The equinox signals the start of autumn, and Neptune reaches opposition. Find out what other astronomy highlights are coming up in our weekly stargazing guide for 18 to 24 September 2023.
Chris: Hello and welcome to Star Diary, the podcast from the makers of BBC Sky at Night magazine. You can subscribe to the print edition by visiting skyatnightmagazine.com or to our digital edition by visiting iTunes or Google Play.
Ezzy: Greetings listeners and welcome to Star Diary. A weekly guide to the best things to see in the northern hemisphere's night sky. As we are based here in the UK, all times are in BST. In this episode, we'll be covering the coming week from 18 to 24 September. I'm features editor Ezzy Pearson, and I'm joined today by reviews editor Paul Money.
Hello Paul!
Paul: Hello, Ezzy how are you doing today?
Ezzy: I'm doing well. Can you tell me and our listeners what we have to look forward to in the next week?
Paul: Well, we start off in the evening sky, but the thing about this is that it's actually in the bright twilight, so you must make sure the Sun's set. In fact, if you look about for 30 minutes after sunset on 18 September, we're looking towards the west southwest, see if you can pick out a really slim crescent Moon.
It's not the slimmest you can get, but it's always quite a pretty sight when it's low down and you've got the Earthshine on it as well and you're... setting quite quickly after the sun. So it's one of those things because of the ecliptics or there's a case of grabbing it when you can, but it's quite a pretty sight, so I thought we'd start off with the crescent moon really low down. I'm looking at about 7:40 but times do vary of course depending on where you are in the UK. There we are a nice evening one, but it is in bright twilight. You have to bear that in mind.
Right. Now onto the 20th, well, we had the opposition of Saturn the other month, well, now we've got the opposition of Neptune. On the 20th, Neptune reaches opposition, which means it's visible all night. It rises as the sun sets, and sets as the sun rises. So effectively, it's visible throughout the hours of darkness. It is magnitude +7.8 , and it also lies below the circlet of Pisces, so that gives you a general area of where to look.
But in actual fact, we're quite lucky at the moment, because it's also quite close to the star 20 Piscium. So, that's about magnitude +5.5, so it's almost two full magnitudes brighter than Neptune. Because it's quite close, you can, if you use 10x50 binoculars and put them on 20 Piscium, the object, the dot, just to the upper right of the star is actually Neptune. And it's quite close. And it stays like that for a few months in actual fact. So it's quite a handy guide to finding Neptune.
So yeah, use 10x50 binoculars and pick out the planet Neptune. It isn't naked eye. It's round about magnitude +7.8, but, you know, it doesn't take a lot of optical sort of like, telescopes or binoculars to pick it out.
So, I say, you could even try 7x40 binoculars to see if you can see it. Be interested to know what's the smallest pair of binoculars that you can actually pick it out. But it does look blue in small telescopes. I've always noticed that. It is a very pale blue. So, and of course, planets are quite steady.
They're, they're a disc, technically. So, stars tend to sort of twinkle a little bit because of the atmospheric scintilation, but the planets being a disc can actually look steady. So often that's a clue. If you're looking at something and you think, is it a star? Hang on. It's quite steady. That means it's the planet. So there we are. The opposition of Neptune on 20th.
Ezzy: I remember when I was doing my, my degree, we were always taught that planets don't twinkle. And they do sometimes if the atmosphere is particularly unsteady.
Paul: Yes, I've seen that myself. There's always exceptions, isn't there?
Ezzy: It is probably one of those situations where if the planets don't look terribly steady, it's probably not the best day to be doing astronomy.
I mean, you can always see something. Unless it's completely overcast, you can always see something. It's always worth persevering, I think.
Paul: The good news is, we don't have to stop there on 20th, because 20th to 21st, we're back to the Moon.
Now, we're always fascinated with the Moon, well it goes round us, it's our nearest celestial body, and it's forever changing, so it's always going to hold our attention. And it's a thicker crescent now. And on 20 and 21st, it lies either side of the red supergiant star Antares in Scorpius. So, on 20th, it's to the right, and it's nearly level, actually, with the star.
If you're looking towards the southwest, obviously, the Moon will be the guide. If you can't find the Moon worry. Although you might have, but like me, I don't have a very good horizon all around. So, I've got, you know, a housing estate, an industrial estate as well. So, you know they... it's a shame, because they stop me seeing things rise and set at the moment, because I can remember when we first moved here.
It was a completely clear Eastern horizon. We had just fields and it was gorgeous watching the Moon rise, which had never been had to do... I couldn't do that when I lived at my original home. So of course, you know, it was great. And then of course they built the industrial estate and put a big bank up to hide it.
So I miss that. I do miss that sort of thing. If we ever moved, I want to make sure if we ever moved, we definitely have a nice, clear horizon all around us.
