e evaluated for the correct 
      epoch. 
 
 
      Target body -- Sun vector 
      ------------------------- 
 
      All surface features on the target body will appear in a
      measurement made at `et' as they were at the target at epoch
      et-lt.  In particular, lighting on the target body is dependent
      on the apparent location of the Sun as seen from the target body
      at et-lt.  So, a second light time correction is used in finding
      the apparent location of the Sun.

 
   Stellar a