hd2pa.c

#include "erfa.h"

double eraHd2pa (double ha, double dec, double phi)
/*
**  - - - - - - - - -
**   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 2023-10-11
**
**  Copyright (C) 2023 IAU ERFA Board.  See notes at end.
*/
{
   double cp, cqsz, sqsz;


   cp = cos(phi);
   sqsz = cp*sin(ha);
   cqsz = sin(phi)*cos(dec) - cp*sin(dec)*cos(ha);
   return ( ( sqsz != 0.0 || cqsz != 0.0 ) ? atan2(sqsz,cqsz) : 0.0 );

/* Finished. */

}
/*----------------------------------------------------------------------
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