altazimuth coordinate system or horizon coordinate system, astronomical coordinate system in which the position of a body on the celestial sphere is described relative to an observer's celestial horizon and zenith. The coordinates of a body in this system are its altitude and azimuth. Altitude is measured from the celestial horizon along the vertical circle through the body and the zenith of the observer. Azimuth is measured along the celestial horizon from the observer's south point (the point on the horizon directly south of him) to the point where the body's vertical circle intersects the horizon. Because the earth rotates on its axis, the altitude and azimuth of a celestial body are constantly changing.