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Green Flash

Big Bear and Pepe Millard

Sat 21 Mar 2009 23:17

Green Flash

If we are to believe the old legend that Jules Verne recounts in his novel The Green Flash, it is a magic light ray that endows those who are lucky enough to see one with the power to look deeply into their own heart and recognise true love. Well, seeing a green flash may not bring you heightened powers of self knowledge, but its existence is not just a myth, either. We have been lucky enough to see two this week.

A Green Flash as opposed to all you saddos that thought I meant Green Flash.

Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible for a short period of time above the sun, or a green ray shoots up from the sunset point. Green flashes are actually a group of phenomena stemming from different causes, and some are more common than others. Green flashes can be observed from any altitude (even from an aircraft). They are usually seen at an unobstructed horizon, such as over the ocean, but are possible over cloud-tops and mountain-tops as well.

Explanation

The reason for a green flash lies in refraction of light (as in a prism) in the atmosphere: light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the Earth. Higher frequency light (green/blue) curves more than lower frequency light (red/orange), so green/blue rays from the upper limb of the setting sun remain visible after the red rays are obstructed by the curvature of the earth. Green flashes are enhanced by mirage, which increase the density gradient in the atmosphere, and therefore increase refraction. A green flash is more likely to be seen in clear air, when more of the light from the setting sun reaches the observer without being scattered. We might expect to see a blue flash, but the blue is preferentially scattered out of our line of sight and remaining light ends up looking green. With slight magnification a green rim on the top limb of the solar disk can be seen on most clear-day sunsets. However the flash or ray effects require a stronger layering of the atmosphere and a mirage which serves to magnify the green for a fraction of a second to a couple of seconds.

Blue flashes - Very occasionally, the amount of blue light is sufficient to be visible as a "blue flash". The term should not be confused with the similar usage of blue flash referring to the blue light seen in nuclear accidents.

Green rim

Upper rim is green and lower rim is red while the sun is setting behind the Golden Gate Bridge

As an astronomical object sets or rises, the light it emits travels through the atmosphere, which works as a prism separating the light into different colors. The colour of the upper limb of an astronomical object could go from blue to green to violet depending on the decrease in concentration of pollutants as they spread throughout an increasing volume of atmosphere. The lower limb 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, but sometimes the conditions are right to see a green rim just above the horizon. The following quote describes probably the longest green rim, which at times could have been a green flash, observation. It was seen on and off for 35 long minutes by members of the Richard Evelyn Byrd party from the Little America exploration base. Often a green rim changes to a green flash and back again during the same sunset.

Green flashes in fiction

The green flash in Pirates of the Caribbean: At World's End

This phenomenon features as a major plot device in the Disney's film Pirates of the Caribbean: At World's End. In the movie's back story, the green flash is fictionalized, becoming an occurrence of legendary quality that is rumored to signify a soul returning from the dead. Later in the film, this concept is further expanded, and the flash is revealed to signal when a trapped spirit escapes from its imprisonment in Davy Jones's Locker, a feat achieved by the film's main characters. However, the size and scale of the flash depicted in the film is greatly exaggerated.

This phenomenon is also referred to in a novel called The Green Ray by Jules Verne - outlined above, a novel called Smith and the Pharoahs by H. Rider Haggard, a novel called Don't Stop the Carnival by Herman Wouk . The green flash is also briefly mentioned in M. K. Wren's "A Gift upon the Shore", a mythic story of survival after a nuclear holocaust. In the book Flush seeing the green flash is described as a family goal. In the book The Pride and the Peacock by Victoria Holt, a large opal named "The Green Flash at Sunset" is the titular treasure. The opal is so named due to a glint of green that may be rarely seen in certain light and at a certain angle to the gem. In a film by Eric Rohmer called Le rayon vert and a 2008 film directed by Paul Nihipali, Green Flash.