 Greetings and welcome to the Introduction to Astronomy. One of the things that I like to do in each of my introductory astronomy classes is to begin the class with the astronomy picture of the day. From the NASA website that is apod.nasa.gov.apod. And today's picture for October 23 of 2023. Well, it is titled Moon Io from Spacecraft Juno. So what do we see here? Well, this is one of the four large moons of Jupiter and the innermost of those known as Io. Now, it is a moon that is about the same size as our own moon. And interestingly enough, it orbits about the same distance from Jupiter as our moon orbits from Earth. However, there are some very big differences with this. One is that it is very close to the much larger gravity of Jupiter and that makes Io an extremely active place volcanically. So while our moon has been dead for billions of years, Io is still active and is actually the most volcanically active object in our solar system. Now, why does that happen? Well, tidal forces from Jupiter are a big part of that. Jupiter has stronger gravity, so it stretches one side of the moon more than it does the other. Now, if it weren't for other moons, that wouldn't make too big of a difference because it would just orbit in elongated shape with that elongation pointing towards Jupiter. But because of forces from the other moons as well that continually affect its orbit, it gets twisted and turned. And essentially, Jupiter and the moons are needing Io like a big ball of clay. If you take a lump of clay that is cold and you work it for a while, it will become very pliable. Well, this is what has been happening to Io for billions of years, making it extremely active volcanically so that most of its surface is covered with lava. So there is lava on a lot of the surface and the rest is frozen out rock, but it constantly resurfaces itself on a very short time scale. So not... What that means is that we essentially see no craters on Io because they don't last long enough to be visible. They get wiped out very quickly over the course of, just say, years and decades and centuries. They don't last for incredibly long amounts of time as they do on other objects such as our own moon. So here we get to see a very close-up image of Io as imaged by the Juno spacecraft, which is orbiting Jupiter. Now, this is one really nice image that gives us a better chance to study the volcanic surface of this moon, but there are a couple more coming up, one in December and one in February, that where it will actually be many times closer, 10 times closer, giving us some of our best views we have ever had of this moon. So we look forward to those over the coming months to give us an even better understanding of this unusual moon about the size of our own moon. So that was our picture of the day, for October 23rd of 2023. It was titled Moon Io from spacecraft Juno. We'll be back again tomorrow for the next picture, previewed to be Eclipse Sky. So we'll see what that is about tomorrow. And until then, have a great day, everyone, and I will see you in class.