Ordinary Meeting, 2004 March 31

 

The Spring Sky

Starting close to home, Mr Mobberley discussed the prospects for the forthcoming total lunar eclipse on May 4, though the UK was unfortunately not going to be a good observing location this time around. The eclipse would be visible at moonrise at 21h00 UT, already having reached totality. The Moon would still only be peeking above the south-eastern horizon at an altitude of 11° at the close of totality, so it seemed that only UK observers with particularly flat horizons would see it.

Further from home, NASA's rover Opportunity had taken a break from surveying Martian geology to observe transits of the Martian moons across the solar disk. First, Deimos had transited the Sun on March 4, photographed by Opportunity's PanCam at 3h04 UT. Phobos, the larger Martian moon, had later eclipsed the Sun's edge on March 7, captured on camera at 2h46 UT. Never before had mankind made extraterrestrial eclipse observations. Also from Mars, the first images of the planet's landscape from Europe's Mars Express satellite were truly stunning. Craggy cliff faces towered around impact craters, the scars of past landslides clear to be seen. Even the tiniest details could be resolved, each pixel representing a distance of only 10m. By imaging some features at oblique angles, the landscape came to life, appearing to reach off towards the horizon.

Back on Earth, few could have failed to notice the particularly bright intruder which had appeared in the evening sky in recent weeks, shining brilliantly at mag –4. Though the surface of Venus was fairly featureless, the new apparition brought with it a renewed challenge for amateurs to image the so-called Ashen Light. Many visual observers claimed to have observed this apparent brightening of the shadowed side of Venus, as if the planet were itself glowing. However, none had yet been able to convincingly image it, and it remained without theoretical explanation. As the apparition drew to a close, the prospects for observation would improve as the shadowed portion of the surface grew larger. Around April 3, Venus would skirt within 30' of the Pleiades, and the speaker pointed out that at this time, both objects, as well as Mars, would all be within 2° of the same declination, making easy observing for those with equatorial mounts.

The Association's resident supernova hunters had been hard at work, Mark Armstrong having five new discoveries, and the President three, bringing the UK total to 130 discoveries. In addition, Nishimura and Liller had discovered a nova at mag 8.2 near the Sagittarius teapot on March 18, an area where there had been a number of similar events in recent times. Also notable was Peter Birtwhistle's fourth asteroid discovery, 2004 DN25, on February 22, around mag 20-21. It seemed that he had beaten the automated searches of LINEAR and NEAT to it, and was particularly remarkable since in the past, great discoverers like Brian Manning had never picked up such objects until they were much brighter, typically mag 16-17.

A great deal of media excitement had stirred with the discovery of the most distant asteroid found to date, provisionally named 'Sedna'. This was the latest in a flurry of discoveries of transneptunians (asteroids lying beyond the orbit of Neptune) in recent years. With the exception of Pluto, the first known example was 1992 QB1, discovered only twelve years ago. Since then, over 800 had been identified, five larger than 900km across, but the recent discovery of 2004 DW on February 17 was exceptional in that its size was estimated to be around 1,500km, at least half the size of Pluto. The possibility remained that it might be substantially larger since Pluto's unusually high albedo made size comparison difficult by comparative brightness alone. If this were confirmed, 2004 DW would be the largest minor planet known, larger than Charon, and bringing Pluto's status as a planet further into question. The speaker showed an image of 2004 DW on March 7 by Peter Birtwhistle, noting with amazement that the Moon had been one-day past full that night, 41° away. In the stacked image, of total exposure 42 minutes, the estimated magnitude of the minor planet was 18.8.

The discovery of 'Sedna', officially named 2003 VB12, had been announced on March 15 based on observations from the Palomar Observatory on 2003 November 14. Its highly eccentric orbit had perihelion at 76 AU, around twice the distance of Pluto, and aphelion at a remote 990 AU. It was presently close to perihelion, and appeared around mag 21. At 1,500km across, it was the second largest known transneptunian only to Pluto, but its distant aphelion was not the furthest known: the orbit of 2000 OO67 receded as far as 1010 AU from the Sun.

