Horizons

Horizons are the apparent boundary between the earth and the sky as viewed from a location. In everyday use, a horizon is what you see when you look toward the distant sea, land, or flat terrain; it seems to separate ground from sky.

Geometrically, the horizon is the circle where the tangent plane to the Earth's surface at the observer intersects the sky. Its position depends on the observer’s height above the surface and the curvature of the Earth. Because the atmosphere refracts light and because the terrain may rise above sea level, the horizon you actually see—the apparent horizon—can differ from this geometric line.

Two common senses of horizon exist: the geographic (or ground) horizon, which is the boundary formed by the visible surface of the Earth meeting the sky, and the celestial horizon, defined as the intersection of the celestial sphere with the local horizontal plane. The celestial horizon is used to measure altitude (angle above the horizon) and to set the observer’s azimuth (direction along the horizon).

Horizons are important in navigation, surveying, and astronomy, where they help determine position, time, and visibility conditions. In photography and art, the horizon line helps structure composition. Culturally, horizons function as metaphors for limits of knowledge, future prospects, and exploration.