The equatorial coordinate system is a spherical coordinate system used for directions in the celestial sphere, fixed to the stars/constellations as seen from Earth. Elements:
It (and the closely related International Celestial Reference System) is commonly used for planetary orbit calculation and its coordinates are those generally used in identifying and naming objects (e.g., J designators).
The Earth's precession results in a slow but never-ending shift in the equatorial coordinates that indicate a point on the celestial sphere: on the order of an arcsecond per month. This is a prime reason for specifying an epoch.