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# fk5hz

erfa.fk5hz(r5, d5, date1, date2)

  Transform an FK5 (J2000.0) star position into the system of the Hipparcos catalogue, assuming zero Hipparcos proper motion.

  Parameters
    r5 double array
    d5 double array
    date1 double array
    date2 double array
  Returns
    rh double array
    dh double array

  Notes

  Wraps ERFA function `eraFk5hz`. The ERFA documentation is:

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

      Transform an FK5 (J2000.0) star position into the system of the
      Hipparcos catalogue, assuming zero Hipparcos proper motion.

      Given:
         r5           double   FK5 RA (radians), equinox J2000.0, at date
         d5           double   FK5 Dec (radians), equinox J2000.0, at date
         date1,date2  double   TDB date (Notes 1,2)

      Returned:
         rh           double   Hipparcos RA (radians)
         dh           double   Hipparcos Dec (radians)

      Notes:

      1) This function converts a star position from the FK5 system to
         the Hipparcos system, in such a way that the Hipparcos proper
         motion is zero.  Because such a star has, in general, a non-zero
         proper motion in the FK5 system, the function requires the date
         at which the position in the FK5 system was determined.

      2) The TT date date1+date2 is a Julian Date, apportioned in any
         convenient way between the two arguments.  For example,
         JD(TT)=2450123.7 could be expressed in any of these ways,
         among others:

                date1          date2

             2450123.7           0.0       (JD method)
             2451545.0       -1421.3       (J2000 method)
             2400000.5       50123.2       (MJD method)
             2450123.5           0.2       (date & time method)

         The JD method is the most natural and convenient to use in
         cases where the loss of several decimal digits of resolution
         is acceptable.  The J2000 method is best matched to the way
         the argument is handled internally and will deliver the
         optimum resolution.  The MJD method and the date & time methods
         are both good compromises between resolution and convenience.

      3) The FK5 to Hipparcos transformation is modeled as a pure
         rotation and spin;  zonal errors in the FK5 catalogue are not
         taken into account.

      4) The position returned by this function is in the Hipparcos
         reference system but at date date1+date2.

      5) See also eraFk52h, eraH2fk5, eraHfk5z.

      Called:
         eraS2c       spherical coordinates to unit vector
         eraFk5hip    FK5 to Hipparcos rotation and spin
         eraSxp       multiply p-vector by scalar
         eraRv2m      r-vector to r-matrix
         eraTrxp      product of transpose of r-matrix and p-vector
         eraPxp       vector product of two p-vectors
         eraC2s       p-vector to spherical
         eraAnp       normalize angle into range 0 to 2pi

      Reference:

         F.Mignard & M.Froeschle, 2000, Astron.Astrophys. 354, 732-739.

      This revision:  2021 May 11

      Copyright (C) 2013-2021, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.