The International Celestial Reference System (ICRS)

A reference system is a set of specifications that define a coordinate system, together with anything required to define positions and velocities of objects in space and time. The ICRS coordinate axis have its origin at the baricenter (center of masses) of the Solar System, and its orientation is defined by the directions to a large number of very distant objects.

ICRS was adopted by the International Astronomical Union (IAU) as the fundamental astronomical reference system. In modern star catalogs, coordinates are expressed in terms of the ICRS, so an ICRS-based position can be directly compared to catalog positions. The ICRS is described in more detail in the document Sistemas de referencia en Astronomía (in Spanish).