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

A guide to September 2021’s night sky in the northern hemisphere. This month features Jupiter, Saturn, Mercury, asteroids Pallas, as well as the last chance to see Spica this year.

There’s no shortage of stargazing opportunities in the September 2021 night sky in the northern hemisphere.


The month starts off with a series of occultations with stars like Mebsuta.

Then the planets Jupiter, Saturn, Mercury and Neptune are all well positioned later in the month.

To find out how to observe these sights and more, listen to the latest episode of Star Diary below.

Bookmark the Star Diary podcast page and never miss an episode!


Ezzy Pearson Greetings, listeners, and welcome to Radio Astronomy’s guide to the best things to see in the Northern Hemisphere night sky in September 2021. I’m news editor Ezzy Pearson. And I’m joined on the podcast today by reviews editor Paul Money. He’s going to be telling us the best sites to catch this month. So, Paul, what are your recommendations for September 2021?

Paul Money Well, we’ve got a good month. So the thing about September is that we really start getting back into the dark skies. Yay! I, I’m not a great sunbather, you understand. So give me the night skies any time. So now we’re back to September. It’s great. And we kick off actually with an unusual pair of events because it’s not often you get an occultation of two different stars, one after the other on successive mornings. And it just so happens that the path of the moon going around us is tilted to the ecliptic and it goes through Gemini. And as it happens, there’s quite a few decent stars that have the potential to be occulted. And that’s what happens this time. So we’ve got Epsilon Geminorum on 2 September. Now, these are I mean, I have to say, you have to get these Ezzy. I know. I know. I mean, set an alarm or what not, but these are early morning ones. So I’ll just it’s easier for me just to stay up because if I go to bed, I’m doomed. You know, that saying I’ll hit the alarm and go back to sleep. So I have to stay up.

Ezzy Pearson The astronomer’s conundrum – to stay up all night or to get up early.

Paul Money Exactly. So, I mean, the choices we have to make is terrible, isn’t it? So yes. But this is worth getting up for because, you know, I mean, it’s a bright star for a start. And so you… it’s actually called Mebsuta, And it takes about an hour or so. And I always like to say, look, start before it. So you really have to be looking in the morning sky. I’d start at about 1:00 a.m. It’s always best to get familiar with the area, although I have to say, if you have difficulty finding the moon, then you might need to take up a new hobby. I’ve always thought of any event involve the moon. If you can’t find the moon, yes, you need to do something else or it’s probably completely overcast. So I always like to give myself plenty of time before these events because anything could go wrong. You know what it’s like with equipment sort of thing. You might see this with binoculars. Now, the thing about binoculars is that the disappearance is on the daylight side for both events, for both mornings. So this is 2 September with Mebsuta, as I look around about 1:00 a.m. and keep glancing at Mebsuta and the moon and you’ll see the moon creeping closer. I love these because it shows the solar system in motion, doesn’t it? You know, you really realise that, you know, things actually do move. We’re so used to the night sky not appearing to move. That much will change that. When you see the moon creeping up to the star, you feel, “look out, star, there’s a moon behind you”. Well, it’s actually going to be in front because then it crosses over the star. So the disappearance being on the bright side is interesting because it’s harder to see the exact moment when it is gone. Whereas if it’s on the night side sort of thing, the dark Terminator is a lot easier. So you’d think the reappearance just over an hour or so later would be easy because it’s on the dark side. It will suddenly appear. But the trouble is you have to really work out where it is on the limb because it might catch you. You might be looking at the wrong part of a limb of the moon sort of thing. Of course, it’s dark, so it’s very difficult to say. But you hope that there might be a little bit of Earthshine there by now because this is a thick waning crescent moon. So it’s one of those things that there might be enough of a glow with it for you to work out roughly where it’s going to appear and then keep your gaze looking at this and time it, because these timings are still… their fascinating, because it gives you an idea of the clockwork mechanism of the solar system. So that’s on the 2nd September. Then the next morning we treated to another one, Keppa Geminorum is actually occulted. And so this one’s a bit late. I’ve got to make the decision. Do I now set the alarm? Because they start looking about 3:30 in the morning. And again, it’s about an hour. These vary slight. So, again, depending on where you are in the UK are plus or minus ten minutes to the time sort of thing just to make sure. And so you’re looking from about 3:30 till about 4:40, 4:45 in the morning. But the skies are still relatively dark there. So that’s two occultation to kick us off with. And it’s not often say you get to occultation on successive mornings like this. So and I say I like watching them because they are the clockwork mechanism of the solar system, the solar system in action itself. Now not content with that if we… Like I mean, I’m sticking with the early morning, even though, you know, I know how hard it is to get up.

