c2ixy¶

erfa.c2ixy(date1, date2, x, y)[source]¶

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

Parameters:

date1double array
date2double array
xdouble array
ydouble array

Returns:

rc2idouble 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.

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c2ixy¶