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:
Note that the relationship between right ascension and longitude shifts with time, its most obvious shift occurring over a daily cycle. In contrast, the relationship between declination and latitude is basically constant.
The equatorial coordinate system (and the closely related International Celestial Reference System) is commonly used for the position of planets in the celestial sphere and its coordinates are commonly used for identifying and naming objects beyond the solar system (e.g., J designators).
The Earth's precession of the equinoxes slowly changes the coordinates of points on the celestial sphere on the order of an arcsecond per month. This is a primary reason for specifying an epoch.