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Sky notes: 2013 August & September.

(Written for 22:00 UT on 2013 September 1.)

In the north Ursa Major has almost completed its descent towards the horizon meaning that on the opposite side of the pole, Cassiopeia is climbing and Cepheus is approaching culmination not far from the zenith. There are a number of open clusters that fall within the borders of Cepheus, the brightest being NGC 7160 which lies inside the quadrilateral that makes up the main body of the constellation. At magnitude 6.1 it requires only a small telescope or binoculars. Cassiopeia also has its share of such clusters, the best known of which are M52 (magnitude 6.9) and NGC457 (magnitude 6.4).

Now is a good time to identify the convoluted shape of Draco the dragon, which begins with its tail between the two Bears and ends with its head close to Hercules. The third star in the tail of the dragon is Thuban, otherwise known as Alpha Draconis although it is not the brightest star in the constellation. Nearly 5,000 years ago it had the distinction of being the Pole Star, but due to the Earth's precession it has lost that role, one that it will not return to until approximately 20350 AD. The brilliant Capella is now visible some 15[degrees] above the north eastern horizon.

Looking east we see that the autumn favourites of Pegasus, Andromeda and Perseus are making their presence felt. The asterism that we know as the 'Square of Pegasus' is of course something of a misnomer because the top left star is Alpheratz or Alpha Andromedae. Pegasus is home to M15, a globular cluster that lies on the constellation's western fringes close to the border with Equuleus. At magnitude 6.3 it is an easy object in binoculars but to resolve individual stars requires a telescope aperture of at least 120mm. Below Pegasus we find one of the fishes of Pisces, a constellation that has been, and will continue until 2017 to be, the home of the planet Uranus. It contains a smattering of galaxies, the brightest of which is the face-on spiral M74 at magnitude 9.2, although its surface brightness will be lower. It is best located by star hopping from eta Piscium which lies 1[degrees] 20' away to the west.

Towards the south the bright star Deneb lies on the meridian at an altitude of more than 80[degrees] meaning that the Pelican and North American nebulae are well positioned for imaging. Below and slightly to the west of Deneb we find one of the other members of the Summer Triangle--Altair in Aquila, which is always easy to identify because of the two stars ([beta] and [gamma] Aquilae) that attend it on either side.

Lower, but still on the meridian, is Capricornus which contains the globular cluster M30, visible in larger binoculars although a moderate sized telescope is needed to begin resolving individual stars. Lower still lies the small and insignificant Microscopium. On a really clear evening it is worth trying to locate its brightest stars; gamma ([gamma]) at magnitude 4.67 and epsilon ([epsilon]) at 4.71 despite their being at an altitude of just over 6[degrees].

Turning to the west, the final member of the Summer Triangle, brilliant Vega in Lyra, is at a commanding altitude of 65[degrees]. Beta ([beta]) Lyrae is a much observed naked eye variable with a period of thirteen days and a magnitude range from 3.3 to 4.3. Also in Lyra is the lovely planetary nebula M57 at magnitude 8.8 lying roughly on a line between beta ([beta]) and gamma ([gamma]) Lyrae and slightly closer to the former. An aperture of 120mm will show it well but the central star at magnitude 14.8 will require something with considerably more light gathering power such as a 300/350mm instrument.

Below Lyra and skimming the south western horizon are the two indistinct constellations of Serpens and Ophiuchus, both of which were members of Ptolemy's original 48. The former has been divided into two (Serpens Caput and Serpens Cauda) by the large and faint Serpent-Bearer. M5 is a magnitude 5.7 globular cluster in Caput which should be just visible to the naked eye given a dark site and good air quality. It is an excellent sight in a moderate telescope and is well worth observing despite its low altitude.

