Editing Green flash (section)

===Green rim===

[[Image:Green rim of the setting sun.jpg|thumb|The upper rim is green while the lower one is red, as the sun sets behind the [[Golden Gate Bridge]]]]
[[Image:Green rim and green flashes in SF.jpg|thumb|The green rim and flashes of a setting sun]]

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 (optics)|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 [[Air pollution|pollutants]] as they spread throughout an increasing volume of atmosphere.<ref name="Dispersive refraction">{{Cite web |url=http://www.webexhibits.org/causesofcolor/13B.html |title=Dispersive refraction |website=webexhibits.org}}</ref> The lower rim of an astronomical object is always red. 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,<ref name="Green and red rims"/> 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)|Little America exploration base]] in 1934:

{{quote|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.<br />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.<br />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 {{sic|[[antenna (radio)|antanea]]}} post.<ref name="byrd">{{cite news |first=Russell|last=Owen |title=Explorers see phenomenon in Pole area: Byrd party watches 'Green Flash' blaze in sky at Little America |newspaper=San Francisco Chronicle |page=5 |date=23 October 1929}} as quoted in {{cite web |
url=http://mintaka.sdsu.edu/GF/bibliog/bibliog.html|title=Annotated bibliography of mirages, green flashes, atmospheric refraction, etc.|
first=Andrew T.|last=Young  |access-date=2011-02-22}}</ref>}}

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.<ref name="Green and red rims">{{Cite web |url=http://mintaka.sdsu.edu/GF/explain/simulations/std/rims.html |title=Green and red rims |author=Andrew T. Young |website=mintaka.sdsu.edu}}</ref> That is too early to use any [[magnification]] like [[binoculars]] or a [[telescope]] to look directly at the Sun without potential harm to the eyes. (Of course, a magnified image might be projected onto a sheet of paper for safe viewing.) As the Sun gets closer to the horizon, the green rim becomes fainter due to atmospheric reddening.<ref name="Green and red rims"/> According to the above, it is probably correct to conclude that although a green rim is present during every sunset, a green flash is rarer because of the required [[mirage of astronomical objects|mirage]].