The eccentricity of an ellipse is a measure of how nearly circular the ellipse. Eccentricity is found by the following formula eccentricity = c/a where c is the distance from the center to the focus of the ellipse a is the distance from the center to a vertex

**Formula** for the Eccentricity of an Ellipse

#### The special case of a circle's eccentricity

A circle is a special case of an ellipse. Analogous to the fact that a square is a kind of rectangle, a circle is a special case of an ellipse. In terms of the eccentricity, a circle is an ellipse in which the eccentricity is zero.