Angle between two points on a sphere. (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 angle, in radians, between the two points. It is always positive.
slaDcs2c P.T.Wallace Starlink 31 October 1993