rc2i = 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. IAU 2000.
date1,date2 double TT as a 2-part Julian Date (Note 1)
x,y double Celestial Intermediate Pole (Note 2)
rc2i double[3][3] celestial-to-intermediate matrix (Note 3)
- 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.
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The Celestial Intermediate Pole coordinates are the x,y components of the unit vector in the Geocentric Celestial Reference System.
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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.
- Although its name does not include "00", This function is in fact specific to the IAU 2000 models.
eraC2ixys celestial-to-intermediate matrix, given X,Y and s
eraS00 the CIO locator s, given X,Y, IAU 2000A
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.