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# c2i06a

erfa.c2i06a(date1, date2)

  Form the celestial-to-intermediate matrix for a given date using the IAU 2006 precession and IAU 2000A nutation models.

  Parameters
    date1 double array
    date2 double array
  Returns
    rc2i double array

  Notes

  Wraps ERFA function `eraC2i06a`. The ERFA documentation is:

      - - - - - - - - - -
       e r a C 2 i 0 6 a
      - - - - - - - - - -

      Form the celestial-to-intermediate matrix for a given date using the
      IAU 2006 precession and IAU 2000A nutation models.

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

      Returned:
         rc2i        double[3][3] celestial-to-intermediate matrix (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 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.

      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
         eraC2ixys    celestial-to-intermediate matrix, given X,Y and s

      References:

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

      This revision:  2021 May 11

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