CIO – Celestial Intermediate Origin

The Celestial Intermediate Origin (CIO) is a reference point on the celestial equator that defines Earth’s rotation angle precisely and without ambiguity. It forms part of the modern Earth Orientation Parameters (EOP), linking the International Celestial Reference Frame (ICRF) and the International Terrestrial Reference Frame (ITRF). Maintained by the International Earth Rotation and Reference Systems Service (IERS), the CIO replaces the traditional equinox-based system to ensure long-term accuracy in Earth rotation and reference frame alignment.

More Info: https://geodesy.science/item/earth-orientation-parameter