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i. In general, the apparent or visible junction of the earth and sky, as seen from any specific position. Also called a visible or local horizon.
ii. When an apparent boundary is modified by refraction, terrain, or other factors, it is called an apparent horizon.
iii. A small circle on the celestial sphere, the plane of which passes through the observer's eye and is parallel to the observer's rational horizon, is called a sensible horizon.
iv. A path followed by a radar beam when it is tangential to earth is a radar horizon.
v. The locus of the point at which direct rays from a terrestrial radio transmitter become tangential to the earth is a radio horizon.
vi. A line resembling the visible horizon but above or below it is called a false horizon.
vii. An artificial horizon is a gyroscopic instrument for indicating the attitude of an aircraft with respect to the horizontal. See artificial horizon.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved