text: #include "sofa.h"
  
  double iauS06(double date1, double date2, double x, double y)
  /*
  **  - - - - - - -
  **   i a u S 0 6
  **  - - - - - - -
  **
  **  The CIO locator s, positioning the Celestial Intermediate Origin on
  **  the equator of the Celestial Intermediate Pole, given the CIP's X,Y
  **  coordinates.  Compatible with IAU 2006/2000A precession-nutation.
  **
  **  This function is part of the International Astronomical Union's
  **  SOFA (Standards Of Fundamental Astronomy) software collection.
  **
  **  Status:  canonical model.
  **
  **  Given:
  **     date1,date2   double    TT as a 2-part Julian Date (Note 1)
  **     x,y           double    CIP coordinates (Note 3)
  **
  **  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 quantity s remains below 0.1 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.  This
  **     function requires X,Y to be supplied by the caller, who is
  **     responsible for providing values that are consistent with the
  **     supplied date.
  **
  **  4) The model is consistent with the "P03" precession (Capitaine et
  **     al. 2003), adopted by IAU 2006 Resolution 1, 2006, and the
  **     IAU 2000A nutation (with P03 adjustments).
  **
  **  Called:
  **     iauFal03     mean anomaly of the Moon
  **     iauFalp03    mean anomaly of the Sun
  **     iauFaf03     mean argument of the latitude of the Moon
  **     iauFad03     mean elongation of the Moon from the Sun
  **     iauFaom03    mean longitude of the Moon's ascending node
  **     iauFave03    mean longitude of Venus
  **     iauFae03     mean longitude of Earth
  **     iauFapa03    general accumulated precession in longitude
  **
  **  References:
  **
  **     Capitaine, N., Wallace, P.T. & Chapront, J., 2003, Astron.
  **     Astrophys. 432, 355
  **
  **     McCarthy, D.D., Petit, G. (eds.) 2004, IERS Conventions (2003),
  **     IERS Technical Note No. 32, BKG
  **
  **  This revision:  2019 June 23
  **
  **  SOFA release 2021-01-25
  **
  **  Copyright (C) 2021 IAU SOFA Board.  See notes at end.
  */
  {
  /* Time since J2000.0, in Julian centuries */
     double t;
  
  /* Miscellaneous */
     int i, j;
     double a, w0, w1, w2, w3, w4, w5;
  
  /* Fundamental arguments */
     double fa[8];
  
  /* Returned value */
     double s;
  
  /* --------------------- */
  /* The series for s+XY/2 */
  /* --------------------- */
  
     typedef struct {
        int nfa[8];      /* coefficients of l,l',F,D,Om,LVe,LE,pA */
        double s, c;     /* sine and cosine coefficients */
     } TERM;
  
  /* Polynomial coefficients */
     static const double sp[] = {
  
     /* 1-6 */
            94.00e-6,
          3808.65e-6,
          -122.68e-6,
        -72574.11e-6,
            27.98e-6,
            15.62e-6
     };
  
  /* Terms of order t^0 */
     static const TERM s0[] = {
  
     /* 1-10 */
        {{ 0,  0,  0,  0,  1,  0,  0,  0}, -2640.73e-6,   0.39e-6 },
        {{ 0,  0,  0,  0,  2,  0,  0,  0},   -63.53e-6,   0.02e-6 },
        {{ 0,  0,  2, -2,  3,  0,  0,  0},   -11.75e-6,  -0.01e-6 },
        {{ 0,  0,  2, -2,  1,  0,  0,  0},   -11.21e-6,  -0.01e-6 },
        {{ 0,  0,  2, -2,  2,  0,  0,  0},     4.57e-6,   0.00e-6 },
        {{ 0,  0,  2,  0,  3,  0,  0,  0},    -2.02e-6,   0.00e-6 },
        {{ 0,  0,  2,  0,  1,  0,  0,  0},    -1.98e-6,   0.00e-6 },
        {{ 0,  0,  0,  0,  3,  0,  0,  0},     1.72e-6,   0.00e-6 },
        {{ 0,  1,  0,  0,  1,  0,  0,  0},     1.41e-6,   0.01e-6 },
        {{ 0,  1,  0,  0, -1,  0,  0,  0},     1.26e-6,   0.01e-6 },
  
