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Sky Notes by Neil Bone 2005 August & September

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Sky Notes by Neil Bone 2005 August & September Sky notes by Neil Bone 2005 August & September below Castor and Pollux. Mercury is soon ing June and July, it is still quite possible that Sun and Moon lost from view again, arriving at superior con- noctilucent clouds (NLC) could be seen into junction beyond the Sun on September 18. early August, particularly by observers at The Sun continues its southerly progress along Venus continues its rather unfavourable more northerly locations. Quite how late into the ecliptic, reaching the autumnal equinox showing as an ‘Evening Star’. Although it August NLC can be seen remains to be deter- position at 22h 23m Universal Time (UT = pulls out to over 40° elongation east of the mined: there have been suggestions that the GMT; BST minus 1 hour) on September 22. Sun during September, Venus is also heading visibility period has become longer in recent At that precise time, the centre of the solar southwards, and as a result its setting-time years. Observational reports will be welcomed disk is positioned at the intersection between after the Sun remains much the same − barely by the Aurora Section. the celestial equator and the ecliptic, the latter an hour − during this interval. Although bright While declining sunspot activity makes great circle on the sky being inclined by 23.5° at magnitude −4, Venus will be quite tricky major aurorae extending to lower latitudes to the former. Calendrical autumn begins at the to catch in the early twilight: viewing cir- less likely, the appearance of coronal holes equinox, but amateur astronomers might more cumstances don’t really improve until the in the latter parts of the cycle does bring the readily follow meteorological timing, wherein closing weeks of 2005. possibility of recurrent, relatively quiescent northern hemisphere autumn begins at the start Very much returning to prominence is auroral displays for observers at, for exam- of September. With the nights steadily draw- Mars, brightening from mag −0.5 in early ple, the latitudes of northern Scotland. The ing in, and becoming properly dark from early August to mag −1.6 at the end of September weakened solar magnetic field associated August onwards, there is certainly a feeling as the distance between Earth and the planet with coronal holes allows the escape of broad that summer is coming to its close. After the shrinks ahead of November’s opposition. By streams of energetic particles which may equinox, the hours of darkness exceed those of early August, the Red Planet − seen against sweep across Earth for several days at a time, daylight, and longer sessions of night-time ob- the stars of the Pisces/Aries border − is al- returning at roughly 27-day intervals corre- serving become possible. ready rising around 22h UT, and is unmis- sponding to the Sun’s rotation. Coronal hole Sunspot activity should reach its mini- takeable as the brightest object apart from streams can energise the auroral ovals, pro- mum for the current nominal 11-year cycle the Moon in late-evening skies. By mid-Sep- ducing displays consisting of stable, quiet (no. 23 in the series) in the next 12 months. tember, Mars is up by the end of evening arcs or bands, with only occasional rayed Remarkably, occasional large late-cycle spot twilight, and with its apparent disk diameter activity, at higher latitudes. Encounter be- groups continue to appear and daily obser- now greater than 15 arcseconds, the planet tween such streams and Earth is particularly vation by the safe method of projection re- may reveal some surface markings in even favoured around the equinoxes, and the au- mains productive. quite modest amateur telescopes (100− tumn of 2005 might be a productive time for The Moon is new on August 5 and Sep- 150mm aperture) under good conditions. the quiet ‘Scottish aurora’, to borrow a term tember 3, placing the darkest night-time con- Jupiter becomes lost in the evening twi- from Aurora Section Director Ron Livesey. ditions in the opening fortnight of either light during September, and is not well-placed − month good news for Perseid-watchers. for observation in this interval. Saturn returns Full Moon falls on August 19 and Septem- from solar conjunction during August, now ber 18, the latter being popularly known as seen against the stars of Cancer (near the Meteors the Harvest Moon. The shallow inclination Praesepe open cluster M44). By mid-Sep- of the ecliptic to the eastern evening horizon tember, mag +0.4 Saturn rises around 01h UT, Absence of moonlight favours this year’s at this time means that although the Moon and the planet will be high enough for reason- return of the ever-popular Perseids, peaking moves