Amateur astronomy is one of my many hobbies. I taught in a planetarium for about ten years in the 1980s-90s and have written several books on skywatching. I find that my skywatching hobby goes well with lighthouse hunting.
(Photo above shows the green flash behind La Perdix Lighthouse on the Brittany Coast of France and is from Astronomy Picture of the Day by Laurent Laveder http://apod.nasa.gov/apod/ap120510.html)
Lighthouses usually have wonderful skies over them and unobstructed flat horizons beyond them. They often afford the chance to observe the skies at their finest—dark, looming, and touched by the hand of an ocean atmosphere that produces intriguing special effects.
One interesting phenomena I’ve seen several times from lighthouses is the Green Flash. You’ve got to be patient and observant to see it, and sky conditions have to be just right. Lighthouse locations can provide those “just right” conditions. You’ll need a clear view of the flat western horizon—no trees, mountains, hills, buildings, or any other elements. And Green Flashes occur most often when the air is still and no clouds obscure the horizon.
If such a serene day winds down at the shore, park yourself with your favorite drink, relax, focus, and wait. Wear dark sunglasses too, or better yet, look at the sunset through #14 welder’s glass, as staring at a bright horizon a long time will cause discomfort and may even harm your eyes.
(Photo below is from Colin Legg Photography. It appeared on www.earthsky.org and shows a sun dome with a green flash setting behind Rottnest Island Lighthouse in Western Australia. What an amazing shot! Find more of Colin Legg's fantastic images of the sky on https://www.facebook.com/ColinLeggPhotography)
Catch it if you can, and you’ll never be deceived by a lover! So says an old Irish superstition concerning the Green Flash. That seems reason enough to look for it. Even more, it’s one of nature’s special magic shows. Now you see it; now you don’t!! Your eyes can catch it. So can your camera.
The Green Flash is, just as its name implies, a momentary flash of green light that appears just as the Sun's orange disk disappears over the horizon at sunset. It also occurs at sunrise but is less often observed at this time, due to the difficulty of predicting the exact moment when the upper rim of the Sun will rise. But go for it anyway, especially on the East Coast at your favorite lighthouse. Just watching the sunrise at a lighthouse is beautiful enough. Add the Green Flash, and you'll really experience something memorable.
A morning or evening following a cleansing rainstorm is good for Green Flash conditions, as are days when a cold front has just moved down from Alaska or Canada. Although the ocean horizon works best because of its flat line, the Green Flash can appear on any flat horizon. Some people have seen it on mountaintops and over buildings, but this is very rare. Your best chance of catching it is at the coast over the ocean horizon. Summer and winter are the choicest times to look, when the angle of the Sun’s descent is shallowest.
If you’re using welder’s glass to dim the bright sun (and I highly suggest you do!!), you might also see some sunspots, which appear as small dark flecks on the Sun. These are cooler areas on the Sun’s surface and are associated with its magnetic field. Very near the moment when the sunset disk is about to disappear over the horizon, look at it intently through the welder’s glass. If conditions are right, you’ll see green color effects just as the disk disappears. Depending on its intensity, the Green Flash may be bead-like, spiked like a star, or elongated into a dash or ray.
What causes this momentary flash of emerald light on an otherwise tangerine Sun? Unknown to your eyes, the upper edge of the Sun’s disk is rimmed by a green arc of light. The reason your eyes can’t see the green when the Sun is still up is that green occupies a much narrower part of the visible spectrum than other colors. Yellows, oranges, and reds have much longer wavelengths and dominate your field of view. But at the final moment of sunset, the shorter wavelength green gets its chance to shine. It shows through in a jewel-like flash.
When you see it, consider yourself lucky. Not many people catch this elusive phenomenon. Science can’t guarantee that seeing the Green Flash will keep your love life intact, as the Irish believe, but observing it will automatically admit you to a unique circle of skywatchers.
If you happen to observe it at a lighthouse, consider yourself even luckier: You’ll see a manmade beacon in the lighthouse and a nature-made beacon on the horizon! Don't despair if the elusive Green Flash doesn't appear. Trade the green "flash" of a lighthouse instead--maybe Portsmouth Harbor's handsome old beacon, seen below. (Lens photo by Bruce "Boondog" Robie, who caught his reflection in the lens as a bonus. Bottom photo by Matt Currier.)
To find out more about the Green Flash, check out this website from astronomer Andrew Young -- http://mintaka.sdsu.edu/GF/.
To buy a piece of welder's glass go here: http://www.amazon.com/Welders-Glass-Solar-Eclipse-Viewer-Pleasing/dp/B007O10Z80