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erfasrc/src/sepp.c  view on Meta::CPAN

#include "erfa.h"

double eraSepp(double a[3], double b[3])
/*
**  - - - - - - - -
**   e r a S e p p
**  - - - - - - - -
**
**  Angular separation between two p-vectors.
**
**  Given:
**     a      double[3]    first p-vector (not necessarily unit length)
**     b      double[3]    second p-vector (not necessarily unit length)
**
**  Returned (function value):
**            double       angular separation (radians, always positive)
**
**  Notes:
**
**  1) If either vector is null, a zero result is returned.
**
**  2) The angular separation is most simply formulated in terms of
**     scalar product.  However, this gives poor accuracy for angles
**     near zero and pi.  The present algorithm uses both cross product
**     and dot product, to deliver full accuracy whatever the size of
**     the angle.
**
**  Called:
**     eraPxp       vector product of two p-vectors
**     eraPm        modulus of p-vector
**     eraPdp       scalar product of two p-vectors
**
**  Copyright (C) 2013-2020, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{
   double axb[3], ss, cs, s;


/* Sine of angle between the vectors, multiplied by the two moduli. */
   eraPxp(a, b, axb);
   ss = eraPm(axb);

/* Cosine of the angle, multiplied by the two moduli. */
   cs = eraPdp(a, b);

/* The angle. */
   s = ((ss != 0.0) || (cs != 0.0)) ? atan2(ss, cs) : 0.0;

   return s;

}
/*----------------------------------------------------------------------
**  
**  
**  Copyright (C) 2013-2020, NumFOCUS Foundation.
**  All rights reserved.
**  
**  This library is derived, with permission, from the International
**  Astronomical Union's "Standards of Fundamental Astronomy" library,
**  available from http://www.iausofa.org.
**  
**  The ERFA version is intended to retain identical functionality to
**  the SOFA library, but made distinct through different function and
**  file names, as set out in the SOFA license conditions.  The SOFA
**  original has a role as a reference standard for the IAU and IERS,
**  and consequently redistribution is permitted only in its unaltered
**  state.  The ERFA version is not subject to this restriction and
**  therefore can be included in distributions which do not support the
**  concept of "read only" software.
**  
**  Although the intent is to replicate the SOFA API (other than
**  replacement of prefix names) and results (with the exception of
**  bugs;  any that are discovered will be fixed), SOFA is not
**  responsible for any errors found in this version of the library.
**  
**  If you wish to acknowledge the SOFA heritage, please acknowledge
**  that you are using a library derived from SOFA, rather than SOFA
**  itself.
**  
**  
**  TERMS AND CONDITIONS