Altitude, Azimuth & the Local Horizon: Structures & Vocabulary

Altitude and azimuth form a local coordinate system that describes where an object appears in an observer's sky at a particular place and time. Altitude measures angle above the horizon, while azimuth measures direction around the horizon.

Altitude runs from 0° at the horizon to 90° at the zenith. Azimuth is measured around the horizon from a chosen zero direction, commonly north through east in astronomy. The horizon, zenith, and meridian define the local reference frame.

Telescope pointing software converts catalog coordinates and time into local altitude and azimuth. Direct measurements with calibrated mounts or simple angle tools reproduce the predicted motion.

The local horizon system is built from the observer's location and gravity-defined vertical. The zenith is overhead, the horizon is perpendicular to that direction, and the meridian divides the sky through north and south. Latitude determines the altitude of the celestial pole, while time determines which equatorial coordinates map to each altitude and azimuth. The same star can therefore have very different local coordinates for observers in different cities.