The Green Flash

Now is a great time for sunset watchers to seek the green flash. Contrary to widespread belief, the green flash does exist and it is best seen when the sun goes down behind a distant horizon such as you get at sea or over the lake. But by fall, as the sun begins to trend south, it sets over the land at many spots by the lake shore making the green flash unlikely. So May through early September are prime green flash months.

The green flash is a genuine optical phenomenon. It has been measured as a spectrum and it has been captured on film ( check out plate six in Dr. Marcel Minnaert's book Light and Color in the Outdoors for one color photo of it). It’s best seen on a clear night when the sun stays sharp and distinct up until the very end of the day as the last tiny glimmer slips over the hard edged horizon of earth's curve. You won't get a green flash on a sultry evening of haze and cloud when the sun becomes a brilliant red round ball and then fades into the gray murk hanging on the horizon.

The green flash, I read in Dr. Marcel Minnaert's book, can appear in several forms. Sometimes it seems to be a brief slice or blob of color seen atop the regular sun as it sets. Or it can appear in more spectacular settings as a slender column or candle flame shaped ray of green light, lasting perhaps three seconds immediately after the sun has vanished. It is usually emerald green but sometimes it may appear to be blue or violet. It has been seen from airplanes and during an expedition to the South Pole it was observed for 35 minutes. This happened because the sun was moving exactly in line with the horizon, allowing for refraction to separate out the green light.

Different colors of light bend different amounts as they pass through the atmosphere. Rays of green light bend slightly more than do red rays, so this makes visible two solar disks superimposed and just slightly out of alignment. The blue green one appears a bit higher than the red one. Conditions causing mirages are good for especially strong refraction, explains Minnaert, and can distort and magnify the green rim into an actual brief green ray, shooting up several sun diameters from the horizon.

I've watched dozens if not hundreds of sunsets over the water during thirty odd years of sailing that would seem to fit the criteria for the green flash and I have yet to see it. But Dr. Minnaert explains a possible reason for my failure. My sunset watching technique needs modification. According to Light and Color in the Outdoors, the trick is not to look directly at the sun until the last moment just before it slips under. Turn your back to it until someone lets you know when its nearly gone. Then watch for a flash or green ray in the last second or two before it utterly vanishes. If you start watching too soon, you'll burn the sun's image into your eye and get an after image (which also looks green). This prevents you from seeing the true green flash.

I haven't been able to watch a good clear sharp sunset since reading this, but next time I do so, I plan to try this trick. Dr. Minnaert writes with a bit of practice you can frequently see the green rim of the sun and when large temperature differences exist between land and water, conditions are especially favorable to see a flash.

Conditions were right but no flash appeared to my camera this evening-

Dr. Minnaert says you can see the flash or a green rim of the sun at dawn too if you know exactly where the sun will first peak over the horizon. Since I don't get up for that early morning shift at the cannery anymore, I don't catch very many sunrises, so someone else will have to look for this one. If anyone does spot a true green ray or flash, ( not a round green spot retinal after image,) drop a line with the date and place and I'll post it. Happy sunset watching!