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