ellipse

Definition: ellipse: A flattened circle or oval. That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Kepler based on the careful observations by Tycho Brahe. An ellipse is essentially a circle that has been stretched out of shape. When describing ellipses, the eccentricity defines how "stretched out" it is. In the case of the moon (diagram), when it is closest it is at perigee and when it is farthest from Earth is is at apogee.