Elusive ‘green flash’ at sunset starts, ends in a blink
From June 25, 2004:
I guess a lot of you have been going to San Diego lately, because all of the sudden I’m getting questions about the green flash. I’ve had three or four in the past week or so.
This is the deal with the green flash, that quick burst of green light some people think they see just as the sun drops below the horizon: Chances are you think you saw it, but you didn’t.
It’s rare.
Now, first of all, when you see the sun setting on the horizon, it really isn’t there.
It has already set and is below the horizon by about one diameter’s worth. The light you are seeing is sunshine bent by refraction in the atmosphere. It’s also the thickest part of the atmosphere.
So the sun’s rays are being bent and dispersed and broken by the atmosphere, and colored arcs of light are visible just above and just below the disk of the sun, which is all kind of orangish.
Now because of the different wavelengths of the different colors in the spectrum and the way refraction works, you’re going to see a band of red under the sun’s disk and possibly a band of green, the color that is being bent the most, along the top of the disk.
There are astronomers who will tell you a true green flash is too small to be seen with the naked eye.
Others will tell you the brilliance of its color makes up for its size.
But I am pretty sure that you are very lucky if you really, truly have seen the green flash.
It lasts no longer than the blink of an eye, and conditions have to be perfect: no clouds, no haze, no junk in the atmosphere.
I have a feeling that a lot of people who think they have seen the green flash actually have been staring at the sun for too long, which isn’t a good idea.
When you do that, you kind of get a green afterimage burned onto your retina.
So if you’re going to the beach to try to see the green flash, don’t look at the sun until that very last instant before it sets.