Night Sky Watch for October

Written by David Pugh

Dear Colleagues,

October can often provide steady conditions for observing night sky objects and the nights continue to draw out, providing longer observing conditions if the clouds part.

Although Mercury is at greatest eastern elongation on October 1, it will not be visible in the evening from the UK. However, Venus is a dazzling spectacle at mag -4.1 in the morning skies, rising in the east 3 hours before sunrise. On the mornings of October 2 & 3 Venus passes very close to the mag 1.4 star Regulus in the constellation Leo and should be a treat in both binoculars and a telescope for early risers. Seen in a telescope Venus's disk shrinks from 16 to 13 arc secs and the phase, (the amount lit by sunlight), increases from 72% to 81% as it heads towards the far side of the Sun. On Oct 3 you should be able to see Venus and Regulus in the same telescopic field of view, a rare sight. With the naked eye Venus and Regulus will appear close together through the first week of October. In the pre-dawn sky of October 13 and 14 Venus is fairly close to the waning crescent Moon which should make a beautiful naked eye or binocular view.

Mars, however, is really the planet of the month as it reaches its very closest to Earth, (62 million kilometres) on October 6 and perihelic opposition 8 days later on October14. It will shine at a dazzling - 2.6 in Pisces, outshining Jupiter, its prominent orange-red colour dominating the eastern and southern sky through October nights, being on view for 13 hours a night. Mars only comes this close every 2.2 years so it is a special opportunity for telescope owners to view Mars surface markings. Mars disk is a spectacular 22.6 arc secs wide. The south polar cap is getting quite small but features such as the Syrtis Major may be visible if there are no prominent dust storms. At 6am on October 3 Mars is only 0.8 degrees NW of the full Moon and, at 7pm on October 19 only 3.3 degrees above the nearly full Moon.

Although telescopically I would suggest that you take every opportunity to view Mars this month, it is often the case unfortunately that a crystal clear night may result in poor "seeing" conditions for planetary observing or imaging. If this is the case you may well experience soft focus rather than a sharp image or the planet appearing to dance about or the disc appearing to expand and contract in the worst apparent conditions. Always open up your telescope tube at least half hour before you intend to use it to allow internal temperatures to equalise with the outside air. Often if there is hazy or passing cloud, whilst this is far from ideal for deep sky observing, it can result in steadier seeing for planetary and lunar observing. At least Mars will reach an appreciable height well above the horizon so should be less affected by adverse conditions than Jupiter and Saturn located close to the south-south west horizon in Sagittarius.

In fact by the end of the month Jupiter is setting by 10pm with Saturn, a few degrees to its east, being not far behind, so it is your last chance to view the gas giants this month. The Moon gets spectacularly close to Jupiter on October 22. Uranus is at opposition on October 31st. It offers the most favourable opposition for decades, due to its very accessible location in Aries. Through the month Uranus is between 15 and 22 degrees east of Mars, so the red planet is a good starting point for seeking out mag 5.7 Uranus, which is a binocular object but obviously needs a telescope to make out its small (3.7 arc sec) greenish disk. Neptune is an evening object that was at opposition in September. At mag 7.8 it is at its highest at 11pm mid month. It requires a good star chart to find but it is just half a degree below mag 5.7 98 Aquarii during the first half of the month.

Peaking on October 20/21, the Orionid Meteor Shower, associated with its famous parent comet Halley, is a reasonable shower. It produces about 20 fast meteors per hour in ideal conditions, often leaving persistent trains. They appear to radiate from the Orion / Gemini border which rises after 10pm. The waxing crescent Moon should have long set by the time that the radiant is well placed so conditions should be favourable.

In terms of deep sky observing, most of the summer constellations and objects mentioned last month are still on view, though Aquila will be sinking down toward the SW horizon. Naturally though, the autumn constellations are now well placed, headed by the "signpost" asterism of the Square of Pegasus which leads us to many of the autumn constellations, such as Andromeda, Triangulum, Aquarius, Pisces, Pisces Australis, and Aries. In Pegasus, apart from the nice globular cluster M15, mentioned last month, if you have a telescope you could try the impressive spiral galaxy NGC 7331. At mag 9.5 and 47 million lys away it is one of the very largest spirals known with a diameter of 130,000 lys.

Moving NE to Andromeda there is, of course, our Local Group giant spiral M31, the famous Andromeda Galaxy. It appears as a fuzzy patch to the naked eye on a clear night and was observed this way for the first time by the famous persian astronomer Al Sufi in the 10th century. Spanning an enormous 3 x 1 degrees, to see the whole galaxy you are best using binoculars or a small refractor or rich field telescope. Such instruments will also reveal M31's two satellite elliptical galaxies M32 and M110 (NGC205). M31, being located some 5 degrees NW of Mirach (Beta Andromeda), it is an interesting coincidence that the other Local Group spiral galaxy M33 the "Triangulum Galaxy" lies some 5 degrees SE of Mirach, i.e they are on opposite sides of this second magnitude red giant star at similar angular distances from it! However, M33 is much more of a challenge. Although it has an integrated total magnitude of 5.7, its light is spread over an area of 67 x 41 arc minutes. So its light is equivalent to a 6th magnitude star spread over an area equivalent to more than two full Moons, making it extremely dim! This face on spiral also has just a tiny nucleus. Binoculars or a telescope with a low power eyepiece on a dark night are your best chance at spotting it.

Other worthwhile objects to observe include the two globular clusters M2 in Aquarius and M30 in Capricornus, as well as the beautiful double star Almach (Gamma Andromedae).