Robert Avent's book suggestion, Heuvelmans' "In the Wake of the Sea
Serpent," is an excellent one. Heuvelmans may be gullible, but his method
is scientific and readable at the same time.
 
The green flash is a spark of yellowish-green light, the last glimpse of
the sun as it vanishes behind the horizon, seen only under special
circumstances. The horizon must be distant, unobstructed by trees or
buildings. The air must be very clear, and free of clouds where the sun
sets. (This condition is now rare in the eastern United States owing to
widespread air pollution.) And it lasts only for a moment, so you have to
look at the right time to see it. Under these circumstances, and with the
observer ready for it, the green flash is not all that rare; I have seen it
at least four times, twice in California and once each in Alabama and
Nevada. Except in Nevada, I saw it over the ocean each time.
 
How can you watch the bright sun at sunset without burning out your eyes?
In fact, you cannot and must not watch the sun continuously under these
circumstances, which are very dangerous. What you can do, is to keep track
of the sun using your peripheral vision, checking it only occasionally as
you watch other parts of the sky. When the sun is ready to dip beneath the
horizon, look at it momentarily. Usually, there is no green flash, but once
in a while, the last bit of an orange or red sun will suddenly brighten
into a green spark and then vanish.
 
Shellers often place themselves in situations where the green flash is
visible, but if you aren't looking at just the right time, you can spend a
long lifetime without seeing it. The same is true of many other natural
optical effects, such as odd rainbows, haloes, sundogs, glories, etc. I
recommend a short Dover book, "The Nature of Light and Colour in the Open
Air", for those who are interested.
 
A professor once required each member of his class to go outside and scan
the entire sky once a day and record any optical effects they saw. Since
the students chose any time they pleased, the professor ended up with a
complete record of the optical effects during each daylight period for an
entire term. He, and the class, discovered that optical effects are
surprisingly common, but generally pass unnoticed.
 
At the end of a long career, an eminent geologist was asked what he would
do during his retirement. "I have spent my life looking at the ground," he
said. "Now I am going to look at the sky." And he did. He watched clouds
and birds, without regretting the time he had spent looking at the ground.
I often think of this while collecting shells in the beautiful outdoors,
and take a moment or two to scan the sky.
 
Andrew K. Rindsberg
Geological Survey of Alabama