 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy we are going to talk about orbital eccentricity and what that means for orbits both in our solar system and beyond. So what do we mean by eccentricity when we talk about that in terms of an orbit? Well the eccentricity is a measure of how flattened the orbit is. So an eccentricity of zero is a circle. So here we have an eccentricity of zero and we go up by tenths here and then even less to point nine five here the closer we get to one the more flattened this circle becomes. So it's a circle here and then we have various ellipses that are greatly greater or lesser amounts flattened by the amount of the eccentricity. So we can have again with an orbital planet orbit within the solar system if it was perfectly circular you'd have an eccentricity of zero. Well that's not going to happen you're always going to have some very slight eccentricity. The Earth's eccentricity is a little bit greater than zero it's got a slightly. So the amount of eccentricity between zero and one tells you how flattened that ellipse is. Now this was given to us by Johannes Kepler who determined that the orbits of the planets were not circles but were actually ellipses. Isaac Newton was able to modify that and improve upon it and showed us that there are other possibilities for these as well and let's take a look at these and what we have is that there also are not only circles shown here in red an example of an elliptical orbit in green but you can also have unbound orbits you can have an eccentricity of one which is a parabolic orbit just barely escaping outward so the object comes in and then escapes it is not a closed orbit where the object would return. You can also have an eccentricity greater than one which is a hyperbolic orbit shown here in the purple. So we can see those different types of orbits and this is how Isaac Newton generalized this in that the orbits are not just ellipses but are actually conic sections. So what is a conic section? Well you imagine a plane being sliced through a cone and the different things that you can get would be a circle and we'll see an example of that that's where this starts when it's come the plane is flat and actually parallel to the base you get a circle as you start to tilt that you get ellipses of various sizes so let's go ahead and take a look at the ellipses here so now we're getting ellipse and it's getting more and more flattened as we stretch it down eventually it'll break the bottom becoming parabolic and then finally continue as we tilt downward and become a hyperbolic orbit so any orbits that we find not just in the solar system comets sometimes have parabolic or hyperbolic orbits but anywhere else in space would also be one of these types of orbits possibly perfectly circular very difficult to get that exactly equal to zero generally you would find that the orbits would be either elliptical or hyperbolic since you need exact numbers to get a circular or a parabolic orbit so again that applies not just to the solar system but to any object orbiting in the universe so let's go ahead and finish up with our summary and what we've looked at is the eccentricity of an orbit describe how much it deviates from a perfect circle eccentricity of zero is a circle an eccentricity of less than one would be a closed orbit if it's zero at circular anything greater than zero but less than one would be elliptical and an eccentricity greater than or equal to one would be an open orbit and that would be an orbit that does not return to the same to the object so something that comes in and visits us on once and never comes back again like some comets equal to one parabolic orbit greater than one hyperbolic orbit so that concludes this lecture on orbital eccentricity 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