green flash


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green flash

or

emerald flash,

a refractive phenomenon of the atmosphere where the top edge of the setting (or, less frequently, rising) sun will momentarily turn emerald green. The green color lasts from a fraction of a second to two seconds. It is usually seen over a low distant horizon, such that as of the ocean or a prairie, when the sky is clear and free of clouds. The phenomenon was explained by James Prescott JouleJoule, James Prescott
, 1818–89, English physicist. His scientific researches began in his youth when he invented an electromagnetic engine. Joule made valuable contributions to the fields of heat, electricity, and thermodynamics.
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 in a letter to the Manchester Literary and Philosophical Society in 1869 and popularized in Jules Verne'sVerne, Jules
, 1828–1905, French novelist, originator of modern science fiction. After completing his studies at the Nantes lycée, he went to Paris to study law. He early became interested in the theater and wrote (1848–50) librettos for operettas.
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 novel Le Rayon Vert (1882; tr. The Green Ray, 1883).

The green flash occurs primarily because the atmosphere acts like a weak prism, refracting sunlight and separating it into different colors. As the sun sets, red and orange light, which are refracted the least, disappear first. Although green light is in roughly the middle of the spectrum, it is usually the last color to be seen by someone watching a sunset because blue and violet light are practically all absorbed by the contamination in the atmosphere, which scatters blue light and removes it from the line of sight. Under extraordinary conditions, however, a "blue flash" may be seen. At sunrise the phenomenon is reversed, with the green flash appearing first.

green flash

A shortlived phenomenon sometimes observed immediately before sunset whereby the last segment of the Sun's limb turns green just as it is about to sink below the horizon. The effect, resulting from refraction and absorption by the Earth's atmosphere, is sometimes followed by the appearance of a vertical green ray at the very instant of sunset. The same phenomenon may sometimes be observed at the moment of sunrise. The green flash is best seen in an area where the horizon is uninterrupted, typically at sea.

Green Flash

 

a flash of green light over the disk of the sun as it is setting, observed for several seconds at the moment when the upper limb of the solar disk disappears below the horizon. The origin of the green flash is associated with the refraction of the sun’s rays in the atmosphere. Since the atmosphere is denser in the lower layers than in the upper, light rays traversing through the atmosphere are bent and broken up into the primary colors, since the refraction of the red rays is somewhat smaller than that of the green and blue; here the angle of refraction of the rays increases as the sun approaches the horizon. When the atmosphere is in a quiescent state, the “stretching” of the spectrum from the upper (violet) to the lower (red) end measures as much as 30”. Along the long path of the solar rays through the lower layers of the atmosphere, most of the yellow and orange rays are absorbed by water vapor and O2 molecules, while the violet and blue rays are considerably attenuated as a result of scattering, so that basically the green and red rays remain. As a result, two solar disks, green and red, are visible, largely but not entirely over-lapping each other. Therefore, at the last instant before the total disappearance of the solar disk, when its red image is below the horizon, the upper limb of the green image is visible for a short time. The green flash is observed only when the air is very clear, most often over the maritime horizon. Sometimes if the air is very pure, a blue flash is also visible. The green flash can also appear at sunrise.

REFERENCES

Minnaert, M. Svet i tsvet v prirode. Moscow, 1969. (Translated from Dutch.) [9–-1352–1]

green flash

[′grēn ¦flash]
(astronomy)
A brilliant green coloration of the upper limb of the sun occasionally observed just as the sun's apparent disk is about to sink below a distant clear horizon. Also known as blue flash; blue-green flame; green segment; green sun.
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