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# s06a

erfa.s06a(date1, date2)

  The CIO locator s, positioning the Celestial Intermediate Origin on the equator of the Celestial Intermediate Pole, using the IAU 2006 precession and IAU 2000A nutation models.

  Parameters:
    date1 double array
    date2 double array
  Returns:
    c_retval double array

  Notes

  Wraps ERFA function `eraS06a`. The ERFA documentation is:

      - - - - - - - -
       e r a S 0 6 a
      - - - - - - - -

      The CIO locator s, positioning the Celestial Intermediate Origin on
      the equator of the Celestial Intermediate Pole, using the IAU 2006
      precession and IAU 2000A nutation models.

      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.

      Called:
         eraPnm06a    classical NPB matrix, IAU 2006/2000A
         eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
         eraS06       the CIO locator s, given X,Y, IAU 2006

      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.

         Capitaine, N. & Wallace, P.T., 2006, Astron.Astrophys. 450, 855

         McCarthy, D. D., Petit, G. (eds.), 2004, IERS Conventions (2003),
         IERS Technical Note No. 32, BKG

         Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981

      This revision:  2021 May 11

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