The?horizon?or?skyline?is the apparent line that separates?earth?from?sky, 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. When looking at a sea from a shore, the part of the sea closest to the horizon is called the?offing. The word?horizon?derives from the?Greek?”?????? ??????”?horiz?n kyklos, “separating circle”,from the verb???????horiz?, “to divide”, “to separate”, and that from “????” (oros), “boundary, landmark”.
Historically, the distance to the visible horizon has long been vital to survival and successful navigation, especially at sea, because it determined an observer’s maximum range of vision and thus of?communication, with all the obvious consequences for safety and the transmission of information that this range implied. This importance lessened with the development of the?radio?and the?telegraph, but even today, when flying an?aircraft?under?visual flight rules, a technique called?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 his or her?spatial orientation?by referring to the horizon.
In many contexts, especially?perspective?drawing, the curvature of the?Earth?is disregarded and the horizon is considered the theoretical line to which points on any?horizontal plane?converge (when projected onto the picture plane) as their distance from the observer increases. For observers near?sea level?the difference between this?geometrical horizon?(which assumes a perfectly flat, infinite ground plane) and the?true horizon?(which assumes a?spherical Earth?surface) is imperceptible to the naked eye dubious???discuss but for someone on a 1000-meter hill looking out to sea the true horizon will be about a degree below a horizontal line.