Ordinary Meeting, 2005 December 17

 

The December Sky

Mr Mobberley opened his final Sky Notes of 2005 with a review of the year's celestial events. Turning first to comets, 51 had been discovered to date since January 1, excluding the Sun-grazing discoveries of the SOHO satellite. Two had been amateur discoveries: 2005 T5 and 2005 N1, by Broughton and Juels-Holvercem respectively. LINEAR, once the bane of amateur patrollers, had recorded a mere eight discoveries, and NEAT a near-insignificant two; both had been overtaken by the 22 discoveries of the Catalina Sky Survey's 68-cm Schmidt. The dominance of Catalina was perhaps obscured by the lack of comets bearing its name; its discoveries were typically named after the telescope's human operators instead.

Over 330 supernovae had been discovered thus far in 2005, including over 130 by the Sloan Digital Sky Survey (SDSS) and over 65 by the Lick Observatory Sky Survey (LOSS). UK amateurs were still finding events as well, though – a total of 14 over the year – eleven by Tom Boles, two by Mark Armstrong, and one by Ron Arbour; the UK tally of amateur discoveries now stood at a once-unimaginable 176. Nine galactic novae had been recorded, as well as one – Nova Liller – in the nearby Large Magellanic Cloud.

On October 3, an annular solar eclipse had been visible across Portugal, Spain and northern Africa; a partial eclipse to a maximum of 66% had been visible from London. Galleries of images by Nick James from Valencia and Pete Lawrence from Madrid were shown, and Mr Mobberley dwelled briefly upon Lawrence's images of third contact to demonstrate how the shape of the Bailey's Beads as they appeared matched that predicted from lunar topography.

Turning to the present sky, the speaker remarked upon his frustration at the lack of comets. Perhaps the best prospect was 29P/Schwassmann-Wachmann, which in coming months would appear to be casting a 4° circle around Hamal. Though typically a meagre mag ~16, its occasional outbursts often reached mag ~12. 2005 E2 (McNaught), near Nashira in Capricornus at present, was brightening towards perihelion on February 23, and might reach mag 10, but was sinking into evening twilight, already visible at an altitude of only 15° in true darkness. Perhaps the best CCD-target of the winter would be 2005 B1 (Christensen) at mag 14, presently around a degree south of Rastaban in Draco, which would spend coming months tracking eastward toward Cygnus.

A final mention was given to 101P/Chernykh, which Christensen, of the Catalina Observatory, had discovered on November 30 to have shed a mag 18 fragment, presently around a day ahead of main comet. This was not the first fragmentation for 101P; a similar event had been seen in 1991.

It being several months since the previous Sky Notes (September), there were several supernova discoveries to report. Tom Boles had found three: 2005ej on September 23, 2005io on November 3 and 2005ip on November 5 – his 93rd discovery. Mark Armstrong had made his 72nd discovery on December 9 – 2005ls in MCG+07-07-01.

Mr Mobberley moved next to the planets, and first to Mars, presently drifting through Aries, and due to pass into Taurus in the second week of February. On the evening of the Meeting, it would transit at 20h30 UT, and though its disk had now shrunk to a mere 14" as compared to 20" at opposition in October, Solis Lacus would be well shown. A sizable dust storm had blown up in mid-October, but alas it had been mostly unobservable from the UK – the similarity of Mars' rotation period to our own had meant that on several consecutive nights the affected region had been turned to the Earth only in UK daytime; US observers, by contrast, had had a fine view. The storm appeared to have started around October 17 in the vicinity of Chryse, before spreading northward to fill Valles Marineris a couple of days later and flowing over Mare Erythraeum shortly thereafter, later surrounding Solis Lacus. It appeared to subside towards the end of the month. Despite the difficulty of observing it from these shores, Damian Peach had managed to capture some images as it just rotated into view at 03h30 UT on October 23, using the 15" Newtonian in Sir Patrick Moore's Selsey observatory.

The speaker remarked that one prominent feature of this apparition had been the bright appearance of Olympus Mons in the northern hemisphere, which stood out in Damian Peach's images. In passing, he remarked upon two photographs which he had found of John Fletcher, first observing Mars with the 60" Mount Wilson reflector, and later with the 36" Lick Observatory reflector; this seemed to show that just occasionally there were chances for planetary observation with large telescopes.

Saturn was well placed in Cancer at present, visible for much of the night. In Spring 2003, its rings had reached their 15-yearly maximum inclination to the Earth, providing a good view of Saturn's South Pole, and the southern face of its rings. Its northern polar region had been entirely concealed behind the rings, only re-emerging in late 2004. A comparison of Damian Peach's images from then and now illustrated the decline in tilt over the intervening time; an easily-observable slither of the northern polar region was now visible.

