This month heralds the beginning of winter; the cold and dark months which might not be to everyone’s taste, but they are what astronomers like best. There is more time to go stargazing! Brave the winter chills this month and enjoy Comet Wirtanen, as it slowly glides through the night sky. It should be visible to the naked eye throughout the month, reaching magnitude +3.0 as it comes closets to the Sun on the 12th of the month. It will just outside Earths orbit: 12 million kilometres away from us. It will be an unforgettable sight on the 16th, when the comet passes the Pleiades star cluster. On the 23rd December the comet skims only 12 arcminutes from Capella. It could look like a bright star that has sprouted a tail.
Not forgetting the regular brilliant constellations of winter. All the main guides, Ursa Major, or the Plough, Orion and Cassiopeia are on view. Orion, the main constellation of winter, is getting higher in the sky and will be at its best after Christmas. Of the stars we see near Orion, only Sirius the Dog Star is difficult to find, as it is still very low in the sky. You can use the three stars in Orion’s belt to point to the lower left to locate Sirius.
The Plough is now standing on its handle in the north. Capella, the bright yellow star, is not yet at the overhead point but it is very high up and cannot be missed. The twins of Gemini, Castor and Pollux, are much higher. Now is a great time to look for the Pleiades, or Seven Sisters, which are in the sky, looking south. This is a cluster of stars moving through space together. If you have good eyesight it is easy to see the seven brightest members of the Pleiades, using binoculars or a telescope however you would discover that there are actually about two hundred and fifty stars in the cluster.
Of the summer triangle Altair is no longer visible, Vega is very low in the sky, and Deneb can still be found in the north west.
On the night of the 6th/7th December we may witness a rare meteor shower; the Andromedids. Dust shed by Comet Biela in 1649 impacts the Earth.
During the course of the night of 14th into the early hours of the 15th of this month, will be the maximum of the spectacular Geminid meteor shower. When the Earth hits a stream of interplanetary debris from the asteroid Phaethon. They will be best observed after the Moon has set.
On the night of the 20th of the month, the Moon moves in front of the Hyades star cluster after midnight, and is next to Aldebaran by dawn.
As for the planets this December you will find Saturn low in the south west after sunset. Lying in Sagittarius, at magnitude +0.5, sets around 5pm. By the months end the ringworld disappears into the twilight glow.
Mars lies higher in the evening sky. Setting about 11pm. The distinctive reddish glow fades from magnitude 0.0 to magnitude +0.5 during this month as the planet treks from Aquarius into Pisces.
On the 8th of the month, Mars acts as a convenient guide to finding faint Neptune. Use binoculars or a small telescope to spot a faint dot 15 arcminutes below the red planet, and a thousand times fainter, at magnitude +7.9. It’s a conjunction of Solar Systems reddest and greenest planets. Neptune lying in Aquarius, sets around 11pm.
Uranus at magnitude +5.7 resides on the fringes of Aries and Pisces, setting around 3am.
Venus is rising around 4am as the brilliant morning star, at magnitude –4.5. It appears dazzlingly bright in the totally dark sky. Travelling from near Spice in Virgo through to Libra.
To the lower left of Venus, around 7am, you will find Mercury and Jupiter, just above the south east horizon. At the beginning of the month fainter Mercury at magnitude +0.5 lies just above Jupiter. The gas giant is second only to Venus at magnitude –1.7. The two planets converge, passing less than a degree apart in the night of the 21st to 22nd, in the constellation of Ophiuchus, when Mercury has doubled in brightness to magnitude –0.4.
We are due for a serious celestial display on the night of December 13th and 14th, around 100 meteors per hour might be observed from around 10 pm onwards. As the grains of dust are from an asteroid they are slightly larger than those from a comet, so the Geminids can produce many bright white coloured fireballs in the sky. Look up in any direction and you might be lucky to see at least one Geminid.
Phases of the Moon for December:-
December 7th – New Moon
December 15th – First Quarter
December 22nd – Full Moon
December 29th – Last Quarter
The Shortest Day
The winter solstice occurs on December 21st. This is the time when we have the shortest day of the year. From this point on the Sun will slowly get higher in the sky and daylight will lengthen.