The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on ...
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or near the surface of the relevant body. This curve divides all viewing directions based on whether it intersects the relevant body´s surface or not.
The true horizon is a theoretical line, which can only be observed to any degree of accuracy when it lies along a relatively smooth surface such as that of Earth´s oceans. At many locations, this line is obscured by terrain, and on Earth it can also be obscured by life forms such as trees and/or human constructs such as buildings. The resulting intersection of such obstructions with the sky is called the visible horizon. On Earth, when looking at a sea from a shore, the part of the sea closest to the horizon is called the offing.