horizon

The horizon is the apparent boundary between the sky and the surface of the Earth or sea as seen from a given location. It marks the intersection of the observer’s local horizontal plane with the celestial sphere. The exact position of the horizon depends on the observer’s height above ground and the lay of the land or water; the higher the observer, the farther the horizon appears. A common geometric relation gives the distance to the horizon as d ≈ sqrt(2Rh), where R is Earth's radius and h is the observer’s height above the surface.

There are several related notions. The geometric horizon is the true line of tangency between the surface and the imaginary plane through the observer; the apparent or visible horizon is the line as seen through the atmosphere, which can be altered by refraction, weather, and terrain. On flat seas the horizon often appears as a smooth line, while mountains or trees can create a jagged boundary.

In navigation, the horizon serves as a reference for measuring altitude; historic instruments such as the sextant use the horizon to determine the angle between a celestial body and the horizon. In art and photography, the horizon line is used as a compositional anchor, guiding balance and perspective. In astronomy, the term celestial or astronomical horizon denotes the intersection of the local horizontal plane with the celestial sphere.

Etymology: the word horizon derives from Latin horizon and from Greek horizōn, meaning "bounding" or "limit", from horos "boundary".

Beyond ordinary observations, "horizon" also appears in cosmology as the cosmic horizon, the limit of the observable universe set by the finite age of the universe and the expansion of space.