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The "old Scottish Legend"
No story is more widespread, nor more false, than the
"ancient" legend Jules Verne introduces in his 1882 novel Le
Rayon-Vert, according to which, one who has seen the Green
Ray is incapable of being "deceived in matters of
sentiment," so that "he who has been fortunate enough
once to behold it is enabled to see closely into his own
heart and to read the thoughts of others."
This is the stuff of 19th-century romantic French novels,
not Scottish folklore. Anyone who spends an afternoon
in the folklore section of a good library will find not only
that no such legend exists; but that the Scots, along with
the rest of the Celtic people, regard green as a color
associated with evil spirits, death, and misfortune.
The after-image myth
The notion that green flashes at sunset are
after-images, or due to contrast effects in the eye, seems
plausible to many people who are unaware of green flashes at
sunrise, which in fact were obseved from the earliest
times. Today, of course, it takes only a color
photograph to show that real green flashes exist.
Nevertheless, there is a physiological component to sunset
flashes (and not to sunrise ones): and this may help explain
the persistence of this faulty explanation. The effect
is bleaching of the red-sensitive retinal pigment.
Thus, the appearance of sunset green flashes is uaually a
mixture of physics and physiology; but sunrise flashes are
almost pure physics. And, even when physiology lays an
important part, the flash is not an after-image.
The sun shining through the waves
Another popular incorrect explanation attributes the
flash to the sun shining through the waves on the
ocean. Apart from the fact that this is contrary to
the laws of optics--light entering a wave is bent downward,
into the water, and cannot escape again--observations of
flashes over land horizons show this is not a correct
explanation. Furthermore, the mock-mirage flashes
appear well above the horizon, when a considerable part of
the sun is still up; clearly, this green light has nothing
to do with the waves. Indeed, close examination shows
that even the inferior-mirage flashes are slightly above the
apparent horizon, and thus cannot be due to light shinging
through sea-water. You can see this both in a
photograph and in a simulatin of such a flash.
The path length confusion
Many people think the flash has something to do with a
long air path. One often sees remarks to the effect
that it's caused by the large amount of air you have to look
through at the horizon. It's hard to see where this
idea came from. Maybe these people are thinking of the
removal of the orange light (mostly removed by ozone in the
upper stratosphere). The long air path explains why
flashes are more often green than blue or violet: a
combinatin of Rayleigh and aerosol scattering selectively
weakens the shortest wavelengths, so that green is usually
the last color seen at sunset, and the first to appear at
sunrise. In polluted urban air, the long air path
often removes the green as well, so that the flash is an
unspectacular yellow. Green flashes are by-products of
mirages; so it is the mirage, rather than the path length,
that should receive attention.
The textbook fallacy
Though the connectoin with mirages was apparent to
several careful obsevers, it never got into the
textbooks. They uniformly give the impression that
"normal" refraction alone will produce a flash. The
idea was just that the normal green trim would be cut off by
a sharp edge, such as a distant horizon; the remaining
"green segment" would be the flash itself. The picture
was reinforced by Mulder's name "green segment" for one of
his three main classes of flashes. This is still the
explanation you find on most Web pages, too.
But this notion was soundly refuted by Gerhard Dietze, whose
theoretical treatment of this textbook model in 1955 showed
that it produces a green flash ten times smaller than the
naked eye can see. Unfortunately, Dietze published in
an East German meteorological journal, so his work was
overlooked by O'Connell (an astronomer) and indeed by
everyone else for some 40 years.
The Byrd expedition
One often reads that Admiral Byrd saw a green flash
lasting 35 minutes. This story has a factual basis;
but the story isn't a fact.
Byrd's brief descriptin of this episode says that the polar
sunset was "prolonged" by the sun appearing and reappearing
from behind the barrier surface. Hence they had a
series of sunsets. The "green sun," lasted 35
minutes. Even from this brief account it is obvious
that the green was not seen continuously for 35 minutes, but
only off and on. A fuller account was published by the
expedition's meteorologist, W. C. Haines, who explained
that
"The sun was skirting the southern horizon, its disk
disappearing at intervals only to reappear again a few
moments later. ... The irregularities in the
snow surface permitted the upper limb of the sun to appear
in one or more starlike points of light from adjacent
notches. These points or flares of light would
sometimes have a greenish color on their appearance or
disappearance. The length of time during which the
green flare was visible varied from a fractin of a second to
several seconds ... When the sun sank too low to be seen
from the ground, it was still visible from elevated points
such as the anemometer post or radio towers. The above
effect was seen at intervals during a perod lasting over
half an hour. Conditions were more favorable for its
occurrence when first observed. Later the green
appeared for shorter and less frequent intervals, and the
orange and red flares increased in frequency."
This account is much different from the one broadcast by a
radio program on Sept. 29, 1999: "Richard Byrd, on one
of his expeditions to the south pole, saw the green flash
for an amazing 35 minutes, as the rising sun slid across the
horizon at the end of the long, dark winter."
Transcript:
http://www.mountwashington.org/notebook/
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