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The night sky for December 2021

Northern Hemisphere

Ian Morison tells us what we can see in the Northern Hemisphere night sky during December 2021.

The Planets

Highlights of the Month

Haritina Mogosanu and Samuel Leske from the Carter Science Centre in New Zealand speaks about the Southern Hemisphere night sky during December 2021.

In the southern hemisphere it is summertime but everything else is the same, we too are preparing for Christmas and to celebrate the anniversary of Apollo 8 - the first people to ever go around the Moon on Christmas Day 1968.

Speaking about the Moon, this month, the New Moon is on the 4th of December - this is when it is great to go deep-sky observing. First quarter is on the 11th of December - which means the Moon is in the sky in the first part of the night, and is setting after midnight. Full Moon occurs on the 19th of December - we hope you like long walks under the moonlight, because no deep sky objects will be easily visible in the sky as full Moon does make light pollution, and you will use a moon filter if you want to observe the Moon. And finally last quarter on the 27 of December, which means the Moon is in the sky in the last part of the night so rising after midnight. This is very helpful to plan your stargazing accordingly.

Venus, Saturn and Jupiter are beautifully aligned at sunset and visible at the beginning of the month just after sunset. Venus is the closest to the horizon followed by Saturn and then Jupiter. Towards the end of the month, Venus will get too close to the Sun, Neptune and Uranus are visible all night long. Mars is in Libra and moving into Scorpius by the end of the month, which means it will be visible in the morning sky then.

I often get comments like, who can remember oh, so many stars, or is it really hard to remember all those constellations and so on. While this is true, many cultures - we all know that, invented stories about different stars in the sky and that made things easier to remember. Some cultures, in particular, had seasonal asterisms, and some of these asterisms encompassed the entire sky. How to remember all of it? Is by linking to things that are memorable or make your own stories!

With that in mind, let’s look at the sky in December in New Zealand.

December is very famous for Christmas and Christmas is very famous for Christmas trees. As I was learning the south celestial night sky, one night it occurred to me that right now in December, the entire region of the south celestial pole looks like a giant Christmas tree. Of course I am biased, and I love Christmas trees as well, but this trick helped me remember where everything was. Let’s look at that patch of the sky.

The sky looks like a Christmas tree only in December, just after sunset, if you look at the southern part of the sky. I will start at the top with Achernar, we can give that magical tree topper function.

Is very high in the sky about 75 degrees from the horizon. Then, just a bit lower down, the Magellanic Clouds are like two patches of snow one third town the tree and at the base, Look under the Southern Cross, those two stars, Gamma Centauri (Muhlifain) and Delta Centauri, together with the Southern Cross and Musca are the trunk.

All the other stars in Centaurus and Vela are decorations on the lower branches. Can you see the giant celestial Christmas tree? We might not have snow this time of the year in the Southern Hemisphere but it is like nature compensated for that with a celestial Christmas Tree just in time for Christmas.

Then, in the North, the Great Square of Pegasus – is the only horse that looks like a square and one of the very few northern constellations that is not upside-down.

Pegasus pulls a sleigh – we can make that up out of the Pleiades, Hyades and Orion. Draw a line between these asterisms and you can see the sleigh. Orion is the back of the sleigh where the driver sits and the Pleiades is the front where the runners are curved. Can you see the sleigh? We don’t have reindeers in New Zealand so Pegasus the flying horse will have to do.

Running behind the sleigh, on the ground , is a Dog - this is the Dog Star, Sirius. He’s very happy on the snowy road, which is the Milky Way. We can imagine that the sleigh has just come up from behind the Christmas tree and is flying across the sky to the Northern Hemisphere. But there’s one more star, my favourite star here, Canopus. I call this the Cat Star as every Sirius astronomer in New Zealand who wants a cat should contemplate this name first.

Halfway through from the top of the Christmas tree which is the star Achernar, our cat, Canopus jumps to catch the back of the sleigh. By the looks of it, it will probably land on the dog.

So we can turn the night sky in December, in New Zealand into a big Christmas Scene. We have a horse, sleigh, and a big Christmas Tree.

Draw a line from Sirius to Canopus – which are two very bright stars, it will lead you to the Magellanic Clouds. Sirius is in fact the brightest star in the sky, Canopus is the second brightest star in the sky and to the right of the imaginary Christmas Tree is Alpha Centauri, the third brightest star in the sky.

Lower on the Northern horizon, underneath the galloping hind-legs of Pegasus, Andromeda Galaxy is a smidge of light. It is the furthest object we can see with the naked eye, 2.5 million light years distance from us. When the light that we see now from Andromeda left the galaxy, on Earth, some of our hominid ancestors, facing food shortages, developed larger brains, as an evolutionary strategy. This led to the genus Homo, which first arose 2.5 million years ago. Homo habilis developed as the ice ages began, a time known as Pleistocene. And who knows, maybe the first human memories of winter too.

Some cool binocular objects this month lay low around the horizon. Best objects to start with are obviously the Moon, the planets and double Stars (Alpha Centauri, Gamma Velorum, Beta Muscae and Upsilon Carinae, The Pleiades and Andromeda Galaxy are all great binocular targets. Then, of course, the Magellanic Clouds are great to look at.