     /* 11-20 */
        {{ 1,  0,  0,  0, -1,  0,  0,  0},     0.63e-6,   0.00e-6 },
        {{ 1,  0,  0,  0,  1,  0,  0,  0},     0.63e-6,   0.00e-6 },
        {{ 0,  1,  2, -2,  3,  0,  0,  0},    -0.46e-6,   0.00e-6 },
        {{ 0,  1,  2, -2,  1,  0,  0,  0},    -0.45e-6,   0.00e-6 },
        {{ 0,  0,  4, -4,  4,  0,  0,  0},    -0.36e-6,   0.00e-6 },
        {{ 0,  0,  1, -1,  1, -8, 12,  0},     0.24e-6,   0.12e-6 },
        {{ 0,  0,  2,  0,  0,  0,  0,  0},    -0.32e-6,   0.00e-6 },
        {{ 0,  0,  2,  0,  2,  0,  0,  0},    -0.28e-6,   0.00e-6 },
        {{ 1,  0,  2,  0,  3,  0,  0,  0},    -0.27e-6,   0.00e-6 },
        {{ 1,  0,  2,  0,  1,  0,  0,  0},    -0.26e-6,   0.00e-6 },
  
     /* 21-30 */
        {{ 0,  0,  2, -2,  0,  0,  0,  0},     0.21e-6,   0.00e-6 },
        {{ 0,  1, -2,  2, -3,  0,  0,  0},    -0.19e-6,   0.00e-6 },
        {{ 0,  1, -2,  2, -1,  0,  0,  0},    -0.18e-6,   0.00e-6 },
        {{ 0,  0,  0,  0,  0,  8,-13, -1},     0.10e-6,  -0.05e-6 },
        {{ 0,  0,  0,  2,  0,  0,  0,  0},    -0.15e-6,   0.00e-6 },
        {{ 2,  0, -2,  0, -1,  0,  0,  0},     0.14e-6,   0.00e-6 },
        {{ 0,  1,  2, -2,  2,  0,  0,  0},     0.14e-6,   0.00e-6 },
        {{ 1,  0,  0, -2,  1,  0,  0,  0},    -0.14e-6,   0.00e-6 },
        {{ 1,  0,  0, -2, -1,  0,  0,  0},    -0.14e-6,   0.00e-6 },
        {{ 0,  0,  4, -2,  4,  0,  0,  0},    -0.13e-6,   0.00e-6 },
  
     /* 31-33 */
        {{ 0,  0,  2, -2,  4,  0,  0,  0},     0.11e-6,   0.00e-6 },
        {{ 1,  0, -2,  0, -3,  0,  0,  0},    -0.11e-6,   0.00e-6 },
        {{ 1,  0, -2,  0, -1,  0,  0,  0},    -0.11e-6,   0.00e-6 }
     };
  
  /* Terms of order t^1 */
     static const TERM s1[] = {
  
     /* 1 - 3 */
        {{ 0,  0,  0,  0,  2,  0,  0,  0},    -0.07e-6,   3.57e-6 },
        {{ 0,  0,  0,  0,  1,  0,  0,  0},     1.73e-6,  -0.03e-6 },
        {{ 0,  0,  2, -2,  3,  0,  0,  0},     0.00e-6,   0.48e-6 }
     };
  
  /* Terms of order t^2 */
     static const TERM s2[] = {
  
     /* 1-10 */
        {{ 0,  0,  0,  0,  1,  0,  0,  0},   743.52e-6,  -0.17e-6 },
        {{ 0,  0,  2, -2,  2,  0,  0,  0},    56.91e-6,   0.06e-6 },
        {{ 0,  0,  2,  0,  2,  0,  0,  0},     9.84e-6,  -0.01e-6 },
        {{ 0,  0,  0,  0,  2,  0,  0,  0},    -8.85e-6,   0.01e-6 },
        {{ 0,  1,  0,  0,  0,  0,  0,  0},    -6.38e-6,  -0.05e-6 },
        {{ 1,  0,  0,  0,  0,  0,  0,  0},    -3.07e-6,   0.00e-6 },
        {{ 0,  1,  2, -2,  2,  0,  0,  0},     2.23e-6,   0.00e-6 },
        {{ 0,  0,  2,  0,  1,  0,  0,  0},     1.67e-6,   0.00e-6 },
        {{ 1,  0,  2,  0,  2,  0,  0,  0},     1.30e-6,   0.00e-6 },
        {{ 0,  1, -2,  2, -2,  0,  0,  0},     0.93e-6,   0.00e-6 },
  
