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 versus refraction 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
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. Here's how Jules Verne describes an inferior mirage sunset.
All eyes were again turned towards the west. The sun seemed to sink with greater rapidity as it approached the sea; it threw a long trail of dazzling light over the trembling surface of the water; its disk soon changed from a shade of old gold, to fiery red, and, through their half-closed eyes, seemed to glitter with all the varying shades of a kaleidoscope. Faint, waving lines streaked the quivering trail of light cast on the surface of the water, like a spangled mass of glittering gems. Not the faintest sign of cloud, haze, or mist was visible along the whole of the horizon, which was as clearly defined as a black line traced on white paper. Motionless, and with intense excitement, they watched the fiery globe as it sank nearer and nearer the horizon, and, for an instant, hung suspended over the abyss. Then, through the refraction of the rays, its disk seemed to change till it looked like an Etruscan vase, with bulging sides, standing on the water. On very rare occasions the mirages of astronomical objects other than the Sun and the Moon might be observed. An apparent magnitude of an astronomical object should be low enough (that is, bright enough) in order to see it as not only a real object, but also a miraged one.
Mock mirage
A mock mirage of astronomical objects is much more complex than an inferior mirage. While an inferior mirage of astronomical objects can produce only two images, a mock mirage can produce multiple miraged images. The shapes of the miraged object are changing constantly and unpredictably. In order for a mock mirage to appear, the cooler air needs to be trapped below the inversion. Several inversion layers produce multiple pancake-like shapes. It is possible that the solar anomaly mentioned in the Book of Joshua may have been an example of a mock mirage. In that tale, Joshua launched a surprise attack on the Amorites following a night march, causing the Amorites to panic and flee as far as Beth-horon, but they did not find a safe haven there. "...they were more who died with the hailstones than they whom the children of Israel slew with the sword." Hailstones are a rare event in deserts and are a good precondition for creating a mock/superior mirage of the setting sun. Inferior mirage is the most common mirage in the deserts. When the Israelites went from a hot desert to a hail-covered desert to fight the Amorites, the inversion layers could have created a mock mirage of the setting sun. To the Israelites, the sun would then have appeared to stand still. A poem is quoted from the Book of Jasher, which states that the Sun stood still at Gibeon, and the Moon in the valley of Ajalon, in order that Joshua could complete the battle.
10.12 Then spoke Joshua to the LORD in the day when the LORD delivered up the Amorites before the children of Israel; and he said in the sight of Israel: 'Sun, stand thou still upon Gibeon; and thou, Moon, in the valley of Aijalon.10.13 And the sun stood still, and the moon stayed, until the nation had avenged themselves of their enemies. Is not this written in the book of Jashar? And the sun stayed in the midst of heaven, and hasted not to go down about a whole day.
Novaya Zemlya effect
Due to a normal atmospheric refraction, sunrise occurs shortly before the Sun crosses above the horizon. Light from the Sun is bent, or refracted, as it enters Earth's atmosphere. This effect causes the apparent sunrise to be earlier than the actual sunrise. Similarly, apparent sunset occurs slightly later than actual sunset. In ordinary atmospheric conditions, the setting or rising Sun appears to be about half a degree above its geometric position. But sometimes, very unusual atmospheric circumstances can make it to be visible when it is really between two and five degrees below the horizon. This is called the Novaya Zemlya effect, because it was first observed in Novaya Zemlya, where the Sun was seen when, according to astronomical calculations, it should have been two degrees below the horizon. However, due to changes in air pressure, relative humidity, and other quantities, the exact effects of atmospheric refraction on sunrise and sunset time cannot be predicted. Also note that this possible error increases with higher (closer to the poles) latitudes.
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