Catch the Green Flash!

One day many years ago, I visited a southern New Jersey beach and watched the Sun set. The sand glowed with a warm gold. A vast expanse of Delaware Bay spread before me with crimson-tinged waves. The huge, flattened globe of the Sun had dimmed enough for me to look straight at it as its bottom edge touched the distant horizon.

Green Ripples
From high-elevation vantage points, you may see green flashes ripple off the top of the setting Sun distorted by temperature layers in the atmosphere.
Peter Aniol
I was filled with the grandeur of the scene, thrilled to watch as that fiery globe sank from view — going . . . going . . . almost gone. Suddenly the loveliness was pierced by a stronger beauty so intense that it left my jaw hanging. For just a second or two, the last tiny piece of Sun gleamed a vivid green. I shouted in surprise and joy on the empty shore. I had just seen the legendary green flash.

While people often tell me about their sightings of meteors or halos around the Moon, hardly anyone reports seeing the green flash. Why haven’t more people witnessed this astonishing phenomenon? The biggest impediment has been misinformation. Many sources claim that the green flash is very rare, or only visible in the tropics, or seen only at sunsets over the ocean.

In reality, mild versions of the green flash are not uncommon in most climates, and even its most spectacular forms don’t necessarily require a sea horizon to observe. Furthermore, the phenomenon only rarely appears as a flash. Far more often you’d see a rapid coloration of the last tiny blip of Sun as it sets.

Literary Beginnings

The green flash didn’t gain public attention until 1882, when it appeared as an important topic in the Jules Verne novel Le Rayon Vert — “The Green Ray,” as the phenomenon is sometimes known. Verne waxes floridly poetic about the color of the flash, describing it as “a most wonderful green, a green which no artist could obtain on his palette, a green of which neither the varied tints of vegetation nor the shades of the most limpid sea could ever produce the like! If there is a green in Paradise, it cannot but be of this shade, which most surely is the true green of Hope.”

One of Verne’s characters also recalls a Scottish legend that claims that whoever has seen the green flash will never again err in matters of the heart. It’s a charming idea. Unfortunately, researchers have never found any trace of the legend in Scottish folklore. So it seems likely that the imaginative French author made it up himself.

A rash of green-flash sightings and speculations followed in the wake of Verne’s book. But these were often made by uncritical observers and writers. They in turn probably helped set off a skeptical — and incorrect — theory about the cause of the green flash. According to some scientists of the late 19th and early 20th centuries, the green flash did not arise in Earth’s atmosphere at all, but only in the viewer’s eyes. The green flash, they claimed, was the complementary-color afterimage created by staring too long at the bright setting Sun. You can produce this effect yourself by staring at a red object, say a ripe tomato, in a brightly lit room. After about 15 seconds, switch your gaze to a white surface and you’ll see a ghostly bluish green “anti-tomato.”

The True View

Veteran observers pointed out, however, that the green flash could not be an afterimage because it had been seen many times at sunrise, when the green appears before the red of the Sun. And by the middle of the 20th century, photographs finally verified once and for all that it was physical, not physiological. How then does the atmosphere produce this marvel? There are two optical effects at work: refraction and dispersion.

Atmospheric Refraction
Sunlight bends — or refracts — as it travels through denser layers of our atmosphere. Cooler (and denser) air over hot ground will produce a false image of the sky — a mirage, as shown in the top panel. Refraction also lifts the Sun when it’s low above the horizon (lower panel), as sunlight passes through much more atmosphere than when it’s overhead.
Night Sky

When light enters Earth’s atmosphere it is refracted, or bent, in the direction of the denser air — in other words, downward. The amazing result of such refraction makes the image of any celestial object we see appear slightly higher than the true position it would occupy if Earth had no air. The effect is greatest at the horizon, where light takes the longest pathway possible through the atmosphere.

The typical amount of atmospheric refraction at the horizon is about ½° — the apparent diameter of the Sun or full Moon. This means that when we see the Sun’s bottom edge touch the horizon, the Sun’s top edge has really just gone below the horizon! What we see is the refracted image of the Sun displaced upward by one diameter.

But refraction is just part of the story. Have you ever seen a rainbow or used a glass prism to spread sunlight into its spectrum of colors? This occurs because sunlight is composed of a range of wavelengths (colors), each of which is refracted by a different amount. The shorter (bluer) the wavelength, the more it’s refracted by the atmosphere.

Splitting Sunlight
Air can act just like a prism to spread sunlight into its component colors. The degree to which light beams refract depends on their color (wavelength). Refraction is strongest when the Sun lies low above the horizon, creating a blue-green fringe on the top edge of its disk.
Night Sky

So the Sun’s normal white or yellow-white image is really composed of disks of different colors at ever-so-slightly different heights in the sky: the blue-light Sun sits a bit higher than the green-light Sun and so on, with each spectrum color until the red-light Sun, which is lowest. This dispersion of wavelengths is small enough, however, that the Suns of different colors overlap and produce white — except at the bottom and top of the Sun’s disk. The very bottom edge is red; the very top edge is blue.

