
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 $\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 Bcoordinate 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.