Caoimhín's Content

As promised last time, we are going to be looking far ahead into the future to take a look at Halley’s Comet. Halley’s Comet has a period of about 75 years, though this varies due to the planets effect as it passes them. Right now, we have to wait 37 years for it to return in 2061, so we are about halfway through Halley’s orbit. If we take a look for the comet right now, we’re looking 35 AU away, 35 times further from us than we are from the Sun. As the Earth and the Sun are comparatively close together, the difference between our distance and the Sun’s distance to the comet is just one-third of an AU. As I mentioned last time, 1 AU or Astronomical Unit is the distance from the Earth to the Sun. At the moment the comet is way out, roughly in the direction of Canis Minor, not too far in the sky from Procyon, the brightest star in Canis Minor. At such a great distance, the comet is almost impossible to see, given that it is a reasonably small lump of rock and ice, just 10 kilometers across. This of course would be a huge comet if it crashed into us, but it is tiny compared to the planets and dwarf planets.

Even over the next few years, the comet won’t seem to change position much, given that it is in the slower, distant, portion of its orbit. Moving through time day by day for 37 years would be mostly boring, but as it comes close to the Sun in 2061, it will develop it’s spectacular tail. Moving forward to that year and looking at the comet during the day, without any atmosphere, we can see how close it seems to get to the Sun. With it’s orbital path traced out, we can see the tight turn it takes as it orbits close to the Sun, before flying back out towards the outer solar system. Without zooming in at all, its phenomenal tail clearly stretches across the sky, very different from the faint tail we were able to see of comet 12P/Pons-Brooks last time. While C12P/Pons-Brooks was just about visible to the naked eye, usually requiring binoculars at least to see the tail, C1P/Halley will have a clear tail, visible to the naked eye, even from a city, potentially for a week or more. The name 1P/Halley indicates that this was the first comet confirmed to be periodical. As it only returned every 75-ish years, few people got to see it twice, and as it varies a little from year to year, people didn’t realize it was the same comet. It has probably been observed since at least 204 BC, but it was only in the 1700’s that Edmund Halley correctly calculated its period.

Looking at the comet at sunrise, with the atmosphere in the way, its tail remains visible. It is certainly visible and impressive from a Dark Sky, but even with some light pollution its tail remains visible to the naked eye. At least with as much light pollutions a city like Cork, this comets tail will be visible to the naked eye. This is one of the reasons that the comet gets so much attention. Not only was it the first to be understood, it is one of the brightest and most impressive, occasionally appearing brighter than Venus, according to historical reports at least. 75 years may feel like a long time, but this is technically a short period comet, compared to the comets whose periods are measured in 100’s if not 1000’s of years. Although a person could certainly see this comet twice, if you manage to see it young enough, for most people this is a once in a lifetime event. You need to be lucky enough to be born just before the comets return, preferably with enough time to be old enough to remember it, but still young enough to be likely to live another 75 years until it returns.

The Comet itself, and its tail, will be at their brightest when they are closer to the Sun, this is also usually when the tail is at its longest. The tail will slowly build up to its peak, and then fade as it passes its perihelion and begins to move away from the Sun again. This does mean that when the comet is in the sky for the longest amount of time, it is going to be fainter. When it’s at its brightest, it is usually in the sky for the least amount of time. In the Stellarium simulation, you can see two tails, one broader and shorter, the other longer and thinner, overlapping but pointing in slightly different directions. This can also be seen a little in images from 1986 and 1910, the last two times it appeared. I mentioned this in the previous article as well, but heavier particles and less charged particles falling off the comet will be less effected by the solar wind, allowing them to follow the motion of the comet itself a little more. We will be seeing the comet pretty much the middle of the summer in 2061, giving us slightly shorter days to observe the comet. Luckily, given its incredible brightness, it can easily outshine the glow of both sunrise and sunset. For most of its closest approach, the comet will appear to be almost over the Sun from our perspective, letting us see it both morning and evening. We can only predict to certain degrees of accuracy, but this comet is famous for its brightness. It is the comet that appears in the Bayeux Tapestry, along with various other works of art and ancient records, always being described as an incredible event. Although I was not yet born in 1986 when it last returned, I have heard that it was an amazing sight, and the next return is predicted to be even better.

Of course we do still have a long time to wait for Comet Halley to return, but there is another event, slightly less rare, coming up much sooner. We do still have to wait until 2032, but just 8 years is certainly less of a wait than 37. On November 13 of that year, we will get to see a transit of Mercury. This is a more frequent event than the return of Halley’s Comet, but it is much briefer and more difficult to observe. The comet is going to be visible to the naked eye for a week or more, and with binoculars and telescopes for even longer. The transit of Mercury however only lasts a few hours, it is a much much quicker phenomenon, quick enough that it often is only visible from certain parts of the world, similar to an eclipse. Luckily, it will be visible from Ireland, although we aren’t the best viewing location. Both of the most recent transits of Mercury, in 2019 and 2016, were visible from Ireland, and I was lucky enough to observe one of them, thanks to a solar telescope called a Coronado. Mercury is so small, that when it is sitting in front of the Sun, it may still be too small to easily see or recognize through eclipse glasses or similar eye protection. Some magnification is almost always necessary, and it generally isn’t safe to point a telescope at the Sun. However, some telescopes are built for exactly this purpose. Studying the Sun is still a part of astronomy, such as observing its corona without waiting for an eclipse, or tracking sunspots appearing on its surface. With this kind of special equipment, observing transits in detail becomes viable. Pinhole projection, often used for eclipses as well, is also a viable option for observing transits.

The next transit of Mercury is going to happen very early in the morning for us here in Ireland, and in fact it begins a little before the Sun rises for us. I don’t usually say that it is necessary to get up very early, for example if Saturn is up at 3 o’clock in the morning, you can always wait for it to move into the evening sky, as it eventually will. However, this transit will only be visible for a few hours, and you can’t observe it until the Sun has risen. Given that we will be in winter time, this means about 9 o’clock in the morning, so that’s not too bad. Watching Mercury cross over the disk of the Sun helps reinforce the size difference, with the little black dot of Mercury potentially being dwarfed by the Sun’s sunspots. Sunspots are hard to predict, so I can’t guarantee that there will be any in 8 years time, but it is certainly possible. The transit will begin at about 6:45 in the morning, before the Sun has risen for us here in Ireland. It will then drift across the disc of the Sun, before finally moving off the disk at about 11 o’clock, just before midday. This means that there will be a big window to observe the transit, though for us in Ireland it will only be visible for about 2 hours. The planet doesn’t move across the very middle of the Sun, but quite high on the disc, which makes the transit last longer. The complex interaction between our orbit, Mercury’s orbit, and how we see the transits also means the gap between them can vary, with just three years between 2016 and 2019, but 13 from then till 2032. You can certainly see a few of these in your lifetime, especially if you are willing to travel, just like the eclipses. Transits of Venus are significantly rarer, but we will leave that topic for another time.

I hope you get a chance to see Comet 1P/Halley in 2061 and the transit of Mercury in 2032. I will, of course, talk about these events in more detail when we are closer to them occurring, assuming that I’m still going to be making posts like these that far in the future. If you want to be notified when I do, and help make sure that I can continue doing this for that long, you can subscribe to this website and my YouTube channel. There will be plenty more posts between now and then, and hopefully I’ll see you back here for those.