At this time of the year the Milky Way is a beautiful sight as it makes its way through Perseus, Cassiopeia, and Cygnus before it meanders to the horizon via Aquila and Sagittarius. At the time of Perseid maximum, meteor watchers at dark sites will be able to admire its majesty particularly at times of good transparency. A pair of binoculars should always be to hand to sweep it looking for those jewels that a simple pair of 7x50s can so often reveal.

Planets and dwarf planets

Mercury reached greatest western elongation on the last day of July, but despite this is poorly placed for observation during August. Superior conjunction occurs on August 24 but the situation doesn't improve during September with the planet setting only 30 minutes after the Sun at month's end. The early second week of October. However, providing caution is exercised, Mercury can be found in daylight with Goto telescopes of moderate size although 'sweeping' for the planet this near the Sun should most certainly not be employed.

Venus suffers from the same problem in that it will always be close to the horizon. It begins August 32[degrees] from the Sun but setting just 1% hours after it. With the Sun 6[degrees] below the horizon (the start of nautical twilight) Venus is just over 4[degrees] above it, so despite having a magnitude of -3.8 will be an extremely difficult object. If we fast forward to the last day of September we can see what changes there have been. The angular separation of the two bodies is now over 44[degrees], but Venus still sets around 1% hours after the Sun and is still approximately the same height above the horizon when nautical twilight begins. Things do, however, improve as the year wears on and although the angular distance between Venus and the Sun decreases, the planet's height above the horizon starts to increase.

Earth reaches the Autumnal Equinox on September 22 at 20:44 UT when its axis is tilted neither towards nor away from the Sun.

Mars is becoming a more obvious morning object whose brightness remains steady at +1.6 whilst its apparent size increases slightly. It begins the period in Gemini (in the company of Mercury and Jupiter) although its direct motion carries it into neighbouring Cancer before the month's end, finding itself amongst the stars of M44, the Beehive cluster, on September 9. By the end of September it has made its way across the border and into Leo. At the start of August Mars rises 2/4 hours ahead of the Sun whilst by the end of September this has become nearly 4/ hours although it will be next January before it becomes an evening object.

Jupiter is a morning object that remains in Gemini throughout August and September. Its magnitude increases slightly from -1.9 to--2.1 during the period with this brightening continuing for the rest of the year. At the beginning of August the planet rises more than 2/ hours ahead of the Sun whereas by the end of September it has technically become an evening object rising 15 minutes before midnight. It is fascinating to watch the dance of the four Galilean satellites as they cross in front and disappear behind the body of the planet. A full list of occultations and transits can be found in the Handbook of the BAA, and more information is available on the Jupiter Section website at http://www.britastro. org/jupiter/

Saturn begins the period in Virgo but its direct motion carries it across the border into Libra on September 1. At the start of August the planet is visible in the SW sky at an altitude of 17[degrees] with the Sun 6[degrees] below the horizon. The two bodies are, however, moving gradually closer together and by the end of September will be 'only' 32[degrees] apart with Saturn just 5[degrees] above the horizon at the start of nautical twilight.

The planet's north pole increases its angle of tilt from 17.5[degrees] to 19.2[degrees] during the period making Saturn a superb sight particularly in moderately sized instruments. The amount that the pole is inclined towards us continues to increase throughout the rest of this year. Saturn's largest satellite, Titan at magnitude 8.7, is an easy object particularly when at elongation. These occur on August 7, 15, 23 and 31 and also on September 8, 16 and 24. Planetarium programs will soon identify the retinue of fainter attendants that cluster around the ringed planet.

Uranus is currently in Pisces and rises at a little before 22:00 UT as the period begins. By the end of September it culminates at midnight UT at a very respectable altitude of 42[degrees]. At magnitude 5.7 it is at the boundary of naked eye visibility, but is an easy object in binoculars with a small greenish-blue disk observable in larger amateur telescopes.

Neptune spends the whole of August and September in Aquarius at magnitude 7.8. On the last day of the period it lies approximately mid way along a line drawn from sigma (o) Aquarii (mag 4.8) to 38 Aquarii.

