Have you ever seen the sun set at the seaside? Yes? And did you follow it until the top edge of the sun’s disc just touched the horizon and then started to disappear? Probably. But did you observe the phenomenon that occurs at the instant of the last ray of light when the sky is perfectly clear? Perhaps not. Well, the first time that the opportunity for such an observation offers itself (it is very rare), take it and you will see that it is not a red ray, or rather flash, but a green one; a wondrous green that is not found anywhere else in nature. If there is green in Paradise, it must be this green: the true green of hope!

Jules Verne, Le Rayon Vert (1882)

The sequence above consists of three frames from a short sunset video recorded by the Boon Companion in the Caribbean recently. Something odd happens in the last frame—the rim of the sun turns apple green. The effect was visible to the naked eye, and lasted something less than a second.

This phenomenon is called the green flash, and during ten suitable Caribbean sunsets (clear horizon, yellow sun), I managed to see it six times (though four of those were rather feeble offerings). Its cause is superficially simple, but there are complications.

When I wrote about the shape of the low sun recently, I described how light rays travel in a curved path through the atmosphere, lifting the image of the setting sun, like this:

So that, at sunset, the sun is actually below the geometrical horizon, but lifted into view by atmospheric refraction:

What I didn’t mention is the phenomenon of atmospheric dispersion—the refractive index of air is slightly different for different wavelengths of light. Shorter wavelengths are bent more than longer ones. This means that the red image of the sun is lifted slightly less than the green image of the sun. (The blue image of the sun would be lifted even more, but blue wavelengths are very efficiently scattered by the atmosphere before they reach our eyes.) So the true story of atmospheric refraction looks like this:Which means that the image of the sun arriving at our eyes is actually a little smeared in the vertical direction, with more green at top and red at the bottom:In reality, the effect is so small that it isn’t visible to the naked eye under most circumstances. O’Connell calculated that the green rim is only 10 seconds of arc wide—that’s about a 180th of the width of the solar disc, and a sixth of the normal resolving power of the human eye, which is about 60 seconds of arc. (For a discussion of visual resolution see this previous post.) So the green and red rims are simply lost in the yellow overall colour of the solar disc. But when the sun has dropped so far below the visual horizon that only the green rim is visible, then all that can reach our eyes is green light. Even at the equator, with the sun dropping vertically below the horizon, that narrow green rim will be visible on its own for 2/3 of a second. At higher latitudes, with slower sunsets, the visibility will be longer. And that’s the common explanation of the green flash—the momentary visibility of the upper green edge of the solar disc after the rest of it has dropped below the horizon.

But: what can you actually see, if the green rim is narrower than the resolving power of your eyes? You’ll find website claiming that it will simply be invisible, and that there must be some other explanation for the green flash. But a moment’s reflection suggests that’s not true. Every star in the night sky is so far away that its disc is below the resolving power of the human eye, but they’re not invisible. As a concrete example, the star Betelgeuse, in the shoulder of Orion, has a disc 0.05 seconds of arc in diameter—that’s 200 times smaller than the green rim of the setting sun, but we not only see it very well, we can clearly distinguish its orange-red colour. What happens is that your eyes smear out the light of Betelgeuse into a little blob about sixty seconds of arc across, and it becomes apparently dimmer in proportion. If our eyes could resolve its tiny disc, it would be a spark of light with the eye-watering surface brightness of the filament in an old-fashioned incandescent light-bulb.

So that’s what happens to the green rim of the sun. At the horizon, it’s horizontally wider than the eye’s resolving power, but blurred vertically so its apparent brightness is reduced. It forms a horizontal bar of green light, brighter in the middle and fading out towards the ends. And when it’s that close to the horizon, its light is also significantly dimmed by atmospheric scattering. What we see is then determined by how well that blurred bar of green light stands out against the bright background of the sunset sky. Conditions are going to vary from sunset to sunset, but it seems that the green rim could plausibly be responsible for a rather anaemic and very short-lived green flash.

Sometimes, though, the flash is a bright, pure, emerald green. I saw two of these out of my six Caribbean green flashes. So there really must be more to the story.

There’s a hint of the cause in this picture, taken just before the video sequence at the head of this post (you may need to click on it to see the full resolution):

If you look carefully, the setting sun has a tiny flared skirt immediately above the horizon. This is actually a reflection of the solar disc. It’s generated by a mirage—surface air has been warmed by the ocean, producing a low-density layer that refracts light back to our eyes, as if there were a mirror hovering just above the distant waves.

Below are some more extreme examples of the same effect. An “Etruscan vase” sunset:

And an “omega” sunset:

In each case the sun is setting behind a “reflective” mirage layer positioned just above the horizon.

When I turned my (filtered) binoculars on the sun just as the green flash started to develop, I saw something that looked like this:

The upper part of the ellipse is the direct view of the sun and its green rim; the lower part is a miraged image. As the sun set they closed together like a winking eye, until they formed what Marten Mulder called a green segment.

This green segment is deeper than the green rim—depending on the optics of the mirage, several times deeper. And that’s what increases the intensity of the light so that we see a proper, vivid green flash.

There are other kinds of mirage that similarly enhance the green flash, but this inferior mirage is by far the most common.

If you want to prolong your view of the green flash, you have two options. One is to move upwards to match the speed at which the sun is setting, so that you can keep the green rim continuously in view. Marcel Minnaert describes running (presumably backwards) up the slope of a Dutch dyke while watching the setting sun. He was able to keep the green flash in view for about 20 seconds. The other option is to head to polar latitudes, where the sun rises and sets by skimming along the horizon. In 1929, from Richard Byrd‘s Antarctic camp “Little America“, the green flash was observed during a long polar sunrise—it flickered in and out of existence for 35 minutes as the upper rim of the sun skated along the irregular horizon.