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# c2txy

erfa.c2txy(tta, ttb, uta, utb, x, y, xp, yp)

  Form the celestial to terrestrial matrix given the date, the UT1, the CIP coordinates and the polar motion.

  Parameters
    tta double array
    ttb double array
    uta double array
    utb double array
    x double array
    y double array
    xp double array
    yp double array
  Returns
    rc2t double array

  Notes

  Wraps ERFA function `eraC2txy`. The ERFA documentation is:

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

       Form the celestial to terrestrial matrix given the date, the UT1,
       the CIP coordinates and the polar motion.  IAU 2000.

       Given:
          tta,ttb  double         TT as a 2-part Julian Date (Note 1)
          uta,utb  double         UT1 as a 2-part Julian Date (Note 1)
          x,y      double         Celestial Intermediate Pole (Note 2)
          xp,yp    double         coordinates of the pole (radians, Note 3)

       Returned:
          rc2t     double[3][3]   celestial-to-terrestrial matrix (Note 4)

       Notes:

       1) The TT and UT1 dates tta+ttb and uta+utb are Julian Dates,
          apportioned in any convenient way between the arguments uta and
          utb.  For example, JD(UT1)=2450123.7 could be expressed in any o
          these ways, among others:

                  uta            utb

              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 and MJD methods are good compromises
          between resolution and convenience.  In the case of uta,utb, the
          date & time method is best matched to the Earth rotation angle
          algorithm used:  maximum precision is delivered when the uta
          argument is for 0hrs UT1 on the day in question and the utb
          argument lies in the range 0 to 1, or vice versa.

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

       3) The arguments xp and yp are the coordinates (in radians) of the
          Celestial Intermediate Pole with respect to the International
          Terrestrial Reference System (see IERS Conventions 2003),
          measured along the meridians 0 and 90 deg west respectively.

       4) The matrix rc2t transforms 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.

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

       Called:
          eraC2ixy     celestial-to-intermediate matrix, given X,Y
          eraEra00     Earth rotation angle, IAU 2000
          eraSp00      the TIO locator s', IERS 2000
          eraPom00     polar motion matrix
          eraC2tcio    form CIO-based celestial-to-terrestrial matrix

      Reference:

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

       This revision:  2021 May 11

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