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slalib/pa.f

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+      DOUBLE PRECISION FUNCTION sla_PA (HA, DEC, PHI)
+*+
+*     - - -
+*      P A
+*     - - -
+*
+*  HA, Dec to Parallactic Angle (double precision)
+*
+*  Given:
+*     HA     d     hour angle in radians (geocentric apparent)
+*     DEC    d     declination in radians (geocentric apparent)
+*     PHI    d     observatory latitude in radians (geodetic)
+*
+*  The result is in the range -pi to +pi
+*
+*  Notes:
+*
+*  1)  The parallactic angle at a point in the sky is the position
+*      angle of the vertical, i.e. the angle between the direction to
+*      the pole and to the zenith.  In precise applications care must
+*      be taken only to use geocentric apparent HA,Dec and to consider
+*      separately the effects of atmospheric refraction and telescope
+*      mount errors.
+*
+*  2)  At the pole a zero result is returned.
+*
+*  P.T.Wallace   Starlink   16 August 1994
+*
+*  Copyright (C) 1995 Rutherford Appleton Laboratory
+*
+*  License:
+*    This program is free software; you can redistribute it and/or modify
+*    it under the terms of the GNU General Public License as published by
+*    the Free Software Foundation; either version 2 of the License, or
+*    (at your option) any later version.
+*
+*    This program is distributed in the hope that it will be useful,
+*    but WITHOUT ANY WARRANTY; without even the implied warranty of
+*    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*    GNU General Public License for more details.
+*
+*    You should have received a copy of the GNU General Public License
+*    along with this program (see SLA_CONDITIONS); if not, write to the 
+*    Free Software Foundation, Inc., 59 Temple Place, Suite 330, 
+*    Boston, MA  02111-1307  USA
+*
+*-
+
+      IMPLICIT NONE
+
+      DOUBLE PRECISION HA,DEC,PHI
+
+      DOUBLE PRECISION CP,SQSZ,CQSZ
+
+
+
+      CP=COS(PHI)
+      SQSZ=CP*SIN(HA)
+      CQSZ=SIN(PHI)*COS(DEC)-CP*SIN(DEC)*COS(HA)
+      IF (SQSZ.EQ.0D0.AND.CQSZ.EQ.0D0) CQSZ=1D0
+      sla_PA=ATAN2(SQSZ,CQSZ)
+
+      END