# Horizontal Coordinate System

Celestial coordinate system

A celestial sphere is an abstract sphere centered on an observer. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere.

Horizontal Coordinate System

Horizontal Coordinate System is a celestial coordinate system that uses the observer’s local horizon as the fundamental plane. Coordinates of an object in the sky are expressed in terms of altitude (or elevation) angle and azimuth.