SLA_BEAR

Direction Between Points on a Sphere

ACTION:
Returns the bearing (position angle) of one point on a sphere seen from another (single precision).
CALL:
R = sla_BEAR (A1, B1, A2, B2)
GIVEN:

A1,B1

R

spherical coordinates of one point

A2,B2

R

spherical coordinates of the other point


RETURNED:

sla_BEAR

R

bearing from first point to second


NOTES:
(1)
The spherical coordinates are [α, δ], [λ, ϕ] 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 ±π. The sense is such that if [A2,B2] is a small distance due east of [A1,B1] the result is about + π/2. Zero is returned if the two points are coincident.
(3)
If either B-coordinate is outside the range ±π/2, the result may correspond to “the long way round”.
(4)
The routine sla_PAV performs an equivalent function except that the points are specified in the form of Cartesian unit vectors.