The right ascension of an object is its angular distance measured eastward along the equator from some defined reference point where the right ascension value is set to zero.This reference point, the origin of right ascension, has traditionally been the equinox: the point at which the Sun, in its yearly circuit of the celestial sphere, crosses the equatorial plane moving from south to north.Therefore, in practice, we use a specific reference frame—a set of fiducial objects with assigned coordinates—as the practical representation of an astronomical reference system.The scheme is completely analogous to how terrestrial reference systems are established using survey control stations (geodetic reference points) on the Earth's surface.In fact, the very definitions of these planes are problematic for high-precision work.
A reference frame consists of a set of identifiable fiducial points on the sky (specific astronomical objects), along with their coordinates, that serves as the practical realization of a reference system.
The accuracy with which we know the motions of the objects (unless they are assumed zero) is an essential factor in this computation.
Astrometric star catalogs list proper motions, which are the projection of each star's space motion onto the celestial sphere, expressed as an angular rate in right ascension and declination per unit time.
The Sun's apparent yearly motion lies in the ecliptic, the plane of the Earth's orbit.
The equinox, therefore, is a direction in space along the nodal line defined by the intersection of the ecliptic and equatorial planes; equivalently, on the celestial sphere, the equinox is at one of the two intersections of the great circles representing these planes.