What's in the night sky in the week of 10 to 16 April, 2023 in our weekly stargazing guide. This week offers the chance to see Venus glide past the Pleiades open cluster, and an occultation of the star Alniyat by the Moon.


Chris Bramley Hello and welcome to Star Diary, the podcast from the makers of BBC Sky at Night Magazine. You can subscribe to the print edition of the magazine by visiting www.skyatnightmagazine.com or digital edition by visiting on iTunes or Google Play.

Ezzy Pearson 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 10 to 16 April. I'm as he appears in the magazine features editor and I'm here to take you through this week's stargazing highlights. And actually we start this week with a carry on from last week's episode. If you go back and listen to that, you'll remember that we were talking about Venus, the planet Venus passing by the Pleiades, which is an open cluster that looks a little bit like a miniature version of the plough, but it's surrounded by this beautiful nebulosity on 10 April, Venus is going to be making its closest approach to the Pleiades when it's only going to be 2.7 degrees away. It'll remain fairly close on 11 April as well. So if you don't manage to get to see it on 10th or the weather, isn't that great? There is a second opportunity to see it on the 11th. From then on, it will continue moving out across the night sky, away from the Pleiades and towards a different cluster, the Hyades. So the Hyades is a V-shaped cluster. It's not quite as spectacular as the Pleiades. It's not got as many members to it, but it is still quite an interesting thing to be able to see. There's also a bright star that appears to be in the Hyades, and that is Aldebaran which is a plus 0.9 magnitude star. So fairly bright star, but this isn't actually associated with the Hyades. So when you get an open cluster like these, these are all stars that are gravitationally bound together, usually because they were formed in the same stellar nursery and they haven't quite moved away from each other yet. But Aldebaran is 150 lightyears away from this cluster. That's a very long way. It's not actually gravitationally associated with them at all. It just happens to be along the same line of sight. And on 13 April, the planet Venus is going to be between the Pleiades and the Hyades open clusters right there in the middle. It will remain in the vicinity of the Pleiades until 14 April, at which point it will start really moving across onto other targets on the night sky. But Venus is very bright at the moment. It is still a -4.0 magnitude planet, so it is still going to be the brightest thing on the sky after the Moon. So keep an eye out for that on 10 and 11 April and then keep an eye on it right the way through to the 14th as well.

Then on 10th April, we have an occultation. An occultation occurs when a foreground object, in this case the Moon, appears to pass in front of a more distant background object, which is going to be the star Alniyat otherwise known as Sigma Scorpii. Alniyat a +2.9 magnitude star. So naked eye visible but definitely not one of the brightest in the sky. And in fact, not one of the brightest in Scorpius even. But what you should see, if you look towards the Moon on 10 April at around about 3:30 in the morning. So this is going to be an early one if you want to catch this occultation, as you should see that the Moon and the star are very close together. They'll be about 11 degrees from the southern horizon is where you want to be looking for this pair. And around about 3:30 – where we are, it's exactly 3:33 BST – the star will disappear behind the Moon. The Moon will carry on its way across the night sky and then at about 4:30, what you should see is the star appears to blink back in. And that time it will appear on the dark limb. So when it disappears, it's going to be on the lit side of the Moon, which means that it does kind of blur out that that sudden disappearance. But when it reappears, it's going to be on the dark limb. So it'll be suddenly from the darkness appears this bright point of light again. If you want to catch this event, do set up with plenty of time. You really want to make sure that you've looked up exactly when it's going to happen in your particular location, because it does vary a little bit. Make sure you're set up 20 minutes, half an hour beforehand so everything's good to go when the event does roll around and make sure you're looking towards it for about 5 minutes before you expect it to happen, to make sure that you don't miss it. It is very much a blink and you'll miss it moment, so be sure that you are paying attention.

