Red Sunset

Sunsets are reddened because for sun positions which are very low or just below the horizon, the light passing at grazing incidence upon the earth must pass through a greater thickness of air than when it is overhead. Just before the sun disappears from view, its actual position is about a diameter below the horizon, the light having been bent by refraction to reach our eyes. Since short wavelengths are more efficiently scattered by Rayleigh scattering, more of them are scattered out of the beam of sunlight before it reaches you. Aerosols and particulate matter contribute to the scattering of blue out of the beam, so brilliant reds are seen when there are many airborne particles, as after volcanic eruptions.

The equivalent phenomenon can be seen at sunrise

This shows an early morning sun with the exposure dark enough so that the light from the Sun does not saturate the CCD detector. The color is indeed very red.
When an attempt was made to lighten the image so that the surroundings could be seen, the light from the Sun saturated the medium and appeared white.
This image of the red morning sun was shot about 7:30 AM near Atlanta. A small amount of morning fog enhanced the red by Rayleigh scattering, which would scatter more blue light out of the sight line. Successive exposures at about 1 stop difference were taken and the sun from the darker one transferred over the sun image in the lighter one.

The redness of the sun is about as it appeared to the eye, but the remainder of the sky is much darker than it appeared to the eye, pointing to the remarkable contrast handling ability of human vision.

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Green Flash

The famous but seldom seen "green flash" or "emerald flash" which occurs just before the last part of the sun disappears from view at sunset is caused by the same atmospheric refraction and scattering effects which produce the red sunset.

A rich subject for debate over the years, the green flash is rarely seen, but its observers wax eloquent about the brilliant green or emerald color when it is seen. In uniform air, the dispersion is apparently so small that the separation of red and green images is not visible. It takes more unusual layering of the atmosphere to enhance the separation.

Such a seldom seen and dramatic effect as the green flash tends to collect myth, so some care must be taken to separate fact from myth. I had reported from another reference that the perceived brilliance of the green might be heightened by the low-light enhancement of green vs the red end of the spectrum (see "Rods do not see red!" in color puzzles). Andrew Young contests this, stating that sunsets are so bright and provide so much light even in the green that significant bleaching of the pigment for both red and green may occur, certainly not the conditions for the scotopic or low-light vision. Young maintains an excellent website of resources about green flashes, "An Introduction to Green Flashes".

The index of refraction for red is 1.000292 and that for blue is 1.000295. Out of a total refraction of about 0.53°, the dispersion is only 0.006° or about 20 arc seconds, compared to a 120 arc sec resolution for the eye. Thus under normal conditions the eye would not see this.

Some historyMore numerical detail
A hint of a green flash
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Atmospheric optics concepts

References
Schaaf

Greenler

Meinel & Meinel

Minnaert



Green Flash

by Andrew Young

Le rayon vert
 
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Green Flash History

Jules Verne's 1882 novel "Le Rayon Vert" (The Green Ray) popularized the green flash, described as

"a green which no artist could ever obtain on his palette, a green of which neither the varied tints of vegetation nor the shades of the most limpid sea could ever produce the like! If there is a green in Paradise, it cannot be but of this shade, which most surely is the true green of Hope"

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Green Flash Details

The index of refraction of air for red is 1.000292 and that for blue is 1.000295. Out of a total refraction of about 0.53°, the dispersion is only 0.006° or about 20 arc seconds, compared to a 120 arc sec resolution for the eye. Thus under normal conditions the eye would not see the separation of red and blue images of the sun.

The green or blue would be seen only after the red image is gone, and the passage time for this 20 arc second band is reported by the Meinels to be about 1.4 seconds.

The successive red, green, and then blue images can be seen more distinctly by viewing the planets Venus and Jupiter with a telescope as they pass below the horizon. The ease of viewing the colors in these cases has to do with their smaller angular diameters.

At right is a rough sketch of the color separation in a photograph of Venus reproduced in Greenler's "Rainbows, Haloes and Glories". It is described as a photograph taken by Raymond F. Newell, Jr. in New York when Venus was 2 to 3 degrees above the horizon. Also displayed there is a photograph taken when Venus was 10 degrees above the horizon which has a very different appearance. At 10 degrees the upper portion is bluegreen, with a thin green region, a white region, a yellow region and then a very thin red edge on the bottom.

The Green Flash
Index

Atmospheric optics concepts

Reference
Greenler
Plate 7-9
 
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Twilight Wedge

The "twilight wedge" is the visible shadow to the earth cast upon the sky by the setting sun. It can be seen as a blue-gray line or wall which rises in the east after a clear sunset. It is often tinged with pink.

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