Monday, March 15, 2010

The Green Flash

In Griffith's great book Blue Water, he writes that the voyager is "a connoisseur of sunsets, a seer of the green flash." Light from the sun consists of all colors, and each color corresponds to different wavelengths, longest for red, shortest for blue and ultra-violet. As light from the sun passes through the atmosphere, it refracts, or bends, and the amount of refraction depends on the wavelength of the light. Red is bent least, blue and ultra-violet the most. The shortest blue wavelengths are so refracted as to scatter amongst the air molecules and across the sky - we see light of that wavelength in every direction, and the sky is blue.

The green is refracted not quite so much as to scatter, and when the sun has set or nearly set, there may, under optimal conditions, occur a moment when the only light refracted sufficiently to reach the observer consists of green wavelengths. Sometimes the flash is bluish, and sometimes, very rarely, violet.

For this explanation I am again indebted to my 1966 Bowditch.

When I have a clear horizon at sunset I watch for the green flash, and I have seen it but twice. It lasts longer at high latitudes, as does the sunset, and Bowditch informs me that at mid-latitudes it lasts about .7 seconds. That seems right to me - a flash, but a long discernible flash.

On my cutter I always watched for the green flash, and my persistence became a matter of shipboard humor. It was generally held even by the captain that the green flash was a myth. One perfect evening we were off Jamaica, cruising downwind in the Trades, just beautiful. It was after dinner and there happened to be a few officers on the bridge, including the C.O. I had the watch, and I stood on the bridgewing and as usual watched for the green flash. The captain and some others joined me, the usual jokes being made.

Just as the upper limb of the sun sank the horizon, there appeared at that spot a brilliant emerald light, obvious to everyone. It stayed for a long moment, and then it was night.

1 comment:

  1. Your post brought to mind a book in my library called "Light and Color in the Outdoors" by M.G.J. Minnaert, who painstakingly and lucidly explains the science behind many of the these atmospheric phenomena. There are four pages alone on the green flash. You can browse it here on Google Books:

    Warm regard,


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