Caoimhín's Content

Today, we are going to be taking a look at the planet Mercury. Some of this will cover general features of the smallest planet in the solar system, but we are going to be looking at Mercury in particular because of its greatest elongation coming up on the 19th of August.

From midday on the 19th, we need to move backwards in time so that we can catch sunrise. This is Mercury’s greatest western elongation, so it is west of the Sun, meaning we see it in the east at sunrise. Just before the Sun comes up we will get Mercury, nice and clear, reasonably high above the horizon. This is one of the best occasions that we’ll get to take a look at Mercury. It never rises particularly high in the sky and it’s not quite as bright as some of the other planets, but it will be in a reasonably good position, far from the Sun, later this month.

We also have Venus, Jupiter and Saturn in the sky, so we’ve got quite a lot of the visible planets all visible here at the same time. Moving back in time to the 12th brings Jupiter and Venus together in a conjunction, and we can see the Moon passing by all of these planets as well, being particularly nice on the 20th. As we move forward from the 19th, Mercury gets closer and closer to the horizon at the same time of the morning. However, it’s also getting darker at the same time. As we move through August and into September, we’re going to be moving away from the summer solstice even more, we’re going to be moving closer and closer to the autumnal equinox. This means shortening days, and later sunrises. Even besides this, Mercury does get a little bit brighter as we move into the future from this opposition, so even though it isn’t in the sky for as long, and even though it won’t get quite as far away from the Sun in the sky, it does look brighter, and that does make it easier to see.

Mercury is still visible, barely, as we get into September. We will be after the greatest elongation, putting Mercury lower in the sky, but moving later in the morning keeps Mercury visible. It will be visible later in the month even at times when it would have disappeared closer to its greatest elongation. Mercury is so small and so close to the Sun that when we’re looking at a half Mercury, not much of the planet is illuminated and it’s not particularly close to us, so it’s not particularly bright in the sky. We can see at least nearly a half Mercury on the 19th. To me, it looks like it is exactly a half on the 21st. We know that Mercury is around its greatest elongation, because it must be forming something like a 90 degree angle with the Earth-Sun system for us to see the sun illuminating exactly half of it. Mercury looks its furthest away in the sky a little before it actually reaches a half, but it’s always very close. As we keep moving into the future, Mercury gets more full. We’re seeing more of Mercury’s illuminated face and that increased illuminated surface area means that Mercury is reflecting more light back to us. Even though it’s not getting as far from the Sun in the sky, it is brighter after its elongation. That’s going to keep it visible for longer and make it a little bit easier to see.

We can use the magnitude numbers in Stellarium to track the change in brightness. Around the 25th, Mercury’s magnitude is around minus 1. If we move back towards its greatest elongation, that number starts getting higher, which means brightness is lowering. If we take a look at the reduced or adjusted magnitude, it’s just 0.65, reduced by around 5 atmospheres. This is causing some reduction, but it’s not much for Mercury, because it’s nice and high in the sky compared to usual. The reduced magnitude varies with how high an object is in the sky, and so especially with the planets it varies by latitude due to the angle of the ecliptic. If we keep moving forward its brightness is going up so much that even though the amount of air masses extincting it are also going up, the reduced magnitude does as well. As mercury gets lower in the sky, the atmospheres rise to 12 and a bit. However, Mercury is reduced to 0.3 instead of 0.65 because we’re reducing from a much higher magnitude. That’s part of what’s keeping Mercury more visible and making it easier to see. If we move back to the greatest elongation, the 19th even though Mercury looks more like a half to me when we’re up to the 21st, we can see Mercury’s magnitude is just negative 0.02. It’s extincted by seven air masses because it’s not that high in the sky. If we keep moving forward, we’ll see that even though the amount of air masses that are blocking it out get much higher, its magnitude gets much higher as well, to minus 1.19 there on the 30th of August.

Mercury’s variation in brightness can be a bit counterintuitive. Even though it’s at its furthest on a particular date, it’s not going to be as bright on that date. If we move before the greatest elongation, Mercury gets even fainter. Just on the 14th, only a few days before the greatest elongation, also reduced by 12 air masses, it’s reduced to 2.55 magnitude from 0.87, not even negative magnitude. As we’re before the greatest elongation, Mercury’s phase is just a narrow crescent there. Even though it is closer to us, so much more of it is in darkness that we’re not seeing it as bright in the sky. As we move forward, Mercury seems to be getting further away from us and it gets fuller. That’s because it is curving back around the Sun.

