[x, y] = ERFA.bpn2xy(rbpn)
Extract from the bias-precession-nutation matrix the X,Y coordinates of the Celestial Intermediate Pole.
rbpn double[3][3] celestial-to-true matrix (Note 1)
x,y double Celestial Intermediate Pole (Note 2)
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The matrix rbpn transforms vectors from GCRS to true equator (and CIO or equinox) of date, and therefore the Celestial Intermediate Pole unit vector is the bottom row of the matrix.
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The arguments x,y are components of the Celestial Intermediate Pole unit vector in the Geocentric Celestial Reference System.
"Expressions for the Celestial Intermediate Pole and Celestial Ephemeris Origin consistent with the IAU 2000A precession- nutation model", Astron.Astrophys. 400, 1145-1154 (2003)
n.b. The celestial ephemeris origin (CEO) was renamed "celestial intermediate origin" (CIO) by IAU 2006 Resolution 2.
This revision: 2021 May 11
Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.