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### Einstein Online is provided by the # ellipse

A geometrical figure. Choose a plane and, within this plane, two different points (each is called a focus of the ellipse). Also, choose a specific distance value larger or equal to the distance between the two chosen points. The ellipse is the collection of all the points in the plane for which the following holds true: If we measure the distance of such a point to the first focus and the distance to the second focus, and if we add up the two values, then the result is the distance value chosen in advance.

Special cases of an ellipse are a circle (in this special case, both focus 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 aroudn a central star is an ellipse, according to Newton's theory of gravity. 