  / 
                           .   |  e. \ / 
       *             &lt;--------------- *  surface point on 
    viewing            vector            target body 
    location           to viewing 
    (observer)         location 
 

   Note that if the target-observer vector, the target normal vector
   at the surface point, and the target-sun vector are coplanar, then
   phase is the sum of incidence and emission.  This is rarely true;
   usually 

      phase angle  &lt;  solar incidence angle + emission