Popular Astronomy

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What's Up for December 2017

It's clear out, it's a rotten night on telly, and you really ought to get out there and learn a few constellations. Oh OK, it's a good night on telly, but that’s what recorders and catch-up are for. Save it up for a rainy evening and get out there now!

Now it can get pretty cold in December, so the smart astronomers make sure they put on more than just two T-shirts. Who cares what you look like – this is astronomy, not a fashion show. Being cool is one thing, being absolutely frozen is another. Bobble hats, anoraks, even stretch a point and wear gloves. The more comfortable you are, the better you will enjoy stargazing.

You will need a star map, and you will need to know how it works. This may seem obvious, but there is a knack to understanding these things. Here is our map for this month:

Sky for December 17

Wrong way round?

Now don't write in and tell us that we have the points of the compass all wrong. This is a map of the sky, so you have to hold it over your head. When you do this, east and west will be the right way round. How you hold your computer above your head is up to you!

It shows the whole sky, so the scale is quite small. Normally you turn to see different parts of it, so to see the view looking north, for example, hold the map upside down with north at the bottom.

The map shows the sky in the middle of the month at about 8 pm, at the start of the month at 9 pm, or by the end of the month at 7 pm.

TIP If you aren't sure of the direction of north from your location, click here for a page on Getting Your Bearings.

Making sense of the stars

One constellation or star group that many people recognise straight away is the Plough. This is low down in the north at the moment, so turn the map upside down (once you've printed it out, of course, clever clogs) and you will see the seven-star pattern near the horizon. OK, it might not look much like one of those spiky things you see rusting away in the corner of a field, but these names were given a long time ago, when ploughs were, well, different. If you're American, you probably call it the Big Dipper – and dippers aren't around these days either.

So think of it as a saucepan instead. The distance across the saucepan to the tip of the handle is just about the same as your outstretched hand at arm's length. Once you have found this, you will get an idea of the scale of the map compared with the sky.

Next turn round so you're facing south and look over to the west for the three stars marked Deneb, Vega and Altair. Vega is also one of the brightest stars in the sky. Altair has two fainter stars on either side of it, though in a poor sky you may only see the upper one. The three bright stars, Vega, Deneb and Altair, are called the Summer Triangle, even though they are still visible well into autumn and even winter, like now.

Once you've found the Summer Triangle, look to the south and find the Square of Pegasus, which is shown in greater detail on the map below.

December sky looking south

Things to look for

This bit of the sky is not exactly teeming with bright stars, so unless you live in the country you may not see much at all. But you should be able to see the Square of Pegasus, quite a way up in the sky and larger than it appears on the map. If you want to be picky about it, it's not really a square, but don't just expect too much. The stars are not too bright, either, but it's a useful signpost to other constellations at this time of year, and particularly to the Andromeda Galaxy (see below).

But while on the subject of constellations (star groups), use the map to pick out the three bright stars of Andromeda, one of which is shared with the Square. They lead on to Perseus, at the top left.

Below Andromeda are three stars in Aries, quite close together, and below that are the stars of Cetus. From towns only Diphda is likely to be visible.

And to the left, which is the southeast, you can see the fantastic star cluster called the Pleiades – that's pronounced Ply-a-deez. They are also known as the Seven Sisters. Count them – if you have good eyes there are nine, and a lot more fainter ones. Nine of the stars have names, but two of them are the parents of the seven sisters, you see, so it all works out in the end. The Pleiades and Aldebaran are in Taurus.

If you want a map with all the constellation names on it, click here.

Photo of M31 copyright Robin Scagell

M31 as it appears with a small telescope

Finding the Andromeda Galaxy, M31

This really is quite easy to find once you know the trick, and even if you live in a city you should be able to find it using binoculars. You start from the Square of Pegasus, count two stars to the left and two stars up, and there it is. Look at the map and you will see which stars to count.

Unless the sky is very clear, you may need to use a trick to actually see it. This is called averted vision, which means not looking directly at it but a short distance away from where you know it to be. This works because your eyes are more sensitive to light away from the centre of vision. M31 is a faint oval shape, more or less as it appears on the map.

