l ellipsoid, the limb is always
   an ellipse.  The limb is determined by its center, a semi-major
   axis vector, and a semi-minor axis vector.

   We note that the problem of finding the limb of a triaxial
   ellipsoid is mathematically identical to that of finding its
   terminator, if one makes the simplifying assumption that the
   terminator is the limb of the body as seen from the vertex of the
   umbra.  So, this routine can be used to solve this simplified
   version of the problem of finding the t