Ordinary Meeting, 2008 November 22

 

The November Sky

Dr Hewitt opened with a summary of the current circumstances of the Sun, Moon and planets. The Sun continued to show very little sunspot activity, as had been discussed in several previous Sky Notes talks. A few small sunspots were occasionally visible, but there was still no sign of the start of a new sunspot cycle. The Moon was currently waning and would be new on November 27, full on December 12, and new again on December 27. As a stalwart member of the Deep Sky Section, the speaker joked that members would have no excuse for not observing the deep sky with their new telescopes at Christmas.

Among the planets, Saturn was currently the best placed, and Venus would be creeping into the evening sky over the coming months. Of the remaining planets, Mercury was not currently visible – it was about to pass superior conjunction on November 25 – but would return into the evening sky in the latter half of December, reaching maximum solar elongation on 2009 January 4. Mars was also near to the Sun presently and would pass solar conjunction on December 5; it would not be visible again until late summer in 2009. Jupiter's present apparition was now well past its best, having passed opposition on July 9; it too would not be easily visible until summer 2009. Uranus and Neptune remained visible in the early evening sky in Aquarius and Capricornus respectively, but would soon disappear into evening dusk.

Returning to talk about Venus' coming apparition, the speaker explained that though it remained low on the western horizon in the evening sky at present, it would become a major sight within the next few weeks. Its gibbous disk measured 15.6" across at the time of the meeting, and its magnitude was around –4.1. By mid-December its disk would grow to 18" across and brighten to mag. –4.2, and by the end of the year it would reach 20.8" across and mag. –4.3. As its orbit brought it closer to the Earth over the next few months, its phase would wane to a crescent, passing half-phase at around New Year, though as its disk correspondingly grew in angular size, it would continue to brighten for some weeks to come, reaching maximum brightness in mid-February and remaining visible until mid-March.

In the more immediate future, Venus was heading towards two conjunctions at the end of the month. On the evening of November 30, Jupiter and Venus would pass within around 2° of each other, low on the western horizon at dusk, not far from the handle of the teapot in Sagittarius. On the following afternoon, Venus would be occulted by the Moon, and although the occultation would begin in broad daylight, at 15h47 UT, and end in dusk, at 17h17 UT, the speaker thought it would be readily visible through a pair of binoculars. Venus would be seen to disappear behind the dark limb of the three-day-old Moon, and reappear from the illuminated limb. Given the relatively large size of Venus' disk, its appearance and disappearance would not be instantaneous, but would take several seconds.

Turning to Saturn, the speaker explained that it was presenting an unusual image at present on account of its rings being aligned very close to edge-on to our line of sight. As a result, its visual brightness was around a magnitude fainter than usual, and some of the appeal of its telescopic majesty was missing, but the speaker lamented that there seemed to be so few amateur images in circulation of the planet when in this configuration. He added that with the rings now obscuring a minimal area of the planet's 15.5" disk, this was an ideal time to study surface detail, and that the edge-on orientation of Saturn's system of moons also afforded a rare opportunity to observe transits and occultations, as the moons passed in front of or behind the planet, and mutual shadow events, when the moons cast shadows upon one another.

Turning to discuss comets, Dr Hewitt explained that there were no especially bright objects in the sky at present. Perhaps the best prospect was C/2006 W3 (Christensen), which had been discovered by the Catalina Sky Survey on 2008 November 18 and which was presently well placed in the circumpolar constellation of Cepheus at mag. 10. C/2006 OF2 (Broughton) was currently a little fainter at mag. 11, lying in northern Lynx, close to the border with Camelopadalus. Rather less well placed was C/2008 A1 (McNaught), reportedly at around mag. 8, but low down in Ophiuchus and fading fast.