Ezzy Pearson Sometimes the night sky just wants you to not go to sleep.

Paul Money Exactly. It just…

Ezzy Pearson it really doesn’t like your sleep schedules.

Paul Money Exactly. It’s conspiring against this isn’t Ezzy, but the next morning, 4th September it’s not an occultation, but in actual fact, the moon will be directly above, And it’s a really thin crescent moon now, and it’s about 4:00 a.m. Look at the Moon and below It will be the Beehive star cluster, Messier 44, Praesepe as it’s known. So that’s an extra bonus sort of thing. So if you’ve got used to get it on the first two mornings, then then you should be easy to get up on the next one, says he’s going to be fast asleep.

Ezzy Pearson If  your sleep schedules already ruined, you might as well stay up another night.

Paul Money Exactly. Sort of thing. You know, I have to admit if I do an all nighter now, it does get to me. I think he’s my age. We won’t go into that any more about my age. But I’m beginning to realise why it’s often easier to set an alarm now because staying up all night is not so much fun. Now, I used to enjoy them sort of thing, but and of course in the end. So most guys, if you stay up all night, it doesn’t feel like such an ordeal. But as we September, because we’re heading towards the equinox, which is on the 22nd this year and so equal day, equal night and we’ll have longer nights. So half the night sort of thing will be dark. So long that out of style sort of thing. So yeah, I think I might set an alarm for 4th September Beehive and Moon conjunction as we see here. But let’s, let’s get back to the evening sky because I mean, I have to say the vast majority of people will tend to look in the evening sky. And we just can’t get rid of it. I mean, Venus is still lingering that it’s a quirk of the orbit and the tilt of its orbit and the angle of the ecliptic. And Venus just lingers in the twilight and it’s been there months and it’s still going to be there for quite a few more months yet. So you need to be looking around about.. Say, about thirty to forty minutes easily after sunset. So the star is getting at least reasonably dark. And then you’ll spot Venus in the west southwest quite low down. But don’t leave it too long because of course it sets so you lose it. But the thing about this is that in actual fact, on the 5th so we’ve got a run of… This is on the evening of the 5th so don’t get up in the morning for this, Venus isn’t in the morning sky. It’s In the evening twilight now, but on the 5th is actually above Spika or Spicer (Spica). Depends on how you want to pronounce it. But the thing about this is that that’s Alpha Virginis is probably your last chance to see Spica actually at this time of the year. After that it will be lost. It’ll be too close to the glare of the sun. So Venus is a brilliant guide, you know, and the evening stars sort of thing, which is not as dark as a planet and it’s directly above Spica. Now, give it a few more days because as it happens, we get the crescent moon back into the evening sky and it’s always try looking around about the 7th or 8th. See if you can see a really, really, really slim crescent moon. Because the thing about this is that it’s a very ephemeral when it’s a really thin crescent. But then on the 9th and 10th, it’s either side and above Venus. So you’ve got a nice pair in there. But of course, on the 9th, it’s also quite reasonably close to Spica. So that might be a very, very, very last chance to see Spica as well. But obviously you’ll see Venus. But again, you’ll be wanting to be looking about the same sort of time. So Venus will have the moon, crescent moon to its right on the 9th and to its upper left on the 10th. And of course, on the 10th, the crescent will be a bit thicker because the moon is gradually building up its phase sort of thing. We’re seeing more and more of the illuminated side. So at least that’s in a decent time, isn’t it? Sort of thing. For us to actually observe. actually does depend on your own local circumstances because it’s very difficult for me because I have lots of buildings. So I’d have to travel out, find somewhere in the Licolnshire Woldes to actually observe it. So I’ve got a nice, decent flat… I have got a little site a couple of miles away so I can go to, so that’s not too bad.

