Ordinary Meeting, 2005 April 23


The April Sky

Mr Mobberley opened with an update of the number of UK supernova discoveries, presently standing at 170. Since the previous meeting, Tom Boles had contributed three – one on April 3, and two on April 11 – bringing his tally to 89. He wondered when Mr Boles would reach his one-hundredth discovery, and speculated that based on past performance, he might reach that milestone before the end of September. Those interested in observing supernovae for themselves were recommended to try 2004dj, which, despite having been discovered seven months previously, on 2004 July 31, still remained at mag ~15.7 (as measured on April 10); at a declination of +69°, it was readily UK-observable. Another possibility was 2005ay in NGC 3938 in Ursa Major, discovered by Doug Rich on March 27, which remained an easy CCD target at mag ~15.

In what he believed to be the first mention of a double-star event in the fifteen-year history of his Sky Notes, the speaker next wished to draw members' attention to the forthcoming periastron (close approach) of the two mag-3 stars of Porrima, also known as γ-Vir. 39 light-years away, its components were in a 169-year orbit, with average separation 40 AU – comparable to that of Pluto from the Sun. However, its orbit was of high eccentricity, ~0.9, and at periastron the companions would be very close – based on data from the previous close approach of spring 1836, perhaps less than 4 AU apart, corresponding to 0"3 on the sky. On that previous occasion, the closest half (180° in position angle) of the orbit had been completed in a mere six years, while the remaining 180° had taken 163 years; recent re-analysis indicated that at the moment of closest approach, the rate of change of position angle had been as high as 120°/yr.1,2 At the time of the forthcoming periastron, around mid-May, it would not be possible to resolve the two stars, but over following months there would be scientific interest in monitoring when the pair first became noticeably elongated, and then distinguishable. In 1836, there had been putative evidence that its rate of change of position angle had continued to rise after periastron, peaking a short while later, in violation of Kepler's Second Law. If correct, this could be explained by the presence of a third unseen mass in the system – Kepler's Laws applying only to two-body systems – opening the possibility for quite unexpected behaviour this time around.

Moving onto novae, the speaker gave an update on Nova Cynus 2005 (V2361 Cygni), although its solar elongation hindered observation at present. Discovered at mag 9.7 on February 10, it was placed at mag 17-18 by the latest observations, Mr Guy Hurst reported. It had dimmed rather rapidly, suggesting that a thick dust cloud might have enshrouded it early in its outburst, and that it might be a DQ Her Type intermediate polar.

The brightest comet in the sky remained Machholz (2004 Q2), fading at mag 8. Having passed close by the celestial north pole in March, it was now heading southward through Ursa Major, and would pass into Canes Venatici on May 18. Looking ahead, the most exciting prospect of coming months would be 9P/Tempel, brightening at mag 11.5, and likely to reach at least mag 9.5 before July 4, when it would be impacted by a 370-kg projectile as a part of NASA's Deep Impact mission, whilst situated ~3° north-east of Spica. Throughout July it would remain ~7° above the UK south-western horizon in evening twilight, making it a challenging target at its usual brightness, but were it to brighten significantly after the impact, it might become prominent.

Three fainter comets were briefly mentioned: firstly, 141P/Machholz, at mag ~13, was moving eastward through Taurus, and to pass through northern Orion into Gemini in mid-May. 62P/Tsuchinshan, in Coma Berenices was a little fainter at mag 14. Lastly, the speaker set members a dawn CCD challenge to image 21P/Giacobini-Zinner – comparatively bright at mag 12, but plunging eastward into dawn twilight, and to pass through the Square of Pegasus in mid-May. He asked Jonathan Shanklin whether he knew of any visual observations; he did not, but thought them possible were it not for astronomers' general dislike of early mornings.

Mr Mobberley reported that the oppositions of two bright minor planets were imminent: Ceres on May 11 at mag 7 in Libra, and Pallas on April 27 at mag 7.2 in Coma Berenices. He then remarked that the time seemed ripe for a detailed look at Jupiter, given its favourable placement, transiting with Virgo at 23h30 BST. Starting with a brief tour of its dark belts and lighter coloured zones, he showed a schematic of its surface, showing the two most prominent dark bands, the North and South Equatorial Belts (NEB/SEB), straddling the Equatorial Zone (EZ), with the Great Red Spot (GRS) lying on the southern edge of the SEB. Outside this complex lay a myriad of tropical and temperate belts and zones, many of which were transitory.

In the NEB, the speaker noted a new white spot (WS), first reported by Chris Go on March 12, and a long, dark, blue-tinted, plateau that proceeded it, first reported by Frank Melilo on March 15. In the S.S. Temperate region, the long-lived chain of five anticyclonic white ovals (AWOs) was still evident, presently prograding past the GRS. Cyclonic eddies were often visible between them, and in recent months two had become notably bright: those between A2 and A3, and between A4 and A5.

Turning to the SEB and neighbouring S. Tropical Zone (STZ), it was noted that several SEB jetstream spots had been apparent in February, some of which had entered the Red Spot Hollow (RSH), but which had now become less conspicuous. Two notable dark spots were visible in the STZ; history suggested that they would last 1-2 years before drifting into the RSH. Whilst in that region, Mr Mobberley added that a new S. Tropical Belt had begun to emerge from the proceeding end of the GRS in late March. Moving to the S. Temperate region, the famous Oval BA remained large and well defined.

To the north, he noted that cyclonic reddish-brown spots (barges) and AWOs (portholes) were appearing in the NEB, as predicted following its 2004 classical expansion event. John Rogers reported that six barges and seven portholes could presently be tracked; all but one were new this apparation, the exception being white spot Z. Further north, there had been a vigorous outbreak of N.N. Temp. B. jetstream spots, joining two long-lived AWOs in the N.N. Temp. Z.; similar spots had previously appeared methane bright, but the speaker noted a lack of methane-band observation of late, urging more.

He added that Damian Peach, perhaps the Association's finest imaging talent, had taken his Celestron 9.25" to the clear skies of Barbados, from where he would be returning in mid-May, and so he anticipated a fine display of Jovian images in next month's Sky Notes.

Among the other planets, Saturn remained visible in the evening sky, transiting at 18h30 BST. To continue the theme of objects rarely mentioned in his Sky Notes, the speaker added that Pluto would reach opposition on June 14, close to ξ-Serpens; it would be an easy CCD target. Turning briefly to asteroid occulations, he remarked that three events had been forecast for Europe's skies on the night of April 10/11, but regretted to report that after solid cloud cover, no observations had been reported.

To close, he turned to the hybrid solar eclipse of 2005 April 8, inviting Mr Mike Maunder and Ms Val White to report their observations. Mr Maunder had observed from the decks of a Discovery cruise, near Pitcairn Island, where totality had lasted 30 seconds. He reported that he had not been planning any serious photography, but that this had changed when he had met an imaging enthusiast by the name of Miloslav Druckmüller on ship, who had been developing new software to create smooth movies by stacking time-lapsed images. Taking all of the images from those on the cruise, he had created an eclipse movie which, in the speaker's view, had surpassed what the eye could see. He was eager to see what details it might be able to reveal in planetary imaging.

Ms White had travelled on a Sky & Telescope expedition to Panama, where the annular phase of the hybrid eclipse had been visible for 14.9 seconds. It was reported that Bailey's Beads had been visible almost permanently.

After the applause for Mr Mobberley's presentation, Mr Boles introduced the afternoon's final speaker, Mr Doug Ellison.




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