Caoimhín's Content

Today we are going to take a quick look for the planet Mercury. In a recent video, I just about spotted Mercury just above the sunset towards the end of June. Now that we are into June, of course the sunset is going to be getting later, as late as it can get. That means that we’re going to have to stay up pretty late to catch anything that is only visible at sunset, as Mercury is.

I definitely saw Mercury around the sunset towards the end of the month, but Mercury’s greatest elongation is in July. So to see exactly when we’re going to see Mercury and when it’s going to look its best, we need to take a look at the sky without any atmosphere, we can see the Sun and Mercury at the same time and how far away they are from each other. Coming back a little bit closer to beginning of June, Mercury is already to the side of the Sun. Mercury is above the horizon when the Sun is setting, but of course, from our location here in Ireland, there’s still far too much sunlight there at sunset for Mercury to be visible. We’ll have to keep moving forward, and we can see Mercury getting further and further from the Sun as we move closer to July. It reaches its greatest eastern elongation, then turns around and comes back the other way. It definitely looks like Mercury is making that turn around the 31st of June or the 1st of July, but those aren’t the dates that I saw for Mercury’s greatest elongation. This could be down to the angle at which we’re seeing it from here in Ireland. Even towards the middle of June, still a little bit into the future, there’s still plenty of light there creeping above the horizon, so Mercury and Jupiter aren’t visible. At least, not with any low hills or buildings on the horizon.

We’ll keep moving forward towards the end of the month, which gets Mercury further from the Sun. It is still low, so it may end up behind trees or buildings, but it’s at least a significant distance from the Sun. Hopefully it will still be visible above the horizon even with the atmosphere on, and it is, but barely. Mercury is just about visible above the horizon. If we take a closer look at Mercury, we should see a half Mercury. When Mercury is at its greatest elongation, it should appear as a half Mercury through a telescope. By the end of the month it is close to its greatest elongation, so we’re seeing very close to a half Mercury. The amount of the surface of the planet that’s lit up tells us what side of the planet is facing the Sun relative to the side of the planet facing us. For Mercury and Venus, this means that they never look fully illuminated from the Earth. Of course, Venus can sometimes be an exception to this, as we saw with the evening and morning Venus just a short time ago. Either their dark side will be facing towards the Earth when they are on our side of the Sun, or they will be on the far side of the Sun with their fully illuminated face hidden by the glow of the Sun. As we push closer to the elongation, Mercury should be continuing further away from the Sun, but for us here in Ireland, it seems to be dropping under the horizon.

To fix this we’ll head to the zero horizon, perfectly flat with nothing in the way. Of course, there isn’t anywhere where the horizon will be completely flat all the way around, as the zero horizon in Stellarium will simulate, but you don’t need the sky to be completely flat the whole way around. As long as you’ve got a clear view out to the west, practically the northwest, that is all you need to get a good glimpse of Mercury. Coming back from the greatest elongation, Mercury actually looks a little bit brighter around the 23rd, but again, this is for us here in Ireland. It may reach further from the Sun, but at the wrong angle for us to see it from here. Either way, coming back earlier in the month, Mercury is still visible just above the horizon on the 10th. This is only possible due to how low Mercury remains unobscured. If we come back a little bit more, Mercury and Jupiter have a conjunction, though not a very close one. They are only both visible when they just above the horizon. Close to the horizon, the atmosphere is going to be extincting these things, making them seem fainter, and anything obscuring your view of the low horizon there towards the northwest is going to make things pretty impossible to see.

In a reasonably recent piece, we took a look ahead to sunrise in October 22nd 2028, and that far in the future, Mercury and Jupiter are close enough together that you can see Jupiter’s moons and Mercury in the sky together. It looks like that’s not going to be possible for us for the early part of June and Jupiter will only be in the sky with Mercury for the early part of June. As we continue to move into the month, Jupiter ends up behind the Sun. Jupiter is moving, slowly, but really it is the Earth’s motion causing this. As we continue to move, it is in such a way as that Jupiter ends up behind the Sun. Mercury on the other hand is going to keep pushing further and further out until it reaches its furthest elongation, turns around and starts coming back in again.

As Mercury continues out to its elongation, it’s position will sort of curve across the sky. There is a very good chance, a better chance than not, that we are seeing Mercury’s angle of inclination relative to the ecliptic. It can happen that Mercury appears to follow the ecliptic perfectly, but this is not usually the case. With an ecliptic drawn across the sky, it is clear that Mercury is very much above the ecliptic when it comes out from behind the Sun. Then it seems to cross the ecliptic as we come forward to early July, but you should also see the angle of the ecliptic is changing pretty drastically. As we move into the past, the angle of the ecliptic at sunset is a lot steeper, whereas as we move back up to today and into the future, the angle of the ecliptic is far more shallow. This is partly due to the time of sunset changing, but also due to Earth’s axial tilt. That means that even though Mercury is definitely as farther from the Sun, it’s closer to the horizon and closer to that glow of sunlight, so it’s not going to be as visible. To make it worse, Mercury also gets fainter as it comes up to its greatest elongation.

