horizon system

The horizon system in astronomy is a coordinate system used to describe the position of celestial objects based on an observer's local horizon. It uses two coordinates: altitude, which is the angular elevation of an object above the horizon, and azimuth, which is the angle measured clockwise around the horizon, typically starting from north. This system is fundamental for celestial navigation and terrestrial surveying.1

The horizon is the apparent boundary where the sky meets the ground or sea. Directly overhead is the zenith, and directly underfoot is the nadir; these two points are the poles of the horizon. The distance to the visible horizon depends on the observer's height above the surface. For mapping stars, coordinate systems fixed to the celestial sphere, like the equatorial or ecliptic systems, are generally more practical than the horizon system.2341