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#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 2021-05-12
**
** Copyright (C) 2021 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. */
}
/*----------------------------------------------------------------------
**
**
** Copyright (C) 2013-2021, NumFOCUS Foundation.
** All rights reserved.
**
** This library is derived, with permission, from the International
** Astronomical Union's "Standards of Fundamental Astronomy" library,
** available from http://www.iausofa.org.
**
** The ERFA version is intended to retain identical functionality to
** the SOFA library, but made distinct through different function and
** file names, as set out in the SOFA license conditions. The SOFA
** original has a role as a reference standard for the IAU and IERS,
** and consequently redistribution is permitted only in its unaltered
** state. The ERFA version is not subject to this restriction and
** therefore can be included in distributions which do not support the
** concept of "read only" software.
**
** Although the intent is to replicate the SOFA API (other than
** replacement of prefix names) and results (with the exception of
** bugs; any that are discovered will be fixed), SOFA is not
** responsible for any errors found in this version of the library.
**
** If you wish to acknowledge the SOFA heritage, please acknowledge
** that you are using a library derived from SOFA, rather than SOFA
** itself.
**
**
** TERMS AND CONDITIONS
**
** Redistribution and use in source and binary forms, with or without
** modification, are permitted provided that the following conditions
** are met:
**
** 1 Redistributions of source code must retain the above copyright
** notice, this list of conditions and the following disclaimer.
**
** 2 Redistributions in binary form must reproduce the above copyright
** notice, this list of conditions and the following disclaimer in
** the documentation and/or other materials provided with the
** distribution.
**
** 3 Neither the name of the Standards Of Fundamental Astronomy Board,
** the International Astronomical Union nor the names of its
** contributors may be used to endorse or promote products derived
** from this software without specific prior written permission.
**
** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
** POSSIBILITY OF SUCH DAMAGE.
**
*/