horizon
American Heritage Dictionary:
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ho·ri·zon
(hə-rī'zən) 
n.
n.
- The apparent intersection of the earth and sky as seen by an observer. Also called apparent horizon.
- Astronomy.
- The sensible horizon.
- The celestial horizon.
- The limit of the theoretically possible universe.
- The range of one's knowledge, experience, or interest.
- Geology.
- A specific position in a stratigraphic column, such as the location of one or more fossils, that serves to identify the stratum with a particular period.
- A specific layer of soil or subsoil in a vertical cross section of land.
- Archaeology. A period during which the influence of a specified culture spread rapidly over a defined area: artifacts associated with the Olmec horizon in Mesoamerica.
[Middle English orizon, from Old French, from Latin, from Greek horizōn (kuklos), limiting (circle), horizon, present participle of horizein, to limit, from horos, boundary.]
Wiley Book of Astronomy:
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horizon
(1) Cosmological horizon: the maximum distance from us that light has traveled since the beginning of the universe; objects farther away are invisible to us because there has not been enough time for light to travel from them to Earth. (2) Astronomical horizon (or sensible horizon): the great circle formed by the intersection of the celestial sphere with a plane perpendicular to the line from an observer to the zenith; in other words, the great circle whose poles are the nadir and zenith.
Britannica Concise Encyclopedia:
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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.
For more information on horizon, visit Britannica.com.
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The visible horizon is the apparent boundary line between sky and earth or sea. The astronomical horizon is the great circle of the celestial sphere 90° from the zenith and the nadir. See also Astronomical coordinate systems.
Roget's Thesaurus:
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horizon
noun
- The extent of one's perception, understanding, knowledge, or vision: ken, purview, range, reach, scope. See ability/inability, knowledge/ignorance, see/not see.
Oxford Dictionary of Geography:
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horizon
soil horizon
A distinctive layer within a soil which differs chemically or physically from the layers below or above. The A horizon or topsoil contains humus. Often soil minerals are washed downwards from this layer. This material then tends to accumulate in the B horizon or subsoil. The C horizon is the unconsolidated rock below the soil. These three basic horizons may be further subdivided. Thus, Ah horizons are found under uncultivated land, Ahp horizons are under cultivated land, and Apg horizons are on gleyed land. The B horizons are also subdivided by means of suffixes: Bf horizons have a thin iron pan, Bg horizons are gleyed, Bh horizons have humic accumulations, Box horizons have a residual accumulation of sesquioxides and Bs horizons are areas of sesquioxide accumulation. Bt horizons contain clay minerals and Bw horizons do not qualify as any of the above. Bx horizons, or fragipans contain a dense but brittle layer caused by compaction. C horizons are also subdivided: Cu horizons show little evidence of gleying, salt accumulation, or fragipan; Cr horizons are too dense for root penetration; and Cg horizons are gleyed. Additional suffixes may be used. Some soil scientists use the term D horizon for the consolidated parent rock.
In addition to these soil horizons, other layers are distinguished. Thus, the layer of plant material on the soil surface is classified as: the L horizon (fresh litter); the F horizon (decomposing litter); the H horizon (well decomposed litter); and the O horizon (peaty). A leached A horizon is termed an E horizon or eluviated horizon.
The apparent or visible junction of the earth and sky, as seen from any specific position.
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[Ge]
In American archaeology this term refers to patterns of locally distinct phases or cultures that are linked together into bigger groups through recurrent cultural patterns and/or distinctive artefacts. Stone tools or pottery types provide typical features defining widespread horizons. The term was introduced by G. Willey and P. Phillips in 1955.
Columbia Encyclopedia:
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horizon
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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(DOD) In general, the apparent or visible junction of the Earth and sky, as seen from any specific position. Also called the apparent, visible, or local horizon. A horizontal plane passing through a point of vision or perspective center. The apparent or visible horizon approximates the true horizon only when the point of vision is very close to sea level.
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A layer of soil in the soil profile. See also soil profile.
Word Tutor:
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horizon
IN BRIEF: The line where the earth seems to meet the sky. Also: The range of a person's experience.
We all live under the same sky, but we don't all have the same horizon.
