Star Diary Podcast: What’s in the night sky, December 2021

A guide to December 2021’s night sky in the northern hemisphere.

In December 2021 set yourself a challenge and attempt to see every major planet in the Solar System. Throughout the month keep an eye out from Comet A1 Leonid which is expected to make an appearance. Then on December 25th, Christmas Day, see if you can find the Red Planet Mars passing by the Red Star Antares.



Ezzy Greetings, listeners and welcome to Radio Astronomy’s Star Diary – Our guide to the best thing to see in the Northern Hemisphere’s night sky in the month of December 2021. I’m news editor Ezzy Pearson and I’m joined on the podcast today by reviews editor Paul Money who is going to tell us the best things to catch in this month’s night sky. So Paul, what are your recommendations for December 2021?

Paul Hi, Ezzy. Well, I mean, you know, December we’ve got the dark skies. Yeah! Finally. The bad news is, of course, is the winter solstice and then the light nights start… The night start to God… No, we can’t have that. You know, light at night is good. Great, but let’s enjoy the December skies as we can because we’ve got a host of events, some in the morning and some in the evening as well. We’ve got a gathering of evening planets, especially towards the end of the month, which will be interesting. So we start actually with the morning sky and this particular case sort of thing, we’re looking at Mars. Mars has been a bit absent for a while because, of course, it had its conjunction with the Sun, so it’s not observable. In fact, it affected the rovers, if you remember, because they can’t communicate with the rovers, can they, when it’s so close to the Sun because of the line of sight. But Mars is back in the morning sky now, and of course, rover operations have all resumed as well. So from December the 1st to the 3rd, we’re looking in the morning sky and it’s really you’re looking at about sort of like twilight time. So as the onset of Twilight, what you’ll see is the crescent moon on December 1st is to the left of Spica and is Spica of course in Alpha Virginis, in the constellation of Virgo. And so this is emerging into the morning sky as well the next morning. It’s a slimmer crescent for the Moon. I love watching out for these that they’re just the cereal. They just hang in there this lovely little slim crescent. And you often see the Earth shine as well. The dark side of the moon feebly illuminated of course by light from the Earth. The light bounced off the Earth’s atmosphere back onto the Moon. So its second hand sunlight really, isn’t it? So on the second, it’s actually just above Alpha Libra. Now that’s Zubenelgenubi, that’s a bit of a mouthful, isn’t this sort of thing? But it’s a nice double star, you know, very good for binoculars and small telescope. So I always like it in the Moon’s closest targets like this because it draws our attention to them. So, you know, probably if you’d never seen this double star before, that’s the morning to do it if you’re actually out observing. Now on the third, what you find is the really, it would be really difficult because the the crescent moon was really low to the Horizon. But if you do spot it, you’ll see to its upper right Mars. Mars will be emerging out of the glare of the sun, so into the morning sky now. And in fact, Mars will be playing piggy in the middle because Zubin will be one side and the crescent moon will be the other. But you’ve got to be careful as you’re watching the Crescent Moon this time because obviously the sky will be getting lighter and what you mustn’t do is leave it too long and then accidentally catch a glimpse of the Sun. Obviously, you’ve got to be very, very careful and certainly with optical aid, you know, you have to be really careful and and really pay attention to watching out for when sunrise occurs. So, so there we are in the morning sky. Then December, the first of the third that’s an encounter we Spica, then Alpha Librae and then Mars all with the Crescent Moon. But also the last one will be a lot harder to say a really slim crescent moon.

