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Concept
Mirage of astronomical objects
Résumé
A mirage of an astronomical object is a meteorological optical phenomenon, in which light rays are bent to produce distorted or multiple images of an astronomical object. The mirages might be observed for such celestial objects as the Sun, the Moon, the planets, bright stars, and very bright comets. The most commonly observed of these are sunset and sunrise mirages.
Mirages are distinguished from other phenomena caused by atmospheric refraction. One of the most prominent features of mirages is that a mirage might only produce images vertically, not sideways, while a simple refraction might distort and bend the images in any way.
The distortion in both images displayed in this section was caused by refraction, but while the image on the left, which is a mirage, demonstrates only vertical distortion, the image on the right demonstrates distortion in all the ways possible. It is easier to see the vertical direction of the mirage not even at the mirage of the Sun itself, but rather at the mirage of a sunspot. As a matter of fact, it is at least a three-image mirage of a sunspot, and all these images show a clear vertical direction.
Inferior mirage of astronomical objects is the most common mirage. Inferior mirage occurs when the surface of the Earth or the oceans produces a layer of hot air of lower density, just at the surface. There are two images, the inverted one and the erect one, in inferior mirage. They both are displaced from the geometric direction to the actual object. While the erect image is setting, the inverted image appears to be rising from the surface.
The shapes of inferior mirage sunsets and sunrises stay the same for all inferior mirage sunsets and sunrises. One well-known shape, the Etruscan vase, was named by Jules Verne.
As the sunset progresses the shape of Etruscan vase slowly changes; the stem of the vase gets shorter until the real and the miraged Suns create a new shape – Greek letter omega Ω. The inferior mirage got its name because the inverted image appears
below the erect one.
Source officielle
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Concepts associés (3)
Atmospheric optics
Atmospheric optics is "the study of the optical characteristics of the atmosphere or products of atmospheric processes .... [including] temporal and spatial resolutions beyond those discernible with the naked eye". Meteorological optics is "that part of atmospheric optics concerned with the study of patterns observable with the naked eye". Nevertheless, the two terms are sometimes used interchangeably. Meteorological optical phenomena, as described in this article, are concerned with how the optical properties of Earth's atmosphere cause a wide range of optical phenomena and visual perception phenomena.
Atmospheric duct
In telecommunications, an atmospheric duct is a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals (and light rays) are guided or ducted, tend to follow the curvature of the Earth, and experience less attenuation in the ducts than they would if the ducts were not present. The duct acts as an atmospheric dielectric waveguide and limits the spread of the wavefront to only the horizontal dimension.
Mirage
vignette|Mirage chaud sur le Grand Lac Salé, Utah, États-Unis|alt=Mirage chaud sur le Grand Lac Salé. vignette|Mirage froid : Double soleil sur le Lac Supérieur.|alt=Mirage froid sur le Lac Supérieur. Le mirage (du latin miror, mirari : s'étonner, contempler) est un phénomène optique dû à la déviation des faisceaux lumineux par des superpositions de couches d'air de températures différentes. En fait, il s'agit d'une propagation anormale de la lumière dans une atmosphère où la température, la pression et l'humidité ne varient pas verticalement selon la normale.