dence angle at `spoint', as seen from
               `obsrvr' at time `et'.  This is the angle between the
               surface normal vector at `spoint' and the spoint-sun
               vector.  Units are radians.  The range of `solar' is [0,
               pi].
 
               Note that if the target surface is non-convex, a solar
               incidence angle less than pi/2 radians does not imply
               the surface point is illuminated. See the description of
               `lit' below.


  