Paul Now, December, the 1st of the 31st first, I, you know, the whole month gone. Now We’ve harped on about this for quite a few months, and it sounds like we’re a stuck record. You know what? Venus is still there. Yeah, it’s a quirk of the tilt of the Earth that the ecliptic over the. A couple of months are starting to get steeper, and that has little interest in effect because the effect remains that Venus, which has been very low, has actually improved in visibility, even though it is slowly dropping back towards the glare of the Sun. So it’s dropping deeper into the twilight, but the steepening of the ecliptic means it’s keeping it above the horizon. So we’ve still got Venus. You ‘d have expected to have lost it several months ago, but that’s down, I say to the time of year on the steepness of the ecliptic. So it will actually look as if it’s gradually catching up with Jupiter and Saturn, and Jupiter is receding further from Saturn. And so Saturn is piggy in the middle at the moment with Jupiter and Venus. But Venus motion stops and then it starts to head back to the solar glare. So it looks like he’s going to have a close encounter with Saturn, but he doesn’t get one. Oh, that’s a shame. But he does happen other years, so we’ll have to watch out for them. So as it happened, they’re all in the south western evening twilight skies. And the reason why I mention it for the whole month is that we’ll have them all month in this sort of position and gradually dropping more and more into the twilight. And as such, in the last week of the month, we get Mercury joining Venus as well. But we’ll come back to that a little bit later on. When we look in on December 6th and 7th, what we find is the crescent moon then lies either side of Venus. In the early evening, twilight is passed through the new phase. Remember, on the third, it was really low to the horizon in the morning sky, so it passed through the new phase and now it’s into the evening sky. And because Venus is in the evening twilight, then is going to have the first encounter with that planet. And it’s a lovely… I mean, again, if you could take a picture, I think this is an interesting experiment. Take a picture of Venus and the Moon on December 6th and then take another picture on the 7th. Venus again will be like playing piggy in the middle. A little bit of an isosceles triangle, but the crescent moon, will be to the lower right of Venus on the sixth and then off slightly to the upper left of Venus on the 7th. But there’s another bonus because you’re almost describing a W in the sky because on the 7th it’ll actually be to the lower right. This is the moon. The Crescent Moon will be to the lower Saturn. So you can see how Venus crept up closer to Saturn, getting them in the same apparition with the Moon. So December 6th close to Venus, then the other side of Venus on the 7th, but below Saturn. So I think so there we’ve got Saturn. Then on the 8th, the moon plays the piggie in the middle and forms a triangle with Saturn and Jupiter. So we’ve got a whole string of planetary encounters with the Moon from December the 6th to the 9th. On the 9th, the  thicker crescent moon by now will be to the lower left of Jupiter. But on the 8th, just as an extra, I mean, always like these lectures. Again, starting on the eight, the crescent moon will be just above the star Zeta Capricorni. So if you’ve never seen that star before, which it’s a naked eye star in theory you should have done, but you know it’s easy to just look up and say, Oh, that’s just a star. Well, you’ll have to identify this particular star because on December 8th, the Crescent Moon will be just above Zeta Capricorni. So there we are, a little string of planets for you in the evening sky at first, but We’ll, we’ll come back to this later on.

Paul Now for the whole month the question is – will we have a bright comet? We’ve got Comet A1 Leonard. Now the problem with this is comet’s are fickle. They really fickle. We’ve had so many comets that we thought were going to be all this is absolutely going to be brilliant and then they just fizzle away to absolutely nothing or I’ll do the naughty on us and disintegrate. How dare they disintegrate on us? You know, when we want a really good comment. So you know, we we we are we are at the mercy of them, aren’t we? As because, you know, we just never know. We were so blessed with Comet NEOWISE last summer. You know, I mean, last summer, it’s hard to imagine starting the first year of lockdown. And so it gave us something to look at Didn’t it? Take a mind off things, and it was an absolutely brilliant comet. Now we can’t promise that A1 Leonard will actually do that. We just don’t know. We start off December on the sixth it’s sort of above Arcturus and this is in the morning sky, so it seems fairly high up. But it is dropping rapidly. It’s heading towards the horizon. And so December 6th, close to Arcturus, that’s one way of checking it, but it will be around about nine, nine and a half magnitude. Both estimates really do vary wildly with this comet, and you can’t really tie down comet magnitudes very well in advance because as they can be so fickle, it could fizzle away completely. So we’re keeping an eye on this, but it rapidly drops down through Serpens and then into Ophiuchus as we head towards the middle of the month. But because he’s dropping down, he’s getting deeper and deeper into the morning twilight. It should be brightening, so we’ll all be watching out to see what it does. Will it develop a tail? You know, as we just don’t know, you know, so we’ll have to wait and see how active, how bright it will be. I mean, some predictions suggest that its brightness could be plus +4, which is as bright as the Andromeda Galaxy. But we don’t know because in Twilight, the Andromeda Galaxy isn’t easy. So, you know, it’s a, you know, it’s swings and roundabouts as to how good it really be. It could complete surprises and go absolutely mad. You know, we could have a really dramatic one. It could also, as they completely disintegrate. So it’s a real pain.

Ezzy I’m going to be honest, I quite like how unpredictable comets are because there’s so many things in astronomy, which I like. You can predict where Mars is going to be in two thousand years. You know, everything runs exactly like clockwork. And then you’ve got something like a comet and you don’t know what it’s going to do. And that sort of element of surprise, I think, is quite nice to have sometimes.

