 observer.
 
      phase angle              Angle between the vectors from the
                               surface point to the observing body and
                               from the surface point to the Sun.
 
 
   The diagram below illustrates the geometric relationships defining
   these angles.  The labels for the solar incidence, emission, and
   phase angles are &quot;s.i.&quot;, &quot;e.&quot;, and &quot;phase&quot;.
 
 
                                                    * 
                  