I can hear you asking, “Then shouldn’t the last speck of the setting Sun be a blue flash?” When the atmosphere is exceptionally clear, yes. But our atmosphere scatters (redirects) short-wavelength light very well — very little of the Sun’s blue light ever reaches our eyes directly. The proof of this is all around you on a clear day: the blue sky. Scattering is usually enough to remove blue from beams of sunlight and leave green light to predominate at the Sun’s top edge.

Blue Flash
On rare instances of extremely clear skies, the Sun may produce a blue flash. This sequence was captured in Finland. The same situation can happen in reverse at sunrise, but to see it you’ll have to determine exactly where and when the Sun will rise.
Pekka Parviainen

In addition, when the Sun hangs low, water vapor in the atmosphere absorbs most of the yellow and orange light, leaving only some green and a whole lot of red — the typical red Sun of sunrise or sunset. But suppose you see the Sun turn deeply red and noticeably dimmed many minutes before sunset? If that happens, the atmosphere must be humid (or hazy) enough to absorb even the green light. On such a day, the green flash will not occur.

So when the Sun stays bright and yellow-white until it is very low, that’s your cue to look for a green flash. But will it be visible on any such day? According to experts, yes. There is, however, a catch.

Looking at Sunset Safely

It turns out that on most clear, unhazy days the green flash is such a tiny sliver of light that you’d have to use optical aid to detect it. Yet looking through binoculars or a telescope at the Sun without a proper solar filter is normally extremely dangerous — you are likely to suffer eye damage from doing so! An observer absolutely should not look at the Sun with even small binoculars unless two conditions are met:

  • First, the bottom of the Sun must already touch a very distant horizon, not just be passing behind a mountain or building.
  • Second, the Sun must be dimmed enough for the naked eye to look at the solar disk quite comfortably.

Most beginners seeking the green flash would be better off relying on eyes alone (no binoculars or telescope) and waiting until the Sun is right on a distant horizon.

Distorted Sunset
A distorted yellow setting Sun is a good portent for a green flash. It means that you ’re looking through air layers of differing temperatures that may cause anomalous refraction. If the Sun appears deep red, however, there isn’t enough green light left to cause a flash.

An obvious, eyes-only green flash happens when the atmosphere has significant temperature variations and produces anomalous, or greater than normal, refraction. This can occur over land, but it’s more likely over water. In fact, the most common kind of green flash takes place when the surface is distinctly warmer than the layer of air just above it. This happens most often over a body of water, and that’s why green flashes are often associated with seashore sightings.

That condition — air colder than water at sunset — is common in winter and after the passage of cold fronts in late summer and autumn. But it can occur at any time of year in most climates, and when it does you should be alert for a green flash visible to the unaided eye.

Such temperature conditions often produce mirages, and seeing them is a good predictor of a green flash. Have you ever seen a mirage? You may think you haven’t, but there are many types of mirages besides the classic imaginary oasis that fools a weary traveler in the desert. Almost all of us have driven down a road on a warm, sunny day and noticed what seem to be puddles of water on the street ahead — puddles that disappear as we approach them. The “water” is really a view of the sky refracted to us because the air just above the road is so much cooler than the very hot road surface. This is an example of an inferior mirage — “inferior” means that the phantom image appears below the normal image.

Watch That Mirage!

The final predictor of the most common variety of green flash is when an inferior mirage image of the Sun itself — sometimes called the “countersun” — seems to peek up above the horizon just as the true Sun’s bottom edge is about to touch the horizon.

Green Flash
A green flash typically lasts only an instant. This series of photos from the tropics spans just 1 second. The Sun sets more slowly at high northern latitudes, thus prolonging the event a bit.
Peter Aniol

In a matter of seconds, the countersun merges with the real Sun, creating something that looks a little like the Greek letter Omega. When the last piece of the merged pair is left on the horizon, it’s an enlarged combination of the green edges of the Sun and countersun. It’s easily large enough for your eye to detect, and it appears green — for just one or two breathtaking seconds.

To see the green flash caused by an inferior mirage you must usually be standing on a wide expanse of flat ground. There’s another kind of mirage that can produce an enhanced green flash if you are higher above ground level, perhaps on a mountain or in a building. That one requires a temperature inversion (when warm air lies over cold air). If you see a pair of spikes sticking out of the sides of the Sun and these drift up the Sun’s disk, get ready for them to detach from the top of the Sun and vanish in a last-second gleam of green.

Catching your first strong green flash may take considerable patience, but it will be worth it. And along the way you’ll see many otherwise spectacular sunsets and wonders of the sky. Everyone loves beautiful sunsets — why not become a knowledgeable connoisseur of them? One of your rewards will be the emerald or blue-green solar flame whose sight is more wondrous than even Jules Verne could ever hope to convey.

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Fred Schaaf is the author of A Year of the Stars: A Month-by-Month Journey of Skywatching (Prometheus Books, 2003) and many other books about observing the sky.