Caoimhín's Content

Today, we are going to be looking at the planet Mercury. We’re going to start just a couple of days ago, when Mercury was at its greatest elongation. We’re coming back to sunrise on the 7th of December, when Mercury reached its greatest elongation. Thankfully, it will be close to this greatest elongation for a couple of days, and still above the morning horizon for a couple more. Unfortunately, it’s still not in a particularly easy to see location from here in Ireland, but it is at its best condition to see so far this year, this is one of the best opportunities we have to see Mercury.

Looking at the 7th, Mercury is definitely above the horizon, rising at just after 6:30 in the morning. It’s very low in the sky when it first rises, but it remains visible, getting higher in the sky as the sky brightens. By 7:20, it’s much higher in the sky, but the sky is still reasonably dark. By this time we’re getting some glow of sunrise, but Mercury is pretty much at its most visible. Pushing up to 7:50, Mercury is again higher in the sky, but the sky is much, much brighter and that’s going to make Mercury more difficult to see. Mercury is disappearing from the city sky at just about 8 o’clock. Mercury will stay visible for longer if you’re in the countryside, but unfortunately you won’t be able to see it earlier. Mercury is already visible when it comes above the horizon. It doesn’t really matter how much light pollution you have, unless you have sufficient light pollution to block out Mercury entirely.

From a dark countryside sky, we’re able to see Mercury for an extra 10 minutes or so, maybe a little bit longer if your eyesight is good. If we push back to Mercury’s rising, it is getting extincted by the atmosphere, but it’s visible at the same time, about 6:30. This is Mercury at its greatest elongation. If we move forward a couple of days, Mercury gets lower in the sky and in few more days it significantly drops back in towards the Sun, in apparent retrograde motion. On the 24th Mercury is just risen, possibly still behind bushes or low walls, at about 7:40, so about an hour of a difference in rising time. If we move back to just before the greatest elongation, Mercury appears to get a little closer to the Sun, but also higher in the sky. On the 2nd of December Mercury is about as far from the horizon as we’ll get to see it, but it’s not as far from the Sun. As Mercury orbits the Sun, it appears to follow a curved path, bringing it out and down from the Sun. This means that Mercury rises at different times and stays visible for different lengths of time depending on where it is around the Sun.

Brightness also influences visibility, and its greatest elongation isn’t necessarily its brightest. Looking the 7th again, Mercury is magnitude -0.39. If we move back in time, the magnitude goes down. If we move forward in time, past the elongation, it goes up. If the magnitude is going up, that means Mercury is going up back around the back of the Sun. This is something we can double check by taking a closer look. We’ll come back to just the first of December and we’ll zoom in on Mercury. What we should see is a less than half of Mercury. As we move ahead to its greatest elongation, Mercury moves further away from us, but it also shows us more of its illuminated face, making it brighter. As we keep moving forward, Mercury continues getting further away from us, it’s going around the back of the Sun, but it gets up to magnitude -0.53. Mercury is continuing to get brighter as it gets further from us because we’re seeing more of its face illuminated.

This elongation of Mercury is about the best one that we’re going to get here in the Northern hemisphere, thanks to the winter mornings providing dark conditions without being too early. Mercury takes 88 days to orbit the Sun, but the Earth is moving around the Sun as well, we don’t get 88 days between elongations. If we start moving back through time day by day, we will lose Mercury in front of the Sun. Mercury came out in front of the Sun for that particular elongation, and we know that because as we moved ahead it’s brightness and distance from us increases. As we move back in time, the Sun comes up due to the earlier sunrises. We need to get rid of the atmosphere, otherwise we won’t be able to see Mercury easily. Coming all the way back to the last western elongation puts us back in August. We can tell that it’s the elongation because it visibly gets further from the Sun until it stops and turns around again. If we take a closer look at Mercury it should be close to a half Mercury. Seeing Mercury at exactly a half isn’t exactly the same as its greatest elongation. With Mercury so much closer to the Sun, when we’re seeing half of its face illuminated is when it’s making about a 90 degree angle between us and the Sun. This is a little bit off of its greatest elongation in the sky, which is a little closer to inferior conjunction, with Mercury in front of the Sun.

Once we’re at elongation in August, we’ll bring back the atmosphere and come closer to sunrise to get Mercury in view again. We’ve traveled over 100 days to get from early December back to mid-to-late August. This is because of our motion around the Sun compounding with Mercury’s motion around the Sun. Even though for Mercury in a given location, it takes just 88 days for it to get the whole way around the Sun, if we’re also calculating where it appears to be in the sky for us, it’s going to take closer to 100 days to come back to the same relative position. This is thanks to our motion around the Sun also being a contributing factor. Coming all the way back around to the 7th of the 12th, Mercury is under the horizon pretty thanks to changes in the relative position based on our tilt and the change in the seasons.

