A measure of the degree to which the shape of a planet, or other body, deviates from a perfect sphere. Planets and stars that are rotating tend to bulge at their equators to a degree that increases with speed of rotation and also depends on whether the body is solid or fluid. The shape taken up by the body is described as an oblate spheroid. A cross-section through the body that cuts through both poles is elliptical. The semiminor axis of the ellipse is the body’s polar radius, Rp, and the semimajor axis of the ellipse is its equatorial radius, Re. The ellipticity is defined as Re - Rp/ Re.