Have you ever seen 'The Green Flash'? Jonty Pearce gets to the bottom of the phenomenon, and how to catch a glimpse
How many times have we sat watching the sun dip down to the horizon on a beautiful summer’s evening? The sky turns golden, the sun appears to expand, and an orange glow spreads out from the sinking globe of fire. Finally, with the first touch of the horizon, the sun diminishes surprisingly quickly as a quarter, then a half, and finally the last segment sinks below the sea level. And then, quite abruptly, its light is gone and all that is left are the amber memories of sunbeams backlighting the evening sky. The air starts to chill, and the magical moment has gone. A few more sips at your favourite drink, and it’s time to turn and go indoors to the warmth. Or if you’re on a boat, to put another layer on.
But aren’t we missing something? Legend has it that a wink of light can follow the sun’s demise – the so-called Green Flash. Many times have I sat and watched for it and been disappointed, and I had started to suspect that it was a mythical tale borne of overactive imaginations. But now, in the words of the Monkees, I’m a believer.
For I saw it clearly two nights in a row. The sun sank, and disappeared as usual. And then, just as one would turn away thinking that the show was over, a momentary clear flash of green leapt out to surprise us and disabuse our misbelief. I was not expecting the delay of several seconds before the separate and distinct green beam shot out; the reports I have read suggest more of a green rim or peak to the dying sun rather than the darkness before a separate green light that we saw. The gap was so pronounced that, much to our (sympathetic) amusement, crew member Johnny had turned away at the critical moment thereby missing the phenomenon.
When the conditions proved favourable again the following sunset, we were all ready and watching. Just as the flash shone out, I regrettably exclaimed ‘There!’ at which point Johnny, whose hearing is not amongst the most acute, turned to me, saying ‘What?’ thereby missing the event again. I’m afraid the green flash did not make another appearance on our trip, though we relived the humorous moment many times until Johnny grew a little weary of the repeated cries of ‘What?’
Our sighting of the green flash took place in the middle of the Atlantic during a period of stable high pressure and glorious weather with an unimpeded sharp horizon. Green flashes are most commonly seen in stable, pollution-free clean air with a cloudless horizon; most of our sunsets dipped into a thin layer of cloud clinging just above the sea level that obscured the horizon proper and preventing any view we might have had of the phenomenon. It is caused by diffraction of the sun’s rays into a vertical rainbow of colours; a blue flash could therefore be expected since that colour is refracted most of all. In practice, while the blue component of the sun’s light is actually the last to disappear below the horizon, the blue is weak and scattered out of the line of sight leaving the remaining stronger light to appear green. Rarely, when the air is especially clear, enough of the blue or violet light rays can make it through the atmosphere for a blue flash to occur.
Jules Verne speaks of an ancient legend in his 1882 novel Le Rayon-Vert that suggests that those who see the Green Ray are 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’. Unfortunately this is false 19th Century romantic embroidery invented to sell literature. But it would be nice if it were true.