Definition
The horizontal system uses the observer's local horizon as fundamental plane. Altitude (h) is angular height above the horizon (−90° to +90°). Azimuth (A) is the compass direction measured eastward from north (0°–360°).
Topocentric
Horizontal coordinates are topocentric — they depend on the observer's latitude, longitude and time. The same star has different alt-az at different locations or times.