The speaker then invited Dr Richard Miles, in the audience, to outline briefly an observing programme which he was currently undertaking with the Faulkes Telescope, targeting Comet 17P/Holmes following its dramatic and unexpected outburst to visual magnitudes in 2007-8. Dr Miles explained that Comet Holmes had now faded to mag. 20, and that it now took a 45-minute exposure with the 80-inch Faulkes Telescope to resolve the comet and its tail. However, he was continuing to image it in the hope that as its coma began to disperse, it would become possible to resolve its nucleus and perform photometry to determine its rotation period. Dr Miles explained that he suspected that the nucleus might rotate unusually slowly, because all of the other comets which were known to have undergone outbursts were slow rotators, but that he needed observations to back up this theory. He added that he would be discussing his work in more detail at a future meeting on 2009 March 31.

Dr Hewitt briefly mentioned that the next major meteor shower would be the Geminids, due to peak on December 13, but added that since the Moon would be full on the preceding day, this normally rich shower would be rather spoilt this year.

Two well-known variable red giant stars would be approaching maximum in the next few weeks. Mira, also known as Omicron Ceti, would reach a maximum brightness of mag. 3 on around December 30, with a few days' uncertainty either side, and would be visible to the naked eye from now on. The speaker added that this star, 420 light years distant and varying with a period of 330 days, had been one of the first variable stars to have been discovered; its existence had certainly been known to David Fabricius as early as 1596. Dr Hewitt added that in modern times, this star was also remarkable for having been the third star, after the Sun and Betelgeuse, to have had its radius measured – a task which had been made easier by its large disk, measuring 3 AU across. Before Mira's forthcoming maximum, another notable but slightly fainter red giant star, R Aquarii, would be coming up to maximum on around December 17, expected to reach around mag. 6.5.

Dr Hewitt then turned briefly to discuss the triple star system o2 Eridani, also known as 40 Eridani. At a distance of only 16 light years, this was the eighth closest of the naked eye stars, and it was an easy target to find, situated around 14° to the west of Rigel. Despite its proximity, it nonetheless appeared a comparatively moderate mag. 4.5 star to the naked eye on account of its low mass, around 80% that of the Sun, which also gave it a reddish spectral type of KI. Of greater interest, however, were its two companion stars, both situated around 83" from the primary and easily telescopically resolvable at a separation of 8". The first of these, 40 Eri B, was the only white dwarf star to be easily visible through the eyepiece of a moderate-aperture telescope at mag. 9.5. The second, 40 Eri C, was an exceptionally low mass red dwarf star, having only 20% of the mass of the Sun. Whilst these two stars were almost certainly specimens of the most common stellar types in the Universe, their intrinsic faintness made them very difficult objects to see at any great distance, and it was only on account of the closeness of the 40 Eri system that it was possible to see these rare examples.

Turning to his home ground of the deep sky, the speaker mentioned that four fine variable nebulae were visible at this time of year. The best known of these, Hubble's Variable Nebula, NGC 2261, was to be found in Monocerus. Hind's Variable Nebula, NGC 1555, was associated with the mag. 9.5 variable star T Tauri. Less well known, and more recently discovered, were Gyulbudaghian's Nebula, associated with the variable star PV Cephei, and McNeill's Nebula in Orion. The speaker recommended that observers try to take time-sequenced images of these nebulae with consistent instrumentation, as it could often be difficult to distinguish the intrinsic variability of the nebulae from effects resulting from the different sensitivities and wavelength responses of different cameras when comparing disparate images.

To close, the speaker showed a range of recent images taken by members of the Deep Sky Section. Of the autumn galaxies, M33 was a comparatively easy target, but M74, in Pisces, a much greater challenge, having perhaps the lowest surface brightness of any of the Messier objects. The speaker remarked especially upon an image of NGC 925 by Jeremy Shears, taken with a CCD through a 102-mm Takahashi refractor; modern cameras could obtain remarkably fine images through telescopes with surprisingly modest apertures.

Other challenging autumn objects included the planetary nebulae Abell 12, Abell 21 and NGC 7635. However, the speaker closed with a more familiar deep sky gem: the open cluster M41 in Canis Major, showing a fine image taken by Maurice Gavin from Worcester Park.

Following the applause, the President adjourned the meeting until December 13.

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Dominic Ford

Fairfield

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