%N prerotat %F $MIR/src/subs/ephem.for
%D Determine rotation contribution of precession.
%P rjs
%: utilities
%B
        subroutine prerotat(jday,ra,dec,jout,theta)

        implicit none
        double precision jday,ra,dec,jout,theta

  A simple routine to determine the rotation of the coordinate
  system between the mean equatorial coordinates (ra,dec) at "jday",
  and the apparent coordinates as "jout".

  Input:
    jday       The Julian day of the reference time.
    ra,dec     Mean RA and DEC (radians) at the reference time.
    jout       The Julian day of the new date.
  Output:
    theta      The rotation of the coordinate system at time "jout"
               with respect to that at time "jday" (at (ra,dec)).
