A spheroid is a surface in three dimensions obtained by rotating an ellipse about one of its principal axes. If the ellipse is rotated about its major axis, the surface is called a prolate spheroid (similar to the shape of a rugby ball). If the minor axis is chosen, the surface is called an oblate spheroid (similar to the shape of the Earth). The sphere is a special case of the spheroid in which the generating ellipse is a circle. A spheroid is a special case of an ellipsoid where two of the three major axes are equal.