# Dictionary Definition

horizon

### Noun

1 the line at which the sky and Earth appear to
meet [syn: apparent
horizon, visible
horizon, sensible
horizon, skyline]

2 the range of interest or activity that can be
anticipated; "It is beyond the horizon of present knowledge" [syn:
view, purview]

3 a specific layer or stratum of soil or subsoil
in a vertical cross section of land

4 the great circle on the celestial sphere whose
plane passes through the sensible horizon and the center of the
Earth [syn: celestial
horizon]

# User Contributed Dictionary

## English

### Etymology

Derived from ορίζων.### Pronunciation

- /həˈɹaɪzn/, /h@"r\aIzn/

### Noun

#### Derived terms

#### Related terms

#### Translations

horizon

### See also

## French

### Noun

horizon- horizon

# Extensive Definition

The horizon (Ancient
Greek ὁ ὁρίζων, /ho horídzôn/, from ὁρίζειν, "to limit") is the
apparent line that separates earth from sky.

More precisely, it is the line that divides all
of the directions one can possibly look into two categories: those
which intersect the Earth's surface, and those which do not. At
many locations, the true horizon is obscured by nearby trees,
buildings, mountains and so forth. The resulting intersection of
earth and sky is instead described as the visible horizon.

## Appearance and usage

For observers aboard a ship at sea, the true horizon is strikingly apparent. Historically, the distance to the visible horizon 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 their spatial orientation by referring to the horizon.In many contexts, especially perspective
drawing, the curvature of the earth is typically 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. Note that, for observers near
the ground, 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 typically
imperceptibly small, because of the relative size of the observer.
That is, if the Earth were truly flat, there would still be a
visible horizon line, and, to ground based viewers, its position
and appearance would not be significantly different from what we
see on our curved Earth.

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 which have an
altitude
of zero degrees. While similar in ways to the geometrical horizon
described above, 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

The straight line of sight distance d in kilometers to the true horizon on earth is approximatelyd = \sqrt,

where h is the height above ground or sea level
(in meters) of the eye of the observer. Examples:

- For an observer standing on the ground with h = 1.70 m (average eye-level height), the horizon appears at a distance of 4.7 km.
- For an observer standing on a hill or tower of 100 m in height, the horizon appears at a distance of 36 km.

To compute the height of a tower, the mast of a
ship or a hilltop visible above the horizon, add the horizon
distance for that height. For example, standing on the ground with
h = 1.70 m, one can see, weather permitting, the tip of a
tower of 100 m height at a distance of 4.7+36 ≈ 41 km.

In the Imperial
version of the formula, 13 is replaced by 1.5, h is in feet and d
is in miles. Examples:

- For observers on the ground with eye-level at h = 5 ft 7 in (5.583 ft), the horizon appears at a distance of 2.89 miles.
- For observers standing on a hill or tower 100 ft in height, the horizon appears at a distance of 12.25 miles.

The metric formula is reasonable (and the
Imperial one is actually quite precise) when h is much smaller than
the radius of the
Earth (6371 km). The exact formula for distance from the
viewpoint to the horizon, applicable even for satellites, is

- d = \sqrt,

where R is the radius of the Earth (note: both R
and h in this equation must be given in the same units (e.g.
kilometers), but any consistent units will work).

Another relationship involves the arc length
distance s along the curved surface of the Earth to the bottom of
object:

- \cos\frac=\frac.

Solving for s gives the formula

- s=R\cos^\frac.

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).

As a final note, the actual visual horizon is
slightly farther away than the calculated visual horizon, due to
the slight refraction of light rays due to the atmospheric density
gradient. This effect can be taken into account by using a "virtual
radius" that is typically about 20% larger than the true radius of
the Earth.

## Curvature of the horizon

From a point above the surface the horizon appears slightly bent. There is a basic geometrical relationship between this visual curvature \kappa, the altitude and the Earth's radius. It is- \kappa=\sqrt\ .

## See also

- Dawn: the time right before sunrise
- Dusk: the time right after sunset, yielding to twilight
- Landscape
- Landscape art
- Aerial landscape art
- Sextant

## External links

horizon in Arabic: خط الأفق

horizon in Asturian: Horizonte

horizon in Aymara: Chhaqachhaqa

horizon in Bulgarian: Хоризонт

horizon in Catalan: Horitzó

horizon in Czech: Horizont

horizon in Danish: Horisont (geografi)

horizon in German: Horizont

horizon in Spanish: Horizonte

horizon in Esperanto: Horizonto

horizon in Galician: Horizonte (xeografía)

horizon in Korean: 수평선

horizon in Croatian: Obzor

horizon in Bishnupriya: হোরিজোন্টে

horizon in Indonesian: Horizon

horizon in Icelandic: Sjóndeildarhringur

horizon in Italian: Orizzonte

horizon in Lithuanian: Horizontas

horizon in Hungarian: Horizont

horizon in Malay (macrolanguage): Horizon

horizon in Dutch: Horizon

horizon in Japanese: 地平線

horizon in Norwegian: Horisont

horizon in Polish: Horyzont

horizon in Portuguese: Horizonte

horizon in Romanian: Horizonte

horizon in Quechua: Pachapanta

horizon in Russian: Горизонт

horizon in Albanian: Horizonti

horizon in Simple English: Horizon

horizon in Slovak: Obzor (Zem)

horizon in Slovenian: Obzorje

horizon in Serbian: Хоризонт (астрономија)

horizon in Serbo-Croatian: Horizont
(astronomija)

horizon in Swedish: Horisont

horizon in Thai: ขอบฟ้า

horizon in Turkish: Ufuk

horizon in Chinese: 地平線

# Synonyms, Antonyms and Related Words

azimuth, background, clear sight,
color vision, command,
compass, cone vision,
day vision, daylight vision, discernment, domination, eye, eye-mindedness, eyereach, eyeshot, eyesight, false horizon,
farsight, farsightedness, field of
view, field of vision, keen sight, ken, limit of vision, limits, line of sight, naked eye,
night vision, offing,
outlook, outlook over,
perception,
peripheral field, peripheral vision, perspective, perspicacity, perspicuity, photopia, power of sight,
prospect, purview, quick sight, range, reach, remote distance, rod
vision, scan, scope, scope of vision, scotopia, sea line, seeing, sense of sight, sensible
horizon, sight, sightedness, sightliness, skyline, survey, sweep, the distance, twilight
vision, unobstructed vision, vanishing point, view, visible horizon, vision, vista, visual acuity, visual
field, visual sense