Ordinary Meeting, 2005 March 30
The March Sky
Mr Mobberley opened by commenting that the past few months had been exceptionally cloudy, and so the display of observations this month would be rather limited. Another consequence was that there were very few new supernova discoveries by UK amateurs to report, even though this was his first Sky Notes instalment since December. Tom Boles' only discoveries in the intervening months had been a spate of four, all within a space of five days in January, when weather had permitted observation. Despite the slow progress of late, however, with his discoveries now numbering 86, it seemed his century could not be far away. In addition, Ron Arbour had made his fifteenth discovery, 2005au, on March 19.
There had been a spate of novae of late, though none had been readily UK-observable. The first had been Cygnus 2005 (V2361 Cygni), discovered from Kakegawa, Japan, at mag 9.7 on February 10 by Hideo Nishimura. Nick James had succeeded in imaging it from Chelmsford at 19h10 UT on February 12, when it had been setting at an altitude of only ~10° above his north-western horizon. Its light-curve had shown a rapid dimming and reddening, dropping below mag 17 by March 13, suggesting it to be of an interesting type, where a substantial dust cloud had formed around it at an early stage in its outburst.
On March 13, Liller had discovered another, Nova Norma 2005 (V382 Nor), from Chile. It had been mag 9.4 (red band) at discovery, though at declination –52°, not UK-observable. Also on March 13, variable star DO Draconis, an intermediate polar system, had been detected in outburst at mag 11.2. This followed several flares in recent years, most recently on 2004 January 23.
For those interested in observing supernovae, two old events remained readily CCD-observable: the first being 2004dj, discovered in NGC 2403 on 2004 July 31, which had been in a plateau phase at a little below mag 15 since November, and the other 2004et, discovered on 2004 September 27, now holding steadily at mag 13.0-13.5. A more recent event, 2005ay, discovered from Maine by Doug Rich on March 27, would be visible for at least a few more days.
The speaker summarised observations of the March 10 occultation of mag 7.7 star HIP59732 by mag 12.7 asteroid 209 Dido, though there had been cloud cover along most of its track through Europe. Having previously been touted as the occultation of 2005, there had only been three positive observations and timings in the event, all from northern Italy. Hence only three chords could be placed across the asteroid's disk to constrain its shape. However, as some consolation, the speaker looked forward to the October 15 occultation of mag 1.4 Regulus by 35km-diameter asteroid Rhodepe, itself a faint mag 15.4, which, given the brightness of the star, would be naked-eye observable. Regrettably, its path did not cross the UK: observers would need to travel to Portugal, Spain, southern Italy, Greece or Turkey to see it. Given that the maximum duration would be 1.1 seconds, this seemed rather a long journey for a single second of excitement.
Moving onto the comet scene, Mr Mobberley reported that C/2004 Q2 (Machholz), fading at mag 7, remained the brightest comet in the sky, having peaked in excess of mag 4 shortly after its January 6 perihelion. C/2003 T4 (LINEAR), presently at mag 8 and close to its April 3 perihelion, had not performed as well as had been expected; Jonathan Shanklin described his observations as "disappointing". In summary, there were no particularly spectacular comets at present. However, looking back to January, Machholz had provided a very pleasing display, the speaker noting its passing around a degree to the west of the Pleiades in particular, when its tail had crossed the cluster's nebulosity, appearing briefly to merge with it. It was now within a few degrees of the celestial north pole and would descend in declination through April and May, entering Ursa Major on April 22, and then Canes Venatici on May 18.
The speaker noted that another of Machholz's discoveries, 141P/Machholz, was also presently observable at mag 13. In the coming month, it would pass westward into Taurus on April 4, before heading on into Orion on May 4. 62P/Tsuchinshan could be found looping around Coma Berenices, where it would remain until late May.
9P/Tempel 1 was given a final mention, since on 2005 July 4, NASA's Deep Impact mission would be impacting it with a 370-kg projectile, travelling at a velocity of 10.2 km/s (23,000 mph), before monitoring its subsequent activity and resultant cratering. Presently in northern Virgo at mag 12, it would head southward through May and June, passing ~7° to the east of Jupiter around June 7, and ~3° north-east of Spica on the date of impact. Regrettably, it would be beneath the UK horizon at the time of impact, 06h00 UT, which would also be in UK daytime. Throughout July, however, it would remain ~7° above the UK south-western horizon at the end of evening twilight, at around 22h30 UT. During this time, it might brighten dramatically, as a result of the blast's ejection of ice and dust debris, as well as the exposure of a large, fresh area of its surface to sunlight. Mark Kidger, among the bolder of amateur comet enthusiasts, had predicted a rapid brightening to mag 0, followed by a return to normal activity over a period of weeks; others made more modest predictions.
