The Evening Sky in December 2023
Jupiter is the ‘evening star’, appearing in the northeast not long after sunset. It is soon joined by the true stars Sirius, low in the east, twinkling colourfully, and Canopus, a bit higher in the southeast. Almost overhead is Achernar. Midway down the western sky is Saturn, the same brightness as Achernar but cream coloured. At the beginning of the month Mercury will be a bright ‘star’ low in the southwest, roughly where the Sun went down.
Left of Sirius is the constellation of Orion. Bluish Rigel and orange Betelgeuse are Orion’s brightest stars. Between them is the line of three stars marking Orion's belt in the classical constellation. To southern hemisphere sky watchers they make the bottom of 'The Pot'. A faint line of stars above the bright three is the Pot's handle or Orion's sword. At its centre is the Orion Nebula, a glowing gas cloud nicely seen in binoculars.
Left of Orion is a triangular group making the upside-down face of Taurus the bull. Orange Aldebaran, at one tip of the V shape, is one eye of Taurus. The stars on and around the V, except for Aldebaran, are the Hyades cluster. Aldebaran is not a member of the cluster but closer and on the line-of-sight. Further left is the Pleiades/Matariki/Subaru cluster, a tight grouping of six naked-eye stars. Many more stars are seen in binoculars.
Low in the south are the Pointers, Beta and Alpha Centauri, and Crux the Southern Cross, upside down at this time of the year. The Milky Way is wrapped around the horizon. The broadest part is in Sagittarius, low in the southwest (around Mercury in early December.) It narrows toward Crux in the south and becomes faint in the east below Orion.
The Clouds of Magellan, LMC and SMC, high in the southern sky, are two small galaxies about 160 000 and 200 000 light-years* away, respectively. They are easily seen by eye on a dark moonless night as misty patches of light.
Jupiter and Saturn are good targets for any telescope. Binoculars show the disk of Jupiter and maybe one or two of its bright moons close to the planet. Any telescope will show all the ‘Galilean’ moons, but not all four every night as they cross in front of and behind Jupiter and are eclipsed in its shadow. A small telescope will show the disk of Saturn with the ring now becoming edge-on. The Moon will be below Saturn on the 17th and above it on the 18th. It will be near Jupiter on the 22nd.
Mercury ends an evening sky appearance in the first half of the month. At the beginning of the month it is setting nearly two hours after the Sun. It slowly falls lower, night-to-night. It also fades as it moves between us and the Sun and more of its sunlit side is turned away. By mid-month it is setting an hour after the Sun. The thin crescent Moon will be above Mercury on the 14th.
Very low in the north is the Andromeda Galaxy. In binoculars in a dark sky it looks like a spindle of light. It is a bit bigger than our Milky Way Galaxy and nearly three million light-years away.
There is a possibility of a meteor shower on the evening of the Tuesday 12th when a stream of dust from Periodic Comet Wirtanen is forecast to hit the Earth. The prediction is uncertain, but it would be worth watching for between dusk and midnight. This is a one-off event, not a regular annual meteor shower. Shower meteors will appear to radiate from a point west of the zenith.
*A light-year (l.y.) is the distance that light travels in one year: nearly 10 million million km, 10^13 km. Sunlight takes eight minutes to get here; moonlight about one second. Sunlight reaches Neptune, the outermost major planet, in four hours. It takes sunlight four years to reach the nearest star, Alpha Centauri.
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