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slalib/bear.f

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+      REAL FUNCTION sla_BEAR (A1, B1, A2, B2)
+*+
+*     - - - - -
+*      B E A R
+*     - - - - -
+*
+*  Bearing (position angle) of one point on a sphere relative to another
+*  (single precision)
+*
+*  Given:
+*     A1,B1    r    spherical coordinates of one point
+*     A2,B2    r    spherical coordinates of the other point
+*
+*  (The spherical coordinates are RA,Dec, Long,Lat etc, in radians.)
+*
+*  The result is the bearing (position angle), in radians, of point
+*  A2,B2 as seen from point A1,B1.  It is in the range +/- pi.  If
+*  A2,B2 is due east of A1,B1 the bearing is +pi/2.  Zero is returned
+*  if the two points are coincident.
+*
+*  P.T.Wallace   Starlink   23 March 1991
+*
+*  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
+
+      REAL A1,B1,A2,B2
+
+      REAL DA,X,Y
+
+
+      DA=A2-A1
+      Y=SIN(DA)*COS(B2)
+      X=SIN(B2)*COS(B1)-COS(B2)*SIN(B1)*COS(DA)
+      IF (X.NE.0.0.OR.Y.NE.0.0) THEN
+         sla_BEAR=ATAN2(Y,X)
+      ELSE
+         sla_BEAR=0.0
+      END IF
+
+      END