Ordinary Meeting, 2009 January 28

The January Sky

Mr Arditti opened with a review of the circumstances of the solar system bodies. He reported that the Sun remained quiet and continued to show very little sunspot activity, but that some prominences could still be observed on the solar limb in Hα. Of the planets, he reported that Venus and Saturn would be well placed for observation in the coming month, but that Mercury, Mars and Jupiter were not currently observable. He added that whilst it might still be possible to catch Uranus in evening twilight, its present apparition was near its end.

Turning first to Venus, Mr Arditti explained that it presently appeared as a brilliant object in the evening sky and continued to brighten; it would attain a maximum brightness of mag. –4.6 on February 19. He added that as it was presently getting nearer to the Earth, its disk was growing in size – this explained why it was getting brighter – but that the phase of the illumination of its disk was also waning to a crescent. He remarked that whilst the illuminated portion of Venus' disk showed notoriously little detail to the visual observer on account of the thick layers of cloud that enshrouded the planet's surface, some planetary imagers had recently been experimenting in imaging the night side of its disk. By using CCD cameras with near-infrared filters operating at around 1000 nm, some had reported being able to see some clear morphological features on this unilluminated portion of its disk. These were often found to correlate well with surface maps compiled from observations made by the Venus Express spacecraft, suggesting that these observers had, for the first time, successfully made amateur observations of the planet's surface. It seemed likely that this infrared band happened to coincide with a wavelength window where the Venusian clouds were at least partially transparent, and that the surface of the planet was hot enough to glow thermally at these wavelengths, though it had to be conceded that the correspondence of the morphological features in the amateur images with those present in Venus Express observations was not perfect. It consequently seemed likely that at least some of features apparent in the amateur images were actually of atmospheric origin.

The speaker also commented upon the 200-year-old speculation that the planet's night side could be observed to faintly glow – a phenomenon termed the Ashen Light. This effect was exceptionally difficult to observe due to the glare of the nearby illuminated portion of the disk, to the extent that its existence remained controversial to this day, but it had nonetheless been reported very widely by visual observers. To date, no photographic observations had ever been made of it, and the speaker posed this as a challenge to the audience.

The speaker then turned to the Moon, which he explained had passed New Moon two days previously on January 26, would reach Full Moon on February 9, and would reach New Moon again on February 25. On February 4 it would occult the Pleiades between 2h15 and 4h30 UT, though the circumstances were not favourable for observation from the UK since the Moon would be at an altitude of only 7° at the start of the occultation, and would set before its conclusion. The brightest star to be occulted by the Moon in the coming month would be ε-Gem (mag. 3), which would disappear behind the Moon at 19h25 UT on February 6 and reappear at 20h30 UT; at this time, the Moon would be 89%-sunlit and at an altitude of 50° in the UK sky. Finally, the speaker briefly mentioned that in the early morning sky of January 30, the Moon would pass within 1.5° of Venus, forming a photogenic pair.

Mr Arditti reported that Saturn's rings continued to present themselves in an edge-on orientation, as had been discussed in previous Sky Notes talks. He added that they had opened out a little in the past few months, from 1° inclination in December to 2° presently, and that it was now just about possible to resolve the Cassini division. They would, however, narrow again later in its apparition and eventually close completely in August, though this ring-plane crossing would itself not be observable owing to Saturn's proximity to the Sun at the time; it would be only a few weeks away from reaching conjunction on September 17.

Several minor planets were well placed for observation from the UK at present, including 2 Pallas, at mag. 8.2 in Eridanus, and 27 Euterpe, at mag. 9.0 in Cancer, close to the Praesepe Cluster (M44). Early in the evening, 4 Vesta could be found at mag. 8.0 in Cetus, and later in the night, 1 Ceres, could be found at mag 7.4 in Leo.

The principal comet of interest over the next few weeks would be Comet C/2007 N3 (Lulin), which was currently a mag. 6-7 object in Libra. Though it did not rise until the pre-dawn hours at present, it was rapidly moving westward and brightening; it was forecast to pass close by Spica on February 16, and then pass M44 on March 5-6. If it continued to brighten as expected, it was predicted to reach around mag. 4 in late February, becoming an easy binocular target in the UK sky.

The speaker identified ζ-Aur as a variable star to watch over coming months; this was an eclipsing binary whose next eclipse was forecast to begin early in March. The ingress and egress of this eclipse would each take 1.5 days, during which time the star's magnitude would change modestly but detectably from 4.0 to 3.7 in the V-band. Totality was expected to last for 37 days. The speaker also went on to urge members to observe the Variable Star Section's Variable Star of the Year, IP Pegasi, about which more details could be found in the Association's Handbook for 2009.

Turning lastly to the deep sky, Mr Arditti extended warm congratulations to Mr Ron Arbour upon the discovery of his 20th supernova, 2009M, on January 20 in NGC 1028 in Cetus. He then gave a brief review of the open clusters which were accessible in the winter sky. At present, the three famous clusters in Auriga, M36, M37 and M38 were all at high latitudes and relatively easy targets in the UK evening sky. However, he added that M46, M47, M48 and M50 were more challenging and not often observed from the UK on account of their southerly declinations; he closed his talk by showing some sketches of these clusters by Dale Holt.

Following the applause, the President adjourned the meeting until Wednesday March 31.

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Dominic Ford