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:

RETURNED:

NOTES:

(1)

The coordinate frames correspond to $\left[\alpha ,\delta \right]$, $\left[\lambda ,\varphi \right]$ 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.