The difference between the Earth rotation angle and the Greenwich Sidereal Time (GST) of a celestial object. Put another way, it is the angular distance between the equinox and the Celestial Intermediate Origin (CIO), a point on the celestial equator that lies very close to the intersection with the prime meridian of the Geocentric Celestial Reference System. Whereas the path followed by the CIO is always at right angles to the equator, the equinox drifts along the equator because of precession and nutation.