Paul I agree completely. You know, it gives you that bit of unpredictability.  Now there is a sort of it’s sort of good news and bad news because I’ve mentioned that this comet is in the morning sky and it’s heading down towards the east southeast horizon, into the twilight. But ironically, because of its position, it hugs the north west to west to southwest horizon in the evenings. So again, you need to be looking in the evening twilight and this is the problem. It’s in the twilight, so if it needs to get really bright to be spotted in the twilight. So it hugs the horizon all the way through the month sort of thing until about the 24th, and in fact, it passes under Venus. So Venus on the 17th and 18th will be a good guide to see if you can find the comet in twilight with binoculars sort of thing, so its brightness will really play an important aspect as to whether we spot it or not. And my gut feeling is we won’t. But let’s hope it surprises us and does something completely unexpected. And we get this wonderful comet. We can only hope. You know, we wait for these comets, so fingers crossed what will actually happen with it? OK, I’ve just mentioned December the 17th and 18th, well, a day earlier on December the 16th. We have the gibbous moon in the evening sky and it form… I always like it when it forms triangle. I know when you’ve got two other objects, it’s got to be a triangle. We always say this, but this is the Moon forming a triangle with Messier 45, the Pleiades, the Seven Sisters star cluster in Taurus and with Aldebaran in the Hyades star cluster. And of course, Aldebaran is not really a member of the cluster of Taurus. It’s half the distance, so it’s not a true member itself. But the visually it looks like it, doesn’t it? So the Moon forms this wide wide, it’s almost an equilateral triangle… sorry, a right angle triangle. But the bit about this I like is that it’s not too far from the dwarf planet Ceres. So Ceres has been passing through the Hyades cluster.  I did get a night when I spotted Ceres just to the right of Aldebaran sort of thing in binoculars and on a photograph. So Ceres has been moving through the Hyades during November. Now it’s actually well away, but the Moon is a bit of a guide you got to bear in mind. The moonlight will probably wash out Ceres, but it’s worth having a go for it sort of thing. You never know. You might be able to spot it. And it’s the odd dot. If you watch night after night, it’s the one dot that moves. That’s that’s the telltale sign with these minor worlds and dwarf planets. From nights they should show a movement, whereas the stars will remain fixed in relation to their constellations. So last December, the actual 16th… Ceres will be magnitude 7.5. So, yeah, very bright, big, bright Moon there. It will be a bit of a pain.

Paul Now the next night, the 17th, the Moon will be to the right of Tau Tauri. Now I have to say that. If you say to me, you get all tongue tied. But Tau Tauri actually is occulted the Moon for parts of the UK. Not all. So, you know, but it’s worth keeping an eye out because Tau Tauri has got a companion. And I always find it interesting  because in this case, the Moon, which will be getting closer to full, is full on the 19th, so it’s quite a bright moon. But Tau is actually a bright star, so it’s worth looking at because the companion gets occulted first, and I always find that fascinating. It’s fascinating just to see a single star occulted. But when you actually see a companion, you see the Moon creeping closer to the companion. You know it’s going to go first. It adds an extra “value added” to the actual spectacle itself. So that’s sort of in the late evening, we’re talking about 9:20 onwards, So start looking early and not all parts of the UK will actually get to see it. The actual the Moon will miss for some parts of the UK, but even so, worth looking out. I even like the close encounters because it shows the clockwork motion, doesn’t it? You mentioned the clockwork motion. And yes, you’re right, the unexpected is really good. But there’s also this fact that we can make these predictions of occultations of stars really accurately as well and look out for these as well. So they are that’s on the 17th. Now, if we turn to the morning sky again and we go back to Mars as the month progresses, Mars is gradually improving. It’s getting better placed, slightly dark skies as it gets further away from the solar glare. So it’s trying to get out of the twilight and on the the 18th and 19th, on the 18th it’s between Graphius and Disuba, in Scorpius. So this is Beta and Delta Scorpii now on the 18th. On the 19th, which really close to Omega II Scorpii something which light directly below Graphus, Beta Scorpii. So this is Mars actually moving through Scorpius itself. So I always find it fascinating when, again, when a planet is really close to any of the bright stars sort of thing. it’ll be a really close encounter. Omega is actually a double star, very wide naked eye, you know, so worth checking out your eyes to see if you can split them really well, but on the 19th, as they make it to the lower one, will actually have Mars right next to it sort of right underneath it. I love those little encounters. Mars will gradually move northward past the constellation. It technically moves into Ophiucus, because it’s one of those weird things that the 13th constellation of the Zodiac really, you know, because Scorpius is… because the International Astronomical Union, the way they drew the boundaries based on the RA and dec. It’s angular. So you’ve got this small section for Scorpius and then Ophiucus intrudes into this region. And so Mars will pass into a few shifts. But what we find on the 25th and 26th – very important days for certain people, hopefully lots of pressies – that will have it pass in north of Antares. You’ll be several degrees away, but Mars is the Red Planet and Antares is the ruby red heart of the Scorpion. So a great chance this time. If you like getting up in the morning, great chance to see the Red Planet with this really red star as well. So that’s something to look out for and then to cap it all right at the very end of the month, the thin crescent moon ends up hanging below omega and Beta Scorpii as well, with Mars off to its far left. It forms a triangle really with Antares again. And Mars, you’ve got that triangle. This a bit more of a pointy triangle, I have to say as well, and you need to be looking as I say, in the morning twilight and you’re looking at around towards the south east. So it’s better if you got a good, clearer horizon and that is always useful for these sort of things.

