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# xys06a

erfa.xys06a(date1, date2)

  For a given TT date, compute the X,Y coordinates of the Celestial Intermediate Pole and the CIO locator s, using the IAU 2006 precession and IAU 2000A nutation models.

  Parameters
    date1 double array
    date2 double array
  Returns
    x double array
    y double array
    s double array

  Notes

  Wraps ERFA function `eraXys06a`. The ERFA documentation is:

      - - - - - - - - - -
       e r a X y s 0 6 a
      - - - - - - - - - -

      For a given TT date, compute the X,Y coordinates of the Celestial
      Intermediate Pole and the CIO locator s, using the IAU 2006
      precession and IAU 2000A nutation models.

      Given:
         date1,date2  double  TT as a 2-part Julian Date (Note 1)

      Returned:
         x,y          double  Celestial Intermediate Pole (Note 2)
         s            double  the CIO locator s (Note 3)

      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 Celestial Intermediate Pole coordinates are the x,y components
         of the unit vector in the Geocentric Celestial Reference System.

      3) The CIO locator s (in radians) positions the Celestial
         Intermediate Origin on the equator of the CIP.

      4) Series-based solutions for generating X and Y are also available:
         see Capitaine & Wallace (2006) and eraXy06.

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

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

      This revision:  2021 May 11

      Copyright (C) 2013-2021, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.