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A great circle on the celestial sphere having a plane that passes through the center of the earth and is parallel to an observer's horizon. Also called rational horizon.
(Astronomy) See horizon2b
1. the line or circle that forms the apparent boundary between earth and sky.
a. the small circle of the celestial sphere whose plane is tangent to the earth at the position of a given observer, or the plane of such a circle (sensible horizon).
b. the great circle of the celestial sphere whose plane passes through the center of the earth and is parallel to the sensible horizon of a given position, or the plane of such a circle (celestial horizon).
3. the limit or range of perception, knowledge, or the like.
4. Usu., horizons. the scope of a person's interest, education, understanding, etc.
5. a thin, distinctive geological stratum useful for stratigraphic correlation.
6. any of the series of distinctive layers found in a vertical cross section of any well-developed soil.
[1540–50; < Latin horizōn < Greek horízōn(kýklos) bounding (circle), present participle of horízein to bound, limit]