era.pap.md

eraPap

rv = ERFA.pap(a, b)

Position-angle from two p-vectors.

Given:

   a      double[3]  direction of reference point
   b      double[3]  direction of point whose PA is required

Returned (function value):

          double     position angle of b with respect to a (radians)

Notes:

1. The result is the position angle, in radians, of direction b with respect to direction a. It is in the range -pi to +pi. The sense is such that if b is a small distance "north" of a the position angle is approximately zero, and if b is a small distance "east" of a the position angle is approximately +pi/2.

2. The vectors a and b need not be of unit length.

3. Zero is returned if the two directions are the same or if either vector is null.

4. If vector a is at a pole, the result is ill-defined.

Called:

   eraPn        decompose p-vector into modulus and direction
   eraPm        modulus of p-vector
   eraPxp       vector product of two p-vectors
   eraPmp       p-vector minus p-vector
   eraPdp       scalar product of two p-vectors

This revision: 2021 May 11

Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.