Caoimhín's Content

Today we are looking at something that is happening on the date that this article is published, for the second time this week. Normally, I tend to look further into the future on this channel, but there happen to be events coinciding with the dates of posting at the moment. The last piece was about the annular eclipse happening on February 17th, this video is about the greatest eastern elongation of Mercury happening on February 19th.

We begin by taking a look at the sunset. If you are in a similar time zone to Ireland, then we are taking a look at the sunset as it will be just a few minutes after you’ve finished this piece. Looking to the west we have Venus, Mercury, the Moon and Saturn. Neptune’s out in the west as well. Turning around to the south-east we already have Jupiter visible. That means Uranus is up as well, but we need to find it. Just a little after sunset, Rigel and Betelgeuse in Orion will be visible. Moving up and over from them you might spot the red Aldebaran. The Pleiades would be just up a little further, even if they aren’t visible straight away. Uranus is just a bit below the Pleiades. We have quite a few planets in the sky at the same time in this part of February. Venus down low to the west is pretty tricky to see, even without things like trees or telephone poles getting in the way. Venus is very low when it first becomes visible, so it sets very soon after that, before it becomes as amazingly bright as we know Venus can be. A little above Venus is Mercury, so it is a really great time to see it as we come up to 6:50. By then the sky is definitely getting dark, but we still have that glow of sunset telling us where to look. We want to see Mercury at its greatest eastern elongation, which means we’re seeing it in the west, and the west, of course, is where the Sun sets.

The Moon is also there as a helpful marker on the 19th. The Moon is a bit higher than Mercury in the sky, just next to Saturn. It is only a very narrow waxing crescent Moon, very difficult to see in the sunset glow, but it is in the right part of the sky and of course we have Saturn there helping us as well. This is Mercury’s greatest elongation give or take a few minutes. Really of course, Mercury is a continuously moving planet, it’s moving around the Sun just like we are, so it might actually be reaching its greatest elongation while it’s invisible to us here in Ireland. This is very similar to how we might see the Full Moon, but it might reach 100% full when it’s not visible to us here in Ireland. Taking a closer look at how Mercury is going to behave over the course of a couple of days will show how it moves. Mercury’s greatest elongation is when it’s at its furthest reach-out to the side of the Sun from our perspective, but that can sometimes bring it lower and closer to the horizon, if Mercury was higher than the ecliptic when it was coming around the Sun. In this case, Mercury is moving around the front of the Sun and from our perspective seeming to go above the Sun. If we keep moving forward, Mercury is gets closer to us and we’re seeing less of it illuminated. Moving backwards, Mercury was further behind the Sun, so further from us but with more of its surface illuminated.

Thanks to the tilt of Mercury’s orbit, as it gets further from the Sun or further from the Sun from our perspective in our sky, it also seems to get higher, and it continues to seem to get higher as it comes around the front of the Sun. Unfortunately, this makes Mercury much fainter. Even though it’s closer to us, less of its surface is reflecting light back towards us. That makes Mercury more difficult to see even though it’s closer to us, and it makes Mercury easier to see even though it’s further from us when it is before its eastern elongation. Even before the elongation, Mercury is visible in the sky for quite a while and it gets quite bright before it sets. Coming back to the day of elongation, it gets fainter, the sky is brighter at the same time so it’s not going to appear quite as quickly in the sky. However, it’s going to stay visible getting higher in the sky, so higher away from the horizon, at least to begin with, for another few days. It may have the shortest year, but it still takes a while for Mercury to get around the Sun.

Looking ahead to the 26th, this is the date that a lot of publications are advertising as the Parade of Planets. To my eyes here, it’s actually harder to see Mercury and Venus together on the 26th than it is back here on the 19th. Even though Venus is lower in the sky on the 19th, you can see Mercury and Venus together at the same time. As we move forward, Venus does get higher in the sky, Venus is also moving out towards its elongation, but Mercury gets harder to spot. Even though it’s higher in the sky and closer to us we’re seeing so little of its illuminated side that it’s not visible until the sky gets much darker. The darkening sky is also the setting of the planets, so that also puts Mercury pretty low in the sky and Venus practically under the horizon. Really, any time that we’re observing Mercury, we need to be at the coast or at least at an elevated position where things like trees and bushes aren’t going to get in the way too much. Mercury is only visible close to the Sun because it is, physically, very close to the Sun.

