Coordinated horizontal

Calls also "altazimutali coordinates", are reported to the observer, presupposed immovable, and therefore for every star they vary continuously, and they depend on the position of the observer. They are taken like references:

* the horizon, the maximum circumference that separates the visible celestial hemisphere from that invisibile;
* the meridian, the maximum circumference passing for the Zenith of the observer and the poles, that it meets the horizon in the points North and South;
* the foot of the star, point of the horizon more close to the star.

The height (h) is the angular distance of the star from the horizon, and varies between -90° (Nadir) and +90° (Zenith).
The Azimuth (a) is the angular distance between the point North and the foot of the star, measured in hour sense, and varies between 0° and 360°.

The smaller circumferences formed from the points of equal height are the height circles or almucantarat. The maximum semi-circumferences comprising the points of a data Azimut are called vertical.

To times, in place of the height the zenith distance is used them (z), that it is the angular distance of the star from the Zenith of the observer and that varies from 0° to 180°. In any case, z + h = 90°.