The horizon is the line that separates earth from sky. More precisely, it is the line that divides all of the directions you can possibly look into, into two categories: those which intersect the Earth, and those which do not. At many locations, the horizon is obscured by trees, buildings, mountains, etc. However, if you are on a ship at sea, the horizon is strikingly apparent. When flying an aircraft under Visual Flight Rules, the horizon is even more apparent. A technique called attitude flying is used to control the aircraft, where the pilot uses the relationship between the aircraft's nose and the horizon to control the aircraft. He also retains his spatial orientation by referring to the horizon.
The distance in km of the horizon on earth, in a plain (standing on the ground or on a tower, or from a plane) or on a hill or mountain surrounded by plains is approximately √(13h), where h is the height in meters of the eyes.
In astronomy the horizon is the horizontal plane through (the eyes of) the observer. It is the fundamental plane of the horizontal coordinate system, the locus of points which have an altitude of zero degrees. The regular horizon is a little below that.
The first version of this article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See