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 time-dependence, e.g., for specifying locations of stars, but to relate an observation to one of them, the position must be converted between the other system and this one. This system is straight-forward for a telescope with an altazimuth mount, and modern research telescopes use computer control to handle the aiming and conversion.
Elements of the horizontal coordinate system:
Some terms based upon these, describing the position of astronomical objects in the sky: