Our Night Sky: December 2021

As it’s December, the month of the Winter Solstice, we have our Shortest Day with under 7 hours of daylight, so you don’t have to be out late to go stargazing. Although most of the bright stars are missing from our sky this month, it is a good time to spot the Andromeda Galaxy and also three bright planets, Venus, Saturn and Jupiter, which line up in the southern sky as darkness falls. We also have the best meteor shower of the year, the Geminids, which peaks on the night of 13-14 December it may be possible to see up to 120 meteors an hour in the few hours before sunrise on 14 December.

Reach for the Stars I - Sailean nan Cuileag, Salen, Ardnamurchan
The night sky, full of stars and with the streak of the Milky Way showing directly overhead and through a canopy of Scots Pines.

​The Constellations

This month, the sun sets around 4:00 pm and the stars and the constellations start to become visible from about 6:00 pm onwards. As with November, there is a great signpost, in the form of the Square of Pegasus, to help you find your way around the sky. It is a huge square made up of four stars of nearly equal brightness: Scheat, Alpheratz, Markab and Algenib and, in the early evening, you should be able to see it high up in the south, above and to the left of Jupiter, which will be sitting in the south-west.
 
The top left star of the Square is Alpheratz. It is the brightest star in the constellation of Andromeda, which is marked by a line of three more stars running up and to the left of it. You can use this line to find the Andromeda Galaxy, one of the most famous features of the night sky and the nearest spiral galaxy to our own, the Milky Way. The best way to find it is to start Alpheratz and go a further two stars along the line, turn right and count another two faint stars along, and the Andromeda Galaxy (M31) will be close to the second faint star. If you are in a town, you’ll probably need a pair of binoculars to spot it, but if you’re in a dark place out in the country, you should be able to pick it out without using binoculars. 

​Below Andromeda and to the right of the Square is the Constellation of Aries (the Ram), whose three main stars form an easily recognised triangle. Above this, you will find another more regular triangle shaped constellation, called Triangulum (the Triangle), which contains the Triangulum Galaxy (M33), our next closest galaxy after the Andromeda Galaxy. This will be visible with binoculars if you are under a good dark sky.
 
Over to the left of Andromeda, you will find the Constellation of Taurus (the Bull), which can be readily found by spotting Aldebaran, the bright orange giant star that marks the eye of the Bull whose face is depicted by a V-shape of stars to the right. Look further over the left and you will find Orion in the east, which is most easily spotted by picking out the line of 3 stars that makes up Orion’s Belt. 

The Constellation of Andromeda
Image Credit: Till Credner

There will be more about Taurus and Orion next month, but for the moment, draw a line using the three stars in Orion's belt and follow it through Aldebaran and you’ll find the Pleiades star cluster (M45) a bit beyond it. This cluster is the brightest open constellation we can see in the night sky and is a grouping of stars, seven of which are visible to the naked eye, but the entire constellation has more than 1,400 stars. The name comes from the early Greeks who referred to the constellation as the Pleiades, or the Seven Sisters, who were daughters of Atlas and Pleione. These stars are mentioned three times in the Bible and are linked to origin stories for many American Indian tribes.

Southern Night Sky - December 2021
Created using SkyPortal Mobile App

Northern Night Sky - December 2021
Created using SkyPortal Mobile App

​If you look north, you’ll see the same stars as you would in any other month, but their orientation varies from month to month. In December the familiar Plough asterism, which many people recognise, is moving from the north to the north-east. If you follow its two right-hand stars upward, veer a little bit to the right and you’ll find Polaris, the Pole Star. If you look high up over to the north-east, you can see Castor and Pollux, the two brightest stars in the Constellation of Gemini (the Twins), which represent the heads of The Twins. Meanwhile, low down in the North-West, you will find Vega, the brightest star in the Constellation of Lyra (the Lyre). It is the fifth-brightest star in the night sky, and the second-brightest star in the northern celestial hemisphere after Arcturus.

​The Moon

​At the very start of the month, we are close to a new Moon (no Moon) as this occurs on 4 December and if it’s clear a couple of nights later, on 6 December, you will find the slightest of crescent Moons below and to the right of a bright shining Venus as it sets in the south-west at around 5:30 pm. On the following evening of 7 December, the crescent moon will to the left of Venus and will set about an hour later at around 6:30 pm. At sundown on 8 December the moon will be found directly south, hanging in-between Venus and Jupiter, while on 9 December, it will have moved to the left of Jupiter.

