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Today’s video, we are going to take a look at a couple of Saturn’s moons. Saturn is on its way back into the morning sky. For many people, those a little closer to the equator, Saturn is already back into the morning sky, but this early in May, Saturn is still just barely out of view for us. We’ll bring the Sun above the horizon and get rid of the atmosphere just to see how close we are. Pretty close, Saturn is quite far from the Sun, it’s definitely quite close to being in a position where we will be able to see it, but the Sun’ is still just a little bit too bright’s light is still blocking it out. We’ll move forward a little bit later in the month and bring the Sun back down, showing Saturn visible in the morning once we’re through to the 25th of May. It is still quite low in the sky, it’s not up in a particularly good level of darkness, but it will be getting easier and easier to see as we keep moving later. We are going to be looking not so much at Saturn and not even at these more famous moons, but at a couple of moons that are closer to the planet.

Saturn has a lot of moons, and some of them are almost an afterthought when people take a look at Saturn through a telescope. Titan is always a target, thanks to being so large and easy to observe, and people may look for some of the other larger moons like Rhea, Dione and Tethys. Enceladus, of course, with its icy shell and liquid water trapped beneath, is a very popular target at the moment. Out of the even further out moons, Iapetus and Hyperion, Iapetus is a potential target, but Hyperion is very small and difficult to see. The moons that are practically within Saturn’s rings can be even harder to spot, but taking a close look there are Janus and Epimetheus, those will be our targets today. Prometheus is another moon, connected of course to Epimetheus. These smaller moons are visible in Stellarium thanks to the labels, but with those labels removed, many of these moons are invisible to most telescopes. Even if we take a much closer looks, those moons are so small, they don’t show up without help from the software that we’re using. They might show as dark specks backlit by Saturn as these tiny little rocks, these moons that are closer to Saturn, are really more comparable to small asteroids than a moon like ours. Epimetheus is just 116km in diameter. 116km would be a reasonably sized island, but it’s not a very big moon.

There are so many moons orbiting Saturn, and some have interesting relationships. Some moons are in harmony with other moons, with smaller, inner moons orbiting Saturn three or four times for every one or two orbits of the outside larger moon that is influencing them. Some of the moons quite close to each other influence each other as well and Janus and Epimetheus are a pair of moons that have an interesting relationship. What we’re going to do is head to Saturn and more specifically to Janus or Epimetheus, I think we’ll go with Janus first. Thankfully, Stellarium’s location window allows us to journey to locations other than the Earth, including Janus. On this little moon, we’re going to go pretty close to the equator and see what we can see. We are looking out into space, into the southern hemisphere’s sky being pretty close to the equator of Janus that makes sense. We can see some of Saturn’s other moons and there’s a great view of the Milky Way without any light pollution or atmosphere. The Sun is up and so is Saturn’s ring. It’s at such a tight angle, it almost looks like an artifact of Stellarium. There are several of the other moons out in that direction, but we’re looking for one moon in particular. Moving through time a little bit the rings appear to do some very interesting things when you’re practically inside of them, seeming to wiggle as the moon’s inclination causes it to move across the equatorial line. Just behind the ring is Epimetheus, and Epimetheus is what we want to look at from Janus.

What we’re looking for is going to take a while, about four years, so four to eight years depending on how many times through the cycle we decide to go. I’m going to get rid of the ground as well, which doesn’t get rid of Saturn, so Saturn is going to get in the way for some of this. We’re going to zoom in here on Epimetheus. We have to zoom through the rings at the moment, but we’re getting a much better view of Epimetheus now that we are closer to it, so much closer to it in our position here on Janus compared to from the Earth. If we start moving through time, the view presents some very interesting things visually. We behind Saturn, looking at its nighttime side relative to the Sun, with Epimetheus just on the other side of the rings from us, but not behind the planet. Currently, it looks like Epimetheus is getting away from Janus. As we keep moving ahead in time, it slides behind Saturn, it ends up on the opposite side of Saturn from Janus. As I said, this will take about four years so going day by day it is going to take a little bit of time. Continuing forwards, eventually Epimetheus reappears from behind Saturn. Epimetheus is slowly catching up to Janus, but it’s not going to catch up the entire way.

