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 spherical coordinates of one point

A2,B2 D spherical coordinates of the other point

RETURNED:

sla_DBEAR 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 unit vectors.