 International Celestial Reference System and its realizations

The International Celestial Reference System (ICRS) is a fundamental framework used in astronomy to define the positions of celestial objects. It provides a standardized reference system for the sky, similar to how the International Reference System (IRS) provides a reference for positions on Earth. ### Key Features of the ICRS: 1. **Fixed Reference Frame**: The ICRS is designed to be a quasi-inertial reference frame, meaning it doesn't move relative to distant stars.