 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy, we are going to talk about the concepts of apheleon and perihelion and what these mean in terms of orbits. So let's go ahead and look at what these mean. What are apheleon and perihelion? Well, let's first basically talk about the orbits and we know that the planets orbit our sun in elliptical orbits and that means that the distance between the sun and planet changes. So we call the closest approach of the planet to its star perihelion and the greatest distance of a planet from the sun is apheleon. Now we can see those in the orbit here as we watch this move around and its closest approach here on the left hand side, that would be perihelion. That is when the planet passes closest to the sun and on the opposite side we would have apheleon when the planet is furthest away from the sun. Now these are the general terms. We can look at these in terms of another orbit. Let's take one look at another orbit here and we can see them labeled and we see the perihelion on the left hand side, same as in the previous one. So that is perihelion when the distance between the planet and sun is the least. Apheleon over here when they are the greatest. So that's the largest distance. Now these are general terms for these are periapsis and apoapsis. That applies to any object. If you're just talking about an orbit, we use the general terms of periapsis and apoapsis. Perihelion and apheleon apply with the suffix here that perihelion for helios or sun apply to things orbiting the sun. When we talk about things orbiting Earth, we use perigee and apogee. And objects orbiting Jupiter, such as the Juno spacecraft, will have a perijove and an apojove. So when Juno comes closest in, it has perijove when it's closest into the surface and when it's furthest away it would have apojove. And similar for satellites orbiting the Earth, they are a G when they are closest and apogee when they are furthest away. So really the two terms are related to the closest and furthest distances of a planet from the sun, although they can be generalized to talk about orbits around any other object as well. So let's finish up with our summary here. And we talked about how planets follow elliptical orbits around the sun. And that means they're going to change their distance. Closest approach is perihelion. And the greatest distance away is called apheleon. So that concludes this special topic on apheleon and perihelion. We'll be back again next time for another special topic in astronomy. So until then, have a great day, everyone. And I will see you in class.