Editing Horizon (section)

== Appearance and usage ==
[[File:Sailing_Boat_Horizon.JPG|thumb|View of the ocean with two ships: one in the foreground and one to the left of it on the horizon]]
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 [[Telegraphy|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. Pilots can also retain their [[Spatial disorientation|spatial orientation]] by referring to the horizon.

In many contexts, especially [[Perspective projection|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 unaided eye. However, for someone on a {{convert|1000|m|ft|abbr=on}} hill looking out across the sea, the true horizon will be about a degree below a horizontal line.

In astronomy, the horizon is the horizontal plane through the eyes of the observer. It is the [[fundamental plane (spherical coordinates)|fundamental plane]] of the [[horizontal coordinate system]], the locus of points that have an [[altitude (astronomy)|altitude]] of zero degrees. While similar in ways to the geometrical horizon, in this context a horizon may be considered to be a plane in space, rather than a line on a picture plane.