Sky notes by Neil Bone 2006 August & September

month. Full on August 9 will se- September 1, then Mercury moves into the and Moon verely hamper observations of the Perseid evening sky where it is very unfavourably meteor shower. The September 7 Full placed for observers at northern latitudes. Sunspot cycle 23 is now very close to mini- Moon undergoes a small partial eclipse in Venus’ stint as ‘Morning ’ is drawing mum, and on many days recently the pro- Earth’s shadow. The event is part-visible to a close. Never spectacularly easy this jected solar disk has appeared blank. The from the British Isles; first umbral contact time around, despite its brilliant mag –4, few spot groups which have broken out in is at 18h 05m UT, and the eclipse ends at Venus rises about 1h 40m ahead of the Sun the last few months have mostly been quite 19h 37m UT. From UK locations, during early August, but by the month’s small. Regular observers are now on the look- moonrise is around 18h 40m UT, close to end it will be pretty much lost from view. out for the first spots of cycle 24, which mid-eclipse when 19% of the Moon will Through September, the ’s elongation are expected to soon appear at high solar be immersed in shadow – the northern lu- west of the Sun diminishes rapidly, and latitudes. nar limb really only just clips through the Venus will not be visible again until about From the terrestrial perspective, the Sun outer edge of the umbra. The ‘bite’ should, Christmas when it begins a much more fa- is now heading southwards on the , at least, be noticeable, albeit diminishing vourable evening showing. its apparent annual path against the star in extent as the Moon rises higher. The long wind-down of Mars’ current background. At 04h 03m Universal Time Traditionally, September’s Full Moon is apparition continues, with the red planet (UT, equivalent to GMT; BST minus 1 the Harvest Moon, its strong light for sev- now essentially lost into the bright evening hour) on September 23, the Sun sits on the eral evenings after Full being welcome in sky close to the Sun. Setting about an hour celestial equator: this is the moment of the times past as extra natural illumination for after the Sun, Mars is now a dim, distant autumnal equinox. Thereafter, the hours of bringing in the crops. In both August and second-magnitude object with a tiny (4 darkness exceed those of daylight for those September, the angle between the ecliptic arcsec diameter) apparent disk. at the latitude of the British Isles, ushering and the eastern evening horizon is shallow, Jupiter, seen against the of Libra, is in the prime observing season. with the result that the Moon’s daily east- still reasonably-placed in the southwestern The Moon is new on August 23 and Sep- wards motion (about 13°/day) makes mini- sky for an hour or so in the early evening tember 22, giving the darkest night-time mal difference to the time between succes- during August. The mag –2 planet has a skies in the last ten days or so of either sive moonrises, and right up to last quarter disk diameter close to 35 arcseconds, and (August 16, Septem- should show a fair amount of detail in small ber 14) the Moon is telescopes. During September, Jupiter be- up well before mid- comes lost into the evening twilight, setting night. This is a fa- an hour after the Sun. vourable circum- Saturn reaches conjunction on the far side stance for observers of the Sun on August 7, emerging into the wishing to view fea- morning sky during September. Now lo- tures in the heavily- cated in Leo, a little west of Regulus, the cratered lunar south- mag +0.5 ringed planet rises around 02h ern hemisphere under UT – 2h 30m ahead of sunrise – in mid- local sunset condi- September, providing reasonable telescopic tions at a user- views for early risers. As its orbital motion friendly hour. carries it southwards along the ecliptic, Sat- urn’s rings begin to close in their presenta- tion towards us: observers will find the rings markedly less open than they were during spring 2006. The The dearth of naked eye is some- what compensated by the favourable pres- planets entation of Uranus and Neptune – each visible in 10×50 binoculars – during this Mercury is in the interval. The brighter of the two at mag morning sky in early +5.7, Uranus reaches opposition – 180° August. At greatest from the Sun in Earth’s sky – on Septem- elongation 19° west ber 5, when it will be a little more than a of the Sun on August degree due east of the mag +3.7 star Lambda 8, it will rise only Aquarii, SSE from the ‘Water Jar’ about 90 minutes in Aquarius. ahead of sunrise, and Neptune, somewhat trickier at mag +7.8, at magnitude 0 will be is at opposition on August 11, a degree to quite difficult to find the north of mag +4.3 . in the brightening Both these outer ice & gas giants are too northeastern sky. Su- remote to show much more than tiny (3−4 perior conjunction, arcsecond) disks in amateur telescopes. on the far side of the Their favourable presentation in August Sun, is reached on and September, and the comparative short-