Ezzy: I think the only way you can ever sort of make sure of that permanently, as if you're out looking over the sea or the ocean. They're probably not going to build much on the sea.
That is one thing that I think people sometimes forget about when you're looking for an observing site, is it's not just "is directly overhead, lovely and wonderful and clear." It's also can you see these horizons? Can you see these things rising up in the night sky? Because there are some absolutely wonderful events.
And you just need to be able to see the horizon to be able to see them.
Paul: Exactly. Of course the ecliptic is quite low down, so it runs from Scorpius to Sagittarius, and that's why the Moon lingers a lot.
It actually helps the farmers, of course, at this time of year, because the Moon, you know, rises a similar time each successive night, when we're dealing with the autumn. The early autumn in particular. So that's why there's always light for the farmers, which is good for them. So, yeah, I reckon I look about eight o'clock.
Towards, the southwest, for the Moon. But you won't be able to miss the Moon. But I say, on 21st, it'll be to the left of Antares. So on 20th, it's just slightly below a level line with Antares at 8 o'clock. And on 21st, it's just slightly above that line. So it's very subtle, I have to say.
Now, well, we've got to do it. We've got to go back to the morning sky. Now we're talking about 22nd. Now in 20 seconds, about, say, 5. 45 in the morning, get Mercury. We've got a great advantage in that Venus is also up. It dominates, I mean, Venus dominates the morning sky at the moment. It is so bright. But this is Mercury's best apparition in the morning sky this year.
So it gets quite steep because as the ecliptic is low down at a shallow angle in the evening sky, it's actually quite steep in the morning sky because we're seeing the spring constellations come in to view. And one of them, in this particular case, is Leo. Regulus is there above Mercury as well.
So, Mercury reaches greatest western elongation, so that's 18 degrees from the Sun. Now that's it, it will start to drop back, but because this ecliptic is quite steep, we've got the advantage: the sky gets darker quicker, twilight appears shorter, and so therefore it doesn't linger, and you get a chance to see Mercury for a lot longer. It's still in twilight skies, but you get a chance to see it a good hour to an hour and a half before the sun rises. And that, that is pretty good for us.
I say we've got the added bonus of Venus. If you spot Venus, look to the lower left and you should see Mercury as well. So this is the best one in the morning. So, if you like morning walks, you know, early morning walks in the twilight, if you have to get up early in the morning to go to work, sort of thing, look towards the east and you've got both inferior planets, Mercury and Venus, to have a look at.
Ezzy: It does certainly sound better that because I think there was some earlier kind of apparitions in the year where it was "Oh, you can see it for about 10 minutes. That's what you've got. That's your window of when it'll be dark enough to see it and, and it's going to disappear out of low the horizon."
So having a full hour, hour and a half is absolutely fantastic, which is obviously because greatest elongation is when the angular separation between the planet, one of the inner planets, I should say, and the sun is greatest. It's not actually... I always used to think it was when the, the planet was at a 90 degree angle. So you get Venus or [Mercury], the Sun in the corner and then Earth at the other end, making a right angle.
But actually it's when it's more at sort of a 45 degree angle, something about that, which is all to do with perspective as you're looking in through the Solar System. But it is when on the... it will have the biggest separation, which means it will be up in the sky the longest after the Sun is set, which is why it's always a great opportunity to look at these inner planets, which are a bit trickier to get.
Paul: Yes, and it's interesting that the morning apparitions for Mercury are best in the autumn time, and the evening best apparitions are always spring. So I always think March, April time, sometimes creeping into May for the evening apparitions for Mercury, which are really good. And the evening ones in the springtime, they always coincide with the wonderful star cluster Messier 45, but you don't get that in the morning for Mercury, when it's out of its apparition. It's always going to be against the spring constellations. And in this case, Regulus, as I say, is to the upper... Regulus is not quite on a line with Mercury, Vena, and Venus. It's sort of slightly forming a bit of a shallow triangle.
It's the apex of a shallow triangle. But it will be prominent. You will be able to see it as well. So don't, don't confuse Regulus with Mercury. Although Mercury will be a lot brighter than Regulus at that stage.
Now, the next morning, well, the next day, is the autumn equinox. So, we reach that point, it's always traditional, we say, oh, it's equal day, equal night. There's a lot more to it, and I won't go into that at this stage. But, the nights will start to become longer. from now on. And so, of course, we rejoice, we astronomers. I think we should have a national holiday for astronomers that night. That day. So that we can actually stay out all night and enjoy the long, the fact that it's going to get longer nights from now on.
Basically, you know, the, now that the nights are getting longer you know, it's one of those things that when we got to this time of year sort of thing, I would, I would be excited and whatnot and say, Oh good, the nights are drawing in and whatnot.
