The horizon (Ancient Greek ὁ ὁρίζων, /ho horídzôn/, from ὁρίζειν, "to limit") is the apparent line that separates earth from sky. The Ancient Greek language is the historical stage in the development of the Hellenic language family spanning the Archaic (c EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The sky is the part of the Atmosphere or of Outer space visible from the surface of any Astronomical object.
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.
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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. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. 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. Visual flight is the control and navigation of an Aircraft by using the view from the aircraft as the primary reference point 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. Perspective (from Latin perspicere to see through in the graphic arts such as drawing is an approximate representation on a flat surface (such as paper of an image as it is perceived In Astronomy, Geography, Geometry and related sciences and contexts a plane is said to be horizontal at a given point if it is locally 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. The horizontal coordinate system is a Celestial coordinate system that uses the observer's local Horizon as the fundamental plane. In Astronomy, a celestial coordinate system is a Coordinate system for mapping positions in the sky 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 approximately
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. The mast of a sailing ship is a tall vertical or near vertical Spar, or arrangement of Spars which supports the Sails Large ships have several masts 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. The weather is a set of all the phenomena occurring in a given Atmosphere at a given Time. 7+36 ≈ 41 km.
In the Imperial version of the formula, 13 is replaced by 1. Imperial units or the Imperial system is a collection of units first defined in the British Weights and Measures Act of 1824 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 Earth 's shape like that of all major Planets approximates a Sphere. The exact formula for distance from the viewpoint to the horizon, applicable even for satellites, is
where R is the radius of the Earth (note: both R and h in this equation must be given in the same units (e. This article is about artificial satellites For natural satellites also known as moons see Natural satellite. 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:
Solving for s gives the formula
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 κ, the altitude and the Earth's radius. It is
The curvature is the reciprocal of the curvature angular radius in radians. The radian is a unit of plane Angle, equal to 180/ π degrees, or about 57 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 metres that is viewed from 56 centimetres. 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.
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
Dawn refers to the Twilight before Sunrise. It is recognized by the presence of weak sunlight while the sun itself is still below the horizon Sunrise is the instant at which the upper edge of the Sun appears above the Horizon in the East. Dusk refers to the period of time following Sunset. Although commonly confused with twilight dusk is the time frame that occurs either before or after a twilight - when the Sunset, also called sundown in some American English Dialects is the instant when the trailing edge of the Sun 's disk disappears below Twilight is the time before Sunrise, called Dawn, and the time after Sunset, called Dusk. Landscape comprises the visible features of an area of land including physical elements such as Landforms living elements of flora and fauna abstract elements such as lighting For the art of designing external spaces see Landscape architecture. (This article concerns painting and other non-photographic media This article is about the sextant as used for Navigation. For the astronomer's sextant, see Sextant (astronomical.Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic