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The horizon is the apparent line that separates earth from sky. It is the line that divides all visible directions into two categories: those that intersect the Earth’s surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon.
Prior to radio and telegraph, the distance to the visible horizon at sea was extremely important since it represented the maximum range of communication and vision. Today, when flying an aircraft under Visual Flight Rules, attitude flying is used to control the aircraft, where the pilot uses the visual relationship between the aircraft’s nose and the horizon to control the aircraft. A pilot can also retain spatial orientation by referring to the horizon.
In many contexts, the horizon is considered the theoretical line to which points on any horizontal plane converge as their distance from the observer increases. For observers near the ground the difference between this geometrical horizon and the true horizon is negligibly small.
In astronomy the horizon is the horizontal plane through the observer. It is the fundamental plane of the horizontal coordinate system, the locus of points that have an altitude of zero. While similar in ways to the geometrical horizon, this horizon may be considered to be a plane in space.
From a point above the surface the horizon appears slightly bent. There is a basic geometrical relationship between this visual curvature, the altitude, and the Earth’s radius. It is the reciprocal of the curvature angular radius in radians. A curvature of 1 appears as a circle of an angular radius of 45° corresponding to an altitude of approximately 2640 km above the Earth’s surface. At the typical cruising altitude of an airliner(10 km) the mathematical curvature of the horizon is about 0.056; however, the apparent curvature is less than that due to refraction of light in the atmosphere and because the horizon is often masked by high cloud layers that reduce the altitude above the visual surface.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.