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DOUBLE PRECISION FUNCTION sla_DBEAR (A1, B1, A2, B2)
*+
* - - - - - -
* D B E A R
* - - - - - -
*
* Bearing (position angle) of one point on a sphere relative to another
* (double precision)
*
* Given:
* A1,B1 d spherical coordinates of one point
* A2,B2 d 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
*-
IMPLICIT NONE
DOUBLE PRECISION A1,B1,A2,B2
DOUBLE PRECISION 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.0D0.OR.Y.NE.0D0) THEN
sla_DBEAR=ATAN2(Y,X)
ELSE
sla_DBEAR=0D0
END IF
END
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