# 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.

<figure><img src="/files/hJyrtP8Qi9kSMXM3clnk" alt="" width="375"><figcaption></figcaption></figure>

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.&#x20;

## 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.


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