SLA_DBEAR - Direction Between Points on a Sphere

ACTION:

Returns the bearing (position angle) of one point on a sphere relative to another (double precision).

CALL:

D = sla_DBEAR (A1, B1, A2, B2)

GIVEN:

A1,B1	D	spherical coordinates of one point
A2,B2	D	spherical coordinates of the other point

RETURNED:

sla_DBEAR

D

bearing from first point to second

NOTES:

1. The spherical coordinates are $[ \alpha,\delta ]$, $[\lambda,\phi]$ etc., in radians.
2. The result is the bearing (position angle), in radians, of point [A2,B2] as seen from point [A1,B1]. It is in the range $\pm\pi$. The sense is such that if [A2,B2] is a small distance due east of [A1,B1] the result is about $+\pi/2$. Zero is returned if the two points are coincident.
3. If either B-coordinate is outside the range $\pm\pi/2$, the result may correspond to ``the long way round''.
4. The routine sla_DPAV performs an equivalent function except that the points are specified in the form of Cartesian vectors.