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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
*
* 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
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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