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