Ezzy Pearson You do want to make sure that you’ve got a nice, clear view of the horizon. If you’re going to be trying to observe those at home.

Paul Money You can almost guarantee there’s a house or a tree in the way or otherwise sort of thing. So can I say WSW is the direction you want to be looking for it. so that’s the key. Now, the other thing we need to look for asteroids, minor planets, depending how you want to describe them sort of thing. I mean, there’s over a million of them now and then the night sky. But the vast majority, of course, you can’t see with an amateur instrument or binoculars. But we get a chance because Pallas, which was the second asteroid to actually be discovered, is actually an opposition on the 11th September. And so we actually see it in Pisces. It is below an asterism called the Circlet, that sort of thing, which is on the right hand side of the Fishes. Pisces, the fishes. It’s below the square of Pegasus, so Pallas comes to opposition, so therefore it means it’s visible all night. Now, it’s not too far from Neptune. So we’ll come to that in a short while because it means if Pallas’s at opposition, it’s reasonably close to Neptune and it may Neptune will soon be opposition as well. But Pallas first. If you’ve never seen an asteroid or a minor planet, then binoculars will get a magnitude +8.5. So well within the reach of ordinary 10×50 binoculars sort of thing. And it is slowly moving away from the Circlet down towards the Aquarius. So it’s worth trying to get this sort of thing now while you can, because after this it starts to fade and opposition is always the time when these things are at their brightest. So magnitude 8.5, easy binoculars sort of thing. But after this it’ll start to fade. And within about a month or so, he’s down to magnitude 8.9. So getting fainter. So it’s worth grabbing now. Now, as it happens the next day and we switch to the morning sky again. Because the thing about Palllas been opposition, by the way, is that it’s visible. Once you reach opposition, you’re opposite the sun in the sky. It rises as the sun sets and sets as the sun rises. In other words, it’s visible all night. And so pick your moment to observe. But after this, it’s interesting that Pallas moves into the evening sky. It’s just a shame that it starts to fade. So I think this is one of the asteroids that actually fades quite well, really, unfortunately. So grab it around opposition time is the best time. But the next morning, so again, you might want to stay up. That’s the 11th. On the 12th we’re actually looking towards the early morning sky. And we’ve got Taurus back I mean. And I can’t wait to when I see Taurus in the morning is going because I know winter is on the way thing. So I’m really cheerful to my friends. You know, when you consider I’m always hankering for the winter sky. But the point about this is that we’ve got the Hyades up, which has its bright star, Aldebaran, depending how you want to pronounce it. But the thing about that is the bright red eye of the bull, although it’s an orange star. I always Think that’s quite funny, but it does look reddy-orange doesn’t it. The thing about this is that Ceres, now that was the first of the asteroids to be found sort of thing, but it’s now classified as a dwarf planet. Ceres, since about mid-August has actually been gradually moving quite close, but under the higher the star cluster and on the 12th – you could say 11, 12 – is actually below Aldebaran. So in Taurus. So again, this is a similar magnitude as it happens, 8.6, at this particular point because it’s not reached opposition position. It’ll be a few months before Ceres reaches opposition and Bright and Ceres can get quite bright. So you can get two minor worlds in one night you could get Pallas at opposition, and then stay up until the early hours and grab Ceres in Hyades or below the Hyades. It will eventually, during October, November, actually pass through the Hyades as well. So it’s staying in this general area. So it’s a nice, easy patch of sky to recognise. So with binoculars you should be able to pick out night after night, one dot of light is moving compared with the other. And again, you get into clockwork motion of the solar system on display. Now we’re still with the 12th because that evening… I said it’s a busy month. We’ve got a lot going on sort of thing.

Ezzy Pearson Got two things in one night.

