Caoimhín's Content

Today, we are going to have a quick review of a couple of the major comets that have come through our sky in the past couple of months and will continue through our sky in a few months to come. We’re also going to take a look at one of those comets, 3I/ATLAS, from a few other locations, from a few other planets.

We’re going to start with Comet C/2025 A6 Lemmon, looking at the sky for early November. This comet is no longer circumpolar, it’s just under the horizon as we come up to midnight on the 8th of November. It is up earlier in the night, but quite close to the Sun there or at least it seems to be quite close to the Sun in our sky. If we move back in time, back into October, the comet comes back towards the Plough, and it stays in the sky later. As we come forward into November, the comet appears to be getting much closer to the Sun, and indeed it is. Its brightness peaked just around the 28th of October, getting down to 10.08 magnitude, so that is an indicator that the comet is around its perihelion. The comet came around its peak by the Sun as we came out of October and so it’s still quite bright as we come into early November. It is low in the sky and it is quite close to the Sun, but it is still there above the horizon at sunset. The version of it shown Stellarium is reasonably faint, just fainter than magnitude 10. It does seem to be a little bit more visible than that in the real sky, so there is a good chance that you will be able to spot it with binoculars, it shouldn’t actually be as faint as this seems to be indicating. That’s Comet C/2025 A6 Lemmon, it’s one of several comets that are coming past us at this time, another is Comet R2 SWAN.

I mentioned in a previous piece that we’ve got both Comet R2 SWAN and Comet F2 SWAN. I explained the differences in their names, it comes from when they were discovered, what half month they were discovered in. It looks like Comet R2 SWAN is the slightly better one at the moment, it looks like it’s going to be a bit more impressive as we come through November then Comet F2 SWAN. By moving back towards October again, Comet R2 SWAN gets brighter and again it visibly moves closer to the Sun. It was closer to the Sun and brighter back in October. However, as it moves away from the Sun, even though it gets fainter and therefore harder to see, it is also going to get higher in the sky and further from the light of the Sun, which should help its visibility. It also ends up in a much easier position to observe, being up by Saturn as we come through November. However, the Moon is pretty close to the comet just after the night of Halloween. By the time the Moon is out of the sky and away from the comet, it is going to be after getting further from the Sun and fainter, but it will be in an easy to spot position, nice and high up by Saturn. I’m not getting any tails for the comets at the moment, but whether or not tails get generated by Stellarium depends on a few factors, how much we know about the comet, how many orbital parameters we have, that kind of thing. There is a good chance that any measurements of the tails of these comets simply hasn’t become available in a way that Stellarium can make use of it to show the tails.

If we take a look for probably the most famous comet that’s coming through the sky at the moment, 3I/ATLAS, we’ll actually wait until we come around to morning time because that is when it’s going to be in our sky in November. if we move back a little bit again towards the start of November, it will be a little bit closer to Venus and the Sun. It’s much fainter than those other objects, just 14th magnitude, and it is continuing to get fainter as we move out into the future, as it moves away from the Sun. However, it is getting closer to us. The Comet 3I/ATLAS is actually going to make its closest approach to the Earth in late December. On the 19th of December it is only 1.798 AU away from us, which is about 269 million kilometers or 167 million miles. This means that it’s not quite as far away from us as twice our distance to the Sun, it’s a little bit closer to us than twice our distance from the Sun, but it is definitely further from us than we are from the Sun, even at its closest approach in December.

Comet 3I/ATLAS is pretty far away from us, however, it did get closer to a few other objects in our solar system. Around November 3rd is when it was at its closest to Venus. Of course, if you were on Venus, it wouldn’t be very good for you. You certainly wouldn’t have a good view of the comet if you were standing on the surface of Venus due to Venus’s thick atmosphere. Thankfully, when we transport ourselves to Venus in Stellarium, we are given a clear view of space. There are a lot of comets and other minor bodies in the potential location list of Stellarium, which can put Venus off the bottom of the screen. We might as well stay roughly in the same relative location, where Ireland would be if we were on Venus. Comet 3I/ATLAS is in the sky and it is much closer than 1 AU to Venus, just 0.65 AU. This is just about it’s closest approach. If we jump through days on Venus, things will get weird because the days are weird on Venus, hundreds of Earth days long. By moving through time a little slower, the comet gets further away, so it looks like the closest is right on November 3rd here on Earth. However, it’s still pretty faint, it’s 11th magnitude, but still much brighter than we’d be seeing it from here on Earth. The comet’s already gone past its perihelion, it’s already gone past its closest point to the Sun, so it’s not going to be as bright as it could be. All the same, we’re still seeing it much closer to Venus than it ever will get to the Earth.

The comet also passed by Mars, so we’ll take a quick jump to Mars to see how close. When the comet made its closest approach to Mars it was still on its way into the solar system, it hadn’t yet made its perihelion with the Sun. Taking a closer look at this comet from Mars, while we’re still in November it’s 14th magnitude, similar to how we saw it from the Earth, and over an AU away, so we’re going to have to come back. The days are different on Mars as well, but they’re not as different. The day on Mars is just 25 hours compared to our 24, rather than being hundreds of Earth days long for one day on Venus. Coming back to the 3rd of October, the comet is just 0.19 AU from Mars. Even though we are are looking at this from Mars, an AU is always the average distance between the Sun and the Earth. The comet is much closer to Mars than it will ever get to the Earth, much closer than it gets to Venus, and it gets down to magnitude 9. This is earlier in October, so this would be before the perihelion. Again, much easier to see from Mars than it will be from the Earth, but this of course is looking into the past.

That is, that is Comet ATLAS viewed from a couple of the other planets in our solar system, a couple of the other planets that it did make, not just close approaches to, but much, much closer approaches to, than us. If we take a look at the distance of to Sun, it’s 1.678 AU. If we start moving forward, it goes down, but the distance from Mars goes up, so that’s another sign that it is moving in towards the Sun. I think 0.197 is about as close as we’re going to get, it does go down to 0.196, so it does go down another little bit, but that’s just about as close as it’s going to get to Mars. Hopping back to the Earth, we’re not going to get the comet that close to us, and of course, it’s not going to get close to us at all until after it moves past its closest to the Sun and indeed until after November. This comet isn’t going to get close to us until well into December, so we still have to wait quite a while for it to, really, not get that close to us at all. However, it is out there, and it is still going to stay in our sky for months to come. It will take a while for this comet to come all the way up to us and then keep heading out of the solar system, so if you have a good telescope or binoculars, it is there as an option. Those other comets, Comet R2 SWAN and Comet A6 Lemon, they are that little bit easier to see.

While we’re still here, we might as well turn to Comet F2 SWAN, but unfortunately it’s not visible from here in Ireland, it’s under the horizon and not coming up for us at all. So unfortunately, that one is out of the way for us, maybe for viewers in other locations, you might get a chance to take a look at Comet F2 SWAN. It’s also magnitude 19, the faintest of all of the comets we’ve looked at today, so definitely a much harder target to acquire.

That is just a brief update on the comets, we will of course, get back to Comet 3I/ATLAS once we’re through to December and it’s actually making its closest approach to us. This was just to give a comparison with how close it got to the other planets in our solar system, it doesn’t seem to be getting quite as close to the Earth. I hope you enjoyed this piece, if you did 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.