Horizon
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Horizon
</dt>
That great
circle of the celestial
sphere midway the zenith and nadir, or a line
resembling or approximating such a circle.
</dd>
That line where earth and sky appear to meet, and the projection of
this line upon the celestial sphere, is called visible or apparent horizon. A
line resembling the visible horizon but above or below it is called a false
horizon. That circle of the celestial sphere formed by the intersection of the
celestial sphere and a plane perpendicular to the zenith-nadir line is called
sensible horizon if the plane is through any point, such as the eye of an
observer, geoidal horizon if through any sea-level point, and celestial or
rational horizon if through the center of the earth. The geometrical horizon
was originally considered identical with the celestial sphere and an infinite
number of straight lines tangent to the earth's surface, and radiating from
the eye of the observer. If there were no terrestrial refraction, geometrical
and visible horizons would coincide. An artificial horizon is a gyroscopic
instrument for indicating the attitude of an aircraft with respect to the
horizontal. A radio horizon is the line at which direct rays from a
transmitting antenna become tangent to the earth's surface. A radar horizon is
the radio horizon of a radar antenna. [[/a>|/a>
]]
References
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use