 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy, we are going to talk about orbital resonances and what that means. So let's go ahead and look at what we mean by an orbital resonance. And we can see a couple different types of these but the first type we want to look at occur when orbits of different objects become related because of their gravitational interactions. Now we see one example of the moons of Jupiter here. Now if you look at the moons of Jupiter, we have Jupiter at the center, then Io, Europa and Ganymede. And those are the next three moons out. Now if you watch their orbits, they're not just random relative to each other and you'll see that flash of green with Io and Europa that occurs every other orbit of Io. So Io orbits around twice and each time Europa orbits once. So every other orbit, they will be lined back up and you will see that green flash that occurs. Now you will see the same thing for Io and Ganymede. So if you watch that, they will flash at the top of the screen and then they will do so again four orbits later. Now you can also see a resonance between Europa and Ganymede. This flash is at the bottom of the screen. So if you watch when Ganymede comes to the bottom, you see that purple flash. That flash just means that they are lined up. We're kind of highlighting where the orbital resonances are happening. Now they are not limited to things like this. We can also look at the Kirkwood belts in the asteroid belt. So what we see is that when we plot out the asteroid belts versus distance, we get a big group here and then a gap. And then another big group here and then a gap. And then a little bit of a smaller group here and another group over here. And there are all these specific gaps. Well, those gaps are related to the orbit of Jupiter. And you can see that this first major gap has a three to one resonance. That means that any asteroid at this location would make three orbits while Jupiter orbits once. And again, like the moons of Jupiter, they will become lined back up. And over time, these very small objects will get pulled out of their orbits. So we will end up clearing this entire region of asteroids. And we see that there are very few, if any, asteroids at the regions of these resonances. Now we can see they are also any other integer multiples, things like five to two, seven to three, and then way out at the edges the two to one resonance with Jupiter. Now we see the same pattern with the rings of Saturn. Let's take a quick look at that. And here is an image of the rings of Saturn. And they are not just a flat sheet. There are a lot of structures within them. All of these structures are caused by the moons. So for example, we see a large gap that occurs here. That would be associated with a resonance with the orbit of one of the moons of Saturn. But all of the little structures that we see are created by different resonances with the moons. So here we have a big region that's been cleared out because of resonances with one of the moons. But all of those little structures and bands that we see are all associated with the moons of Saturn. Now this was one type of resonance. Let's look at another one that we associate with our own moon. And that is what we call a spin orbit resonance. Our moon is in a spin orbit resonance with the Earth. And that is when the rotational period of a body becomes related to its revolution period. The rotation is how long it takes to spin on its axis. The revolution is how long it takes to orbit all the way around the central object. In this case, how long it takes the moon to orbit around Earth. And because these are locked together, that means that the moon always keeps the same side pointing toward Earth wherever it is in its orbit. And we see this same pattern with all of the other major moons in the solar system. So it's not confined just to our moon. We see this with other moons in the solar system. And we actually see a double case of this with Pluto. Pluto and its large moon are completely locked together, where its moon keeps one face towards Pluto. And the Pluto keeps one face toward its moon. So they are actually completely locked together. Now, the last example we wanted to look at would be Mercury here. Mercury also has an orbital resonance. And we can see that here. If you watch the orbit, we see that arrow here on the far right points away when it gets to the far right point. And when it comes around the second time, it's pointing inward toward the sun. And you will see that pattern continually recurs. Jupiter is, sorry, Mercury is also in a spin orbit resonance. And that means that it has its day and year are related, although not a one to one ratio. So it turns out that three days on Mercury are the equivalent of two years. That is another type of resonance that can occur and that we see in our solar system. So let's go ahead and finish up with our summary. And what we've noted is that the orbital resonances occur when there are integer multiples between the orbits of one object and another object. We see this in the moons of Jupiter, the rings of Saturn, and the asteroid belt. And we also looked at the spin orbit resonances, which occur with our moon and other major moons in the solar system, and with the orbit of the planet Mercury. So that concludes this summary of orbital resonances. We'll be back again next week for another special topic in astronomy. So until then, have a great day, everyone. And I will see you in class.