An ellipse is a trajectory where the sum of distance between a moving point and two fixed points on a plane is constant and this constant is greater than the straight line distance between two points. It can also be defined as a trajectory where the ratio of distance to a fixed point and distance to a fixed straight line is less than one constant point. It is one kind of conic curve, i.e. conic and plane transversal. Ellipses play an important role in Kepler's three laws of planetary motion, which states that a star is one of the two focal points of an ellipse.