Caoimhín's Content

Today, the smallest planet is getting a long video dedicated all to it, long for my channel so far at least. Mercury will be coming up on its Greatest Western Elongation soon, so we will be able to see Mercury in the West in the morning, if you’ve stuck around a few of my videos and articles then you’ve heard already, but it’s counterintuitive enough that I like to repeat it.

Mercury is an exceptional little planet, holding many superlatives in our solar system, including the shortest year. Mercury’s year is 88 days long, 88 Earth days, but because Mercury and the Earth are both moving, Mercury will appear to come back around to the same place about every 116 days, but this is very rough, there is a lot of variation. Mercury has a highly elliptical orbit, it has the most eccentric orbit of a planet in our solar system, comets and asteroids can be even more eccentric. This means it is going around the Sun in very much an ellipse rather than a circle, the more eccentric an orbit is the less circular.

Elliptical orbits slowly swing around, almost like a shape being drawn in a Spirograph, they don’t stay “pointed” the same direction in space. Instead they process around the Sun, meaning that we see Mercury sometimes at its closest to the Sun, aphelion, and sometimes at its furthest, or perihelion. As we and Mercury move, the date of its Greatest Elongation varies from year to year. We will usually see a Greatest Western Elongation of Mercury early in the year, but it does vary from early January to late January into early February depending on the year.

We aren’t just going to talk about Mercury as it appears from the Earth, as a little dot. If we were to take a closer look, we’d see that Mercury has this lovely grey surface, similar visually to the Moon. Part of this is because, just like the Moon, it doesn’t really have an atmosphere. A thick blanket of gas can burn up meteors before they hit the surface and become meteorites, and weather in an atmosphere, even just a little wind, can wipe away features like craters. Without those, a planet or a moon will retain cracks and craters for millennia. Mercury is also a rocky body, and this gives it a similarly grey colour. Rather than an atmosphere, Mercury and the Moon have an exosphere. This is a blanket of gas held by the planet, but under such low pressure that the molecules of gas don’t get close enough together to interact. It’s close enough to a vacuum that sound can’t really propagate and heat doesn’t travel the same way. The edge of the Earths atmosphere eventually becomes an exosphere as it transitions into space. A smaller object with weaker gravity can only hold a diffuse envelope of gas.

Mercury is a tricky planet to actually observe. It’s always close to the Sun, it’s only a good distance to the side of the Sun during its elongations, and these only last a few days. Even in space, because Mercury is so close to the Sun, even a lot of very powerful space telescopes like the Hubble Space Telescope, they can’t safely look at Mercury without risking damage to their optics, the same way our eyes can be damaged by looking directly at the Sun. Technically, you can observe Mercury during the day when it’s at its Greatest Elongation, if you have incredibly good safety equipment, for example a solarscope or coronascope, telescopes specially built to be used during the day. Similarly, you can see Mercury with the naked eye during a total solar eclipse, but you shouldn’t be looking at the Sun with your naked eye during a total solar eclipse, you need some kind of protection, something to keep in mind with an eclipse coming up in early April this year.

Of course we can send things to Mercury and this is one of the ways that we study planets that are difficult to see with our eyes, but Mercury is also very hard to land on. The massive gravity well of the Sun needs to be avoided, and aiming for the tiny ball of rock right next to it requires some complex calculations of orbital dynamics. If you get it wrong, an expensive piece of astronomical equipment might miss Mercury and get pulled into the Sun.

When looking at a map of Mercury, some features are obvious and clear, particularly craters, which look similar to the craters on other planets and the Moon. All over the planet there are little white patches that don’t seem t obe craters. These white patches on Mercury are just paler areas that may be evidence of old vulcanism, possibly pyroclastic flows. Mercury doesn’t have much of a liquid mantel anymore, but it does have a giant iron core. This means it does have a magnetic field, but there’s no liquid stuff moving around so the field is quite weak, These white areas are known as faculae and they’re named after snakes. They aren’t named after individual snakes, the way other features are named after astronomers, we don’t know the first names of many snakes. Rather, it’s the word for snake in different languages, including the Nathair Facula, coming from the Irish for snake. Other features, like craters and valleys are named after other things like observatories and astronomers. There are alos many named after artists and authors, including named Chekhov, and I won’t come back to that later.

Mercury orbits the Sun with a 3:2 spin resonance. Thanks to the interaction between Mercury’s elliptical orbit and the slight wobble of the Sun, Mercury rotates all the way around three times for every two full orbits around the Sun. This rotation is calculated from the background stars, if you are standing on Mercury looking at the Sun, it’s more like two Mercurian years for every full Mercurian day, which is a lot longer than the 24 hour day we get on Earth. However, Mercury isn’t tidally locked, it doesn’t keep one face to the Sun all year long. This is what the Moon does with the Earth, the same part of the Moon always faces the Earth. There are other moons that are tidally locked facing their planet and we do believe that there are planets that are tidally locked going around other stars. For a while it was believed that Mercury did that, but it is not the case, Mercury is rotating, it’s just doing so in a particular resonance with the Sun.

If you were standing on Mercury, as is simulated in this video, about half way through, you would see here that the stars appear to be moving behind the Sun much quicker than the Sun is moving across the sky. If you are in the right place, then just about midday it appears to freeze and then travel backwards and then forwards again. This is the retrograde motion of the Sun and it is only happening on Mercury because of the fact that it orbits with its particular resonance. Not many planets have a day longer than their year, although Venus does for a very different reason. The Sun goes eventually set on Mercury and the Sun will appear to move from East to West, as do the stars and other planets, as Mercury is rotating the same direction as the Earth and most other planets.

Returning to the Earth at the end of the video, we stay at the equator, which shows Venus, Mercury and even Mars stadning in almost a vertical line over the horizon. This is how they would look if you were very close to the equator, where as here in Ireland we see the ecliptic, and thus the planets, at a much shallower angle, especially at this time of the year. You do get a slightly better view of Mercury at this time of the year from areas South of the equator, down around Argentina and South Africa, because it is summer time in the southern hemisphere, meaning the Sun, and morning planets, will appear higher in the sky.

Mercury may be the smallest planet in our solar system, but I hope you found it interesting in a big way. Not only will we return to Mercury in future videos, we will perform overviews of all the other planets, and indeed other features of our sky in the future, to break up the previews of what you will see in the sky. So whatever kind of video you prefer, I hope you’ll join me back here again next time.