     /* 11-20 */
        {{ 1,  0,  0, -2,  0,  0,  0,  0},     0.68e-6,   0.00e-6 },
        {{ 0,  0,  2, -2,  1,  0,  0,  0},    -0.55e-6,   0.00e-6 },
        {{ 1,  0, -2,  0, -2,  0,  0,  0},     0.53e-6,   0.00e-6 },
        {{ 0,  0,  0,  2,  0,  0,  0,  0},    -0.27e-6,   0.00e-6 },
        {{ 1,  0,  0,  0,  1,  0,  0,  0},    -0.27e-6,   0.00e-6 },
        {{ 1,  0, -2, -2, -2,  0,  0,  0},    -0.26e-6,   0.00e-6 },
        {{ 1,  0,  0,  0, -1,  0,  0,  0},    -0.25e-6,   0.00e-6 },
        {{ 1,  0,  2,  0,  1,  0,  0,  0},     0.22e-6,   0.00e-6 },
        {{ 2,  0,  0, -2,  0,  0,  0,  0},    -0.21e-6,   0.00e-6 },
        {{ 2,  0, -2,  0, -1,  0,  0,  0},     0.20e-6,   0.00e-6 },
  
     /* 21-25 */
        {{ 0,  0,  2,  2,  2,  0,  0,  0},     0.17e-6,   0.00e-6 },
        {{ 2,  0,  2,  0,  2,  0,  0,  0},     0.13e-6,   0.00e-6 },
        {{ 2,  0,  0,  0,  0,  0,  0,  0},    -0.13e-6,   0.00e-6 },
        {{ 1,  0,  2, -2,  2,  0,  0,  0},    -0.12e-6,   0.00e-6 },
        {{ 0,  0,  2,  0,  0,  0,  0,  0},    -0.11e-6,   0.00e-6 }
     };
  
  /* Terms of order t^3 */
     static const TERM s3[] = {
  
     /* 1-4 */
        {{ 0,  0,  0,  0,  1,  0,  0,  0},     0.30e-6, -23.42e-6 },
        {{ 0,  0,  2, -2,  2,  0,  0,  0},    -0.03e-6,  -1.46e-6 },
        {{ 0,  0,  2,  0,  2,  0,  0,  0},    -0.01e-6,  -0.25e-6 },
        {{ 0,  0,  0,  0,  2,  0,  0,  0},     0.00e-6,   0.23e-6 }
     };
  
  /* Terms of order t^4 */
     static const TERM s4[] = {
  
     /* 1-1 */
        {{ 0,  0,  0,  0,  1,  0,  0,  0},    -0.26e-6,  -0.01e-6 }
     };
  
  /* Number of terms in the series */
     static const int NS0 = (int) (sizeof s0 / sizeof (TERM));
     static const int NS1 = (int) (sizeof s1 / sizeof (TERM));
     static const int NS2 = (int) (sizeof s2 / sizeof (TERM));
     static const int NS3 = (int) (sizeof s3 / sizeof (TERM));
     static const int NS4 = (int) (sizeof s4 / sizeof (TERM));
  
  /* ------------------------------------------------------------------ */
  
  /* Interval between fundamental epoch J2000.0 and current date (JC). */
     t = ((date1 - DJ00) + date2) / DJC;
  
  /* Fundamental Arguments (from IERS Conventions 2003) */
  
  /* Mean anomaly of the Moon. */
     fa[0] = iauFal03(t);
  
  /* Mean anomaly of the Sun. */
     fa[1] = iauFalp03(t);
  
  /* Mean longitude of the Moon minus that of the ascending node. */
     fa[2] = iauFaf03(t);
  
  /* Mean elongation of the Moon from the Sun. */
     fa[3] = iauFad03(t);
  
  /* Mean longitude of the ascending node of the Moon. */
     fa[4] = iauFaom03(t);
  
  /* Mean longitude of Venus. */
     fa[5] = iauFave03(t);
  
  /* Mean longitude of Earth. */
     fa[6] = iauFae03(t);
  
  /* General precession in longitude. */
     fa[7] = iauFapa03(t);
  
  /* Evaluate s. */
     w0 = sp[0];
     w1 = sp[1];
     w2 = sp[2];
     w3 = sp[3];
     w4 = sp[4];
     w5 = sp[5];
  
     for (i = NS0-1; i >= 0; i--) {
     a = 0.0;
     for (j = 0; j < 8; j++) {
        a += (double)s0[i].nfa[j] * fa[j];
     }
     w0 += s0[i].s * sin(a) + s0[i].c * cos(a);
     }
  
     for (i = NS1-1; i >= 0; i--) {
        a = 0.0;
        for (j = 0; j < 8; j++) {
           a += (double)s1[i].nfa[j] * fa[j];
        }
        w1 += s1[i].s * sin(a) + s1[i].c * cos(a);
     }
  
     for (i = NS2-1; i >= 0; i--) {
        a = 0.0;
        for (j = 0; j < 8; j++) {
           a += (double)s2[i].nfa[j] * fa[j];
        }
        w2 += s2[i].s * sin(a) + s2[i].c * cos(a);
     }
  
