SLA_DPAV - Position-Angle Between Two Directions

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

position-angle of 2nd point with respect to 1st

NOTES:

1. The coordinate frames correspond to $[ \alpha,\delta ]$, $[\lambda,\phi]$ 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.