Ordinary Meeting and Exhibition Meeting, 2005 June 25

 

The June Sky

Mr Mobberley opened with a review of the UK supernova scene, which would be exceptionally quick this month: the recent unprecedented lull continued, and there had been no new discoveries since Tom Boles' 89th on April 11. However, supernova enthusiasts might like to try imaging a recent discovery of the Lick Observatory, SN2005cf in MCG 1-39-3 in Libra. At discovery, on May 28, it had been mag 15.5, but by mid-June had brightened to mag 13.5, making it an easy CCD target. It had been confirmed as a Type Ia event.

As a brief interlude, the speaker remarked that the time of year had arrived when, rather to his dismay, arachnidan guests seemed to delight in visiting his observatory uninvited, and one specimen was displayed.

At 10pm on the following evening, June 26, Mercury, Venus and Saturn would all be contained within a two-degree circle – the second closest conjunction of a triplet of planets in the period 1970-2030. Mercury and Venus would themselves lie at a separation of only 0.1°. The proximity of the Sun would necessitate twilight observing, but members were challenged to image it.

Turning to comets, the brightest at present was 2005 K2 at mag 7, discovered unusually close to the celestial pole at a declination of +75° by LINEAR on May 19. Throughout June it had been plunging southwards through Camelopardalis, entering Ursa Major on June 3, then passing through Lynx into Cancer, where it was now below the southern horizon for latitudes northward of +50°. It would reach perihelion in Hydra at an estimated mag 5 on July 5, before curving sharply westward, passing through Canis Major in September. The speaker commented that it might return as a faint northern object in the winter, however doubt had been cast over this by recent reports of a divided nucleus, implying it to have broken apart; Jonathan Shanklin added that the latest observations indicated that it had faded unexpectedly to mag 9, seeming to confirm this.

Among the fainter comets, 2004 Q2 (Machholz) had now faded to mag 11 and was in Canes Venatici. In mid-July it would pass into Boëtes, where it would remain until October, passing 2° south-west of Arcturus around August 19. 161P/Hartley-IRAS, also around mag 11, was heading northward through Casseopeia, to pass into Camelopardalis in early July, and ~8° from the celestial pole later in the month. 21P/Giacobini-Zinner, a little brighter at mag 10, was in Aries, and would pass into Taurus in early July, and within ~1° of Aldeberan on July 20.

However, perhaps the most interesting observing target would be 9P/Tempel, presently in an easily identifiable part of the sky, 5.5° north of Spica in Virgo. On July 4, NASA's Deep Impact spacecraft would collide it with a 370-kg projectile. From the UK, it would not be possible to observe the moment of impact – that would be at 06h00 UT – but its behaviour in the following days would be highly uncertain, a bright flare being possible. However, its southerly declination – -9.5°, and sinking – would render it tricky to observe from the UK: an early evening object, it had been at altitude 40° at the start of evening nautical twilight on May 25, as compared to a meager 13° on July 4. Mr Mobberley concluded that twilight would pose a huge problem unless it flared spectacularly, and that, in any case, a flat southern horizon would be prerequisite for its observation.

Mr Mobberley briefly mentioned that NASA's Space Shuttle Discovery was due to return to flight on July 13, on its first mission since the loss of Columbia, before announcing the discovery of a new nova in Aquilla on June 10 by the ASA3V instrument of the All Sky Automated Survey at the Las Campanas Observatory in Chile (operated by the Carnegie Institution of Washington). At discovery, its V-band magnitude had been 10.5±0.5, though a light-curve constructed by Guy Hurst from UK observations suggested it to have now faded to mag 12±0.5. The speaker remarked upon the number of novae discovered in the region (RA 18h→20h; Dec –10°→30°) around Aquilla: it had hosted seventeen since 1968.

Moving on to planetary observation, the speaker reported that Jupiter, in Virgo, and setting soon after the Sun, remained just observable at altitude 30° at sunset. A number of images by Association members were shown, most notably those by Dave Tyler, who had found that observing only a few minutes after sunset gave the finest images; for such a bright target, twilight proved to be a lesser problem than the deterioration in seeing closer to the horizon. The speaker noted that both Mr Tyler and the great Damian Peach were present in the audience, and members applauded their images.

Mars, now a morning planet, would pass the next six months in Pisces and Aries, and presently rose to 30° altitude before sunrise at 04h30 BST. Mr Mobberley predicted that its apparition this autumn would yield some exceptional UK observations. While its 2003 opposition had been hyped as its closest approach for thousands of years, its –20° declination had placed it very low in the UK sky; by contrast, its declination would be +15° this autumn, reaching +20° in 2006 January, meanwhile the planet's disk, at 20", would not be much smaller than it had been in 2003.

