, 2006X was recommended as a comparatively straightforward target for amateur imaging.

The speaker briefly turned to mention a few asteroid-observing opportunities. BAA asteroid spotters might be amused to know that 3697 guyhurst would be at opposition at mag 16.3 on April Fool's Day. There would be many asteroid occultations in the coming month, and so the speaker picked two of the best prospects which would be happening at sociable times. On April 10, Boliviana would occult a mag 10 star at 21h35 UT for a maximum duration of 4 seconds across south-western England, and on April 19, Nina would occult a mag 8 star at 22h40 UT for a maximum duration of 10.5 seconds, also across western parts of the UK.

The period April 19-25 would bring the Lyrid meteor shower, for which the Moon would be in a favourable waning crescent phase. This shower typically produced rather meagre rates of around ZHR 10; in practice 6-8 meteors would be observed per hour in dark skies. However, in compensation, they did produce a fair abundance of bright events with lingering ionisation trails.

To close, the speaker mentioned the forthcoming total solar eclipse, which would be visible across central Africa and Asia on March 29. Greatest eclipse would be seen in Libya at 10h10 UT, lasting for 4m07s. From the UK a modest partial eclipse would be seen, reaching a magnitude of just under 30% in the far south-east, but barely reaching 15% in north-west Scotland. An Explorers' Tours expedition would be observing from a site in Libya close to the point of maximum eclipse, combining the spectacle with a weeklong Mediterranean cruise.

Following the applause, the President added to Mr Mobberley's comments on Sir Patrick Moore's health that he had sent a card to Sir Patrick at the time of his operation on the behalf of the Association. Sir Patrick had asked him to extend his warmest thanks to the membership for the many kind words of support that he had received from them in recent weeks. The President then invited Dr Stewart Moore, Director of the Deep Sky Section, to present the evening's second talk.

Fairfield

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41.14°N
73.26°W
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