Coming back here to the 19th, whence we also have the Moon in this little arrangement of planets, we are looking at just 6:30 with a perfectly clear sky. Venus, Mercury, Saturn are all together, and looping around to the high south-west is Jupiter as well, all visible to the naked eye. As long as there are no trees and bushes in the way, we can definitely see Mercury and Venus here together at the same time, for Mercury’s greatest elongation. As we can continue moving forward, Saturn will get lower in the sky at the same time, that means it’s getting closer to the glow of sunset and harder to see. The exact same is happening with Mercury. Only Venus is getting further out or further from the horizon from our perspective. For a few days before and after the greatest elongation, all of those planets are visible together, but as we continue moving forward, Mercury ends up out of view, we have to wait until later for the sky to get dark enough for it to become visible again, and as we continue moving forward, eventually we’ll reach a point where Mercury is so low in the sky when it becomes bright enough to be visible that the extinction of the atmosphere will render it practically invisible again.

Taking a closer look at Mercury, its absolute magnitude is -0.6. Moving forward or backward in date, the absolute magnitude stays -0.6. However, the amount it’s getting extincted by the atmosphere really changes, by several atmospheres, and that’s going to change how bright it appears by several orders of magnitude. When I say order of magnitude here, I don’t mean “times 10” like a normal order of magnitude, I mean “times roughly 2.5”, which is the kind of magnitude that the stellar magnitude scale works on. Looking to the 23rd. Venus and Mercury are pretty close together in the sky, Saturn also isn’t too far away, but the Moon has moved across the sky. We’ve got a range of time here where they’re all visible. Coming back to, we’ll say 6:30 for Mercury to become visible, Saturn becomes visible at a pretty similar time, and they stay visible. Venus will already be visible by then, and as we push ahead, Venus touches the horizon at just 7:00. That gives us half an hour with all of them visible in the sky and Mercury doesn’t touch the horizon or get wiped out by extinction until just after 7:30.

That puts all of those planets in the sky with a reasonable chance of seeing them. Unfortunately, this isn’t great for Neptune which is right next to Saturn in our sky. Neptune appears right next to Saturn, but it is of course far behind it and much fainter. As such, it’s not going to be dark enough for Neptune to be visible until both it and Saturn are quite low in the sky, and of course you’ll need a telescope for Neptune regardless. By the 23rd the Moon has shifted across the sky, bringing it very close to Uranus, which is going to make Uranus a little bit tougher to see. Zooming in on the Moon and the Pleiades, Stellarium doesn’t even show where Uranus should be. Removing the atmosphere shows Uranus very close to the Pleiades and the Moon, but the light of the Moon is going to block out Uranus, making it that bit harder to see. Jupiter, of course, is easy, we’re going to see Jupiter nice and easy, really, at any time of the month.

Today is pretty much the ideal day, the 19th is pretty much the best time to see Mercury. It’s a good balance between Mercury being high from the horizon, away from the Sun, and also with a fair amount of the light side of Mercury facing towards us. As it is at its greatest elongation, it should be around a half Mercury, so there’s plenty of light reflecting back to us from the planet’s surface. As we move forward, we can see Venus and Mercury getting closer together in our sky, and Mercury doesn’t drop back to the horizon that much, it kind of loops as it goes over, really in front of, the Sun. It appears to move this this way because there is a tilt, or inclination, to the orbit or Mercury and the Earth, so we do see it at a little bit of an angle. However, as it goes around in this way, it’s coming in front of the Sun, so less of its bright side is visible to us, making it harder to see.

It all works out to be a little bit tricky, but Mercury is at its greatest elongation today. If you are in a similar time zone to Ireland, there is a good chance that you will be able to see it just after you finished this piece. This piece should be going up at 6 o’clock, by the time you finish it it will be maybe 10 minutes later, another 10 to 20 minutes after that, Mercury should be visible along the western horizon, as long as you you have a clear enough view of the western horizon. It’s a good chance to see the other planets that are part of this parade as well, which of course I went over in a previous piece. If you’d like to check out the parade of planets a bit more, you can head back to a previous piece. This piece was mostly about the greatest elongation of Mercury, which I hope you get to catch over the next few days. Of course if you don’t manage to see it, that’s okay, you’ve already heard about it here. I hope that you enjoyed this piece, if you did then please do like it. If you like this kind of content, then please subscribe to this website and my YouTube channel channel. Thank you very much for watching and hopefully I’ll see you back here next time.