Green flashes and green rays are meteorological optical phenomena that sometimes occur just after sunset or right before sunrise. When the conditions are right, a distinct green spot is briefly visible above the upper rim of the Sun 's disk; 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 Earth's atmosphere can cause the light from the sun to separate out into different colors. Green flashes are a group of similar 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.
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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. While observing at the Vatican Observatory in , D. O'Connell produced the first color photographs of a green flash at sunset. Green flashes are enhanced by mirages , 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. The "green flash" description relates to a group of optical phenomena, some of which are listed below: . Some types not listed in the table above, such as the cloud-top flash seen as the sun sinks into a coastal fog , or at distant cumulus clouds , are not understood.
Rarely, the amount of blue light is sufficient to be visible as a "blue flash". As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere , which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants , as they spread throughout an increasing volume of atmosphere.
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A green rim is very thin and is difficult or impossible to see with the naked eye. In usual conditions, a green rim of an astronomical object gets fainter when an astronomical object is very low above the horizon because of atmospheric reddening,  but sometimes the conditions are right to see a green rim just above the horizon.
The following quote describes what was probably the longest observation of a green rim, which at times could have been a green flash. It was seen on and off for 35 minutes by members of the Richard Evelyn Byrd party from the Antarctic Little America exploration base in There was a rush for the surface and as eyes turned southward, they saw a tiny but brilliant green spot where the last ray of the upper rim of the sun hung on the skyline. It lasted an appreciable length of time, several seconds at least, and no sooner disappeared than it flashed forth again.
Altogether it remained on the horizon with short interruptions for thirty-five minutes. When it disappeared momentarily it seemed to have been shut off by a tiny spurt, an inequality in the skyline caused by the barrier surface. Even by moving the head up a few inches it would disappear and reappear again and after it had finally disappeared from view it could be recaptured by climbing up the first few steps of the antanea [ sic ] post.
For the explorers to have seen a green rim on and off for 35 minutes there must have been some mirage effect present. A green rim is present at every sunset but it is too thin to be seen with the naked eye.
Often a green rim changes to a green flash and back again during the same sunset. The best time to observe a green rim is about 10 minutes before sunset. The image above shows a dramatic example of a green flash, as seen from the Canary Islands.
Green Flash: What It Is and How to See It
While this particular atmospheric phenomenon was considered a myth for a long time, its exact causes have been known for an equally long time, and the only reason why the existence of green flash was doubted was that it only lasts for about two seconds or so. It should be noted, though, that no part of the Sun actually turns green when the flash occurs; the flash is purely related to how sunlight is refracted and scattered through different layers of the atmosphere.
The explanation is very simple. Although the green flash is visible in most sunsets, the effect may be too brief to be noticed unless an observer actively looks out for it.
Illuminating the Sunset's Green Flash
However, when a mirage is present along with exceptionally strong layering of the atmosphere, the effect is greatly enhanced by the mirage, and under these conditions, the flash may be visible for up to two or even three seconds, depending on the local conditions. In rare cases, the flash may be seen as blue, but there is as yet no explanation as to why the momentary flash would be blue, instead of green.
The causes of a green rim are largely similar to those that cause the green flash, but most instances of the green rim are too faint for naked eye observation, although they often show up in photographs.