"Green Flash" © 2007 photo by Mila Zinkova

"Green Flash" © 2007 photo by Mila Zinkova

I miss the ocean. My wife Julie and I took the most adventurous trip of either of our lives over the summer, flying to New Providence Island in the Bahamas to spend some time together in paradise.

The Bahamas was a little crass and commercialized, not really our style, but the ocean! We love to walk on the beach, stand in the ocean, feel the Sun and the water and the wind and the sand. I love that line, the line where water meets sky, that amazing, flat horizon that goes on and on and on, where two worlds meet. The ocean is where I belong.

One evening Julie and I stood in the warm water, watching the sunset. The horizon was clear, and as the Earth spun we watched the horizon approach the Sun. We could almost feel the Earth moving under our feet, the way the whole world just seemed to tilt toward the changing Sun. From yellow, to orange, to red, the Sun got lower and lower (or so it seemed), and began to disappear below that beautiful watery line.

And then it happened. Just as the Sun finally disappeared, at the very top of the disappearing Sun, we saw a flash of green. Julie and I both gasped as one. Did you see it? Yes, I saw it, did you? Yes! The green flash. We saw the green flash!

We watched the sky turn from blue to purple to black, then made our way back up the hill to our inn. It was an evening we both will remember for the rest of our lives.

So what is the green flash? It is the perfect Goldilocks effect, a result of something being “just right.” Like Julie. (Sorry, I couldn’t resist.)

The Sun is called a yellow-white star, but really it’s pretty much white when seen in outer space. Here on Earth, on the other hand, the Sun looks yellow for exactly the same reason that the sky is blue.

To understand what that means, first you have to remember what color really is. Color makes life beautiful. Green is my own favorite color – green sea turtles (which aren’t really green, of course), green Christmas trees, green grass on a springtime baseball field. But green is one wavelength, or set of wavelengths, of light. Red is a different wavelength, and blue another. White light doesn’t really exist (I wrote about that here).

Here’s the thing: different wavelengths (different colors) behave differently. Because of the size of pieces of air – molecules like nitrogen and oxygen – blue light, which has a very short wavelength, is scattered. That scattered light makes the sky blue. What’s left? Light from the Sun arrives at our eyes missing some of its blue. That makes the Sun look a little more yellow.

At sunset, light passes through a thick layer of atmosphere, also filled with dust and haze that helps remove even more blue and violet light. By the time that light reaches us, almost all of the blue and violet light has been scattered out. What’s left? Mostly red and orange, with some yellow and green mixed in. When it’s all added together, the Sun looks reddish.

We’re getting closer to the green flash. The light from the Sun passing through that thick layer of atmosphere bends. That’s called refraction. You see refraction all the time over hot pavement in the summer. Light is refracted differently by different-temperature air. That wavy look over hot pavement, a hot grill, etc., is the result of light waves bending through those different temperatures.

OK, we’re almost there. There’s still green in the Sun’s light. Just as in a rainbow, green light bends more than red light.  So the red light, orange light, etc, pass over our heads as the Sun dips below the horizon. The green light, on the other hand, for just a moment, bends just enough to follow the curve of the Earth and pass right into our eyes.

The green photons are of long enough wavelength to not be scattered, but of short enough wavelength to bend just the right amount. Like Goldilocks, the photons are just right. For just a moment, those green photons generated in the Sun, 93 million miles away, photons that are normally completely hidden, washed out, or blended in to all the other colors, are able to stand by themselves, enter our eyes, and build a memory that will last forever.

That ocean is still out there. Someday we’ll make it back.