Paul Money I know, it’s exciting sort of thing. So we’ve got Jupiter. Jupiter and Saturn are much easier to see now in the evening sky. They’ve moved into the sky since their opposition last month in August, Saturn was at opposition on 2 August and then Jupiter on 20 August. So Jupiter is gradually retrograding, and that means effectively for the layperson, it’s moving from left to right sort of thing because we like to use to prograde, and retrograde for us astronomers don’t mean a lot to the general public.

Ezzy Pearson It’s going the opposite direction that it does most of the time.

Paul Money Exactly. So that’s why it’s retrograde, rather than prograde, but as it does so on the 12th, again, it’s actually directly above, several degrees away, but directly above Delta Capricorni. So it’s a bright star Delta and Gamma I always find…. as it moves on, when we get to the 20th, it’s actually directly below the star 45 Capricorni and it’s gradually heading towards the end of the month, forming a triangle, Delta and Gamma Capricorni. So this is a nice pattern anyway, but it’s always great when it’s reasonably close to another star as well. So that’s the evening, the Jupiter line above Delta Capricorni on the 12th and then on the 20th when it will be a lot better place to see because it’s amazing each passing day, or should say evening, it’s amazing how much the sky is moving and creeping. That area is creeping into a better position to observe in the evening sky. So we have a bit of a gap. Two days! Good grief. So we jump to September the 14th and we mentioned now that Neptune was the opposition or will be opposition. September the 14th is actually opposition. But first things first. It’s very hard in the evening sky, but mercury is also its greatest eastern elongation from the sun. The only problem with this is in very, very bright sun… Twilight and it’s setting just 20 minutes after the sun. So I’m always a bit nervous about observing  mercury so close to the sun like this. But as long as you let the actual sun set literally look towards… Again, sort of the WSW, that general area look for it sort of thing, but make sure the sun has set whatever you do. So you might pick out mercury, but it’s very difficult. This is not a very good apparition for this particular time in the US.

Ezzy Pearson As always, if you are thinking about trying to do any kind of astronomy close to the sun, never look at the sun directly, especially don’t look at the sun directly through optics like telescopes or binoculars, you can do yourself a lot of damage. So as Paul said, make sure the sun has really set before you start looking for Mercury.

