A system for specifying coordinates for locating celestial objects. The International Celestial Reference System (ICRS) was adopted by the International Astronomical Union in 1994 as the new fundamental reference system. The ICRS is defined from a Barycentric Celestial Reference System, where “barycentric” means that the system is based on the center of mass of the solar system bodies, and a Geocentric Celestial Reference System, where “geocentric” means that the system is based on the center of the Earth. The systems are established based on specified gravitational potentials for the two origins. Their orientations are the same, and a standard set of equations, known as the Lorentz transformation, can be used to change from one to the other. Though the fiducial point (orientation of the x axis) of the new reference system is arbitrary, it has been chosen to agree with the dynamical equinox of the preexisting system of moving (dynamical) solar system objects adopted for the beginning of the year 2000, known as J2000.0, as closely as its accuracy allows.