pap¶

Position-angle from two p-vectors.

Parameters:

adouble array
bdouble array

Returns:

c_retvaldouble array

Notes

Wraps ERFA function eraPap. The ERFA documentation is:

- - - - - - -
 e r a P a p
- - - - - - -

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) 2013-2023, NumFOCUS Foundation.
Derived, with permission, from the SOFA library.  See notes at end of file.

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