December can bring cold frosty nights. Nonetheless the coldest nights can be when the skies are clear providing opportunities for a good night observing.

December can bring cold frosty nights. Nonetheless the coldest nights can be when the skies are clear providing opportunities for a good night observing. So put on the winter clothing and brave the cold to see some of the delights.

The Planets

Mercury's best chance of been seen is around December 18 in the evening sky to the southwest. However, it is not one of its favourable appearances and will be tricky to spot. On December 18 itself it is just 31/2d away from a thin crescent Moon. Just follow a line down from the moon at about 45d to the right. It should provide the best opportunity to spot this illusive planet.

Venus is not well placed this month as heads towards superior conjunction (that is when Venus is on the opposite side the Sun to ourselves).

Mars can be found in Leo this month and rises around 9:30pm at the beginning of the month and around 7:30pm by the end of the Moon. Mars is now nearing opposition (occurring late in January) and its angular size is becoming respectable, although it is still only about half the size it was for the 2003 opposition.

Jupiter is past its best, but it can still easily be spotted towards the southwest. By the end of the month it will be setting at 8pm.

Saturn is one for the early morning in the constellation of Virgo. It doesn't rise until 2:00am at the beginning of the month, but by the end of the month it will rise just after midnight. The rings are not as prominent as they can be having passed through being edge on from our viewpoint early in September of this year.

Uranus is in Aquila (but near the border to Pisces) and is still high in the sky after dusk and still offers the opportunity to spot this planet.

Neptune is in Capricornus not far from Jupiter. On the night of the 21st December they pass each other by only 1/2d separation (that's about the width of a full Moon). At the same time the crescent Moon will be above the pair about 4d away.

The Moon

Full Moon 2nd and 31st

Last Quarter 9th

New Moon 16th

First Quarter 24th

As you can see there are two full moons this month which as we have seen from a previous article this is usually referred to as a Blue Moon. Of course the Moon doesn't actually turn blue, however at 7:23pm on the evening of the 31st you would be able to see a small part of the moon missing as it experiences the smallest partial eclipse seen from Britain for 40 years.

The Geminids

For about two weeks centred around December 14 is Geminid meteor shower which is one of the most prominent and reliable throughout the year. The maximum activity is set to occur in the early morning of December 14, with the possibility of seeing up to 100 meteors in an hour, and some of these may well be bright and with some colour. A meteor, or sometimes referred to as a shooting star, is caused as a small particle (about the size of a grain of sand) enters the Earth's atmosphere. Meteor showers occur as the Earth passes through a stream of such particles and debris, which are usually left behind by a comet.

Unusually, the Geminids are associated with asteroid Phaethon, however this asteroid has a highly elliptical orbit, a characteristic normally associated with comets. It is suspected that Phaethon was formally a comet, but has now lost all its volatile materials that are necessary to form the characteristic halo and tails of a comet.

If you would like to know more about meteor showers, comets and have the chance to observe the Geminid meteor shower at a suitable dark site, then the North Devon Astronomical in association with Exmoor National Park are hosting a seminar at the Sportsman Inn, Sandyway on Saturday, December 12, at 8pm.

The diagram above depicts the night sky looking south at about 10pm on December 15. The well known constellation Orion is now well above the horizon and easily seen. It is readily identified by seven bright stars that form a sort of H pattern. The two brightest stars are Betelgeuse (1) and Rigel (2). Betelguese is a Red Supergiant and, at 667 times larger than our own Sun, is truly a giant. If it were to be placed where our Sun is now, all the planets out to Jupiter would actually be within its surface. It is extremely luminous as well; over 50,000 times that of our Sun. Rigel is no dwarf either at 92 times the size of the Sun. It has a much hotter surface than Betelguese, however, and this means that it too is over 50,000 as bright as the Sun. The difference in temperature also shows up in the different colours, Betelguese having a distinct reddish to orange hue and Rigel a slight bluish hue.

The three centre stars of Orion (3) make up what are known as the belt stars and are easily identified. They are useful as a guide to other stars and constellations. Follow a line from these three stars downwards and you will come to the bright star Sirius (4). Excluding our Sun, which is, after all, a star, Sirius is the brightest star in the sky. Its intrinsic luminosity is 27 times that of the Sun, owing much of its place as brightest star to the fact that it's relatively close at 8.6 light years away. In comparison Betelguese and Rigel are 430 and 776 light years away respectively.

Follow the belt stars in the other direction and you will come across Aldebaran (5), the eye of the Taurus bull.

It's worth scanning the area with binoculars as the V formation stars adjacent to Aldebaran are part of an open cluster known as the Hyades and a pair of binoculars will bring many more stars of this group into view.

Now keep going and you will come across another, tighter group of stars. With just your eyes and a dark enough sky you should be able to see six stars, possibly seven. This open cluster is known as the Plieades, but it is also known as the Seven Sisters. Again it is worth viewing with binoculars as these will reveal many more stars in the group.

Moving down and to the west of the Plieades into Cetus is to be found M77 (7). M77 is a galaxy estimated to be 60 million light years away. Larger than our own, this galaxy is thoroughly studied as at its centre it harbours an active super massive blackhole. This causes the galaxy to have an unusually bright centre not only at visible wavelengths, but throughout the electromagnetic spectrum.

Our next object is another open cluster, M35 (8) which can be found above Orion in the constellation of Gemini. Although readily seen in binoculars, a telescope will help reveal the several hundreds of stars that go to make up this rich open cluster.

Our final object is considered by many as one of the highlights. Just down from the belt stars in Orion at what is known as the tip of the sword is the Orion Nebula (9). It can be seen with binoculars, but again is best seen with a telescope which will reveal a cloud like bright patch with considerable detail (the word nebula is the Latin word for cloud). This bright 'cloud like' patch is the result of ultra violet radiation from nearby stars causing the gas to fluoresce. It is a site of prodigious star formation with the gas and dust collapsing to form new stars, most of which will have planets. There are already many young stars that have recently (astronomically speaking) formed in and around the nebula. At about 1300 light years distant, the visible part of the nebula spans more than 20 light years, however the true size of the Orion complex is nearer 200 light years.

The North Devon Astronomical Society meets on the first Wednesday of each month at 7.45 pm at the Methodist Hall, Rhododendron Avenue, Sticklepath, Barnstaple. Next meeting is December 2. Everyone is welcome no matter how inexperienced.

Rick Dooley