This little fuzzy blob is a galaxy just like our own Milky Way, but over 2.5 million light years away. It is the most distant object visible to the unaided eye. It contains around a trillion – that is, a million million – stars.

With that many stars, the chances are that there are at least some planets with life forms that are looking at our own galaxy at exactly the same moment as you, and probably wondering how many heads and arms (or slimy tentacles) you have.

Look out for shooting stars

There are certain times of year when shooting stars – known to astronomers as meteors – are particularly likely, and the days on either side of 14 December is one such time when the Geminid meteors put on a show. The Geminids have become more plentiful in recent years, and they are now the best meteor shower of the whole year. This is a good year to see them because the Moon doesn't rise until about 4 am, so it won't get in the way of seeing meteors.Geminid meteor -- Credit Robin Scagell/Galaxy

Even so, don't expect to see hundreds – you are only likely to see one every few minutes or so, even though this is the strongest meteor shower of the year. The trouble with meteors is that they can sometimes be a bit like waiting for a bus that never comes. You know they ought to be there, but somehow nothing happens. Then just as you turn away, whoosh, you just missed one and see it from the corner of your eye – or worse still, someone else starts jumping around and screeching because they have seen a real stonker.

So where do you look? These meteors can appear just about anywhere in the sky, though they come from the direction of Gemini (marked by the star Pollux on the circular map at top) which is rising in the east during the evening. This is why they are known as the Geminid meteors. 

A meteor looks like a brief streak of light, as shown in the picture, usually lasting less than a second, and is caused when a tiny particle of dust from the tail of a comet that went past long ago collides with Earth and meets its doom as it burns up in our atmosphere. There is no danger from meteors – they are always too tiny ever to reach the ground.  It wasn't meteors that saw off the dinosaurs.

You can find out more about the Geminids, and also another shower called the Ursids, in our Meteor Section, where you can also find out how to observe these shooting stars.

The planets

This is a rotten time for bright planets – there just aren't any in the evening sky. Jupiter and Mars are in the morning sky, rising about 6 am before the Sun in the south-east, but all the other bright planets are too close to the Sun to be seen. So if you want to try out the new telescope you're getting for Christmas, it will have to be on the Moon...

What about the Moon, then?

It begins the month just coming up to full Moon, which is on the 3rd. Last quarter is on the 10th, then comes New Moon on the 18th. You might just see the crescent Moon in the western twilight on the 20th about half an hour after sunset, but it will be very low down and thin.  Finally there's first quarter on the 26th. Sorry, no eclipses this month. We have a whole section of the website devoted to the Moon and its features.


Sorry, no eclipses this month.

The full Moon in December is very high up and bright, so here are some full Moon facts (and one that isn't).

  • When the Moon is really high up, like in that nursery rhyme that claims it is as bright as day, you can't see any colour in anything that's illuminated by the Moon only (that is, out in the country where there is no light pollution). This is because your eyes are not sensitive to colour in dim light.
  • The sky close to the Moon appears bluish, just as it does by day. This is what makes you think that moonlight is blue, and is why they use a blue filter in the movies when they want to pretend that the scene is lit by moonlight.
  • Actually, moonlight is slightly redder then sunlight.
  • If you take a time-exposure photo by moonlight, without any light pollution around, the colours appear just as they do by day. The camera can go on building up light and show the true colours, unlike your eye.
  • Those stories that there are more hospital emergencies and murders when the Moon is full are nonsense. The only reason people think this is that the Moon is more noticeable when it is full than at most other phases. And most people think it is full for two or three days on either side of the actual date, so there are several days in the month when it appears to be full. So that increases the chances of people linking anything with the full Moon.

Get more helpful info

OK, you've read all this for nothing, now comes the plug. This page is brought to you by the Society for Popular Astronomy, which is a really great society to join. It's based in the UK but there are members in other countries as well. It doesn't cost much to join, and there is a special rate for Young Stargazers. At least take a look at what we have to offer.

Text by Robin Scagell