Azimuth and altitude
In the common convention used by Horizons, azimuth increases eastward from true north: north 0°, east 90°, south 180° and west 270°. Altitude is measured above the local horizon toward a 90° zenith.
Other software can use different origins or signs, so conventions must be checked. Azimuth is not geographic longitude, and angular altitude is not elevation in metres above sea level.
One catalogue direction, different local positions
Earth’s rotation changes a star’s altitude and azimuth. Changing observer latitude or longitude also changes the local direction at a given time.
Transforming RA and declination consequently requires an observing site and time. Catalogue coordinates alone usually cannot determine where an observer should point a telescope.
Apparent directions and equipment
Horizons apparent azimuth and elevation include relevant light-time and directional corrections, with optional approximate atmospheric refraction. Refracted and airless outputs describe different conditions.
An alt-azimuth mount drives two axes. Near the zenith, azimuth can change rapidly enough to exceed mechanical tracking rates. Coordinate geometry and equipment limits jointly affect continuous observations.