To make a very quick comparison with Venus, the changes will take longer due to the longer year, but the principle is the same. Even though it rotates in retrograde, Venus still orbits the same way as Mercury and all of the other planets. In late August Venus past its greatest elongation, looking more full than a half. If we keep moving forward, it will keep moving away from us and keep getting fuller. We’ll need to move backwards in time for Venus to appear less full, for it to wane. As we do, it comes closer and closer to a half and closer to us. The magnitude of Venus does change while this happens, , but Venus’s magnitude is going down as Venus goes away from us. This means it’s getting brighter, not by very much, but by a little. Venus’s magnitude increases as it gets closer to us. Venus’s brightness behaves a little bit differently, the relationship between Venus’s brightness, and how much of it is full and its distance from the Earth, is pretty much the opposite of Mercury. Venus at its greatest elongation was magnitude negative 4.3, incredibly bright. If we keep moving backwards in time, that number keeps increasing because Venus is getting so much closer to us, down at least to negative 4.74. The opposite happens with Mercury.

This is partly due to Venus’s atmosphere, which is very reflective, so even a small amount of it reflecting light is going to reflect quite a lot of light. However, it it is also because the distances involved are greater. Venus gets much closer to us when it’s at its closest approach and it gets much further from us when it’s at its furthest. Mercury doesn’t have as much of a difference in its smaller orbit. It’s closer to us, of course, when it’s on our side of the Sun and further from us when it’s on the opposite side of the Sun, but the difference isn’t as great between those points as the different points in Venus’s orbit. The amount of Mercury’s surface getting illuminated also makes a difference in how much light is getting reflected back to us. It’s not a very reflective surface, so we do need quite a lot of it to reflect a lot of light. All of this is reflected in the numbers displayed in Stellarium.

The same will happen when Mercury is at its greatest western elongation, but it’s going to take a little bit of time for that to happen. As Mercury’s orbit takes 88 days to go the whole way around the Sun, it will take 44 days, give or take, for Mercury to come around to its opposite greatest elongation. Of course, the Earth is moving as well, and this changes how long it takes for Mercury to appear to be in the same place. Looking at the midday Sun and Mercury moving around it, it’s clear that the furthest it gets from the Sun is the 19th. Moving forward in time, Mercury comes round to its greatest eastern elongation on the opposite side of the Sun, which would be under the ground if we weren’t looking at midday. This is a little bit into the future, through to the 2nd of November. This is a lot more than 44 days, but it is when Mercury will be at its greatest eastern elongation, which means we’ll see it in the west. Except that we probably won’t see it at all from here in Ireland. The angle of the ecliptic at sunset will be pretty shallow, the ecliptic will be low as the Sun is setting. For us, Mercury isn’t even visible in the sky. We can also see that the Sun is significantly lower because we will be all the way into winter.

Traveling all the way out to the other side, Mercury is still the same distance out from the Sun. The size of its orbit hasn’t changed, and Mercury isn’t particularly eccentric. In this sense of eccentricity, Mercury’s orbit isn’t particularly eccentric, and that means it’s not particularly elliptical. Even though all planets have elliptical orbits, none of them are particularly elliptical, at least not in our solar system, not as elliptical as objects like comets for example. All of the planets have much more subtle eccentricities than comets, and that’s the same for Mercury. Its distance to the Sun doesn’t change particularly greatly over the course of its orbit. So we know that it’s going to be about the same distance out from the Sun, whichever elongation we see, but the angle that the ecliptic is making with the horizon can have an effect on visibility. It is so different on August 19th, in this case it’s so much higher, the angle is so much steeper, than it will be for sunset once we get through to the very beginning of November, that we can’t see Mercury for one even though we can for the other. How visible Mercury is in the sky changes drastically from month to month, when in the year its elongation is happening matters a lot.

This one is particularly good, it looks like we’re going to get a reasonable view of Mercury on August 19th. I hope that you do get to see Mercury and I hope that you enjoyed this piece. If you did, 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.