eastwards by about 13 degrees per able telescopic viewing in the couple of hours on August 12−13. Activity should be noted day, it rises only a few minutes later from before dawn. Observers will find the ring sys- from the shower from late July through to night to night in the week or so after Full. tem markedly more closed in presentation than August 20. Also active at this time are the The third weeks in August and September during the opening months of 2005, with Sat- Alpha Capricornids, Delta Aquarids and Iota present excellent opportunities to observe urn now farther south on the ecliptic. Aquarids, from radiants below the Square of lunar features under their local evening/sun- Ideally placed for binocular observation in Pegasus. A detailed prospectus for the set illumination. Even as late as 29 August or the midnight sky are Uranus (in Aquarius, Perseids can be found in the Notes and News 27 September, the waning crescent Moon is south of the Water Jar asterism) and Neptune section on page 189 of this Journal. up before midnight UT. (on the Capricornus/Aquarius border). Nep- September brings the highest background tune, at mag +7.8, reaches opposition (180° sporadic rates of the year for observers in from the Sun in Earth’s sky) on August 8, the northern hemisphere, with up to 10−12 while mag +5.7 Uranus, farther east along the meteors/hr visible from a clear, dark loca- ecliptic, is at opposition on September 1. The planets tion in the latter half of the night. Observ- ers following this activity in September will Mercury is at inferior conjunction between also detect a few Piscid meteors, from mul- the Earth and Sun in early August, then rap- tiple radiants near the ecliptic. Rates are idly emerges into the morning sky. Greatest Noctilucent clouds usually little more than a trickle of one or elongation, 18° west of the Sun, is reached on two per hour, but may be higher around August 23, at which time Mercury will be and aurorae September 8, when a radiant just east of the rising about 90 minutes before sunrise, and Circlet of Pisces reaches its nominal maxi- may be glimpsed by careful observers as a Although their principal season for occurrence mum. Like other showers emanating from magnitude 0 ‘spark’ in the gathering dawn in the northern hemisphere is taken to be dur- the ecliptic region, the Piscids produce fairly 230 J. Br. Astron. Assoc. 115, 4, 2005 This article is copyright © the Journal of the British Astronomical Association, www.britastro.org/journal. If you wish to reproduce it, or place it on your own Web page, please contact the Editor: Mrs Hazel McGee, [email protected] Sky notes slow meteors, mostly rather modest in faintest of the brightness. three, but appear- ances are decep- tive. Intrinsically, this star is one of the most lumi- Variable stars nous in our Galac- tic neighbourhood Having been at peak brightness − usually − it simply ap- around third magnitude − in early May, Mira pears relatively (Omicron Ceti) will probably be fading to dimmed thanks to Summer binocular targets: open clusters in Cygnus, photographed by the limit of naked eye visibility by the time its enormous dis- Nick Hewitt. Left: M39; right: NGC 7058. it re-emerges to view in August, following tance of 1600 light solar conjunction. Binocular observers should years (whereas mag +0.8 Altair and mag 0.0 found around 20 stars ranging from 7th to be able to follow this long period variable Vega are at 17 and 25 light years respec- 9th magnitude, in a triangular outline. At a star down towards minimum later in the au- tively). Deneb is a highly luminous supergiant distance of 7000 light years, M29 is dimmed tumn. Mira is relatively easy to locate, just in the same league as Rigel in Orion. by dust in the line of sight through the Milky west of the triangle of Alpha, Gamma and Albireo is a well-known colour contrast Way plane. Delta Ceti. double star, to which I am sure many of us Observers seeking a challenge might like Also fading, from maximum in early July, return repeatedly for a quick look. The pri- to try for one of Cygnus’ visually faintest, is Chi Cygni, another star of the Mira class mary component is a mag +3.3 orange star, but most celebrated objects, the Veil Nebula of slowly-pulsating variables. Chi may still the secondary a green star of mag +5.1 (other (see page 219). The remnant of a supernova be a faint naked eye object near Eta Cygni, observers may see the fainter star as blue or which exploded about 5000 years ago, the midway along Cygnus’ ‘neck’, in early Au- turquoise). The two have an apparent sepa- Veil is a fragmentary loop of nebulosity some gust, but will probably require binoculars ration of 34 arcseconds, and can be resolved three degrees across, a couple of degrees for observation by September.
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