horizon coordinate system: see altazimuth coordinate system.

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.

celestial horizon, one axis of the altazimuth coordinate system. It is the great circle on the celestial sphere midway between the observer's zenith and nadir; it divides the celestial sphere into two equal hemispheres. The observer may be unable to see all the stars that lie above his celestial horizon because of obstructions such as buildings, trees, or mountains; he may be able to see some stars that lie below his celestial horizon because of atmospheric refraction.

In pedology, a distinct layer of soil forming part of the vertical sequence in a soil profile. Each horizon differs from the one above or below it in colour, chemical composition, texture, and structure. The horizons become differentiated during soil development because conditions vary with depth. There are generally three major layers within any given soil profile, and they are designated, from surface downward, as A, B, and C horizons. The A horizon generally contains more organic matter than the others; it is also the most weathered and leached. The B horizon tends to be a zone of accumulation, since all or part of the mineral matter removed from the A horizon in solution may be deposited in it. The C horizon consists chiefly of the materials from which the A and B layers were derived; called parent materials, these are only slightly altered, because they are in general not subjected to soil-forming processes.

Learn more about horizon with a free trial on Britannica.com.