The modern equivalent of Greenwich Sidereal Time (GST), referred to a point called the Celestial Intermediate Origin (CIO) instead of the equinox. The CIO is a point defined not by its position but by its motion, and at present lies very close to the prime meridian of the Geocentric Celestial Reference System (within 0.1 arcsec throughout the twenty-first century). Unlike GST, which is a complicated function of both UT1 and Terrestrial Time and includes precession and nutation terms, ERA is a simple linear function of UT1 alone.