 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy we are going to talk about retrograde motion and try to understand what that means for motions of planets in the sky. So what is retrograde motion? Well, let's define it first and the definition is the apparent backward motion of a planet relative to the stars. So here we can see an example as a planet moves through the sky so we can watch motion here and it's heading in this direction and then all of a sudden it keeps going and it loops. It turns around and goes backwards and then it starts heading forward again. So this loop is what we call the retrograde loop. Now originally this was thought to be caused by epicycles and that was when we considered the Earth to be the center of the solar system. So we looked at something like this to explain retrograde motion and it works very well. It does explain retrograde motion. You have the Earth at the center here. You have the planet orbiting Earth here in the red and the planet does not orbit around the main orbit here. It orbits on a smaller circle that is an epicycle. Now that epicycle then goes around in a circle here so the planet goes around in circles here while the center of this moves around the Earth and that motion overall gives us that kind of loop in the sky that will occur. So this will account for retrograde motion being visible in the sky so it is a perfectly reasonable explanation for how retrograde motion occurs. However, as we've learned more and more we certainly have learned that Earth is not the center of our solar system and that means we need to find a new method to be able to explain this. So what we see it as now is just that we are passing another object. So the inner planet Earth we see passing the outer planet Mars and as we move along here as Earth goes around in its orbit it passes Mars and then you can see that these lines will appear to go backwards. So this is position 1 for the Earth position 2, 3, 4 and 5 but in the sky that becomes position 1, 2 then it moved backwards 3 and 4 and then it heads forward again in the sky. It is an apparent motion. The planet is not really moving backwards in this so it just makes it look like it's going backwards because we are passing it. So if you've passed an object of car on the highway you can imagine that it looks like it is going backward next to you but in reality it's still moving forward just at a slower speed and that is what we are seeing here with retrograde motion. It is not a planet changing direction. Planets will never change their direction and always orbit in the same direction. So just to emphasize again that is an apparent motion. So let's go ahead and finish up with our summary and what we've looked at today is retrograde motion and what is retrograde motion? Well again it is the apparent backward motion of a planet relative to the stars in the sky. It occurs when an inner planet overtakes the outer planet in its orbit and it is not a true backward motion. Planets always move in only one direction in a circular or elliptical orbit around the sun. So that concludes this lecture on retrograde motion. 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.