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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
*-
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
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