Green flash
 Green flash at sunset during Mercury transit. 09 May, 2016 (Moscow, Russia) |
Green flashes and green rays are optical phenomena that sometimes occur right before sunset or right after sunrise. When the conditions are right, a green spot is visible above the upper rim of the disk of the sun. The green appearance usually lasts for no more than a second or two. Rarely, the green flash can resemble a green ray shooting up from the sunset (or sunrise) point. Green flashes occur because the atmosphere can cause the light from the sun to separate out into different colors. Green flashes are a group of phenomena which stem from slightly different causes, and therefore some types of green flashes are more common than others.
Green flashes may be observed from any altitude. They usually are seen at an unobstructed horizon, such as over the ocean, but are possible over cloud tops and mountain tops as well. They may occur at any latitude, although at the equator the flash rarely lasts longer than a second.
A green flash also may be observed in association with the Moon and bright planets at the horizon, including Venus and Jupiter. With an unrestricted view of the horizon, green flashes are regularly seen by airline pilots, particularly when flying westwards as the sunset is slowed. If the atmosphere is layered the green flash may appear as a series of flashes.
While observing at the Vatican Observatory in 1960 D.K.J. O'Connell produced the first color photographs of a green flash at sunset.
Green flashes are enhanced by mirage, which increase refraction. A green flash is more likely to be seen in stable, clear air, when more of the light from the setting sun reaches the observer without being scattered. One might expect to see a blue flash, since blue light is refracted most of all, and the blue component of the sun's light is therefore the very last to disappear below the horizon, but the blue is preferentially scattered out of the line of sight, and the remaining light ends up appearing green.
With slight magnification a green rim on the top of the solar disk may be seen on most clear-day sunsets, although the flash or ray effects require a stronger layering of the atmosphere and a mirage, which serves to magnify the green from a fraction of a second to a couple of seconds.
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Wikipedia