From beach sands and seaside bars, people scan the horizon and hope to spy a legendary gleam of green. Discover the truth about this sunset mystery.

By Holly Sherwin
April 20, 2004
William C. Minarich

I'm not sure when I first heard the term "green flash," or howmany evenings I sat on the beach at sunset, hoping to see thelegendary gleam. But what I clearly remember is the moment I firstwitnessed it wink at me across Florida's gulf waters.

Driving back to Naples Bay after a successful afternoon ofgrouper fishing, several friends and I debated this sunsetphenomenon and were skeptical. We'd heard that just as the sunslips over the horizon on a clear night, it can send up a spark ofgreen. But none of us had ever seen it. Some beachgoers spend alifetime looking for it. Others say it doesn't exist, that it's anoptical illusion.

So as we puttered along the shoreline toward Gordon Pass, wewatched the reddening sun and hoped that this sunset would bedifferent from hundreds we'd seen before. We counted down theseconds and held our breaths as the last bit of red sunlight fellbehind the water's far edge.

And then, there it was, just as I had imagined: a brilliantemerald-green flash. It flared up for a few splendid moments, thendisappeared almost before we could shout. Glad to have shared thismagical moment with friends, I was even happier that our debate onthe subject had ended.

Some claim that references to the green flash date as far backas 2500 BC. They cite diagrams on an Egyptian pillar depicting thesun as a semicircular disc painted blue at the top and green at thebottom as evidence.

Literature hints at the flash. One of the earliest publishedaccounts comes from W. Swan, who first observed the phenomenon in1865, but did not submit his writings to Nature magazine for almost 20 years. Some speculate thatJules Verne's 1882 romance Le Rayon-Vert (translated "the green ray") sparked awidespread interest in the flash that prompted Swan, and perhapsother observers, to let the public in on their sightings. Verne'saccount includes a quote he attributes to Scottish legend: "He whohas been fortunate enough to behold it is enabled to see closelyinto his own heart."

But as more witnesses came forward, opinion divided about theflash's true nature. Some argued that it was an optical illusionresulting from fatigue of the retina after looking at the red lightof a setting sun. Since green is the complementary color to red,the eye would see green after being dazzled by the bright red sun.But this would not explain the green flash sometimes seen atsunrise, when the retina would not be strained from afternoon rays.Plus, many fleeting flashes have been captured on film, eliminatingthe possibility that it's merely a physiological phenomenon.

Despite its elusive nature, the green flash can be easilyexplained by ordinary laws of optics. The atmosphere acts as aprism, separating the sun's light into a spectrum of colors. Eachcolor forms a separate image of the sun. Near sunset, a red rimappears on the bottom and a blue or green rim on the top. When ourstar is low on the horizon, this separation is greatest.

Since blue light has the shortest wavelength, the top rimscatters the moment it enters the earth's atmosphere, leaving thenext color in the spectrum―green. But for us to see this thinrim of green, it must be magnified. When air masses in the loweratmosphere are heated and cooled, under the right conditions, theair itself becomes a lens through which the sun's image (includingthe green rim) may more than double its original size. Voilà:the green flash.

Despite technical explanations and detailed accounts from aroundthe world, the most intriguing still comes from that purportedScottish legend. This lovely phenomenon is seen only when naturepresents prime conditions, but Jules Verne's story says it's onlyseen by true lovers. At the next sunset, I'll be looking forit.