​The first quarter Moon (a half moon) is on 11 December and will be sitting low down in the south-east as darkness falls, while this month’s full Moon is on 19 December, becoming 100% full at just after 4:30 am. This means that it will appear full on both the nights of 18-19 December and 19-20 December. It will rise in the north-east just before 3:00 pm on the 18 December and just after 3:30 pm on 19 December. 

The first quarter Moon (a half moon) is on 11 December and will be sitting low down in the south-east as darkness falls, while this month’s full Moon is on 19 December, becoming 100% full at just after 4:30 am. This means that it will appear full on both the nights of 18-19 December and 19-20 December. It will rise in the north-east just before 3:00 pm on the 18 December and just after 3:30 pm on 19 December.
 
On 20 December, the moon will be 98.5% full and will rise a little further to the north at just after 4:30 pm to spend the night traversing the sky between the twins of Gemini, which you can find by spotting the constellation’s brightest stars, Castor and Pollux above and to the left of it. Three nights later, on 23 December, you will find the moon close to Regulus, the brightest star in the constellation of Leo.
 
The first quarter moon is on 27 December and on 28 December, you will see it lying very close Spica once it has risen at just after 1:30 am.
 
This month’s full Moon is called the Cold Moon because December is the first month of winter. Its Old English/Anglo-Saxon name is the Moon Before Yule, while another name is Wolf Moon, however, this is more commonly used for January’s Full Moon.

​The Planets

​If you watch the sky in the south-west this month as darkness falls, you will see a diagonal line of three bright planets. Venus will be to the right, Saturn in the middle and Jupiter to the left. As first half of the month progresses, you will notice Venus moving closer to the other two, but just after half-way through the month it will change direction and move back the other way, heading right and downwards.
 
Venus will get brighter and brighter until 7 December, the day on which it reaches its brightest during its appearance in our 2021 evening sky. It will then begin to fade and if you look at it through a telescope, you will be able to see Venus shrink dramatically to a narrow crescent as the month passes. At the start of the month, it sets at 6:30 pm and by the end of the month it drops below the horizon at 5:20 pm. If you do look for it towards the end of the month, you may be able to see Mercury sitting beneath it and moving from its right to its left as each night passes.
 
Above and to the left of Venus, you will find Saturn, a gas giant that is the sixth planet from the Sun and the second largest in our Solar System. Then, if you look further left and up a little, you will see Jupiter, another gas giant that is the fifth planet from the Sun and the largest our Solar System. Saturn sets in the south-west at around 7:30 pm and Jupiter sets at about 9:30 pm. If you use binoculars or a telescope you will be able to see Jupiter’s four bright moons and Saturn’s rings.
 
Go up and left again and you will find Neptune a little bit above the constellation of Aquarius. It will be above the horizon until about 11:30 pm, but it will be very dim and therefore difficult to see, even with binoculars or a telescope. Its near twin in size, Uranus, will be a little bit brighter and can be found above the horizon until about 4:30 am, over to the east about halfway between the constellations of Aries and Taurus.
 
Finally, Mars begins to make an appearance this month and you will find it low in the morning twilight as it rises in the south-east at around 7:00 am.

Meteor Showers

December is the month for viewing the shooting stars of the Geminids Meteor Shower, which unusually are not debris from a comet, but rather debris from an asteroid called Phaethon. The Geminids have become more plentiful in recent years and are considered by many to be the best meteor shower of the year. They are active from 4-17 December and during their peak, which this year is on the night of 13-14 December, it can be possible to see around 120 meteors per hour.
 
Their radiant point is in the Constellation of Gemini (The Twins) and this year it is close to one of its two bright stars, Castor, over in the north-east sky. Although this is where they originate from, you’ll be able to see them at any point across the sky while they burn up in the upper atmosphere, some 60 miles (100 km) above Earth’s surface.
 
They become visible when the radiant point is high up in the sky and it will get there by about 10:00 pm, so begin looking for them from then. However, the best time is normally at about 2:00 am, when the radiant point is at its highest in the sky, but this year, a waxing gibbous moon will be in the sky until about 3:30 am and will drown out the fainter meteors. It will therefore be best to wait until after the moon has set and look for them in the 2-3 hours of complete darkness between then and before sunrise on the morning of 14 December.

Winter Solstice

This year, the Winter Solstice will occur on 21 December at 3:59 pm am and is the exact moment when the northern hemisphere is tilted furthest away from the Sun. However, many people refer to the Winter Solstice as the Shortest Day of the year, when the number of hours of daylight are at their minimum and the number of hours of night are at their maximum. On 21 December this year, our sunrise will be 8:55 am and our sunset will be 3:47 pm, giving us 6 hours and 52 minutes of daylight.