Paul Now, I mentioned right at the start that we’re looking at things like so like a parade of the planets. Now it doesn’t happen often where you get all the proper planets. I’m sorry. Sorry, Pluto, I apologise Pluto. But no, you’re not included in this. Sorry, you’ve been demoted. But the major planet, Mercury out to Neptune are actually all viewable. So we’re looking now from Mercury and Venus in the evening sky. This is the last week starting say from December the 25th, So on the 25th. Very low down in the southwest. In very bright evening twilight, so you’ve got to bear that in mind. As Venus moves from left to right – December 25th to January 1st – Mercury moves up from the right to the left, so it’s almost like the swapping places, or trying to swap places, but you won’t be able to mistake Venus. Venus is the evening star, and it is the one I think pretty much everybody would recognise as the Evening Star. I do wonder if they’ll think about that on Christmas night when they see the actual evening star there hanging above the horizon. I’m sure we’ll have stories about the Christmas star. Was it Venus, et cetera? I’m sure nobody really knows the answer to that. It’s going to be a debate going on for years, but it gives people something to debate, doesn’t it? But I say, well, have both evening planets in the twilight, the evening twilight. So that starts us off. And if you remember Saturn is the next planet out, it’s in Capricornus and then on the lower edge of Aquarius, we’ve got Jupiter. Jupiter and Venus are the two brightest planets I have to be. So you’ve already got four planets, but it doesn’t stop. There could go a bit further on in Aquarius, you have Neptune. Now you will need large binoculars or a small telescope to spot Neptune, but it is not impossible. So that will be in Aquarius as well. And then further up along the ecliptic, you get up to Aries. So really on an area line between Hammel and this sort of like Mu Ceti. And then you find Uranus, so you got Uranus as well. Now binoculars will show that and if you’ve got really keen eyesight and a dark side, it is potentially possible to see as a naked eye object. I’ve seen it a few times, not many. And now I need glasses. I have to say I can’t see it, unfortunately, but at least I’m glad that at some point I did actually manage to spot it a few years ago with the naked eye, I can see why easily got missed. You know, it’s such an inconspicuous thing. It’s just on the borderline. So we’re doing well now. We’ve got six planets and of course, we’ve got Mars in the morning sky. So if you’re really keen, well, I suppose what you could do is just set an alarm, go to bed. But if you’re anything like me, you set an alarm and the alarm goes off and you smash the alarm and then go back to sleep. So the moral of the story, don’t smash the alarm, listen to the alarm and get up! Because then you’ll be able to see the final planet in this parade. But you’ll have seen all the major planets in the Solar System and the course of one night, and you can do that for most of that week from December the 25th to the very end of the year. It’s always like that you actually get this parade of planets sort of thing. It’d be interesting to photograph them wouldn’t it? Take a photograph of each one sort of thing. And you know, so at least you could say I photographed all the planets over the course of one night. And on December the 3rd, you remember that the Moon will be near Mars as well. So you’ll have another planetary body, the Moon, to boot. I suppose you could go the whole hog if you’re into observing the Sun and you have the specialised equipment, you could photograph the Sunrise and at that you can keep going on. We could add Ceres and what? The list is endless.

Ezzy I think at that point, you’re setting yourself a bit of a challenge.

