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Twilight is a time like no other.
Watching people on the beach watch the sun go down can be as interesting as observing the colors and shapes on the horizon. I know
of no other natural phenomenon that produces applause and cheers from throngs of onlookers. The observance of day’s end seems almost instinctive, a throwback to some tribal ritual of long ago.
People watch the sunsets for different reasons. Over the years viewers have squinted into thousands of sunsets attempting to see the elusive “green flash,” sometimes successfully. I am one of those who have been unsuccessful.
Until a few years ago I had always assumed that the flash was a local oddity, something on the order of the “swamp gas” sightings over the Everglades. However, it is actually a worldwide occurrence, even appearing in the dictionary with the following definition: a momentary green appearance of the uppermost part of the sun’s disk that results from atmospheric refraction when the sun sinks below or rises above the horizon. The December issue of the magazine Astronomy devotes an article to the green flash and related optical illusions. Here is the gist of it.
Sunlight is bent as it travels |
through the Earth’s atmosphere – with some colors being bent more than others. Red is bent the least, followed in increasing order by orange, yellow, green, blue and violet. As the light rays are bent, the colors become separated. Sometimes when the light is bent in a certain way, it forms a rainbow. Sometimes, under
different circumstances, it creates images such as the green flash.
When the sun is near the horizon, its light must pass through more of the atmosphere before reaching our eyes, so the light rays are bent much more than when the sun is high in the sky. The result is that, near the horizon, there is a stack of overlapping images of the sun, with the red one at the bottom and the violet at the top.
We normally don’t see the blue or the violet images because they are scattered into the atmosphere, causing the entire sky to be blue. Water vapors in the air usually
absorbs the orange and yellow rays. This leaves the faint green image which penetrates a cloudy or dusty sky far better than any other color.
When the red image sinks below the horizon an irregularly shaped patch of green remains above for an instant, producing the flash. Some connoisseurs say that a
yellow or orange sun produces the best green flashes. Usually it is all over in a second or so.
In order for the flash to be seen, the atmosphere must be extremely clear and the horizon must be distant, well
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defined and flat. A view looking out over water is best.
Don’t stare at the sun too long
before it sets or you may see a false green afterimage on the retina of your eyes. You can create a bogus green flash by raising and lowering your head behind an artificial horizon such as a concrete wall just before sundown.
Some observers on boats have reported seeing the flash more than once during the same sunset as they slowly rocked back and forth on heavy swells. Admiral Byrd once observed the green color for 35 minutes as the sun rolled along the horizon in the Antarctic. These and other tales are in the Handbook of Unusual Natural Phenomena at the local public library. A classic description of the flash is in the January 1960 issue of Scientific American.
Even if you don’t manage to see it, it is a great sport to watch for the green flash. Sometimes the water looks so glassy and otherworldly just after the sun sets it seems you could walk out and look over the edge to take another peek at the rim of the sun where the green should have been. If you don’t see it, there’s always tomorrow.
The sun’s light is split into multiple images when it is near the horizon. Diagram at the left shows the red and green suns. The top of the green sun is sometimes visible after the brighter colors set as shown at right. |