Nearer to home, asteroid 2004 FH had passed within 30,000 miles of the Earth on March 18, the closest recorded pass of an asteroid by the Earth. For comparison, the speaker gave the Moon's distance as 238,000 miles. However, at 20-metre diameter, the rock posed no threat to humanity at all, as if it were to collide with the Earth in the future it would simply burn up in the atmosphere.

Comet observers were eagerly awaiting C/2001 Q4's close pass by the Earth on May 7, dubbed by some "The Comet of 2004". Presently a southern hemisphere object at dec –65° and mag 5, it would surge northwards in the first week of May, becoming visible in the evening twilight shortly before its closest approach to the Earth, and it seemed possible that it might reach a peak brightness around mag 2 in mid-May. It would go on to reach perihelion on May 15, passing within a degree of the Beehive cluster (M44) on that night, grazing it with its tail. The speaker recommended this as an ideal photo opportunity. Remaining an easy northern hemisphere object, it would pass through Ursa Major in June, fading to fifth magnitude by this time. Heading off in a direction close to the north celestial pole, it would become circumpolar from the UK until 2008, and thus be permanently visible in the coming months as it faded out of sight.

The other exciting comet prospect for 2004, C/2002 T7, was no longer observable from northern latitudes, having produced many spectacular photographs during the course of February as it neared its April 23 perihelion, 0.61 AU from the Sun. Though it had been lost to northern observers in mid-March, if it were bright enough, it might just be visible around the time of perihelion, on the horizon in nautical twilight. Making closest approach to the Earth on May 19, it was hoped it would reach mag 0-1 for southern observers. The speaker had been asked whether it might be possible to observe both comets simultaneously from any location. Though difficult, it seemed both might be simultaneously visible from at latitude –10°, though not until both comets were well past their best.

For the more dedicated, C/2003 K4 would be visible throughout the summer, expected to peak at around mag 5 in the autumn. Presently a northern hemisphere object in Vulpecula at mag 11, it would gradually brighten over the summer months passing into Hercules and reaching mag 8 by July 1, before sinking southwards in September, a few weeks in advance of its October perihelion. Also, Comet Tabur would provide a challenging target, lurking just above dawn nautical twilight throughout May at mag 9. 43P/Wolf-Harrington was low in the western evening sky, passing close by Aldeberan on April 12 at around mag 13.

Jupiter and Saturn were well placed for UK observation, transiting at 6pm and 10pm respectively at the time of the meeting. Saturn's rings were wide open, and Damian Peach's most recent images revealed a markedly blue tinge to the southern polar regions. The speaker congratulated Mr Peach upon the fidelity of his images, and members applauded. In the past, there had been suggestions that the poles appeared bluer, rather similar to the hue of Uranus, when the rings were closing after a period of being wide open. The speaker speculated that this might be because when the poles emerged after a long period in shadow, the frigid surface conditions influenced their colour. In July, as the present tilt of the rings closed, the northern polar regions would emerge from behind the rings, and after spending many months in the shadow of the dark side of the planet, it would be interesting to see what colour they appeared. It was not just amateur observers who were returning superb images of Saturn: the Cassini probe was nearing its destination and had taken a number of images in the past few months, as it drew ever closer to an anticipated orbital insertion on July 1.

Variable star observers were urged to watch η-Geminorum, normally mag 3.2-3.4, previously observed to eclipse every 8.2 years. If the trend continued, an eclipse was anticipated in July, and Colin Henshaw had already reported a slight dip in its brightness in February. Finally, the speaker closed with the discovery of a new nebula by Jay McNeal, an amateur astronomer, using only a 3-inch aperture. The new nebula, close to M78 in Orion, remained somewhat of a mystery, having previously only been known in the infrared. Though uncomfortably close to evening twilight, it would be observable from the UK for a few weeks following the meeting, at an altitude of 20° above the western horizon at 20h30 UT.

Following the applause for another lively instalment, the President thanked the speaker before inviting Dr John Rogers, Director of the Association's Jupiter Section to present the evening's final talk.

Cambridge

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