Ellipse

An ellipse is a two-dimensional curve that has two points called foci such that the sum of the distance from each foci to a point on the ellipse is constant. An example of an ellipse in astronomy is the orbits of planets. Each planet or asteroid orbits the sun along an elypictal path. In the case of the Earth-Sun system one of the foci is the center of mass between the Earth and the Sun and the other point in the ellipse is close by. When an ellipse has to foci which are close together it is nearly circular. A measure of how close an ellipse is to a circle is eccentricity.

External Links

Definition of an ellipse from [mathworld]