Then moving on to 11 April. Mercury is going to reach its greatest Eastern elongation when it has its furthest separation on the sky from the Sun. And on the 11th of April, it will be 19.5 degrees away from our Sun in the evening sky after sunset. Mercury itself is about +0.2 in magnitude at the moment, and it sets about 2 hours after sunset. So for Mercury, that's quite a long time that that's going to be up in the sky. And with a separation of 19.5 degrees when the Sun goes down, it is going to be fairly high in the sky. So you've got lots of time to observe the planet, which you don't normally get. Because it is an inner planet, it tends to be quite close to the Sun most of the time, and that means you don't get a lot of time when the Sun isn't in the sky and Mercury is. But on 11th April, you'll have 2 hours to be able to get to see the pair. Mercury is getting dimmer at the moment and it will continue to get dimmer throughout the month. So if you do want to take a look at it, now is a very good time to be able to see that if you're looking with a pair of binoculars, you might also be able to see the phases of Mercury. The inner planets, both Venus and Mercury, they go through phases like the moon because we are seeing them at an angle to the Sun. Whereas all the other planets we tend to see fairly straight on because we're looking back at them rather than in at them. So that might be something to look out for If you are taking the time to look for Mercury on 11 April.

Then on the 12th of April lunar libration is going to bring the Moon's northern limb better into view. So this region, particularly around the northern pole has a big jumble of all sorts of craters. And when it's tipped towards you, that means that the lighting conditions are slightly different to what you would normally get. And also, it just gives you a bit of a better view to be able to view this region. The Moon is going to be about 61% gibbous on 12 April, meaning that the terminator is going to be passing through that big craterous region. And that means that there'll be lots of nice shadows. When you are looking for things on the surface of the Moon around the terminator, when that's passing over is usually a good time because you get much longer shadows, much more contrast when you are looking at it from Earth. It's also a great opportunity to get started in some lunar photography. If you've never done that or even some sketching, lunar sketching of a nice crater region up on the Northern hemisphere. If you want to find out how to do either of those things. We, of course, as always, have guides over on www.skyatnightmagazine.com. And I will put a link to those below in the show notes as well.

On 14 April, Mars is going to be sitting just 9.5 arc minutes away from the star Mebsuta, Epsilon Geminorum. Mars is about +1.2 in magnitude at the moment. So nice and easy to see that one. Mebsuta is a plus three magnitude star and it's Epsilon Geminorum. It's in the constellation of Gemini. It's in fact the hips of the right hand twin Castor, If you're trying to work out where it is. If you're trying to find Gemini at all, it's over the left shoulder of Orion, which is probably a better known constellation. Most people can recognise Orion in the sky. If you want to find Mebsuta then you can do that by looking at Betelgeuse, which is Orion's left shoulder. It's a slightly red star. You should hopefully be able to see that colouration. And then if you also look for Rigel, which is the right foot of Orion. So you go from Rigel, the right foot draw an imaginary line up to Betelgeuse and then carry on that line about the same distance again. And you should find that it's pointing at the star Mebsuta. So that's how you can find that. Should be pretty obvious on 14th, because it will be next to Mars. So if you see what looks like a fairly bright, slightly reddish 'star' in that region, that's Mars. And the slightly dimmer object next to it is Mebsuta. Mars isn't going to set until around about 3:00AM, so there's lots of time to be able to catch those two in the night sky. It's also a great opportunity if you're not very comfortable still finding your way around the night sky, you've got Mars helping you go from one constellation to another and then maybe look up some nice star hopping guides to try and help you find your way around the other constellations in the region as well. Again, we have all of those guides over on our website www.skyatnightmagazine.com.

