era.c2i06a.md

eraC2i06a

rc2i = ERFA.c2i06a(date1, date2)

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.