### 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 $\left[\phantom{\rule{0.3em}{0ex}}\alpha ,\delta \phantom{\rule{0.3em}{0ex}}\right]$, $\left[\lambda ,\varphi \right]$ 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 $±\pi$. 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 $±\pi /2$, 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.