Celestial Horizon

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 of a spherical coordinate system: altitude and azimuth. Therefore, the horizontal coordinate system is sometimes called the az/el system, the alt/az system, or the alt-azimuth system, among others. In an altazimuth mount of a telescope, the instrument's two axes follow altitude and azimuth.

Horizon

Most commonly, the horizon is the border between the surface of a celestial body and its sky when viewed from the perspective of an observer on or above the surface of the celestial body. This concept is further refined as:

celestial

Adjective

1. Of, relating to, or located in the sky or outer space, where the Sun, Moon, planets, and stars are visible.
2. Of a nose: upturned, as if towards the sky.

Adverb

1. Synonym of celestially (“in a celestial manner (adjective senses)”).

horizon

Noun

1. The visible horizontal line (in all directions) where the sky appears to meet the earth in the distance.
2. The range or limit of one's knowledge, experience or interest; a boundary or threshold.