— Konrad Adenauer (1876-1967)
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i. In general, the apparent or visible junction of the earth and sky, as seen from any specific position. Also called a visible or local horizon.
ii. When an apparent boundary is modified by refraction, terrain, or other factors, it is called an apparent horizon.
iii. A small circle on the celestial sphere, the plane of which passes through the observer’s eye and is parallel to the observer’s rational horizon, is called a sensible horizon.
iv. A path followed by a radar beam when it is tangential to earth is a radar horizon.
v. The locus of the point at which direct rays from a terrestrial radio transmitter become tangential to the earth is a radio horizon.
vi. A line resembling the visible horizon but above or below it is called a false horizon.
vii. An artificial horizon is a gyroscopic instrument for indicating the attitude of an aircraft with respect to the horizontal. See artificial horizon.
ii. When an apparent boundary is modified by refraction, terrain, or other factors, it is called an apparent horizon.
iii. A small circle on the celestial sphere, the plane of which passes through the observer’s eye and is parallel to the observer’s rational horizon, is called a sensible horizon.
iv. A path followed by a radar beam when it is tangential to earth is a radar horizon.
v. The locus of the point at which direct rays from a terrestrial radio transmitter become tangential to the earth is a radio horizon.
vi. A line resembling the visible horizon but above or below it is called a false horizon.
vii. An artificial horizon is a gyroscopic instrument for indicating the attitude of an aircraft with respect to the horizontal. See artificial horizon.
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categories related to 'horizon'
- Processes, Phenomena, Events, and Techniques - horizon: specific position in stratigraphic column that identifies stratum with a particular period; soil layer
- Earth’s Atmosphere and Topographic Features - horizon: point in distance where sky seems to meet curve of Earth; soil layer
- Coastal Land, Islands, and the Sea - horizon: distant point where sky seems to meet sea
- Celestial Phenomena and Points - horizon: great circle on celestial sphere perpendicular to hypothetical line that connects observer’s zenith to nadir; distant line where sky seems to meet Earth
- Soils and Soil Management - horizon: layer of soil with distinct characteristics
See crossword solutions for the clue Wikipedia on Answers.com:
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Horizon
For other uses, see Horizon (disambiguation).
The horizon (or skyline) is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon. When looking at a sea from a shore, the part of the sea closest to the horizon is called the offing.[1] The word horizon derives from the Greek "ὁρίζων κύκλος" (horizōn kyklos), "separating circle",[2] from the verb "ὁρίζω" (horizō), "to divide, to separate",[3] and that from "ὅρος" (oros), "boundary, landmark".[4]
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Appearance and usage
Historically, the distance to the visible horizon at sea has been extremely important as it represented the maximum range of communication and vision before the development of the radio and the telegraph. Even today, when flying an aircraft under Visual Flight Rules, a technique called attitude flying is used to control the aircraft, where the pilot uses the visual relationship between the aircraft's nose and the horizon to control the aircraft. A pilot can also retain his or her spatial orientation by referring to the horizon.
In many contexts, especially perspective drawing, the curvature of the Earth is disregarded and the horizon is considered the theoretical line to which points on any horizontal plane converge (when projected onto the picture plane) as their distance from the observer increases. For observers near sea level the difference between this geometrical horizon (which assumes a perfectly flat, infinite ground plane) and the true horizon (which assumes a spherical Earth surface) is imperceptible to the naked eye[dubious ] (but for someone on a 1000-meter hill looking out to sea the true horizon will be about a degree below a horizontal line).
In astronomy the horizon is the horizontal plane through (the eyes of) the observer. It is the fundamental plane of the horizontal coordinate system, the locus of points that have an altitude of zero degrees. While similar in ways to the geometrical horizon, in this context a horizon may be considered to be a plane in space, rather than a line on a picture plane.
Distance to the horizon
Ignoring the effect of atmospheric refraction, distance to the horizon from an observer close to the Earth's surface is about[5]
where d is in kilometres and h is height above sea level in metres.
Examples:
- For an observer standing on the ground with h = 1.70 metres (5 ft 7 in) (average eye-level height), the horizon is at a distance of 4.7 kilometres (2.9 mi).
- For an observer standing on the ground with h = 2 metres (6 ft 7 in) , the horizon is at a distance of 5 kilometres (3.1 mi).
- For an observer standing on a hill or tower of 100 metres (330 ft) in height, the horizon is at a distance of 39 kilometres (24 mi).
- For an observer standing at the top of the Burj Khalifa (828 metres (2,717 ft) in height), the horizon is at a distance of 111 kilometres (69 mi).
With d in miles[6] and h in feet,
Examples, assuming no refraction:
- For an observer on the ground with eye level at h = 5 ft 7 in (5.583 ft), the horizon is at a distance of 2.9 miles (4.7 km).
- For an observer standing on a hill or tower 100 feet (30 m) in height, the horizon is at a distance of 12.2 miles (19.6 km).
