Horizontal Coordinate System

The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane to define two angles, azimuth (az) and altitude (alt).

This system divides the sky into two hemispheres: The upper hemisphere and the lower hemisphere. The pole of the upper hemisphere is called the zenith. The pole of the lower hemisphere is called the nadir.

Azimuth and altitude/elevation can be useful to track satellites through space. An antenna could use these measurements to follow a satellite and constantly receive data.

Azimuth (az.)

The angle between North, measured clockwise around the observer's horizon, and a celestial body (e.g. moon, star).

Altitude (alt.)

The angle between the object and the observer's local horizon. Also known as elevation (el.)

Celestial Horizon

The great circle separating the upper and lower hemispheres of the celestial sphere. The plane is normal to the local gravity vector.