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# hd2pa

erfa.hd2pa(ha, dec, phi)

  Parallactic angle for a given hour angle and declination.

  Parameters
    ha double array
    dec double array
    phi double array
  Returns
    c_retval double array

  Notes

  Wraps ERFA function `eraHd2pa`. The ERFA documentation is:

      - - - - - - - - -
       e r a H d 2 p a
      - - - - - - - - -

      Parallactic angle for a given hour angle and declination.

      Given:
         ha     double     hour angle
         dec    double     declination
         phi    double     site latitude

      Returned (function value):
                double     parallactic angle

      Notes:

      1)  All the arguments are angles in radians.

      2)  The parallactic angle at a point in the sky is the position
          angle of the vertical, i.e. the angle between the directions to
          the north celestial pole and to the zenith respectively.

      3)  The result is returned in the range -pi to +pi.

      4)  At the pole itself a zero result is returned.

      5)  The latitude phi is pi/2 minus the angle between the Earth's
          rotation axis and the adopted zenith.  In many applications it
          will be sufficient to use the published geodetic latitude of the
          site.  In very precise (sub-arcsecond) applications, phi can be
          corrected for polar motion.

      6)  Should the user wish to work with respect to the astronomical
          zenith rather than the geodetic zenith, phi will need to be
          adjusted for deflection of the vertical (often tens of
          arcseconds), and the zero point of the hour angle ha will also
          be affected.

      Reference:
         Smart, W.M., "Spherical Astronomy", Cambridge University Press,
         6th edition (Green, 1977), p49.

      Last revision:   2017 September 12

      ERFA release 2021-05-12

      Copyright (C) 2021 IAU ERFA Board.  See notes at end.