214 J. Br. Astron. Assoc. 116, 4, 2006 Sky notes age of other planetary targets, might make around August 19, when it should be a faint them interesting subjects nonetheless for naked eye object. Peak brightness is reached observation in telescopes of 100mm aper- at intervals of about 14 months, and Chi ture upwards: seasoned planetary observ- Cyg usually gets as bright as 5th magni- ers might find it interesting to compare the tude: some maxima are noticeably brighter, visual appearance of these two distant as was the case in 2004. Chi appears as an worlds. additional star along Cygnus’ ‘neck’, close to 4th magnitude Eta Cyg, and it will re- main an easy binocular object even as it begins to fade during September. As autumn approaches, the familiar ‘W’ Minor planets of Cassiopeia becomes more prominent in the northeastern evening sky. The middle star of the ‘W’, Gamma Cassiopeiae, is a (1) Ceres is at opposition on August 17, well-known naked eye . Gamma when it reaches visual magnitude +7.6. Its Cas is a young, massive rapidly-rotating B- southerly against the stars of class star, prone to ‘shell’ episodes during Piscis Austrinus, west of Fomalhaut, makes NGC 6572 sketched by Neil Bone using an which outer layers are shed leading to a pro- × it a rather unfavourable target for UK-based longed period of brightening. Currently 80mm f/5 refractor at 40. Unfiltered view. (2005 August 28). observers. around mag +2.3, Gamma reached mag +1.7 Somewhat better placed is 8th magni- during the 1930s, for example. Brightness east-pointing hazy wedge of faint stars. In tude (6) Hebe, moving retrograde against estimates made at weekly intervals may a small telescope at low powers (×20 is the stars of about 5° east of reveal subtle short-term variations, and suit- good), NGC 6633 appears to contain about the wide naked eye pairing of Alpha and able non-varying comparison stars include twenty stars, ranging from mag +7 to +9, in , and reaching opposition Alpha Persei (mag +1.8) and Alpha Cephei a narrow triangle with a long axis of about on August 7. (mag +2.4). 10 arcminutes. A solitary 6th-magnitude star Autumn’s arrival also makes it possible lies to the south. to again follow minima of the eclipsing bi- North of 70 Oph, the 4th-magnitude star nary Algol (Beta Persei), during which the 72 Oph is a good guide (2.5° to its NNW) star dips from its usual mag +2.1 down to for the compact planetary nebula NGC Meteors mag +3.4. This will occur on September 6572, an excellent object for medium-sized 26/27, and in the early evening of Septem- telescopes. In small telescopes, the 15 Normally the summer’s main attraction, the ber 29/30. arcsecond diameter, mag +8.1 disk is essen- Perseids are severely affected by moonlight tially star-like; an OIII or UHC filter will at their August 12/13 maximum in 2006. give away its nature (NGC 6572, emitting Only the brightest meteors will be visible, more or less exclusively at the 500.7 and with glare from the early-rising waning gib- 485.9nm excited oxygen wavelengths, re- bous Moon dominant by the time twilight Deep sky mains undimmed, unlike the stars, when has ended. Conditions will be much, much such a filter is passed in front of the eye- better next . piece). Larger telescopes at higher magnifi- The rich Milky Way fields south of Aquila cations will show NGC 6572 as bluish, and The later parts of August provide some remain well-placed in August and – early in relatively low-key minor shower activity may reveal the 13th-magnitude central white the evening – into September. Another re- dwarf star. which, augmented by reasonable back- gion well supplied with good targets is in ground sporadic rates, can make for pro- eastern Ophiuchus, adjacent to Aquila’s Neil Bone ductive watches. The Kappa Cygnids, western (to the right in British skies) ‘wing’, from a radiant in Cygnus’ more northerly around the triangle of fourth-magnitude stars ‘wing’, have their nominal peak around Aug / 67, 68 and 70 Ophiuchi – sometimes known 17 18, when rates of around 5 meteors/hr as ‘Poniatowski’s Bull’. might be attained. Kappa Cygnids are slow 70 Oph, the star at the eastern point of (meteoroid atmospheric entry velocity 25 the triangle, is a good double for medium- km/s), and in some bright meteors aperture (80−100mm) telescopes. The com- and fireballs have been reported from the ponents are separated by a comparatively shower. easy 5 arcseconds. The brighter star is yel- Towards the very end of August, peak- lowish, and shines at mag +4.2, while the ing around September 1, the Alpha Aurigids mag +6.0 secondary, at position angle 148° produce modest activity from a radiant near to the SSE, is reddish. Capella, well up in the northeastern sky by A couple of degrees to the west, just NNW the early hours. of mag +2.8 Beta Ophiuchi, IC 4665 is a fine, loosely-scattered open cluster, readily visible in 10×50 binoculars. Around thirty stars of mag +7 to +8 are contained in a re- gion 45 arcminutes (1½ moonwidths) across. Variable stars A binocular sweep to the east of Poniatowski’s Bull will turn up another of The long-period (Mira type) variable Chi this region’s good open clusters, NGC 6633. Cygni is expected to be at maximum light Binoculars show this as a prominent north-

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