conic section: ellipse #6

An ellipse is is a geometric figure (shown here in hot red) formed when a plane intersects a cone (green) but fails to cross the base of the cone i.e. the ellipse is a closed figure unlike a parabola or an hyperbola. An ellipse is like a flattened circle and is a very important figure since it describes the orbits of the planets around the sun. When the interesecting plane lies parallel to the base of the cone then the ellipse becomes equal to a circle (a circle is a special case of an ellipse).