A system of coordinates that specifies the angular position of a celestial object relative to the observer's horizon at a given time. The coordinates used are altitude, the object's angular distance above the horizon, and azimuth, the bearing of the object clockwise from north measured parallel to the horizon. Sometimes the zenith distance is used instead of altitude. The coordinates change with the observer's position and as the Earth rotates.
Subjects: Astronomy and Astrophysics.