The Horizontal Coordinate System (az/el system ,Alt/Az system, altazimuth system) is a spherical coordinate system used for directions in the Celestial Sphere, fixed to the horizon and north. It is key to observation from Earth, but time-dependent: the daily rotation of Earth produces the whole sky's constant minute-by-minute change, called the Diurnal Motion. Other coordinate systems avoid this.
Elements of the horizontal coordinate system:
Celestial Coordinate System