Paul Money I mean, we want to keep observing, don’t we? Don’t we do not want to lose our eyesight sort of thing. As always, my perennial  worry ever since I became an astronomer is you worry about your eyesight. I have to say so, yes. Protected as much as you possibly can. So that’s mercury really difficult. But I say September the 14th is also Neptune’s opposition night. So it now becomes visible all night and its magnitude 7, so easily visible in binoculars, so easy to see. It’s interesting because he’s actually passing through from 1st September. It’s actually passing through a sort of like a diamond shape of stars. They’ve all got Hipparcos numbers so I won’t go into them.  But there’s a nice diamond shape of stars are sort of similar brightness and slightly brighter. So I always like to look for we love patterns, don’t we? We’ve got constellations. They just join the dots. But we have a habit as amateur astronomers of joining the dots on smaller and smaller scales, because we are we find patterns easier to recognis. If we can recognise a pattern, We can find some things in them. So in this case, is this nice diamond pattern that Neptune is passing through. At the end of the month, it passes quite close to Hipparcos 115953. So it worth looking at around the 22-24. But Neptune will be at opposition, so it’s visible all night. So again you’ve got another one and it’s so close, it’s only three days after Pallas. That’s why I said when Pallas was the opposition on the 11th. They meant that Neptune was nearby. It had to be opposition within a few days. So things. So any objects within the same Right ascension line, they will reach opposition within a few days of each other. So there we are. So opposition for Neptune. Neptune it is officially the last planet of the solar system. So poor Pluto. But let’s not get into that debate. But the thing is, I do find it looks bluish. I do find it better with a small telescope to look. Binoculars it’s just a dot. But again, if you watch it with binoculars, you can see it moving slowly night after night. So you get an idea of which object actually it is. And that’s why I say on the 22-24th, when it crosses that Hipparcos star, you’ve got something that you can see slowly edging past. So that’s always a good clue. But it may be the last planet, but if you get a chance to put a telescope, if you’ve got a telescope. You know, medium size, six inch onwards sort of thing. Have a look. You might pick out Neptune’s Moon, Triton as well. It is a challenge. Magnitude 13.5, but, you know, it is not an impossibility, so it might be another object, you can take off your little list. I’ve got a list… Getting a big list now, but keep ticking these things off, the objects in the solar system. So we jump another couple of days. And this is a period of the 16th, the 18th. And we’re back to, aren’t we? We often have talked about the moon passing the planets. And it’s been nice not to have the moon mentioned because it seems like some months it’s only the moon doing anything. But we are back to the moon now. And so on the 16th, the moon is to the lower right of Saturn. Now, if you don’t know where Saturn is and you don’t know which object is Saturn, it is actually… it’s and Jupiter, the brightest object in Capricorn. So in theory, shouldn’t be able to mix it up. So when you get the moon down, the moon is to the lower right of Saturn. So when you look at that area, you’ll see the moon, you’ll see a bright star to the left and any fainter stars of the constellation itself. So Saturn should then be quite obvious. The moon then passes below it totally. Its orbit means that at the moment it’s passing quite low below the actual ecliptic. So as it happens, it forms a triangle on the next night. So the apex pointing down between Jupiter and Saturn. So they formed the other corner of the triangle. So the moon’s the 17th and then on the 18th when the moon is to the lower left of Jupiter. So you’ve got this sequence of three nights where the moon passes and it guide you to these planets and of course, to the constellation of Capricorn as well. So say there’s a lot going on, but we haven’t finished quite with the moon because I always like it when it passes the sky objects and guides us. And we’re back to Taurus. But we are in the morning star again and I’ve suggested three to four anywhere from midnight to four a.m., four days on the 26 and 27th. And the moon will be directly below the Pleiades. The seven sisters. Now admitted to the moon light will wash out the body. You’ll see a faint sparkle in the stars. So it’s not as easy, but it will be. Then again, if you use binoculars, you could sweep up from the moon and you’ll be able to sweep up the cluster. I’ll tell you, it’s quite excited. It gives you a bit of a thrill when you start off from the moon sort of thing and you wonder now and then suddenly you come across this cluster of stars that’s just suspended in space. The sun use the moon to guide you to the Pleiades because I say they won’t be as easy with the moonlight, but have binoculars, you’ll be able to find them. And of course, in binoculars, even with moonlight, you should get around about 15 to 20 in 10×50 binoculars. So there’s quite a few bright side. What do you think of the Seven Sisters? But there’s actually two hundred and fifty cluster members at least. Now, I’m not expecting you to say that in binoculars, but…

Ezzy Pearson There’s a bit of a big family reunion.

Paul Money It is a good family reunion. I can’t imagine that many sisters. No, we won’t go there. But the same binoculars, you should get around about 15, 20, and you will get more on a dark night once the moon’s out of the way. But it does mean the moon is actually passing through Taurus. And it’s interesting because, again, it doesn’t occult it, but on the next night, on the 27th in the morning sky, it’s actually quite close to Tau Tauri. I always think that is one of those tongue twisters… Tau Tauri sort of thing, because you’ve got tau, which almost beginning the first three letters of Taurus itself. So Tau Tauri is there and the moon will be quite close to it. So if you’ve never seen Tau Tauri or not realise which one it is, the moon guide you on the 27th. You can’t miss it because there’s this bright star next to the moon itself. So there is a lot going on. We call the main things. So, you know, is there anything you wanted to mention Ezzy

Ezzy Pearson I think you have definitely given us enough to be going on with us? It is. It sounds like it’s a pretty good month for planets both minor and major, as well as a couple of other interesting star events going on as well. Thanks very much for joining us today, Paul.

Paul Money It’s a pleasure, and I hope everybody gets a chance to have some clear skies to observe.


Ezzy Pearson  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 Night magazine, where we have a 16 page Pull-Out Sky Guide, the full overview of everything that’s worth looking up for in September 2021. Would 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 find your way across the night sky. From all of us here at BBC Sky at night magazine, goodbye.