%N precess %F $MIR/src/subs/ephem.for
%D Precess from one mean RA,DEC to another.
%P rjs
%: utilities
%B
        subroutine precess(jday1,ra1,dec1,jday2,ra2,dec2)

        implicit none
        double precision jday1,ra1,dec1,jday2,ra2,dec2

  A simple precession routine, to precess from one set of mean
  equatorial coordinates (RA,DEC), to another at a different epoch.
  This is accurate to order 0.3 arcsec over 50 years.

  Reference:
    Explanatory Supplement to the Astronomical Almanac, 1993. p 105-106.

  NOTE: This does not take account of atmospheric refraction,
  nutation, aberration nor gravitational deflection.

  Input:
    jday1      Julian day of the known epoch.
    ra1,dec1   RA,DEC at the jday1 epoch (radians).
    jday2      Julian day of the new epoch.
  Output:
    ra2,dec2   Precessed coordinates (radians).
