An ellipse is a planar curve. All points on the curve are located around two focal points, such that the sum of the distances to each focal point is equal for all points.

Special cases of an ellipse are a circle (in this special case, both focal points are at the same location) and a straight line joining the two focal points (in this special case, the specific distance chosen is equal to the distance between the two focal points).

As far as gravity is concerned, ellipses are of interest as the orbit of a lonely planet around a central star is an ellipse, according to Newton’s theory of gravity.