Turning to the surface, Dave Tyler's recent images revealed a white spot at system III longitude 118° in the southern polar region; it was unusual to see such a feature so high in latitude. However, even observers without the resolution to pick this out might be interested by a photographic opportunity coming up around February 5, when Saturn would skirt within 30' of the Beehive Cluster (M44). Around 2005 September 15 it had previously skirted ~1° from this cluster; this pass, in retrograde motion, would be closer.

Whilst in that locality, Mr Mobberley mentioned a current Variable Star Section observing campaign target, around 4° east of M44 – a BL Lac object by the name of OJ287, believed to be an unstable accreting black hole in an active galaxy. Stellar in appearance, and typically mag 14-16, it had undergone several outbursts in the past century, on one occasion exceeding mag 12. It appeared to have an 11-12 year period, and given that its last outburst had been observed in 1994-5, another was widely anticipated. The Section would appreciate both visual and CCD observations.

Returning to Saturn, the Cassini satellite was still returning stunning images, all of which were available on the mission website; Mr Mobberley picked a few of his personal favourites to show. Images of Hyperion from Cassini's close pass of September 26 revealed a strange world, pummelled with impact craters, giving an appearance which the speaker compared to that of a bath sponge. One huge impact crater appeared to dominate nearly three-quarters its face. On September 16, the spacecraft had seen Dione partially occult Tethys – an eye-catching meeting of worlds.

On the scientific front, Enceladus had attracted much interest in recent weeks. A fountain-like spray of material had been seen emanating from its southern polar region, towering at least 300 km in altitude; it had been suggested that the source might be geysers erupting from pressurised reservoirs of liquid water beneath the surface. On November 26/7, Cassini had had the opportunity to view this curious moon occult the Sun, and the resulting backlit conditions had been ideal for imaging this vapour plume in scattered sunlight.

A notable feature in several recent images had been Saturn's planet-shine on its moons – analogous to Earthshine on our crescent Moon, but in some cases considerably brighter given Saturn's relative size.

Venus' phase was presently 19% and its diameter 47"; towards Christmas the crescent would narrow to 11% but enlarge to 53" diameter. Recently there had been some debate as to whether it was possible to photograph a shadow cast by Venus; the speaker showed several images by Pete Richard and Douglas Lawrence from Selsey which proved beyond doubt that it was possible. On a similar theme, the same photographers had also captured a particularly atmospheric view of Venus' light twinkling in reflection in the English Channel from the beach at Selsey.

Finally among the planets, Jupiter was now only just observable before sunrise, but would soon be visible earlier in the night. More would be said on this subject at the January meeting.

From January 1-6, the Quadrantid meteor shower would be active, peaking at around 17h00 UT on January 3, making the night of January 3/4 likely to yield the highest rate, perhaps reaching ZHR 100. The Moon would be a favourable four days old.

Two asteroids were singled out for mention: firstly 3 Juno, which had passed opposition on December 12, and was around mag 7.6 in Orion. The second, 4 Vesta, would reach opposition on January 5, and was around mag 6.2 in Gemini. Less readily observable was 25143 Itokawa, a 600-m-long rock whose appearance was compared to that of a gherkin, which had been visited by the Japanese Hayabusa probe in November. To obtain surface samples, the asteroid had been impacted with a series of 5-gram metal balls at 300 m/s, producing sprays of debris, some of which had been collected by the orbiting probe. A small lander called Minerva had also been employed to "hop" around in the asteroid's low gravitational field imaging its surface.

Mr Mobberley closed with a summary of recent asteroid occulations. As advertised in previous Sky Notes instalments, a particularly bright albeit short event had been seen on October 19 – the occultation by 166 Rhodope of mag 1.4 Regulus, for a maximum duration of a mere 1.1 seconds. As an unrelated aside, the speaker remarked what a peculiar star Regulus actually was, spinning much faster than the Sun, with a period of only 16 hours; it would only need to spin 10% faster to fly apart. On October 19, however, its "blinking" had been observed across Europe, confirmed from two sites in Portugal, five in Spain, and one each in Italy and Greece. It had not been observable from the UK, alas. From these timings, Rhodope's diameter had been found to be 63 km by 46 km, in contrast to previous estimates of 35 km.

The speaker also took the opportunity to congratulate Mr Andrew Elliot upon his successful observation of the December 12 occultation of mag 10.0 star TYC 4974-01069-1 by 121 Hermione, with a timing of 5.4 seconds. All of the Association's other observers appeared to have been clouded out. Looking ahead, the next event was only two days away, on December 19, when mag 9.9 TYC 0634-007222-1 would be occulted by 397 Vienna, visible across northern England.

After the applause for Mr Mobberley's lively summary, the President adjourned the Meeting until 2006 January 25, at the new venue of New Hunts House in Guy's Hospital.

Fairfield

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41.14°N
73.26°W
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