     for (i = NS3-1; i >= 0; i--) {
        a = 0.0;
        for (j = 0; j < 8; j++) {
           a += (double)s3[i].nfa[j] * fa[j];
        }
        w3 += s3[i].s * sin(a) + s3[i].c * cos(a);
     }
  
     for (i = NS4-1; i >= 0; i--) {
        a = 0.0;
        for (j = 0; j < 8; j++) {
           a += (double)s4[i].nfa[j] * fa[j];
        }
        w4 += s4[i].s * sin(a) + s4[i].c * cos(a);
     }
  
     s = (w0 +
         (w1 +
         (w2 +
         (w3 +
         (w4 +
          w5 * t) * t) * t) * t) * t) * DAS2R - x*y/2.0;
  
     return s;
  
  /*----------------------------------------------------------------------
  **
  **  Copyright (C) 2021
  **  Standards Of Fundamental Astronomy Board
  **  of the International Astronomical Union.
  **
  **  =====================
  **  SOFA Software License
  **  =====================
  **
  **  NOTICE TO USER:
  **
  **  BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND
  **  CONDITIONS WHICH APPLY TO ITS USE.
  **
  **  1. The Software is owned by the IAU SOFA Board ("SOFA").
  **
  **  2. Permission is granted to anyone to use the SOFA software for any
  **     purpose, including commercial applications, free of charge and
  **     without payment of royalties, subject to the conditions and
  **     restrictions listed below.
  **
  **  3. You (the user) may copy and distribute SOFA source code to others,
  **     and use and adapt its code and algorithms in your own software,
  **     on a world-wide, royalty-free basis.  That portion of your
  **     distribution that does not consist of intact and unchanged copies
  **     of SOFA source code files is a "derived work" that must comply
  **     with the following requirements:
  **
  **     a) Your work shall be marked or carry a statement that it
  **        (i) uses routines and computations derived by you from
  **        software provided by SOFA under license to you; and
  **        (ii) does not itself constitute software provided by and/or
  **        endorsed by SOFA.
  **
  **     b) The source code of your derived work must contain descriptions
  **        of how the derived work is based upon, contains and/or differs
  **        from the original SOFA software.
  **
  **     c) The names of all routines in your derived work shall not
  **        include the prefix "iau" or "sofa" or trivial modifications
  **        thereof such as changes of case.
  **
  **     d) The origin of the SOFA components of your derived work must
  **        not be misrepresented;  you must not claim that you wrote the
  **        original software, nor file a patent application for SOFA
  **        software or algorithms embedded in the SOFA software.
  **
  **     e) These requirements must be reproduced intact in any source
  **        distribution and shall apply to anyone to whom you have
  **        granted a further right to modify the source code of your
  **        derived work.
  **
  **     Note that, as originally distributed, the SOFA software is
  **     intended to be a definitive implementation of the IAU standards,
  **     and consequently third-party modifications are discouraged.  All
  **     variations, no matter how minor, must be explicitly marked as
  **     such, as explained above.
  **
  **  4. You shall not cause the SOFA software to be brought into
  **     disrepute, either by misuse, or use for inappropriate tasks, or
  **     by inappropriate modification.
  **
  **  5. The SOFA software is provided "as is" and SOFA makes no warranty
  **     as to its use or performance.   SOFA does not and cannot warrant
  **     the performance or results which the user may obtain by using the
  **     SOFA software.  SOFA makes no warranties, express or implied, as
  **     to non-infringement of third party rights, merchantability, or
  **     fitness for any particular purpose.  In no event will SOFA be
  **     liable to the user for any consequential, incidental, or special
  **     damages, including any lost profits or lost savings, even if a
  **     SOFA representative has been advised of such damages, or for any
  **     claim by any third party.
  **
  **  6. The provision of any version of the SOFA software under the terms
  **     and conditions specified herein does not imply that future
  **     versions will also be made available under the same terms and
  **     conditions.
  *
  **  In any published work or commercial product which uses the SOFA
  **  software directly, acknowledgement (see www.iausofa.org) is
  **  appreciated.
  **
  **  Correspondence concerning SOFA software should be addressed as
  **  follows:
  **
  **      By email:  sofa@ukho.gov.uk
  **      By post:   IAU SOFA Center
  **                 HM Nautical Almanac Office
  **                 UK Hydrographic Office
  **                 Admiralty Way, Taunton
  **                 Somerset, TA1 2DN
  **                 United Kingdom
  **
  **--------------------------------------------------------------------*/
  }
  