And finally ending the week on 15 April. We have a slightly unusual event that's going to be occurring because 15 April is the day that you want to align your sundials. So why specifically on 15 April? Well, it's because there are two kinds of time. The Sun has been used for centuries, millennia, even, to help people track time throughout the day. But when people started making clocks, they began to realise that actually sun time doesn't keep as clockwork as perhaps you think it might do. Doesn't isn't as clockwork as a clock. And that's because Earth orbits the Sun. Not in a perfect circle, but in an ellipse. And that means the Sun's motion appears to speed up and slow down as it moves across the sky throughout the year. So to measure time consistently, people introduced what was known as the Mean Sun, which is a kind of idealised version of where the sun should be if it moved at a constant rate. And throughout the year, the time difference between these two does drift quite a lot. The Sun time can range from being about 14 minutes and 15 seconds ahead of clock time to being 16 minutes and 25 seconds behind. So that's a big difference, about half an hour difference over the course of the year. You can actually see this difference between the Sun Time and Clock Time. If you go and record the position of the Sun at the same time every day, for instance, if you took a photograph of where the Sun is at midday in UT, obviously you don't want to get confused with daylight savings in there. But if you took a photograph of the Sun at midday, every day, you would see that it appears to move backwards and forwards across the sky throughout the year. It also goes up and down across the sky. But that is because of the tilt of the Earth. Earth is at 23.5 degrees to the Sun, which is why we get a high Sun in the summer and it's low on the sky in the winter and blinds you when you're driving. But the combination of this tilt and the drift between Sun Time and Mean Time means that if you traced out the Sun every single day, you would see that it traces out a figure eight or a sort of bowling pin shape, some people call it. And that's what's known as an analemic curve. But there are four dates throughout the year where SunTime and Clock Time match up. One of those is 15 April. The next is 13 June. There's another one on 1 September and 25 December. So if you did want a date to remember, an easy day to remember when you should be setting your sundials – Christmas Day. Probably not going to have the weather for it, but at least you'll remember it. And on those dates, your sundial should mark the correct time. So as long as you've got an accurate clock, you should be able to position your sundial so that the shadow of the gnomon – which is the part that sticks up and casts the shadow and tells you what time it is – is telling the right time. You can also give yourself a bit of a challenge if you fancy something that's going to last you throughout the entire year. So if you set up your sundial somewhere where it's going to get nice Sun throughout the year. Or if you don't have a sundial, even a stick will do. And if you go out at the same time every day – again, always making sure that you're using the same time systems so always in UT if that's what you're going to be using – and if you do that and mark out the position of where the tip of the shadow is at that same time every day... Well, it doesn't need to be every day. You can have some gaps in between. But perhaps every time when it's nice and sunny. And what you should see if you do that over the course of the year, is that those positions appear to trace out this analemic curve. So there is a nice challenge if anybody at home fancies trying that. And if you want more details on that, perhaps a few more diagrams, be sure to pick up the April issue of Sky Atlantic magazine, where we have that covered in our Sky guide. So that's it for us this week from 10 to 14 April. Venus is going to be passing by the Pleiades and the Hyades. Then on the 10th we have an occultation of the star Alniyat by the Moon. The 11th Mercury reaches its greatest eastern elongation. 12th is a great chance for some lunar Libration features. On the 14th of Mars and Mebsuta in Gemini are going to be coupling up. And then finally, on the 15th, remember to set your sundials. If you enjoyed all of that, please do subscribe to Star Diary to make sure that you never miss another one of your weekly updates on what's going on in the night sky. And we hope to see you all here next week.

If you want to find out even more spectacular sites that will be gracing the night sky throughout the month, be sure to pick up a copy of BBC Sky at Night magazine. Well, we have a 16 page pull out Sky Guide with a full overview of everything worth looking out for. 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 the detailed star charts to help you track your way across the night sky. From all of us here at BBC Sky and Night Magazine. Goodbye.


Chris Bramley Thank you for listening to this episode of Star Diary, the podcast from the makers of BBC Sky at Night magazine. For more of our podcasts, visit our website at www.skyatnightmagazine.com or head to aCast, iTunes or Spotify.


Elizabeth Pearson
Ezzy PearsonScience journalist

Ezzy Pearson is the Features Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.