- For an observer on the summit of Aconcagua (22,841 feet (6,962 m) in height), the sea-level horizon to the west is at a distance of 184 miles (296 km).
Geometrical model
If the Earth is assumed to be a sphere with no atmosphere then the distance to the horizon can easily be calculated. (The earth's radius of curvature actually varies by 1%, so this formula isn't exact even assuming no refraction.)
The secant tangent theorem states that
Make the following substitutions:
- d = OC = distance to the horizon
- D = AB = diameter of the Earth
- h = OB = height of the observer above sea level
- D+h = OA = diameter of the Earth plus height of the observer above sea level
The formula now becomes
or
where R is the radius of the Earth.
The equation can also be derived using the Pythagorean theorem. Since the line of sight is a tangent to the Earth, it is perpendicular to the radius at the horizon. This sets up a right triangle, with the sum of the radius and the height as the hypotenuse. With
- d = distance to the horizon
- h = height of the observer above sea level
- R = radius of the Earth
referring to the second figure at the right leads to the following:
Another relationship involves the distance s along the curved surface of the Earth to the horizon; with γ in radians,
then
Solving for s gives
The distance s can also be expressed in terms of the line-of-sight distance d; from the second figure at the right,
substituting for γ and rearranging gives
The distances d and s are nearly the same when the height of the object is negligible compared to the radius (that is, h ≪ R).
Approximate geometrical formulas
If the observer is close to the surface of the earth, then it is valid to disregard h in the term (2R + h), and the formula becomes
Using metric units and taking the radius of the Earth as 6371 km, the distance to the horizon is
where d is in kilometres, and h is the height of the eye of the observer above ground or sea level in metres.
Using imperial units, the distance to the horizon is
where d is in miles and h is in feet.
These formulas may be used when h is much smaller than the radius of the Earth (6371 km), including all views from any mountaintops, aeroplanes, or high-altitude balloons. With the constants as given, both the metric and imperial formulas are precise to within 1% (see the next section for how to obtain greater precision).
Exact formula for a spherical Earth
If h is significant with respect to R, as with most satellites, then the approximation made previously is no longer valid, and the exact formula is required:
where R is the radius of the Earth (R and h must be in the same units). For example, if a satellite is at a height of 2000 km, the distance to the horizon is 5,430 kilometres (3,370 mi); neglecting the second term in parentheses would give a distance of 5,048 kilometres (3,137 mi), a 7% error.
Objects above the horizon
To compute the height of an object visible above the horizon, compute the distance to the horizon for a hypothetical observer on top of that object, and add it to the real observer's distance to the horizon. For example, for an observer with a height of 1.70 m standing on the ground, the horizon is 4.65 km away. For a tower with a height of 100 m, the horizon distance is 35.7 km. Thus an observer on a beach can see the tower as long as it is not more than 40.35 km away. Conversely, if an observer on a boat (h = 1.7 m) can just see the tops of trees on a nearby shore (h = 10 m), the trees are probably about 16 km away.
Referring to the figure at the right, the lighthouse will be visible from the boat if
where DBL is in kilometres and hB and hL are in metres. If atmospheric refraction is considered, the visibility condition becomes
Effect of atmospheric refraction
Because of atmospheric refraction of light rays, the actual distance to the horizon is slightly greater than the distance calculated with geometrical formulas. With standard atmospheric conditions, the difference is about 8%; however, refraction is strongly affected by temperature gradients, which can vary considerably from day to day, especially over water, so calculated values for refraction are only approximate.[5]
Rigorous method—Sweer
The distance d to the horizon is given by[7]
where RE is the radius of the Earth, ψ is the dip of the horizon and δ is the refraction of the horizon. The dip is determined fairly simply from
where h is the observer's height above the Earth, μ is the index of refraction of air at the observer's height, and μ0 is the index of refraction of air at Earth's surface.
The refraction must be found by integration of
where 
is the angle between the ray and a line through the center of the Earth. The angles ψ and are related by
Simple method—Young
A much simpler approach uses the geometrical model but uses a radius R′ = 7/6 RE. The distance to the horizon is then[5]
Taking the radius of the Earth as 6371 km, with d in km and h in m,
with d in mi and h in ft,
Results from Young's method are quite close to those from Sweer's method, and are sufficiently accurate for many purposes.
