Ask a group of people if they’ve ever seen a green flash, and some interesting responses will follow. Some may say a green flash brings money or luck to those who see it. Others will insist green flashes are the stuff of legends. Many might claim they see green flashes only after downing a few rum punches. And for the fortunate sky gazers who have witnessed a green flash, they must often defend their sightings and endure ridicule and skepticism. But science and a few photographs will silence the nonbelievers. Since an ocean horizon is a favorable spot to see green flashes, and divers spend a lot of time on the water at sunrise and sunset, they’ve got a good chance of witnessing these visual marvels.
According to Andrew T. Young, an adjunct professor of astronomy at San Diego State University, green flashes are real phenomena seen at sunrise and sunset, when some part of the sun suddenly changes color. At sunset, the color usually goes from red or orange to green or blue. The word “flash” refers to the sudden appearance and brief duration of color, which usually lasts only a second or two. There are several different phenomena often lumped together under the name of “the green flash.” This generalization has caused confusion, so the term “green flashes” is more accurate when referring to them. Green flashes involve complex scientific subjects, and it’s easy to be confused when trying to understand them. So put on your thinking cap, and follow along as best as you can.
There are two main factors at work that cause green flashes — refraction and mirages. We’ll start with the first. The Earth’s atmosphere acts something like a weak prism to refract — bend and spread out — sunlight into a spectrum of individual colors — red, orange, yellow, green, blue and violet. Red, at one end of the color spectrum, has the longest wavelengths of light. Violet, at the other end, has the shortest. When the sun’s disk is fully visible above the horizon, the different colors of light rays overlap to an extent where each individual color can’t be seen by the naked eye. But when the sun starts to slip below the horizon, the colors of the spectrum disappear one at a time, going in order of those with the longest wavelengths to those with the shortest. So as the last part of the sun nears disappearing, the red rays slide first below the horizon. A thick cushion of air and water vapor mostly absorbs the next rays, orange and yellow. Blue and violet rays are usually scattered to invisibility. That leaves green as the light ray last seen. Given the right atmospheric conditions, it’s possible to see a green flash. Sometimes, with particularly clear air, enough of the blue or violet light rays make it through the atmosphere, causing a blue flash to be visible. At sunrise, the process is reversed, and a green flash may occur as the top of the sun peeks above the horizon.
If you’ve reached this point and understand why flashes are usually the color green, congratulate yourself. But remember, there are two factors at work, and we’ve only covered the first, refraction. Most explanations of green flashes stop here, but not addressing the second component would be like descending on a dive without learning to ascend. There’s more to this green flash business, so stick with it.
Why is it that the human eye can see a green flash at all? According to Young, the astronomy expert, the missing pieces are mirages. Mirages are often portrayed in cartoons as optical illusions. Think of the thirsty chap trudging through the dry desert chasing after an elusive oasis of water. Or think of the shimmering pools of “water” that can often be seen on hot asphalt roads on sunny days. These mirages are real optical phenomena caused by the distortion of light by alternate layers of hot and cool air. They are not hallucinations caused by thirst or exhaustion. In a mirage, there’s at least one inverted or “mirror” image of an object. The most common mirage is the “inferior” mirage, so named because the mirror image lies below the real one. Inferior mirages include the desert sand and hot road examples, where the sky and its blue color are reflected below the horizon onto these hot surfaces, producing an illusion of water. Another example of an inferior mirage could be a sailing ship. The mirror image of the ship is reflected below the sea horizon and looks to the viewer as if the boat is sailing upside down.
Are you still here? Good. If you understand an inferior mirage, you’re ready to wrap up the explanation of green flashes. The most common green flash is the inferior-mirage flash. Recall that the green ray of light is the last ray seen as the sun sinks below the horizon. If atmospheric conditions create an inferior mirage, the mirror image of the green color will be reflected below the horizon. So in effect, the mirage broadens the sun’s upper green rim enough to see it with the naked eye. And there you have it — green flashes demystified. While the inferior-mirage flash is the most common, there are other types of green flashes caused by different kinds of mirages. But we’ll leave those for another time.
