The equatorial coordinate system is a spherical coordinate system used for directions into the celestial sphere, fixed to the stars/constellations as seen from Earth. Elements:
The equatorial coordinate system (and the closely related International Celestial Reference System) is commonly used for planetary orbit calculation and its coordinates are commonly used in identifying and naming objects (e.g., J designators).
The Earth's precession of the equinoxes 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 primary reason for specifying an epoch.