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# c2ixy

erfa.c2ixy(date1, date2, x, y)

  Form the celestial to intermediate-frame-of-date matrix for a given date when the CIP X,Y coordinates are known.

  Parameters:
    date1 double array
    date2 double array
    x double array
    y double array
  Returns:
    rc2i double array

  Notes

  Wraps ERFA function `eraC2ixy`. The ERFA documentation is:

      - - - - - - - - -
       e r a C 2 i x y
      - - - - - - - - -

      Form the celestial to intermediate-frame-of-date matrix for a given
      date when the CIP X,Y coordinates are known.  IAU 2000.

      Given:
         date1,date2 double       TT as a 2-part Julian Date (Note 1)
         x,y         double       Celestial Intermediate Pole (Note 2)

      Returned:
         rc2i        double[3][3] celestial-to-intermediate matrix (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 matrix rc2i is the first stage in the transformation from
         celestial to terrestrial coordinates:

            [TRS] = RPOM * R_3(ERA) * rc2i * [CRS]

                  = RC2T * [CRS]

         where [CRS] is a vector in the Geocentric Celestial Reference
         System and [TRS] is a vector in the International Terrestrial
         Reference System (see IERS Conventions 2003), ERA is the Earth
         Rotation Angle and RPOM is the polar motion matrix.

      4) Although its name does not include "00", This function is in fact
         specific to the IAU 2000 models.

      Called:
         eraC2ixys    celestial-to-intermediate matrix, given X,Y and s
         eraS00       the CIO locator s, given X,Y, IAU 2000A

      Reference:

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

      This revision:  2021 May 11

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