erfa.hd2pa(ha, dec, phi)[source]

Parallactic angle for a given hour angle and declination.

hadouble array
decdouble array
phidouble array
c_retvaldouble array


Wraps ERFA function eraHd2pa. The ERFA documentation is:

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 e r a H d 2 p a
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Parallactic angle for a given hour angle and declination.

   ha     double     hour angle
   dec    double     declination
   phi    double     site latitude

Returned (function value):
          double     parallactic angle


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.

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

Last revision:   2017 September 12

ERFA release 2023-10-11

Copyright (C) 2023 IAU ERFA Board.  See notes at end.