Epimetheus keeps getting closer and closer, coming around the rings. It’s almost on the same side of the rings as us so we can zoom in without the ring getting in the way. It looks like we’re looking at the nighttime side of Epimetheus so it doesn’t look particularly good, or perhaps it is in Saturn’s shadow rather than being actually in its own nighttime. As Epimetheus comes around the rings, getting closer to Janus, the distance between the two keeps going down and down. As Epimetheus joins Janus on the night side of Saturn, we’re looking at it in darkness, so it doesn’t look amazing. However, Epimetheus passes just inside Janus, and it will do the same thing again, stretch off around the far side of Saturn and eventually swing around, catching back up. We’ll come back to that position where they were very close together, with their lowest distance showing them to be just a thousand kilometers apart. As we’re looking at the nighttime side of Epimetheus, if you were on Epimetheus, you would be looking at the daytime side of Janus, so let’s make a very quick transition out to Epimetheus.

Epimetheus and Prometheus were two titan brothers in Greek mythology, Prometheus being forethought or thinking ahead while Epimetheus was afterthought or hindsight. That is one of the reasons I said these moons are something of an afterthought earlier in the article. From Epimetheus, the daytime side of Janus is just a thousand kilometers away, a bit more than a thousand kilometers away, but they are very close together as they pass by each other. These moons’ orbits are very close to each other and having this frame of reference, where we’re standing on one moon watching the other, we can see them make this kind of pass, coming up close to each other and then separating away to opposite sides of the planet. As we continue moving forward, Epimetheus is now rushing away from Janus. They are getting further and further away from each other. Once Janus looks like it’s almost in the middle of Saturn on the opposite side to Epimetheus, its 302,000 kilometers away, so much further than that closest approach that they just had. As we keep moving forward Epimetheus is going to catch up to Janus again, getting closer and closer.

All the other moons seem to be really whirling past. Saturn’s moons can move quite quickly, and very often if you go out on different nights, you’ll see the moons in different positions. It looks almost like Janus and Epimetheus are moving slower than they really are because we’re looking at their relative speeds relative to each other, Epimetheus is catching up to Janus. Of course, if we were on the Earth, they’d both be going around the planet quite quickly. As we come back to their closest pass, Janus ends up between Saturn and Epimetheus. I think that a lot of this is interesting, the way that the moons move past each other, the way that they interact, but here they have swapped places. Janus is now inside Epimetheus’s orbit around Saturn, whereas the first time we made this alignment it was the other way around. We were standing on Janus looking at Epimetheus’s dark side and now it is flipped. Their positions relative to the Sun, the position of the planet relative to the Sun and its equatorial tilt relative to the Sun, all of that has changed, so we are seeing a little bit of the light side of Janus and Saturn’s rings are still catching that light. We’re on moons orbiting a planet orbiting the Sun, and we’re concerned about how they’re orbiting the planet, but how they’re lit is going to depend on how this whole system orbits the Sun. It does get a little bit more complicated with multiple bodies involved.

Just briefly, right here at the end, we’re going to take a look at this from the solar system observer just to see if we can catch that swap happening a little bit more cleanly. Looking down on the Solar system from above shows the great tilt that Saturn has, it’s not perfectly flat. Even though we’re looking down on the pole of the solar system, we’re not actually looking down at the pole of Saturn. With the orbits of the moons displayed, it’s clear that Epimetheus and Janus are really close together, they appear really close together and their orbits almost seem to blend into one. As we start moving through time they start to drift apart. Janus is orbiting Saturn and its distance from Epimetheus is increasing almost as if Janus is racing ahead. With our view focused on Janus, it almost looks like Janus is hula-hooping Saturn’s rings. We can hop ahead year by year to see the distance increasing and increasing, it does take about four years for this alignment and swap to occur. As we come back around, Janus starts catching up to Epimetheus again. It does, of course, take a little bit of time and the tilt does make it a little bit tough to see, but the orbits do overlap, one orbit comes outside the other. At this alignment, Janus is slightly closer to the planet and as we come back closer towards today, they’ll swap around and we’ll end up with Janus on the outside of Epimetheus again. Just after the alignment, Janus shifts to the exterior orbit as it comes around and that’s going to bring it up to the other side of Epimetheus when it finally catches back up again.

Also out there is Prometheus, so if you’re just watching out for the names it’s very easy to get the two brothers mistaken. The overlap, the swap, the changing of the lanes that happens as Janus and Epimetheus pass by each other is unique in our solar system. When Janus gets close to Epimetheus, Epimetheus technically gets pulled by Janus and it gets pulled into a higher orbit while Janus sort of pulls itself down into a lower orbit and then this swap happens again as they pass by again, over and over.

So those are two of Saturn’s moons that do something particularly weird. The only objects in the solar system that do this weird thing, this weird overlap of their positions and they’re both small moons. They’re difficult to see from here on Earth, but as you now know the view from Janus and the view from Epimetheus can be quite impressive. I hope you enjoyed this description of the strange phenomenon, this strange thing that those moons do and I hope 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 watching and hopefully I’ll see you back here next time.