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 directly related to observation from Earth, but time-dependent: the rotation of Earth produces a constant minute-by-minute change in the whole sky, called the diurnal motion. Other coordinate systems avoid this, but to relate an observation to one of them, the position must be converted between it and this one.
Elements of the horizontal coordinate system:
Some terms based upon these, describing the position of astronomical objects in the sky: