#include "slalib.h"
#include "slamac.h"
double slaDbear ( double a1, double b1, double a2, double b2 )
/*
**  - - - - - - - - -
**   s l a D b e a r
**  - - - - - - - - -
**
**  Bearing (position angle) of one point on a sphere relative
**  to another.
**
**  (double precision)
**
**  Given:
**     a1,b1    double    spherical coordinates of one point
**     a2,b2    double    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.  The
**  sense is such that if a2,b2 is a small distance east of a1,b1,
**  the bearing is about +pi/2.  Zero is returned if the two points
**  are coincident.
**
**  If either b-coordinate is outside the range +/- pi/2, the
**  result may correspond to "the long way round".
**
**  The routine slaDpav performs an equivalent function except
**  that the points are specified in the form of Cartesian unit
**  vectors.
**
**  Last revision:   8 December 1996
**
**  Copyright P.T.Wallace.  All rights reserved.
*/
{
   double da, x, y;

   da = a2 - a1;
   y = sin ( da ) * cos ( b2 );
   x = sin ( b2 ) * cos ( b1 ) - cos ( b2 ) * sin ( b1 ) * cos ( da );
   return ( x != 0.0 || y != 0.0 ) ? atan2 ( y, x ) : 0.0;
}