the line or circle that forms the apparent boundary between earth and sky.
2.
Astronomy.
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.
Also called rational horizon.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.
Usually, horizons.the scope of a person's interest, education, understanding, etc.: His horizons were narrow.
5.
Geology. a thin, distinctive stratum useful for stratigraphic correlation.
6.
any of the series of distinctive layers found in a vertical cross section of any well-developed soil.
[Origin: 1540–50; < L horizōn < Gk horízōn (kýklos) bounding (circle), equiv. to horíz(ein) to bound, limit + -ōn prp. suffix (nom. sing.); r. ME orizonte < MF < L horizontem, acc. of horizōn]
horizon, in astronomy, roughly circular line bounding an observer's view of the surface of the earth where the sky and earth seem to meet. This is the visible horizon. At sea the visible horizon is a perfect circle with the observer at its center, but on land it is irregular due to topographic features. The distance to the horizon varies as the square root of the observer's elevation for small elevations; at four times the height the distance to the horizon is twice as great. The celestial horizon, the principal axis in the altazimuth coordinate system, lies halfway between the observer's zenith and nadir. In geology horizon refers to sedimentary deposits of a certain period, usually marked by characteristic fossils.
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