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# fk54z

erfa.fk54z(r2000, d2000, bepoch)

  Convert a J2000.0 FK5 star position to B1950.0 FK4, assuming zero proper motion in FK5 and parallax.

  Parameters:
    r2000 double array
    d2000 double array
    bepoch double array
  Returns:
    r1950 double array
    d1950 double array
    dr1950 double array
    dd1950 double array

  Notes

  Wraps ERFA function `eraFk54z`. The ERFA documentation is:

      - - - - - - - - -
       e r a F k 5 4 z
      - - - - - - - - -

      Convert a J2000.0 FK5 star position to B1950.0 FK4, assuming zero
      proper motion in FK5 and parallax.

      Given:
         r2000,d2000    double   J2000.0 FK5 RA,Dec (rad)
         bepoch         double   Besselian epoch (e.g. 1950.0)

      Returned:
         r1950,d1950    double   B1950.0 FK4 RA,Dec (rad) at epoch BEPOCH
         dr1950,dd1950  double   B1950.0 FK4 proper motions (rad/trop.yr)

      Notes:

      1) In contrast to the eraFk524 function, here the FK5 proper
         motions, the parallax and the radial velocity are presumed zero.

      2) This function converts a star position from the IAU 1976 FK5
        (Fricke) system to the former FK4 (Bessel-Newcomb) system, for
         cases such as distant radio sources where it is presumed there is
         zero parallax and no proper motion.  Because of the E-terms of
         aberration, such objects have (in general) non-zero proper motion
         in FK4, and the present function returns those fictitious proper
         motions.

      3) Conversion from J2000.0 FK5 to B1950.0 FK4 only is provided for.
         Conversions involving other equinoxes would require additional
         treatment for precession.

      4) The position returned by this function is in the B1950.0 FK4
         reference system but at Besselian epoch bepoch.  For comparison
         with catalogs the bepoch argument will frequently be 1950.0. (In
         this context the distinction between Besselian and Julian epoch
         is insignificant.)

      5) The RA component of the returned (fictitious) proper motion is
         dRA/dt rather than cos(Dec)*dRA/dt.

      Called:
         eraAnp       normalize angle into range 0 to 2pi
         eraC2s       p-vector to spherical
         eraFk524     FK4 to FK5
         eraS2c       spherical to p-vector

      This revision:   2023 March 5

      Copyright (C) 2013-2023, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.