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celestial_core::cio

Module locator 

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CIO locator (s) for the IAU 2006/2000A precession-nutation model.

The CIO locator s positions the Celestial Intermediate Origin on the CIP equator. It’s the arc length from the GCRS x-axis intersection to the CIO, measured along the CIP equator. This small angle (microarcseconds) completes the transformation from GCRS to CIRS coordinates.

§The transformation chain

GCRS -> CIRS requires three pieces:

  1. CIP coordinates (X, Y) - where the pole is
  2. CIO locator (s) - where the origin is
  3. Earth Rotation Angle - how much Earth has rotated

This module provides piece #2.

§When to use this

You need the CIO locator when:

  • Building the GCRS-to-CIRS rotation matrix via gcrs_to_cirs_matrix(x, y, s)
  • Computing equation of the origins (difference between CEO-based and equinox-based sidereal time)
  • Implementing IAU 2000/2006 compliant coordinate transformations

For most uses, CioSolution::calculate handles this automatically.

§Algorithm

Uses the IAU 2006/2000A series expansion with 66 periodic terms across 5 polynomial orders, plus a polynomial part. The full expression is:

s = series(t) - X*Y/2

where t is TT centuries from J2000.0. The -X*Y/2 term accounts for the frame rotation induced by the CIP motion.

§References

  • Capitaine et al. (2003), A&A 400, 1145-1154
  • IERS Conventions (2010), Chapter 5
  • SOFA library: iauS06 function

Structs§

CioLocator
Computes the CIO locator angle s for a given epoch.