And my work colleagues were like, You miserable devil sort of thing, we're still having our holidays for the summer and whatnot. The late summer holidays, exceptionally. And you're talking about dark nights. They always associated dark nights with cold, sort of thing. Well, I do, but I mean, I think Autumn, you get that nice mix of, it may still be reasonably warm, without it being too bitterly cold.
Ezzy: Well, I'm also one of those people, I much prefer the cold to the heat, so autumn's about the right... autumn's the perfect time of year for me.
Paul: It is. I do find, though, we tend to suffer a bit more in terms of mist and fog beginning to develop, and that can spoil the view. I mean, the other night I was supposed to have a clear sky, looked out, they said, right, morning sky will be nice and clear, so I thought, right, I woke up.
And the whole sky was misted over completely, you know, and I just went, Oh! Mind dear, I went back to bed. That's the easiest thing to do, isn't it?
Ezzy: Yes, you can have some lovely crisp nights and you can have some less good nights.
Paul: Yes, the, the weather presenters are very good at saying yes, and tonight will be very clear and, and nine times out of 10 you can sort of look up and as astronomers we have a slightly different definition of clear.
I mean, to clear to us... Clear is clear, you know, not the slightest hint of haze, but to them, if, even if it's slightly hazy, "oh, it was a nice, clear night" and I'm thinking, no! But there again, we're, we're very pernickety, aren't we, us astronomers as such. Anyway, let's finally finish off. We've got to remember that all week, we've actually got a chance technically to see all the planets with the exception of Mars.
There's always one, isn't there? There's always one that's awkward, but Mars is too close to the glare of the sun. It's still in the evening sky technically, but it sets at the same time as the Sun, so you can't actually see it. But all the other planets, and of course you've got the Moon as well, and there is the minor planet Vesta, they're all up during the course of a night.
And because we've got Mercury, I mean that's the added bonus, that's the one we normally wait for, isn't it? That we have to wait for, because it's the one that, you know, quickly comes up in the sky and then disappears again. So it's often quite rare to get them all in one go. And I say, at the moment, we're missing Mars.
But there is quite a parade. We start off with Saturn low down in the morning sky. You get it before it's sets, so about three o'clock, you know, you want to be seeing it. But you get Saturn, then you get Neptune, then you've got Jupiter, then Venus, and of course they're not far from the Pleiades star cluster.
Vesta is actually in the club, top end of the club of Orion. And it's about a similar brightness to Neptune as well, so you can put binoculars on that and keep an eye on that. And then, of course, as we mentioned earlier, we've got Venus, and then Mercury down low as well, so, you know, it's a good chance.
You get, also, the winter sky. I mean, it always amazes me that we tend to think of "oh, yes, we're in autumn now". But if we're in autumn, when you get into the late night, you've got the winter sky, about 5:45, I've set it, and Orion is nearly due south. You know, so Orion is really well up. Sirius is up.
But, then, that's why we've got the spring sky. And I always think, this time of year, you almost get a full parade of the constellations of the seasons. So you've got the summer in the evening, then it turns to autumn.
So, because we've still got the summer sky, but the moon's up at the moment, so it'll cause a bit of a problem. We've got summer constellations, then the autumn, then the winter late in the night, and then just rising we've got the spring constellations. So you nearly get a full year's worth, the season's worth, of the constellations. I think that's quite something. And you've got a parade of planets!
I mean, what more do you want to end the week, eh?
Ezzy: It certainly sounds like there's a lot of great things to look forward to, so thank you very much for taking us through those, Paul.
Paul: It's a pleasure.
Ezzy: And so to summarise, on 18 September, the slim crescent Moon will be low in the western sky. Then on the 20th, Neptune reaches opposition.
You can use the star 20 Piscium to be able to help locate that if you want to. Over 20 and 21st, the Moon will pass across the star Antares. On the 27th, Mercury is at its best for the year. It'll be up for around about an hour, possibly a bit more, and you can use Venus to track it if you want to be able to see it in the sky.
On 23rd, it's the autumn equinox – the day which signals when the nights are going to start getting nice and long and can take advantage of the fact that you'll be able to see some of the summer constellations and the winter constellations, as well as all of the normal autumn constellations over the course of the night, so that's a great opportunity to take advantage of that.
And of course, throughout the week, you can see all of the planets, except for Mars, who's hiding behind the Sun, and even get the minor planet Vesta as well. So lots and lots of things to look out for in the night sky.
If you want to find out what even more things that are coming up over the coming weeks, do subscribe to the podcast and we will hopefully see you back here next week.
If you want to find out even more spectacular sites 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 out sky guide with a full overview of everything worth looking up for throughout the whole month. 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.
Chris: Thank you for listening to this episode of the Star Diary podcast from the makers of BBC Sky Night Magazine. For more of our podcasts, visit our website at skynightmagazine.com or head to iTunes or Spotify.
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