At sunset, another factor that contributes to the ability to see a green flash is the sun’s effect on the eye. Looking at a bright-red sun results in a temporary loss of sensitivity or “blindness” to red. So the brighter yellow stage of a sunset flash that precedes the green stage appears green and not yellow. Thus, the duration of a green flash is, in effect, lengthened, allowing more time for it to be seen by the naked eye. This color distortion only works at sunset and not at sunrise. However, a green flash can still be seen at sunrise due to refraction and the mirage effect, but it’s difficult to know exactly where to look on the horizon for the rising sun, and most stargazers aren’t awake that early.
Myth and Mystery
Without understanding the science behind green flashes, it would be easy to attach superstitious significance to them. The ancient Egyptians, located on the flat desert with a view of the horizon, were in a perfect position to witness green flashes. It is thought that they viewed the green color at sunrise and sunset often, which led to the belief that the sun was green at night. To the Egyptians, night symbolized death. The god of the dead, Osiris, was identified with the nighttime sun and was always painted green. But no one knows if there is a correlation between the green god and green flashes.
Another myth regarding green flashes has been perpetuated by Jules Verne, the 19th-century French author known for his fictional adventure stories. His novel, Le Rayon Vert, revolved around an “ancient legend” probably made up by Verne himself that claims whoever sees a green flash will never be short of luck in matters of the heart. This “ancient legend” has become somewhat of an urban legend, but one must remember that Verne’s intention was to create an interesting story.
And even though there is a scientific explanation for green flashes, not everyone in a group watching the same sunset or sunrise may see one. This, of course, adds to the mystery and disbelief associated with them. After all, if everyone saw a green flash every time he looked for one, the allure would be lost.
Divers positioned near an ocean horizon are in a good location to see green flashes. (See sidebar for advice on looking for green flashes.) And while the setting sun can signal the start of a night dive, it’s also a good time to keep an eye out for these exciting phenomena.
For more information on green flashes, visit the Web site of Andrew T. Young, adjunct professor of astronomy at San Diego State University, at: http://mintaka.sdsu.edu/GF/index.html
Green flashes are real phenomena seen at sunrise and sunset, when some part of the sun suddenly changes color. The word “flash” refers to the sudden appearance and brief duration of color, which usually lasts only a second or two.
Looking for Green Flashes
People who have never seen a green flash don’t know what to look for or where to position themselves. Andrew T. Young, adjunct professor of astronomy at San Diego State University, offers the following advice on looking for green flashes.
Since green flashes are related to mirages, you need to be looking at a horizon lower than your eye level. A sea horizon is guaranteed to be a favorable spot, but you must position yourself 10 to 15 feet (3 to 5 m) above the water for the best visibility. You can also obtain a low enough horizon by standing on a hillside or a tall building with an unobstructed view.
Keep in mind that flashes are not always green, and they are not a “flash” in the sense of a sudden burst of brightness (except at sunrise). Green flashes don’t usually light up the sky. Instead, they are often small, inconspicuous and brief. Duration of one or two seconds is typical.
A requirement for seeing green flashes is reasonably clear air. If the air is full of dust, smog or haze, there won’t be enough green light transmitted at the horizon for you to see a green flash. Try to avoid big cities, particularly those in damp, muggy climates.
Be careful when looking at the sun. The sun is safe to look at when it’s right down on the horizon, but it’s a lot brighter just a few minutes earlier at sunset or a few minutes later at sunrise. A sensible rule to follow: If the sun is too bright to look at comfortably, don’t look at it.
At sunset, keep looking for several seconds after the sun has disappeared. You might see a “green ray” display, or the sun might reappear and give a green flash, owing to waves on the water or in the air. This is more likely at high latitudes than near the equator.