Bearing (position angle) of one point on a sphere relative to another. (double precision)
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. 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 31 October 1993