Description: Learn how the number Pi allows us to relate the radius, diameter, and circumference of a circle.Created by Sal Khan. So the first thing when people kind of discovered the circle, and you just have a look at the moon to see a circle, but the first time they said well, what are the properties of any circle? So the first one they might want to say is well, a circle is all of the points that are equal distant from the center of the circle. All of these points along the edge are equal distant from that center right there.*Teacher Advisor is 100% free.