| Moondark for May: Catching the Green Flash | |
| Ask someone about the green flash, and they’re likely to tell you a good story. In the Pirates of the Caribbean: At World’s End, a green flash means that a soul has returned to the living world. Jules Verne wrote of the pursuit of an indescribable shade of green in the novel “Le Rayon Vert” in 1882. But while the green flash is the stuff of lore and legends, it isn’t a myth. Not if you believe what’s on the internets, not if you believe what you can see with your own eyes. A green flash occurs when some portion, especially the upper rim of the setting sun, turns briefly from red and orange to yellow, green or rarely, blue. Usually, the unexpected color change lasts for less than a second or so, and is aptly called a flash. This term actually applies to several visually distinct phenomena, so reports and descriptions vary. Surprisingly, green flashes may not even be especially rare, if you know what’s happening and the best circumstances to observe them Refraction and scattering of sunlight are responsible for the green flash. Light travels slightly slower in air than outer space, and that difference makes light rays bend: objects near the horizon appear up to half a degree higher than they really are. In fact, when the sun sits just on the horizon, it is actually geometrically below it, and would not be seen at all if not for refraction. Blue light bends a bit more than red, so the setting sun is imperceptibly smeared into overlapping circles of colors, red at the bottom, yellow and green next, finally blue at top. Since blue light is scattered far more than redder colors (that’s why the sky is blue and sunsets red) the blue edge is usually invisible. But as the last vestiges of the refracted sun disappear, the red sun setting first and the blue last, that green rimmed disk may alone remain momentarily as the green flash. Atmospheric inversions may distort the solar disk, create a mirage, magnify the event and probably are a necessary condition for a visible flash. An arc or dot of green may be separate from the solar disk or even flash seconds after the disk disappears. Given the vagaries of the atmosphere, it’s fair to say that no two green flashes are ever the same. I’ve watched a lot of sunsets and have been fortunate to see them from some beautiful locations. On just a handful of those occasions though, I’ve seen the green flash: twice over the Pacific, and three times recently from the Caribbean. Interestingly, among members of a sunset-watching group, some individuals see it, while others don’t. So something more that atmospheric optics seems to be involved, along with skill and a bit of luck. You can improve your chances. Since refraction is greatest near the horizon, a low and unobstructed view is a necessity. A very clear and transparent air mass, with the setting sun almost too bright to look at, and freedom from distant clouds are both required as well. Elevation is not critical, but sunset over the sea or from a high vantage point are best. Fortunately, the time and exact position of the setting sun can easily be anticipated for sunset. Atmospheric physics and some observer reports say that the flash phenomena should occur as well at sunrise, though practically, they will be much more difficult to observe. Clearly, a view of the setting sun from tropical island beach represents ideal green flash viewing conditions. From Delmarva with the sun setting over land to the west, a tree line and ever-present atmospheric haze and pollution, our sunsets are often less than spectacular. Can the green flash be seen from local viewpoints? No harm in trying though, is there? The possibility of green flash surely adds a bit of anticipation (and even mystery) to an otherwise predictable nightly event. Have you ever witnessed a green flash? A quick search on the web returns a wealth of green flash sites, and of them I’d recommend Andrew Young's and Les Crowley’s for authoritative introductions to this phenomenon. While many images and videos can be found on line, it appears the ultimate green flash experience, clouds notwithstanding, would be sunset at the Green Flash restaurant over a Green Flash microbrew. Moondark is written by Douglas C. Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 27 April 2008. Text and images on this web page are free for non-commercial use with attribution under a Creative Commons Attribution-Non-commercial 3.0 License. Ask Doug about other uses. |
