s00a.c

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#include "erfa.h"

double eraS00a(double date1, double date2)
/*
**  - - - - - - - -
**   e r a S 0 0 a
**  - - - - - - - -
**
**  The CIO locator s, positioning the Celestial Intermediate Origin on
**  the equator of the Celestial Intermediate Pole, using the IAU 2000A
**  precession-nutation model.
**
**  Given:
**     date1,date2  double    TT as a 2-part Julian Date (Note 1)
**
**  Returned (function value):
**                  double    the CIO locator s in radians (Note 2)
**
**  Notes:
**
**  1) The TT date date1+date2 is a Julian Date, apportioned in any
**     convenient way between the two arguments.  For example,
**     JD(TT)=2450123.7 could be expressed in any of these ways,
**     among others:
**
**            date1          date2
**
**         2450123.7           0.0       (JD method)
**         2451545.0       -1421.3       (J2000 method)
**         2400000.5       50123.2       (MJD method)
**         2450123.5           0.2       (date & time method)
**
**     The JD method is the most natural and convenient to use in
**     cases where the loss of several decimal digits of resolution
**     is acceptable.  The J2000 method is best matched to the way
**     the argument is handled internally and will deliver the
**     optimum resolution.  The MJD method and the date & time methods
**     are both good compromises between resolution and convenience.
**
**  2) The CIO locator s is the difference between the right ascensions
**     of the same point in two systems.  The two systems are the GCRS
**     and the CIP,CIO, and the point is the ascending node of the
**     CIP equator.  The CIO locator s remains a small fraction of
**     1 arcsecond throughout 1900-2100.
**
**  3) The series used to compute s is in fact for s+XY/2, where X and Y
**     are the x and y components of the CIP unit vector;  this series
**     is more compact than a direct series for s would be.  The present
**     function uses the full IAU 2000A nutation model when predicting
**     the CIP position.  Faster results, with no significant loss of
**     accuracy, can be obtained via the function eraS00b, which uses
**     instead the IAU 2000B truncated model.
**
**  Called:
**     eraPnm00a    classical NPB matrix, IAU 2000A
**     eraBnp2xy    extract CIP X,Y from the BPN matrix
**     eraS00       the CIO locator s, given X,Y, IAU 2000A
**
**  References:
**
**     Capitaine, N., Chapront, J., Lambert, S. and Wallace, P.,
**     "Expressions for the Celestial Intermediate Pole and Celestial
**     Ephemeris Origin consistent with the IAU 2000A precession-
**     nutation model", Astron.Astrophys. 400, 1145-1154 (2003)
**
**     n.b. The celestial ephemeris origin (CEO) was renamed "celestial
**          intermediate origin" (CIO) by IAU 2006 Resolution 2.
**
**     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
**     IERS Technical Note No. 32, BKG (2004)
**
**  Copyright (C) 2013-2015, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{
   double rbpn[3][3], x, y, s;

/* Bias-precession-nutation-matrix, IAU 2000A. */
   eraPnm00a(date1, date2, rbpn);

/* Extract the CIP coordinates. */
   eraBpn2xy(rbpn, &x, &y);

/* Compute the CIO locator s, given the CIP coordinates. */
   s = eraS00(date1, date2, x, y);

   return s;

}
/*----------------------------------------------------------------------
**  
**  
**  Copyright (C) 2013-2015, 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.
**  
**  
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**  
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