# radius vector

The radius vector of a planet in a highly elliptical orbit around a star.

The radius vector is the line joining an orbiting body to its center of motion at any instant, directed radially outward. For a circular orbit, the center of motion coincides with the center of the circle; for a parabolic or hyperbolic orbit, the center of motion is the focus, and for an elliptical orbit it is one of the two foci.