The search for the green flash

After years of trying on every ocean, in numerous ships and on dozens of islands, I finally saw it. On Feb. 6 of this year, from the waterfront patio of the Fort Young Hotel in Dominica, I finally saw the fabled “green flash.” It was bright green, sort of twinkly, very surprising and very brief. I was elated.

Some of you know exactly what I am talking about, but for those who don’t, the green flash is a sunset (or sunrise) phenomenon usually observed from ships and sometimes on shore in the tropics around the world. It is a very brief flash of green that appears right at the sun’s “upper limb” (meaning the upper curve of the sun), as it is called by sailors practicing celestial navigation.

It usually appears only when the sun sets (or rises) over the ocean, hence sailors have a bit of an edge in seeing it. (Some claim that it can be seen over a prairie, but I have never met anyone who saw that.)

My wife, Susan, managed to see the green flash twice, years ago, before I even saw it once. She saw it off Hawaii while I was looking at the same sunset, only I blinked and she didn’t. She said, “It was like a little twinkle of green right above the sun, which appeared just as the top of the sun dropped below the horizon. It only appeared for about a quarter of a second. The flash was much smaller than the diameter of the sun, so it is very hard to see and easy to miss.”

Of course, I believe her. I would never, ever consider the possibility that she made that story up just because she knows I have been looking for the green flash for years, ever since I heard about it.

Conditions for my first sighting were, surprisingly, not very favorable. There were some small clouds above the sun and some thin, distant showers on the horizon that created a golden glow around the sun. There was some humidity in the air, making everything appear rather hazy instead of crisp and clear. But I definitely saw it.

A week later I saw it again. I was chasing the tall ship Star Clipper in a small runabout, taking pictures. Other vessels kept blocking the sun, then clouds moved across the sun, but just at the right moment there was suddenly a direct view of the sun as it disappeared. And, again, just at the top of the sun, there was a little twinkle of green smiling at me for about a third of a second. Once you have seen it and know what to look for, it becomes a bit easier to see it again.

I tried both times to photograph the phenomenon, but both times the flash was so fast and the green area so small that the digital photos didn’t turn out, even when enlarged.

Researching the green flash, I found that it is caused by the refraction of the spectrum of visible light from the sun when it is very low on the horizon. The atmosphere acts as a prism, breaking the sun’s image into several overlapping spheres of light, including red, green and blue. The red is common, but the green sphere, peeking above the red, is rarely glimpsed, and this is the green flash.

For every 10,000 green flashes, there is a very rare “blue flash,” which is similar but bluish-purple in color. I have never seen that. The atmosphere usually scatters the light from the blue sphere to the side and only rarely allows the top of the blue sphere to shine straight through to your eye.

Mirages caused by layers of air with different temperatures can cause the shape of the sun at sunset or sunrise to become hourglass-shaped or even square and can influence the size, shape, color and duration of the flash.

Usually, the green flash is quite small, as I saw it (see figure 1), but very occasionally, when conditions are right (and no one can predict this and it is not very well understood), the flash dramatically extends far out from the sun, several diameters of the sun in extent (see figure 2).

Most people sail and travel for years and see only a few green flashes — or none. But some folks are much luckier.

Ecstatic about my first sighting, I told the manager of the Fort Young Hotel, Mrs. Francica Knight, about my feat.

She gently replied, “Actually, I have seen hundreds of green flashes. I often leave work just before sunset, then stop on a hill overlooking the Caribbean on my way home. I often see the little twinkle of green, but sometimes I have seen the big wide streak of green that flashes out very quickly from the top of the setting sun. Nature is so wonderful, isn’t it?”