Hector Barbossa: “Ever gazed upon the green flash, Master Gibbs?”
Joshamee Gibbs: “I reckon I seen my fair share. Happens on rare occasion. The last glimpse of sunset, a green flash shoots up into the sky. Some go their whole lives without ever seeing it. Some claim to have seen it who ain’t. And some say-”
Pintel: “It signals when a soul comes back to this world from the dead!”
In the film Pirates of the Caribbean: At World’s End, Will Turner, Elizabeth Swan, and the rest of the pirate crew head down to Davy Jones’s locker, world of the dead, to rescue Captain Jack Sparrow. Once they find Jack, they are faced with a new problem- how do they return to the world of the living? All they have is a mysterious map of moving parts and cryptic clues.
In a work of cleverness (or maybe insanity- you never really can tell), Jack solves the riddle. Up is down. As the sun sets in the locker, it rises in the land of the living. They tip the Black Pearl over so it’s upside down in the water. When the sun sets in the afterlife, a brilliant green flash is emitted, signaling that they coming back from the world of the dead. The ship is now right side up in the world of the living and the sun rises.
And then they try to shoot each other, because they’re pirates.
While most of this is purely fantastical fiction, the green flash is a real phenomenon. Under certain conditions, a green light may be visible of the upper edge of the sun as it sets below or rises above the horizon. This is due to the effects of refraction.
The Earth’s atmosphere acts like a prism, separating light into different colors. Light from the sun is refracted, or bent, as it travels at an angle from the fast medium of space to the slower medium of the atmosphere. Shorter wavelengths, such as blue, are bent more than longer wavelengths such as red, separating the colors. When the sun is above your head, the light travels a shorter distance and the different colors in the sun’s light are not well separated. As the sun dips to the horizon, the light must travel further through the Earth’s atmosphere to reach your eyes. With the increased travel length of the light, the colors are separated more.
As the sun sets, it appears red, as red light is the least bent and arrives at a shallower angle. However, with clear conditions, a small amount of green light from the very top of the sun approaches from above the red light, arriving at a steeper angle and creating the green flash. This flash is brief, lasting only for a second or two.
In theory, you should see a violet flash last, as the violet light is bent more than green light. However, air molecules and pollution scatter blue and violet light, so green is typically the last color to be seen. The green flash can also be seen in a sunrise, showing up just before the rest of the red sun peeks over the horizon.
If you want to see a green flash, you need watch a sunrise or sunset from a location where you can see a distant horizon with a distinct edge. This is more easily done over the ocean while on a beach or a boat, but you can see it from a mountaintop, flat prairie, or other location with an unobstructed view. You also need a clear day with no cloud, fog, or other haze on the horizon. If you are trying to catch a green flash at sunset, don’t stare at the sun until it’s nearly gone. Otherwise you’ll burn an afterimage of the sun into your eyes and won’t be able to see the green flash. The green flash doesn’t happen every day, so if you see one, consider yourself lucky.
Now, bring me that horizon.
Keep calm and science on.