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# hfk5z

erfa.hfk5z(rh, dh, date1, date2)

  Transform a Hipparcos star position into FK5 J2000.0, assuming zero Hipparcos proper motion.

  Parameters
    rh double array
    dh double array
    date1 double array
    date2 double array
  Returns
    r5 double array
    d5 double array
    dr5 double array
    dd5 double array

  Notes

  Wraps ERFA function `eraHfk5z`. The ERFA documentation is:

      - - - - - - - - -
       e r a H f k 5 z
      - - - - - - - - -

      Transform a Hipparcos star position into FK5 J2000.0, assuming
      zero Hipparcos proper motion.

      Given:
         rh            double    Hipparcos RA (radians)
         dh            double    Hipparcos Dec (radians)
         date1,date2   double    TDB date (Note 1)

      Returned (all FK5, equinox J2000.0, date date1+date2):
         r5            double    RA (radians)
         d5            double    Dec (radians)
         dr5           double    FK5 RA proper motion (rad/year, Note 4)
         dd5           double    Dec proper motion (rad/year, Note 4)

      Notes:

      1) 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.

      2) The proper motion in RA is dRA/dt rather than cos(Dec)*dRA/dt.

      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) It was the intention that Hipparcos should be a close
         approximation to an inertial frame, so that distant objects have
         zero proper motion;  such objects have (in general) non-zero
         proper motion in FK5, and this function returns those fictitious
         proper motions.

      5) The position returned by this function is in the FK5 J2000.0
         reference system but at date date1+date2.

      6) See also eraFk52h, eraH2fk5, eraFk5zhz.

      Called:
         eraS2c       spherical coordinates to unit vector
         eraFk5hip    FK5 to Hipparcos rotation and spin
         eraRxp       product of r-matrix and p-vector
         eraSxp       multiply p-vector by scalar
         eraRxr       product of two r-matrices
         eraTrxp      product of transpose of r-matrix and p-vector
         eraPxp       vector product of two p-vectors
         eraPv2s      pv-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.