Mr Mobberley moved on to discuss lunar observation – rarely mentioned in Sky Notes, but the outstanding images he had seen in recent months seemed cause for an exception. He explained that the waxing phases of recent moons had been close to their most northerly possible declination of +28.5°, placing them high in the UK sky, where the seeing was good. He added that the Moon's declination oscillated about the equator with a 27.2-day period – the anomalistic month – and that the amplitude of this oscillation itself varied with an 18.6-year period. This amplitude was now close to its maximum, which would be attained in 2006 June, when the lunar declination would peak at ±28.5° each month. In March, the Moon had reached its northerly extreme around First Quarter. The same would be approximately true in coming months, on account of the closeness of its oscillation period to the 29.5-day synodic month (period between like syzygies).
The speaker explained that the 18.6-year cycle resulted from the 5.1° inclination of the Moon's orbital plane to that of the ecliptic, around which it precessed with the aforementioned period due to solar tidal forces. When it was inclined in the same direction as the Earth's equatorial plane, the Moon's declination would oscillate with minimal 23.5 – 5 = 18.5° half-amplitude each month. In 2006 June, however, the plane's inclination would be in the opposite direction, giving rise to a maximal 23.5 + 5 = 28.5° oscillation.
In addition, the speaker added that he had been monitoring wind speeds at a range of altitudes in the Earth's atmosphere1 recently – low speeds usually yielding good seeing conditions – and had noted a period of exceptional stability around March 18/19, coincident with the Moon reaching its high-altitude First Quarter. He proceeded to show a series of exceptionally clear images captured by Jamie Cooper and Damian Peach on that night. Looking ahead, he added that in the forthcoming month, the Moon would once again be highest around the time of its reaching First Quarter.
Moving onto planetary imaging, he reported that Jupiter now transited at around midnight. In recent months, Dave Tyler and Damian Peach had been producing some spectacular images, but perhaps the most notable that he had seen were those by Zac Pujic in Brisbane, Australia – a name which he confessed to not having come across before. He remarked that Pujic's March 4 images were so sharp that at a quick glance they could easily be mistaken for HST images. With Jupiter now in the southern hemisphere, and quite low in the UK sky, Mr Peach had been optimising his imaging techniques for low-altitudes. He used infrared (I), red (R) and blue (B) filters, omitting to image in green (G) light to maximise the integration time in the other colours. He later synthesised a G image by co-adding the R and B frames. The luminance of the final image was set to the sum of the R and I images, while its colouration was taken from the RGB frames. This technique, termed LRGB, produced pleasing results, as images taken in the infrared typically seemed sharper than those taken in visible bands.
Saturn, now in Gemini, transited at 6.30pm, and Mr Peach had recommended observing as early in the evening as possible, even in twilight, to get the steadiest possible image as it sank towards the horizon. The speaker remarked upon the exceptional images taken by several members on the night of its opposition, January 13, in which the rings shone which extraordinary brilliance. He explained that this was a remarkable example of the Opposition Effect. Normally, the Sun's rays illuminated the small particles composing the rings at an angle to our line of sight, causing some of those exposed to our view to be shadowed by other particles, reducing their overall unresolved brightness. However, at opposition, on March 13, our line of sight had drawn within 5' of the direction of the Sun's rays. In this orientation, virtually all of the particles in view had been sunlit, and thus the rings had briefly appeared considerably brighter than usual.
Mars was presently unobservable, rising with Capricornus in dawn twilight, but would soon be visible earlier in the night, becoming an easy target by August.
To close, the speaker mentioned the forthcoming total solar eclipse of April 8, when totality would last a maximum of 42 seconds. It would be a hybrid eclipse – a rare type of total eclipse which reduced to an annular event at the two geographical ends of its path. He explained that this behaviour arose in consequence of the Earth's curvature. Total eclipses were seen when the Earth passed within the Moon's umbral shadow, and annular eclipses when the Moon was in a more distant part of its orbit, where its angular size was less than that of the Sun, and, as a result, the Earth passed behind its umbra, the vertex of that cone-shaped shadowed volume of space falling short of its surface. Hybrid eclipses were an intermediate case, where the curvature of the Earth's surface caused the umbra's vertex to fall beneath it at the centre of the eclipse path, and above it at either end. On April 8, the path of totality would be confined to the southern Pacific Ocean, not crossing land at any point, though annularity would be visible from Venezuela, Colombia and Panama.
Hybrid eclipses, the speaker explained, were around eight times more rare than total solar eclipses; the last such events had taken place on 1986 October 3, when totality had been shorter than a second, and 1987 March 29, when it had lasted 8 seconds. The next would be on 2013 November 3, when totality would last 99 seconds.
Following the applause for Mr Mobberley's talk, the President welcomed the evening's final speaker, Mr Bob Marriott.