(1) Ceres and (4) Vesta both go through solar conjunction in August, and thereafter become morning objects in Leo, rising more than two hours ahead of the Sun as the period ends

Meteors

August sees one of the year's most prolific showers come to maximum [see also Dr John Mason's article on page 187]. The Perseids are active from July 23 until August 20 with the peak of activity being reached on the night of August 12, when possibly one meteor per minute may be seen. This year on that night the Moon is a small waxing crescent that will have set by 21:30 UT.

The meteors (associated with comet Swift-Tuttle) are in many cases fast bright events that regularly leave ionised trains in their wake. The radiant lies close to the 'sword handle' in Perseus, although this is of limited relevance as meteors from the shower can be seen in most areas of the sky. I have seen them myself disappearing below the southern horizon.

Lunar occultations

In the table I've listed events for stars down to magnitude 7.0 although there are many others that are either of fainter stars or those whose observation may be marginal due to elevation. DD = disappearance at the dark limb, RD = reappearance at the dark limb whilst RB = reappearance at the bright limb. There is a column headed 'mm' to indicate the minimum aperture required for the event. There are several interesting daytime occultations, one of which involves alpha (a) Virginis or Spica, and the other epsilon Tauri. Both stars have one event that occurs at the bright limb. Times are for Greenwich and in UT.

Lunar Graze occultations

Two grazing occultations of reasonably bright stars occur during the period in question.

The first is on August 14 at 20:19 UT and involves the magnitude 5.5 star 41 Librae. The track crosses Southern Ireland, South Wales and East Anglia. The second event is on September 2 at 03:15 UT, and on this occasion the mean graze track for the mag 6.5 star, HIP 40177, crosses the tip of South Devon, Portsmouth and the Isle of Thanet in Kent. More details can be found in the Handbook of the BAA.

Brian Mills

Phases of the Moon:
2013 August/September

New     First     Full     Last
        quarter            quarter

Aug 6   Aug 14    Aug 21   Aug 28
Sep 5   Sep 12    Sep 19   Sep 27

Lunar occultations of bright stars

Date      Time    Star             Mag   Ph   Alt
                                              [degrees]

Aug 1     09.15   epsilon Tauri    3.5   DB   54
Aug 1     10.28   epsilon Tauri    3.5   RD   45
Aug 15    20.41   ZC 2394          6.3   DD   15
Aug 17    21.12   ZC 2724          6.3   DD   19
Aug 17    22.49   SAO 161842       6.9   DD   16
Aug 17    23.26   ZC 2733          6.8   DD   13
Aug 18    20.54   ZC 2889          6.9   DD   20
Aug 19    23.41   ZC3051           6.8   DD   25
Aug 27    00.09   ZC 450           6.4   RD   26
Aug 29    00.04   ZC 718           6.0   RD   13
Sept 8    13.56   alpha Virginis   1.0   DD   27
Sept 8    15.08   alpha Virginis   1.0   RB   26
Sept 11   20.01   Psi Ophiuchi     4.5   DD   8
Sept 12   18.49   SAO 185318       7.0   DD   17
Sept 14   18.16   ZC 2828          5.8   DD   18

Date      Time    % illum.   mm

Aug 1     09.15   26         140
Aug 1     10.28   25         70
Aug 15    20.41   66         60
Aug 17    21.12   86         70
Aug 17    22.49   86         110
Aug 17    23.26   86         110
Aug 18    20.54   93         110
Aug 19    23.41   98         110
Aug 27    00.09   63         60
Aug 29    00.04   44         40
Sept 8    13.56   11         40
Sept 8    15.08   11         60
Sept 11   20.01   40         40
Sept 12   18.49   51         70
Sept 14   18.16   73         110
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Title Annotation:Sky notes
Author:Mills, Brian
Publication:Journal of the British Astronomical Association
Geographic Code:4EUUK
Date:Aug 1, 2013
Words:2259
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