Horizon

From Pilots Almanac

Wherever one is located on or near the earth's surface, the earth is perceived as essentially flat and, therefore, as a plane. The sky resembles one-half of a sphere or dome centered at the observer. If there are no visual obstructions, the apparent intersection of the sky with the earth's (plane) surface is the horizon, which appears as a circle centered at the observer. For rise/set computations, the observer's eye is considered to be on the surface of the earth, so that the horizon is geometrically exactly 90 degrees from the local vertical direction.