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A Cape Cod Notebook can be heard every Tuesday morning at 8:45am and afternoon at 5:45pm.It's commentary on the unique people, wildlife, and environment of our coastal region.A Cape Cod Notebook commentators include:Robert Finch, a nature writer living in Wellfleet who created, 'A Cape Cod Notebook.' It won the 2006 New England Edward R. Murrow Award for Best Radio Writing.

The Green Flash

Brocken Inaglory / wikipedia
An example of a "green flash." This image was taken in Santa Cruz, California

Today I’d like to talk to you a bit about the “green flash.” And no, by that I don’t mean the merging of two of my boyhood comic book heroes, the Green Arrow and The Flash. No, I mean the “green flash,” one of the most common yet rarely observed of celestial phenomena.

Many people have observed lunar eclipses, solar eclipses, blue moons and super moons, aurora borealises, meteors, and even - as we saw last summer – a comet, and yet relatively few people have ever witnessed the green flash.  I myself was nearly 60 before I saw one. The lack of sightings is even more strange since the green flash is a phenomenon that occurs somewhere every day.

But first things first. The green flash is a solar phenomenon. It is a small green bud or flattened oval that appears to sit at the top of the rim of the solar disk just before sunset. It generally lasts less than a second – hence the term “flash.” Yet even if you make a determined effort to see one, you are likely to fail.  Here’s why:

As the sun begins to set, light from the diminishing solar disc is refracted, or scattered by the atmosphere.  It’s the same process that causes the sky to appear blue during the day, when all but the blue spectrum is scattered. Because of the different atmospheric conditions that prevail at sunset, it is the green light-waves that remain on the sun’s rim, causing the “green flash.” Got that?  Well, neither do I, but you’ll have to take my word for it.

That still leaves the mystery of why the flash is so seldom seen. The answer lies in the conditions that must be present for the phenomenon to be observed. The green flash is best observed close to sea level on clear days, with an unobstructed view of the horizon and when the water is warmer than the air. Now does that sound like any place you know?  Anyone? Yes, you. That’s right: Cape Cod Bay! During clear autumn days, those of us on the Lower Cape are ideally situated to see the green flash, which makes me think I have just been lazy.

So to test this, one late afternoon in October, my wife and I drove out to the beach at Bound Brook Island. The sun was just dropping out of a low cloud cover, lighting up the faces of the sloping dunes. There was a slight haze in the air. To the north the low silhouette of Provincetown seemed to float and shimmer in a gauzy, rose-orange light, and the entire row of Days Cottages shone with reflected sunlight like a perfect string of gold diamonds.

The tide was out, and the beach, peppered with cobbles, shimmered and shone gold. The sea to the southwest was curled into flat waves by a stiff onshore breeze. As the sun hit the horizon, I climbed up the dune to the top of a bearberry-covered rise and sat down to watch, dangling my feet over the edge. Kathy joined me, kneeling down and wrapping her arms around me from behind. As the last sliver of the sun’s red disc began to disappear below the horizon, I whispered, “Watch for the green flash,” not really expecting one. And then, There it was, indisputably; a definite, pure, lime-green apparition, like a button sitting on top of the sun. It lasted only a second, and was gone. 

Robert Finch is a nature writer living in Wellfleet. 'A Cape Cod Notebook' won the 2006 New England Edward R. Murrow Award for Best Radio Writing.