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 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.
..... Click the link for more information. , the principal axis in the altazimuth coordinate system altazimuth coordinate system (ăltăz`əməth) or horizon coordinate system,
..... Click the link for more information. , lies halfway between the observer's zenith zenith, in astronomy, the point in the sky directly overhead; more precisely, it is the point at which the celestial sphere is intersected by an upward extension of a plumb line from the observer's location.
..... Click the link for more information. and nadir nadir (nā`dər) [Arab.,=opposite], in astronomy, the point on the celestial sphere directly opposite the zenith , i.e.
..... Click the link for more information. . In geology horizon refers to sedimentary deposits of a certain period, usually marked by characteristic fossils.

horizon

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.

horizon

1. the apparent line that divides the earth and the sky

2. Astronomy

a. the circular intersection with the celestial sphere of the plane tangential to the earth at the position of the observer

b. the great circle on the celestial sphere, the plane of which passes through the centre of the earth and is parallel to the sensible horizon

3. a thin layer of rock within a stratum that has a distinct composition, esp of fossils, by which the stratum may be dated

4. a layer in a soil profile having particular characteristics

horizon [hə′rīz·ən]

(astronomy)

The apparent boundary line between the sky and the earth or sea. Also known as apparent horizon.

The distance a light-ray could have traveled since the big-bang explosion at any given epoch in the evolution of the universe.

(geology)

The surface separating two beds.

One of the layers, each of which is a few inches to a foot thick, that make up a soil.

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horizon

horizon