Giving some observing tips, he advised that, as with Jupiter, twilight was a minor consideration in comparison with the improving seeing with altitude. Indeed, with Mars rising so soon before the Sun, some of Mr Tyler's finest images had been taken up to an hour after sunrise (though care had to be taken, pointing telescopes east after sunrise). Secondly, he remarked that Mars' rotation period, 24h37m, was very similar to that of the Earth, and that, as a result, the same face of Mars was visible at nearly the same time on consecutive days, to within 37 minutes. From any given observing location, it consequently took nearly a month to observe the whole surface, even if observations were made throughout the hours of darkness. Each night, new features were visible early in the night, when Mars rose, as its rotation lagged a little from the night before. At the time of the meeting, they appeared on the terminated limb. The speaker warned it was easy to get confused: as the planet rotated throughout the night, it revealed features on the disk's fully-illuminated edge, but from one night to the next, the new features were those on the opposite edge, just disappearing into the terminator as it rose.

Mr Mobberley advised that misty, or even foggy, weather was not a lost cause, and often accompanied anticyclonic systems which yielded steady air masses and very fine seeing. He also urged members, in their excitement to observe the planet itself, not to forget Phobos and Deimos. Finally, to obtain eye-catching sharp colour images, he recommended the use of the LRGB imaging technique described in his March Sky Notes1.

Of recent images, a series by Dave Tyler taken on June 18 at 04h01, 04h07, 04h16, 04h27 and 04h41 UT stood out especially, each having been stacked over 200 seconds. So fine was the resolution that the rotation of the disk was clearly visible when the sequence was viewed in animation. The speaker noted that it was even quite distinct when the 04h01 and 04h07 images were blinked, even though Mars had rotated through only 1.5° in that time interval, shifting even the most rapidly moving features at the centre of the disk by a mere 0"14 across the sky.

Briefly turning to the Cassini probe, the speaker remarked how easy it was, after the initial excitement, to forget its ongoing achievements. On June 9-12, it had completed its first close fly-by of Hyperion, and a compilation of the raw images by John Rogers was shown. On June 8, superb images had also been obtained of Dione – a nearly-spherical rock, not dissimilar in appearance to our own Moon.

Looking ahead, this being the last Sky Notes before the September Out of London Meeting, there were several forthcoming events. Firstly, the Perseid meteor shower, likely to yield occasional meteors throughout the period July 23 until August 20, with maximal activity around 13h00 UT on August 12, at ZHR ≈80. On the night of 11/12, the speaker noted that the seven-day-old waxing Moon would set at 21h26 UT, and so dark skies were possible in the early hours of the morning, when the shower would be at its finest.

Those looking for a holiday with a brief bit of excitement might be interested by the occultation of mag 1.4 star Regulus by mag 15.4 asteroid Rhodope, of 35-km diameter, on October 19 04h23-04h30 UT. The occultation would be visible along a narrow track passing through Portugal, Spain, southern Italy, Greece and Turkey. It was the brightest star to be occulted in the hours of darkness until that of the same star in 2014 March, though with a maximum duration of 1.1 seconds, and the uncertainty in the position of its track making it impossible to guarantee an event at any given location, it seemed a long journey for little reward.

Sixteen days earlier, on October 3, an annular solar eclipse would be visible across Spain, Portugal and parts of Africa. The duration of the European portion of the annular phase would be between 4m10 (Portugal) and 4m20 (southern Spain), and take place between 08h55 UT (Portugal) and 09h05 UT (southern Spain). The African eclipse would commence when the annular phase touched the Algerian capital, Alger, at 09h05 UT. A partial eclipse would be seen in London 07h49-10h19 UT, maximum 66.1%; Edinburgh 07h53-10h13 UT, maximum 58.5%.

Mr Mobberley concluded with the second half of the slideshow that he had started in his previous Sky Notes2, showing the fruits of Damian Peach's recent 21-day observing expedition to the steady skies of Barbados, from which he had returned with 400 Gb of images. The speaker opened with double star Porrima, aka γ-Vir, which had been discussed in detail in his April Sky Notes3, and the components of which were presently separated by less than 0"5. From the UK, they seemed impossible to resolve, but Mr Peach had just managed to do so in Barbados using blue filtered images, and from them, the separation could be estimated to be 0"35.

A sequence of quite remarkable images of Jupiter followed, showing fine filamentary structure within the north/south tropical belts, and resolving fine details within the Great Red Spot. In one image, from April 25, no fewer than 18 anticyclonic circulations, appearing as white spots on the surface, could be seen. On the same morning, at 01h41 UT, an image had revealed Europa transiting in magnificent detail; the speaker thought it just possible to detect a phase in Europa's disk by comparing the brightnesses of his two limbs.

These were followed by images of Mars of similar quality. Mr Mobberley remarked that while these might not appear exceptional, at the time when they had been taken, in late April, the planet had subtended a mere 6"56. It was stunning in itself that they were comparable to older images taken around opposition, when Mars' disk had been several times larger. Following the applause for Mr Mobberley's fine presentation, the President proceeded to introduce the afternoon's final speaker, Mr Roger Dymock.

Ashburn

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39.04°N
77.49°W
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