ycenter at
      the epochs et-lt and `et'.

      If correction for stellar aberration is requested, the target
      position is rotated toward the solar system
      barycenter-relative velocity vector of the observer. The
      rotation is computed as follows:

         Let r be the light time corrected vector from the observer
         to the object, and v be the velocity of the observer with
         respect to the solar system barycenter. Let w be the angle
         between them. The aberration angle