The Night Sky in December
Mars is taking up a favourable position in the night sky. It is visible early evening, due south around 6pm. As it recedes from Earth, the planet is dimming and shrinking. Despite this, it remains easy to recognise, thanks to its distinctive salmon-pink colour. On Dec 1, Mars manages to reach a height of 28 degrees above the southern horizon early evening. By the end of the month this height increases to 36 degrees. The width of your outstretched hand at arm’s length is about 25 degrees.
Throughout December, Mars moves northeast from Aquarius into Pisces, ending the year just southwest of the faint Circlet asterism in Pisces. Mars has a close encounter with binocular planet Neptune on the evenings of Dec 6 and 7 and is joined by a near first-quarter Moon on Dec 14 and 15.
For early risers, Mercury, Venus and Jupiter are putting on a show in the pre-dawn sky. Venus is obvious as a bright beacon low in the southeast, many times brighter than the stars that surround it. Mercury is closer to the Sun by about 24 degrees for much of the month – slightly less than the apparent width of your outstretched hand at arm’s length. Jupiter slowly appears to move away from the Sun and has a close encounter with Mercury on Dec 22, when the planets appear to be less than 1 degree apart. The Moon will appear close to Venus on Dec 3 and 4, rejoining the scene at the end of the month.
The Moon appears as a 33 per cent lit waxing crescent on the evening of Dec 13, an obvious sight 15 degrees (one-and-a-half fist widths at arm’s length) west of Mars. The Moon sets in the westsouthwest around 10pm. Given favourable weather, this leaves the sky dark for the peak of the annual Geminid meteor shower, the most reliable and active shower of the year. The Geminids show activity from Dec 4-17, with a broad peak centred on the night of Dec 13/14.
The peak zenithal hourly rate for this shower is 120 meteors per hour, but realistically, you can expect to see around 50 meteors per hour. Find a dark location, give your eyes at least 20 minutes in darkness to adapt, then look up for 30 minutes for the best results. A sunlounger makes for more comfortable viewing but do wrap up warm! Look at a point roughly two thirds up the sky, in any direction. A good recommendation is to look towards the southeast or south, where Orion will be visible. Geminid meteors appear to emanate from a point near to the star Castor in Gemini.
A reasonably bright comet will grace the night sky during December. In comet parlance, the term “bright” is often misused and generally means it is at least visible in binoculars. Comet 46P/wirtanen should be visible to the naked eye given dark skies if it follows predictions. The comet will be just 7.2million miles away on Dec 16, which will probably make it appear with a large apparent size but a low surface brightness, making it harder to see.
All month, 46P/wirtanen will track rapidly north. In mid-month it passes between the Pleiades and Hyades open clusters in Taurus the Bull. Then, on the night of Dec 23/24 it passes very close to the bright star Capella in Auriga the Charioteer. As luck would have it there is another binocular-bright comet visible over this period too, comet 38P/stephan-oterma. This moves from Gemini into the indistinct constellation of Lynx, coincidentally the same end-of-year destination as 46P/wirtanen.