Caoimhín's Content

Today we are going back in time to 2019, so not particularly far back in time. We’re going back to 2019 to the 11th of November to take a look at a transit of Mercury. On the 11th of the 11th 2019, if we take a closer look at the Sun in the evening before sunset, the transit of Mercury is ongoing. Mercury is visible as a little do in front of the Sun. This transit of Mercury was the last transit of Mercury that we had. We also had one in 2016 and before that it was the much earlier 2000’s, 2006 and 2003. We’ll be getting our next one in 2032.

From here in Ireland we saw the beginning of the transit of Mercury, as Mercury comes onto the face of the Sun. It moved towards the center of the Sun, and then the Sun sets, or at least gets behind the trees and buildings, before we get to see the end of the transit. These transits can last for quite a while, for hours, much longer than a solar eclipse by the Moon. The planets Mercury and Venus transiting the Sun can last for much longer, but aren’t visible to the unassisted eye. As always, never look directly at the Sun.

From Ireland, Mercury came in towards the top of the Sun and then moved across in front of it. If we were to take a look at this eclipse from a different location, we would be able to see all of the eclipse and it would look like it was passing closer to the center of the Sun as well. When we’re looking at things that happen closer to us, we’re often able to see evidence of the phenomenon of parallax, which is things looking like they’re in a different position if they’re closer to us relative to things behind them if we move location. Watching the full transit occurring close to noon, Mercury seems to come up and make a loop before going down the other side. Part of the reason it looks like it’s making this big swinging shape is because we are watching it as it crosses the sky, we’re watching the Sun move past its zenith. Some of the apparent curvature of the path is actually down to the Earth moving, the Earth turning and turning away from the Sun. You might know that something similar happens to the Moon. If we watch the Moon rise, come up to the zenith, and then sink down on the other side, it looks like the Moon’s turning in the sky, rotating as it does so. I might mention that again a little bit later, but what I really want to do is take a look at this transit of Mercury from the Sun.

If we have a transit of Mercury, then from the Sun that means we will have Mercury transiting the Earth. From the surface of the Sun, the Earth and Mercury are practically on top of each other, which is a good sign. That means we are pretty much seeing the transit, but we’ll have to take a closer look. Mercury, of course, is a very small planet, but it looks bigger than the Earth when seen from the Sun. If we move through time, Mercury and the Earth should appear to get closer together instead of further apart. However, it looks like my first attempt put me on the wrong position on the Sun to see Mercury directly in front of the Earth, similarly to how some portions of the Earth wouldn’t be able to see the transit. We’re going to have to change our position, which is easier said than done. We will need to change our position while keeping an eye on the relative positions of the Earth and Mercury to try and get them to line up a little bit better. Stellarium lets you do this using the location window. It is tricky, of course, the Sun is very big, so even small adjustments on this map are actually very large adjustments in real distance across the surface of the Sun. You need to make much smaller adjustments to try and get things to line up. Of course, you don’t want the adjustments to be too small, otherwise things will take too long.

It is interesting that although Mercury is so small compared to the Earth, it looks bigger. Mercury is the smallest thing that we consider a planet, Pluto, of course, is considered a dwarf planet. Mercury is the smallest planet and it’s smaller even than some moons of Jupiter. Nonetheless, from the right location on the Sun, at just after 2 o’clock UTC, there is an occultation of the Earth by Mercury. The Earth is completely being obscured by Mercury. While this is happening, a shadow of Mercury is passing in front of the Sun as seen from the Earth. Any shadow of Mercury cast on the Earth would be such a tiny shadow, I don’t think it would be visible on the Earth. That means we won’t be able to track the passage of the transit across the Earth the way we can with a lunar eclipse. From the Sun, with the perfect alignment, we have the Earth perfectly behind Mercury. It doesn’t look like there is any Earth sticking out anywhere, showing that Mercury looks a little bigger from the Sun. We’re going to go to Mercury, and we’re going to go to the side of Mercury that is facing the Earth during this transit of Mercury in front of the Sun.

We are on Mercury. Initially we are on the daytime side of Mercury. We’ll move around to the nighttime side of Mercury, which puts the Earth almost exactly in the middle of the sky, which is a good sign. As I mentioned, Mercury is so small that I do not think there will be a shadow visible on the Earth, I don’t think we will be able to do the same thing as the eclipse. Turning through the day here, coming up to the peak moment of the transit, or at least where it looked like Mercury was eclipsing the Earth when we looked at it from the Sun, I’m not seeing a shadow of Mercury, which really says how small Mercury is. Even when Mercury blocks out the entire Earth when seen from the Sun, if we look at it from the Earth, Mercury is just a tiny little dot. If we go back to the Sun, I would need to go through all of the tiny adjustments again to get things to line up perfectly, but we’ll still see the two planets fairly close together. It is only a very small region of the Sun that allows you to see Mercury perfectly blocking out the Earth. However, if we return to the default location here on Earth and take a look back at the Sun, we will be able to see the transit of Mercury occurring still. Transits of Mercury do take a while, and because they do last for quite a while they’re visible over a large portion of the Earth. As the Earth turns, we’re able to see Mercury from different locations on Earth.

We’ll come back to the Earth and observe the whole transit. Starting with Mercury just next to the Sun, it touches down at just half 12 UTC. We are looking at this in November, so the time in Ireland is UTC, this isn’t using summertime, which of course Ireland is using at the moment. As it moves across the disk of the Sun, it can be seen that Mercury is smaller than some of the sunspots. Mercury is closer to us than these sunspots and still appears smaller, which should give you an idea of how large those sunspots really are. Taking a closer look at Mercury leaving the Sun, it’s just finishing leaving the Sun at just after 6 o’clock. So that is hours. That is an almost six hour long transit, give or take. That is Mercury as it crosses the Sun, and with no atmosphere or ground it appears to be going more directly across the Sun here. That’s because we’re not watching the Sun come up from one side of the sky and go down on the other, we’re watching the Sun as it’s setting, as it’s going under the horizon, so we’re not getting that kind of turning effect that we get when these objects are high in the sky.

Quickly grabbing the Moon, thankfully the Moon is very close to full here, 98% full. That should be close enough for our purposes. Moving around to moonrise, as the Moon comes up, the Sea of Tranquility is pretty high on the Moon, almost facing upwards into the sky. As we come up to midnight, things sort of straighten up, the football player on the Moon as I describe it looks a little bit more upright. As things go back down, it turns the other way with the Sea of Tranquility sort of facing down, tilting down towards the horizon. That twisting effect is also the cause of the apparent bend in Mercury’s transit.

That was the transit of Mercury in 2019 from a few different viewpoints. I hope that you enjoyed the view. If you did enjoy 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 watching and hopefully I’ll see you back here next time.