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 [α, δ], [λ, ϕ] etc..
(2)
The result is the bearing (position angle), in radians, of point V2 as seen from point V1. It is in the range ±π. The sense is such that if V2 is a small distance due east of V1 the result is about + π/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.