SLA_DBEAR

Direction Between Points on a Sphere

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:

RETURNED:

NOTES:

(1)

The spherical coordinates are $\left[\alpha ,\delta \right]$, $\left[\lambda ,\varphi \right]$ 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 B-coordinate 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.