
ACTION:
 Returns the bearing (position angle) of one celestial direction with respect to another
(double precision).

CALL:
D = sla_DPAV (V1, V2)
GIVEN:
V1  D(3)  vector to one point 

V2  D(3)  vector to the other point 

RETURNED:
sla_DPAV  D  positionangle of 2nd point with respect to 1st 


NOTES:


(1)
 The coordinate frames correspond to $\left[\phantom{\rule{0.3em}{0ex}}\alpha ,\delta \phantom{\rule{0.3em}{0ex}}\right]$,
$\left[\lambda ,\varphi \right]$
etc..

(2)
 The result is the bearing (position angle), in radians, of point V2 as seen from point
V1. It is in the range $\pm \pi $.
The sense is such that if V2 is a small distance due east of V1 the result is about
$+\pi /2$.
Zero is returned if the two points are coincident.

(3)
 There is no requirement for either vector to be of unit length.

(4)
 The routine sla_DBEAR performs an equivalent function except that the points are
specified in the form of spherical coordinates.