Some favourites of ours are visible in the night sky in December. The first of these is M74 which is vey hard to see due to its very low surface brightness. With very dark skies it can be seen from the Wairarapa, you’d need Bortle Scale 2 or 1 to see it. Luckily it’s not all bad for galaxy hunting in December as not too far from M74 is the bright galaxy of M77 – also known as Cetus A. This one is easy to spot even from central Wellington. We won’t see the faint outer regions of the spiral arms but the bright active core is very visible and at 33 Million light years distant the photons from this object have spent a long time making their way to Wellington.

We do have some very impressive galaxies in the Southern Sky. One of these is NGC 253 – also known as the Sculptor Galaxy. This is a large spiral galaxy at an angle to us so it looks like an elongated ellipse. It’s relatively bright and easy to spot if you’ve got plenty of aperture. You’ll have to put your light bucket on the back of your scooter and head to a dark sky location to make out much detail, but if you do, you’ll be in for a treat as you take in the complex shapes and clumps of detail visible on the disk. Sculptor is about 12 million light years away and appears about 27 arc minutes long so is quite big.

Quite close to Sculptor is the tight spiral galaxy known as NGC 300. This is a great galaxy to view as it’s quite close at only 6.6 million light years – for Northern Sky observers it’s a bit like a mini M33. Viewing from Wellington will show the bright core but you’ll have to head to the hills to get any detail out of the spiral arms. Keen astrophotographers will have a better time in Wellington as this galaxy is bright enough to burn through the light pollution and produce quite a nice picture.

The problem with viewing galaxies is that they don’t really look anything like the beautiful photographs people take. They are often just a faint grey smudge in the eyepiece and you have to use your best visual observing skills to get any detail out of what you’re looking at. This is when it’s great to swing the telescope around to the majestic brilliance of the likes of the Tarantula Nebula. This gives you a picture in the eyepiece very similar to what photographers capture, just not in colour. This big giant bright complex of gas clouds and massive stars looks a bit like a spider, hence its name and it is a must see of the Southern Sky and is almost compulsory viewing on any observing evening.

At this time of the year the two galaxy groups of the Fornax Cluster and the Grus Quartet are also in a good position for viewing. As the month advances the position of the Fornax Cluster improves and the position of the Grus Quartet gets worse so get in early to see these four stunning galaxies. Both groups are between 60 Mly and 80 Mly distant with the Grus Quartet being three galaxies visually quite close to each other and another galaxy a little further away. With the right eyepiece you can get all four in the same field of view. With the Fornax Cluster it is possible to get up to 11 galaxies at once in the same field of view. These are mainly elliptical galaxies including the stunning Fornax A.

December in the Southern Hemisphere is generally the unhappy month for the astronomer, unless your thing is solar astronomy, because the nights are short and the temperatures are creeping up. It seems to take forever for the night to get truly dark and forever for the telescope to cool down. Unlike the rest of the population, us astronomers are craving a cold front to blow through and give us a nice cool patch of air to settle the thermals and give us some great seeing. The good news is that this time of year is perfect for an all nighter of astronomy (all 4.5 hours of astronomical night) as the Milky Way passes through the zenith in the early hours and there’s a wealth of deep sky objects to fill your eye piece.

Due East – Taurus Orion region

A good place to start an evening’s viewing is Orion, that majestic constellation that is easy to find in the Northeast at about 35 degrees in elevation. With a pair of binoculars an observer can easily see the Great Orion Nebula (Messier 42 or M42) which is a huge star forming region just above Orion’s Belt, which is made up of the three stars of Alnitak, Alnilam and Mintaka. Mintaka is very dear to navigators as it’s located exactly on the celestial equator.

Orion has a number of very interesting stars including Betelgeuse which is a red supergiant and one of the largest stars in the sky. It is one of the few stars that have been imaged and it’s unusual shape is quite apparent showing it probably has a very unstable atmosphere causing the asymmetric bulging of the star. Because of its massive size Betelgeuse will not live for much longer – maybe only another 1000 years. Or it may have already exploded! But given its 400 or so light years away we might not find out for a while. When it does go it will create quite a spectacle on Earth as it will be a very bright supernova and will probably even be visible in daylight.

M42 is relatively close to us at about 1400 light years which makes it one of the brightest nebulae in the sky. With a telescope the M42 can appear to have a greenish tint, unlike the bright red photos that are often published. It is estimated that M42 is about 24 light years across and that it is part of a much larger structure known as the Orion Molecular Cloud, which extends for about 10 degrees across the whole constellation of Orion. This cloud includes the famous Horse Head Nebula (B33), Flame Nebula (NGC 2024), M78 and Barnards Loop (Sharpless 2-276). If you have access to an infrared telescope, you can also see the brightest object in infrared, that is the Becklin-Neugebauer object. M42 is one of the most photographed objects due to its brightness and visibility in both the Northern and Southern Hemispheres.

These are some of my favourite things about December, other than sleigh bells and snowflakes and warm woolen mittens and from here from New Zealand,

I wish you clear skies so that you can always see the stars and always remember we are made of the same star dust as they are.

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The Night Sky This Month is one part of the Jodcast. The full show contains the latest news, interviews with astronomers, answers to listener questions and more.

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