Curvature of the horizon
From a point above the surface the horizon appears slightly bent (it is a circle, after all). There is a basic geometrical relationship between this visual curvature 
, the altitude and the Earth's radius. It is
The curvature is the reciprocal of the curvature angular radius in radians. A curvature of 1 appears as a circle of an angular radius of 45° corresponding to an altitude of approximately 2640 km above the Earth's surface. At an altitude of 10 km (33,000 ft, the typical cruising altitude of an airliner) the mathematical curvature of the horizon is about 0.056, the same curvature of the rim of circle with a radius of 10 m that is viewed from 56 cm. However, the apparent curvature is less than that due to refraction of light in the atmosphere and because the horizon is often masked by high cloud layers that reduce the altitude above the visual surface.
Projective geometry
In visual geometry, the horizon is a provocative concept. Standing on a floor plane where parallel lines converge toward a point on the horizon, one sees that the point of convergence on the horizon is a vanishing point, which geometers call a point at infinity. Since each point on the horizon corresponds to a convergence point for its own set of parallel lines, the horizon is a line at infinity that represents the various sets of parallel lines. The science of graphical perspective developed in conjunction with formal projective geometry. John Stillwell describes these developments in the chapter titled "Horizon" in his book Yearning for the Impossible (2006). After introducing parallelism through traditional axioms, he introduces coordinates, which are "a natural consequence of the parallel axiom", and the slope of a line. Then moving to visual perspective of a a tiled floor, he reviews the construzione legittima in 1505 by Jean Pèlerin . Projective configurations provide a context to discuss "incidence" as a geometric primitive. There is a short review of literature by Girard Desargues, Etienne Pascal, Abraham Bosse, and Phillipe de la Hire. Projective configurations associated with Desargues and Pappus of Alexandria are illustrated. The role of these configurations as both theorems and axioms is discussed. Stillwell also ventures into foundations of mathematics in a section titled "What are the Laws of Algebra ?" The "algebra of points", originally given by Karl von Staudt deriving the axioms of a field, is considered along with supporting work by David Hilbert (1899) and Ruth Moufang (1932). Concluding the chapter with four axioms for the projective plane that determine Euclidean geometry as well as the laws of algebra, Stillwell writes
- This discovery from 100 years ago seems capable of turning mathematics upside down, though it has not yet been fully absorbed by the mathematical community. Not only does it defy the trend of turning geometry into algebra, it suggests that both geometry and algebra have a simpler foundation than previously thought.[8]
See also
Notes and References
- ^ "offing", Webster's Third New International Dictionary, Unabridged.
- ^ "ὁρίζων", Henry George Liddell and Robert Scott, A Greek-English Lexicon. On Perseus Digital Library. Accessed 19 April 2011.
- ^ "ὁρίζω", Liddell and Scott, A Greek-English Lexicon.
- ^ "ὅρος", Liddell and Scott, A Greek-English Lexicon.
- ^ a b c Andrew T. Young, "Distance to the Horizon". Accessed 16 April 2011.
- ^ In this article, mile refers to a "land" mile of 5,280 feet (1,609.344 m).
- ^ John Sweer, "The Path of a Ray of Light Tangent to the Surface of the Earth", Journal of the Optical Society of America, 28 (September 1938):327–29. Available as paid download.
- ^ John Stillwell (2006) Yearning for the Impossible, pp 47 to 76, A K Peters, Ltd., ISBN 1-56881-254-X
External links
- Derivation of the distance to the horizon. Steve Sque.
- Dip of the Horizon. Andrew T. Young.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Translations:
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Horizon
Dansk (Danish)
n. - horisont, synskreds
Nederlands (Dutch)
horizon, gezichtskring-/ einder, verschiet, kim
Français (French)
n. - horizon, strate, (fig) en vue, horizon (d'idées, d'intérêts)
Deutsch (German)
n. - Horizont, Gesichtskreis, Kulturschicht
Ελληνική (Greek)
n. - ορίζοντας
Italiano (Italian)
falda, orizzonte
Português (Portuguese)
n. - horizonte (m)
Русский (Russian)
горизонт, кругозор
Español (Spanish)
n. - capa, estrato, horizonte
Svenska (Swedish)
n. - horisont (äv. bildl), nivå
中文（简体）(Chinese (Simplified))
地平线, 限度, 眼界
中文（繁體）(Chinese (Traditional))
n. - 地平線, 限度, 眼界
日本語 (Japanese)
n. - 地平線, 水平線, 範囲, 限界
العربيه (Arabic)
(الاسم) الأفق, أفق المرء العقلي
עברית (Hebrew)
n. - אופק, שכבה גיאולוגית או קבומת שכבות
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