an ellipse is the trajectory of a point on a plane where the sum of the distances from a moving point to two fixed points is a constant value and this constant value is greater than the straight-line distance between the two points. it can also be defined as the ratio of the distance to the fixed point to the distance to the fixed straight line is less than 1 the locus of the point of constant value. it is a type of conic section, which is the section line between a cone and a plane. ellipses play an important role in kepler's three laws of planetary motion, which states that a star is one of the two foci of an ellipse.