A geocentric coordinate system based upon the present position of the celestial pole but with right ascensions reckoned from a point called the Celestial Intermediate Origin (CIO) instead of the equinox. The CIO is a point on the celestial equator at present very close to the prime meridian of the Geocentric Celestial Reference System. CIRS directions are the modern equivalent of classical apparent places. To express the direction of a star as a CIRS position, its catalogue coordinates in the International Celestial Reference System are corrected for proper motion and parallax, for the gravitational deflection of light by the Sun, and for stellar aberration, and are then rotated to take account of precession and nutation. Calculating hour angles from CIRS right ascensions requires knowing the Earth rotation angle, which can be thought of as a simplified sidereal time, free of precession and nutation effects. The difference between CIRS right ascensions and classical equinox-based right ascensions is termed the equation of the origins.
Subjects: Astronomy and Astrophysics.