### Horizontal Coordinate System

**(az/el system, Alt/Az system, altazimuth system)**
(celestial coordinate system based upon horizon and north)

The **Horizontal Coordinate System**
(**az/el system**, **Alt/Az system**, **altazimuth system**) is a
spherical coordinate system used for directions in the
Celestial Sphere,
fixed to the horizon and north. It is directly related to observation
from Earth, but time-dependent: the rotation of Earth
produces a constant minute-by-minute change in the whole sky,
called the **Diurnal Motion**. Other coordinate systems
avoid this, but to relate an observation to one of them,
the position must be converted between it and this one.

Elements of the horizontal coordinate system:

**Altitude** - angle from the horizon.
**Azimuth** - angle eastward from north.
**Zenith Distance** - compliment of altitude, which can be used instead.

(*coordinate system,horizon,celestial sphere,Earth*)
http://en.wikipedia.org/wiki/Horizontal_coordinate_system

**Referenced by:**

Celestial Coordinate System

Ephemeris

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