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+.help pa Jun99 "Slalib Package"
+.nf
+
+      DOUBLE PRECISION FUNCTION slPA (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
+  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+
+.fi
+.endhelp