Paul Yes, it is a bit of a challenge. But you know, we do like challenges with the Sky at Night, don’t we? Now, the thing about this is that, you know, I haven’t mentioned meteor showers and you know, I thought, well, I’ll leave them till the end because basically meteor showers, we love them or hate them, because you either go out and you don’t say anything or it’s cloudy or there’s moonlight up. So it’s one of those things that meteor showers this month are actually spoiled a little bit by moonlight. The two main ones are the Geminids and the Ursids. It’s the Geminids on the 14th in the morning, really the early hours of the 14th. So but the problem is the Moon is up. But the good news is the Moon sets about 3am. So I mean, it’s dark skies now because it’s winter. And so really, I mean, I know somebody will say, actually December 14th, you’ve got a week to go before the winter solstice. But you know, it’s it feels like winter as as far as I’m concerned. Meteorologically, they class it as winter from the 1st December. But the point is, you know, after 3am, the skies will get dark. So we have a window of opportunity to keep a lookout for the Geminid because they are a good… those and the Perseids are the two showers, Really, we say look out for because they are the best ones of the year. They have the most meteors and because the zenith hourly rate is quite high, over hundred. And so the thing about that is the high rate means you’ve got a better chance of seeing them. Unfortunately, a few days later, basically a week and a bit later, starting on December the 22nd, the Moon will be just a few days past full and it will probably drown out the Ursids, which is a weaker display. The Ursids is on the 22nd, then I’m probably going to be a loss. Even though it’s a circumpolar zone meteor shower and is visible all night. But I think the Moon will really spoil that one now. Very rare, I don’t often mention really minor showers, but I get the bulletins from the International Meteor Organisation and I noticed the Sigma Hydrids. Now I’d never heard of the Sigma Hydrates, but they peak on the 9th, so the will actually be a chance to see those during the course of the night. The only problem is they have what we call a zenith hourly rate of just Seven. Seven! oh my.

Ezzy That is not high at all.

Paul But I’ve been out and I’ve seen meteor showers. I mean, you have to be patient and there is a chance that you might get some early Geminids as well, because obviously the thing about the meteor showers is that we often talk about observing on the peak night. But the reality is meteor showers often spread over a week or two and sometimes even a month. And some meteor showers actually spread over a really wide range. Hydra is actually rising in the early evening and so it is worth looking at. Is this close to the radiant is close to the head of the Hydra, so it’s worth looking out. And if you can trace them back to Hydra, they are distinctive to the Geminids. The Gemini will be a completely different route, so it’s worth looking out just in case you see one of these meteors for this rather small shower. But you know, as it’s in a dark sky. I always think with the small showers is worth looking out when there’s little Moon because you’ve got a better chance of actually seeing them. So that’s the meteor showers for the month. The Geminids are worth trying, I say, from 3 a.m. on December 14. You can look. I would look 13th the morning, the 14th and the 15th because they are spread out over that period of time. So, you know, and as we go into the 14th and 15th, the Moon will be getting, it will be setting, sort of better. So we’ll have longer to say on the 15th, but 14th is the peak morning really. Perseids are washed out. But these rather unusual Sigma Hydrids are worth trying for on the 9th. So I think in the evening, just keep a lookout. You never know you might spot them. So there’s quite a lot to see. There’s an occultation thrown in for good measure as well. We’ve got the potential for a comet. So you never know Ezzy, you know, fingers crossed. We were all hoping we have some clear skies. That’s the key, isn’t it. it’s the clear skies and we can have all these predictions. As you say, we can predict the Solar System. They’re clockwork, but in the end, we’re at the mercy of the weather.

Ezzy Yeah, we can at least guarantee the dark nights that we can guarantee the long nights, but we can’t guarantee the clear nights.

Paul Exactly.

Ezzy So, yes, there’s lots to see in the December night sky starting off. So, yeah, it sounds like there’s lots to see in the December night sky starting off at the beginning of the month. You’ve got Mars appearing in the morning sky, then moving on throughout the month, you’ve got A1 Leonid, potentially a comet that could be appearing close to the horizon. So keep an eye out on that for the first half of the month. Then on the 25th, on Christmas Day, I’m going on to Boxing Day, Mars, the Red Planet will be passing the Red Star Antares, so that’s definitely one to look out for. And then finally, in the last week of the month, you’ll have all of the major planets visible throughout the night. So if you fancy setting yourself a little challenge, why not try and photograph all of the planets in one single night.  And if you manage it, make sure you let us know at We always love to see all of your pictures. So thank you very much for joining us today, Paul, and telling us all about what’s coming up in the month,

Paul My pleasure Ezzy


Ezzy If you want to find out even more about the spectacular sights that will be gracing the night sky this month, be sure to pick up a copy of BBC Sky at Night Magazine, where we have a 16 page pull up sky guide. The full overview of everything worth looking out for in December 2021. Whether you like to look at the Moon, the planets or the deep sky, whether you use binoculars, telescopes or neither, our Sky guide has got you covered with detailed star charts to help you track your way across the night sky from all of us here at BBC Sky at Night magazine, goodbye.