Thanks to its short year, we’re going to see Mercury in and around the same time of year reaching its elongations. However we’re not going to see it on the same date every year. Just like all of the planets, Mercury is orbiting around the Sun on its own schedule. If we go through years, Mercury isn’t at its greatest elongation on the same date in 2026, if we move through a few days it reaches its greatest elongation just a little bit further back in November. It’s still visible out on that side of the Sun. If we keep moving forward, in 2027 again it’s not at greatest elongation, but far enough from the Sun that we can see it in the sky, and if we move through a little bit of time it comes up to elongation again. We’re seeing Mercury at its greatest elongation, not on the exact same dates, but around a similar season every year. Even though we can’t rely on Mercury being at its greatest elongation every year for a given date, every few years it will be pretty close.

Let’s go back here to 2022, because it looks like we’re pretty far from Mercury’s greatest elongation in this instance, Mercury isn’t even above the horizon. So far we’ve been looking at western elongation, visible in the morning, and moving ahead even a week or two doesn’t get it above the horizon. Mercury is a very difficult planet to observe, so there’s always a chance that it’s not super far from its greatest elongation, but obscured by the sunrise or the atmosphere. To make sure, we’ll get rid of the atmosphere an bring up Mercury and the Sun. Mercury isn’t far from an elongation, but on the wrong side of the Sun until we’re way back into October. There is a chance that this would push the winter eclipse into January, but definitely a bit further from its elongation in 2022 than the dates we we’re seeing in the later 2020’s. Coming back to the 1st of December, we’re a few days early for elongation in 2025, still close in 2026, less so in 2027 but still on the right side of the Sun. Then Mercury’s off to the other side of the Sun in 2028, 2029 and 2030. It’s coming around pretty close to a greatest elongation every winter, but whether it is the greatest eastern elongation or the greatest western elongation we get in December certainly changes.

Back in 2025 and the 19th of August, we’ll do the same thing, we’ll go through a few different years. Mercury is pretty much at inferior conjunction on the 19th of the 8th 2024, which means that it would be a good few days from elongation. We can see it being in eastern elongation for a few years, and then over to its western elongation for a few years, Generally seeing it reasonably far from the Sun except for a couple of years where it appears to be almost at conjunction. Even then, Mercury is regularly coming visible and is regularly visible close to elongation around certain times of year, even though the pattern doesn’t hold perfectly.

Thanks to Mercury’s very short year, even though we can’t rely on exact dates to tell us when we’re going to be able to see Mercury at any particular elongation, we do regularly see it. Anyone who is up looking at the sunrise most days is going to see Mercury at sunrise very regularly, every couple of months. The same thing with sunset, thanks to Mercury’s very quick orbit. However, it is irregular, and one of the reasons is because of Mercury’s great eccentricity. Mercury is the most eccentric planet in the solar system. This makes positions on its orbit and position relative to the Earth just a little bit tougher to pinpoint dates for. Mercury will be going at its quickest when it’s closest to the Sun and slowest when it’s furthest from the Sun, so when we see it at elongation, it may be at its closest to the Sun and moving quickly, it may be at its furthest and moving more slowly. Not only that, but these eccentric orbits process as well. Mercury’s aphelion and its perihelion are moving around the Sun relative to their previous location. Of course, the Earth then is also moving around the Sun, changing when we’re going to see Mercury at its greatest elongation and whether or not we’re seeing it at aphelion or perihelion is going to change over those longer periods of time. All this means that when we’re seeing it and how long it takes for Mercury to go from one elongation to the other can vary based on whether or not it’s passing through its aphelion or its perihelion, or where those points are relative to which elongations being compared.

That’s just a little bit about Mercury, particularly its orbit and how it appears in the sky. I hope you get to see Mercury this year, it’s still visible for the 9th of December, and even if we move a little bit further to the 12th, it’s still visible in the morning. It is the most difficult planet to see, maybe after Uranus. It’s certainly one of the most difficult planets that is visible to the naked eye to actually catch, so I hope you get to catch it. If you enjoyed this piece, then please do like it. If you like this kind of content, then please subscribe to this website and my YouTube channel. Thank you very much for reading and hopefully I’ll see you back here next time.