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I'm not the best person to review this as I don't know the details of the frame (hence the desire for docs lol).

Would @ayshih be interested?

The change log has to be removed and pre-commit has to be made happy, but I want to make sure the content is actually valid. Thanks, all.

@pllim Thank you
@Cadair Below are some content validations for the docstring:

The description has been cross-checked against the following primary sources:

• USNO Circular 179 (§2.6 — "The Celestial Intermediate Reference System"): https://arxiv.org/abs/astro-ph/0602086
  CIP definition, CIO as non-rotating origin, ERA as a linear function of UT1 - all confirmed

• IAU 2000 Resolution B1.3 (CIP & CIO adoption) and IAU 2006 Resolution B1 (P03 precession model):
  IAU 2006 precession + IAU 2000A nutation → micro-arcsecond accuracy - confirmed

• ERFA source (liberfa/erfa — the open-source SOFA): https://github.com/liberfa/erfa
  apco / atciqz routines used for ICRS ↔ CIRS - confirmed

• Astropy CIRS transform code (
  icrs_cirs_transforms.py):
  https://github.com/astropy/astropy/blob/main/astropy/coordinates/builtin_frames/icrs_cirs_transforms.py#L30-L67
  Single apco call for the astrometry context - confirmed
  The "equation of the origins" description and the CIRS vs. GCRS distinction are also consistent with Capitaine et al. (2003), A&A 412.