Mercury is at its brightest there when it’s almost behind the Sun, it’s much fainter as it comes past its point of greatest elongation and back around in front of the Sun. If we take a closer look at Mercury, a little past its elongation we should see a little bit less than a half Mercury. If we come back to the elongation, we’ll come back to that half Mercury, and if we keep moving back, we’ll got closer to a full Mercury. Even though it is fighting with a lot more of the glow of sunset, more of the face of Mercury is sending sunlight back in our direction. Even though Mercury is closer to us when it’s on our side of the Sun, it looks brighter when it’s further from us on the far side of the Sun, and that’s just because of the amount of the face of Mercury that we’re seeing illuminated, we see a lot less when Mercury is almost between us and the Sun.

That’s how Mercury is going to look at sunset from here in Ireland. Shifting our perspective to roughly the equator will give us a better view. Even coming a little bit closer to today, Mercury is visible above the zero horizon on the 7th. Mercury and Jupiter will pretty close together, and from the equator they are much further above the horizon. This whole time we’ve been assuming a sky with light pollution, so this is still pretty much the kind of sky we would get to see from the city. Taking a closer look at the pair of planets, one of Jupiter’s moons, Ganymede, is just about visible. Of course, you’d need a very wide field of view and a lot of magnification to capture this, so that’s going to be very hard. If you do have a spectacularly wide telescope, then maybe you’ll be able to pull it off, but being closer to the equator is likely a must. Being higher above the horizon means less atmosphere in the may, making objects like moons much easier to resolve. Continuing into the future, Mercury will continue to rise higher, crossing the ecliptic around its greatest elongation, which is in the future in July, and then coming back down on the other side.

From the equator, we see the same phenomenon even clearer. Even though Mercury, as it comes down along the ecliptic after its greatest elongation, is getting a lot closer to us, it’s still getting fainter. As Mercury crosses the ecliptic, reaching its furthest point from the Sun and coming around to our side of the Sun, it gets harder to see. If we move a little bit later, Mercury definitely stays visible, but it is still fainter than it appears before the greatest elongation. Moving back in the other direction, Mercury gets more visible even though it’s getting further from us. From Ireland, as Mercury passed its elongation, it got lower in the sky, moving under the ecliptic. From the equator, the ecliptic seems to rise almost vertically, and so Mercury after its elongation isn’t lower, it’s just further south. Even though there is no extra atmosphere making Mercury fainter, it still does fade, showing even more clearly the influence that Mercury’s phase has on its brightness.

Thanks to the lessened effect6 of long summer days, Mercury can be seen pretty much darkness as early as the 12th of June at just 7 o’clock, if you’re down at the equator. Despite the lesser influence of Earth’s axial tilt, moving through dates still causes the angle of the ecliptic at sunset to change a little bit. That of course is because the Earth is a little bit tilted, so as we move around the Sun, the angle at which we see the ecliptic will change no matter where we are, even if you’re at the equator, the ecliptic will have a little swap around the solstice, where it goes from tilting to one side to tilting towards the other. MOving into the past shows Jupiter sticking pretty close to the ecliptic as it moves through the sky, whereas Mercury has a much bigger deviation from the ecliptic as it kind of comes out from behind the Sun. From the equator, it starts out north of the ecliptic, rises up and crosses the ecliptic around its greatest elongation, and then comes down on the south side of the ecliptic, on our side of the Sun. A combination of the face facing us and how much atmosphere is in the way is going to make Mercury look much, much fainter once it comes around to that post-elongation position.

We will quickly take a look from the southern hemisphere, and we’ll go pretty far south, because Mercury at its brightest, is going to be under the ecliptic. Starting with Mercury more or less on the ecliptic, roughly at its greatest elongation in early July, the ecliptic seems to tilt north, the opposite of what we see in the northern hemisphere. If we come back in time before the elongation, we can see that Mercury is under the ecliptic, and that means it’s going to get blocked out by the atmosphere and the glow of the Sun easier. As such, we can’t see Mercury as early from the Southern Hemisphere as we can from the Northern Hemisphere, just because of the angle that Mercury is making as it goes around the Sun, its orbital inclination. Where we see the ecliptic in the sky, especially the angle it makes with the horizon, that has to do with our axial tilt, but Mercury’s position in the sky relative to this ecliptic, that’s down to Mercury’s own orbital inclination. As we push past the elongation, Mercury ends up above the ecliptic in the Southern Hemisphere. Partly thanks to the tilt of the ecliptic that we’re getting when we’re down south, Mercury will still be there, visible at its greatest elongation, and a little after it.

Coming back to the default location, back to Cork City in Ireland, of course there is going to be a tree in the way, but Mercury is up and visible in a location like Cork City. Coming back to the zero horizon, just to show what it’s going to look like if you are down at the, down at the coast, Mercury around its greatest elongation is barely visible. It gets easier as we come back closer to today, further from the elongation. I was surprised seeing Mercury visible above the horizon earlier in June rather than early July, because that is when Mercury reaches its greatest elongation. Especially for planets like Venus, we expect them to be at their most visible when they’re at their greatest elongation, but as we can see from the tilt that Mercury itself has, from the tilt that the Earth has, and from how that all comes together with these angles at the horizon for us, Mercury is still going to be tricky to see when it reaches its greatest elongation, we need to get out earlier.

I hope you heed this warning and get a chance to see Mercury before it gets too faint. Most of all, I hope that you enjoyed this piece. If you did, please do like it, you can also subscribe to this website and my YouTube channel if you’d like to see more content like this, Thank you very much for reading and hopefully I’ll see you back here next time.