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 atmospheric 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. 这显示了一个清晨的太阳，曝光足够暗，使得太阳的光线不会使CCD探测器饱和。颜色确实非常偏红。

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. 当试图使图像变亮以便看清周围环境时，太阳光使介质饱和并呈现出白色。

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. 这张红色清晨太阳的照片是在亚特兰大附近约7:30拍摄的。少量的晨雾增强了红色，这是由于瑞利散射，它会将更多的蓝光从视线中散射出去。随后拍摄了约1档曝光差的连续照片，并将较暗照片中的太阳转移到较亮照片中的太阳图像上。

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.

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.

It is also possible to see the green flash associated with the Moon's rising or setting, as demonstrated by photos taken in Chile's Atacama Desert. This rare 也可以看到与月出或月落相关的绿色闪光，如在智利阿塔卡马沙漠拍摄的照片所示。2011年4月，Gerhard Hudepohl拍摄到了一次罕见的月绿色闪光。如上所示，绿色闪光是一种折射现象，当光线以精确的低角度进入地球大气层时，绿色波长的光会优先到达观众的眼睛。 Moon green flash was captured by Gerhard Hudepohl in April of 2011. As shown above, the green flash is a refraction phenomenon which occurs when the light enters the Earth's atmosphere at a precise low angle which brings the green wavelengths of light preferentially to the viewers eye.

The location of the viewing of this lunar green flash was on the Chile's 8500 ft mountain Cerro Paranal, the home of ESO's Very Large Telescope. The atmospheric conditions are so exceptional there that it is one of the best places to see fleeting events such as the green flash of the setting Sun. The above photo was taken on a clear morning when the setting Moon was crossing the horizon.

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"

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. 右侧是一幅粗略的色分离图，该图是Greenler的《彩虹、光环和光辉》一书中所载的金星照片。该照片由Raymond F. Newell, Jr.于纽约拍摄，当时金星位于地平线以上2至3度。图中还显示了另一张照片，当时金星位于地平线以上10度，其外观与前一张照片截然不同。在10度时，上部为蓝绿色，接着是一小片绿色区域、白色区域、黄色区域，然后是极薄的红色边缘。

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.