right|350px|thumb|Horizontal coordinates use a [[celestial sphere centered on the observer. Azimuth is measured eastward from the north point (sometimes from the south point) of the horizon; altitude is the angle above the horizon.]]
The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane to define two angles of a spherical coordinate system: altitude and azimuth.
Therefore, the horizontal coordinate system is sometimes called the az/el system,
<!-- End of "refs=" -->