Q: Whatever happened to the famous but rare 鈥済reen flash鈥� we used to hear about so often twenty or so years ago? Observers reported a momentary but brilliant green appearance of the Sun as it sank below the horizon, usually over the sea in tropical climates. Sometimes the whole sky turned green as well. Was this green flash real, and if so, what caused it? Why is it so rare, has it been recorded on film and where are the best places for seeing it?
A: The green flash is caused by different amounts of refraction in the Earth鈥檚 atmosphere. Rays of blue light are bent more than green, rays of green are bent more than yellow, and rays of yellow are bent more than red.
This refraction raises the apparent altitude of a celestial object such as the setting Sun, because the difference between red and green rays is about a minute of arc at the horizon. Therefore the upper part of the setting Sun has a green fringe. If the air is very clear and the horizon is sharply visible, the green fringe will be all that is visible of the setting Sun for a second or two as the Sun vanishes over the horizon, causing the distinct green flash referred to above. The flash can also be seen at sunrise if you know exactly where the Sun will appear.
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The air must be completely free of haze and cloud between the observer and the setting Sun in order to witness the phenomenon, a condition that is rarely met, which is why few people have seen the flash.
Under very favourable conditions, the even rarer blue flash can be seen in which the green shade appears to slide swiftly up the spectrum into a pale blue speck just before the Sun disappears completely over the horizon. Blue light is bent more than green light and so this phenomenon is witnessed immediately after the green flash and just before the Sun vanishes.
I saw several green flashes in 1995, either from a mountain-top observatory (where a momentary blue flash was visible) or from sea level looking out over the ocean. Reports that the whole sky turned green are unlikely to be true; the flashes are not especially bright, and afterimages from the dazzling pre-flash setting Sun can make it hard to see the flash well.
The total light from the green flash itself has been estimated to be about the same as from the planet Jupiter at opposition.
Green flashes are reported more frequently in tropical regions or coastal areas because this is where the likelihood of a long clear air path is the greatest. But, given the right weather conditions, they can be seen anywhere.
A few years ago volcanic dust from the Mount Pinatubo eruption reduced the clarity of the atmosphere to such an extent that green flashes were rarely seen, but most of the dust has since dispersed and they are visible once again.
Taking photographs of the green flash requires some special camera equipment, such as a telephoto lens and a tripod, but not many people bother and this explains why photographs of the phenomenon are rare. An example was published in the astronomy magazine Sky and Telescope in 1994, and I have also found a number of earlier ones.
A: As an airline pilot, I have had ideal opportunities to observe the green flash. It can only be seen in certain atmospheric conditions. Crystal blue-green clarity of the sky is essential 鈥� a dusty red sky is useless and I never saw it in two years of flying over Arabia.
The necessary clarity often occurs above a sheet of strato-cumulus cloud with a sharply defined top. The flash happens as the upper limb of the Sun touches the top of the cloud. The visible segment of the Sun at sunset contracts to a point and then turns a brilliant neon-green because the blue-green end of the Sun鈥檚 spectrum of light is refracted most. The Sun must be a brilliant white before it sets 鈥� any hint of redness usually means there is no chance of seeing a green flash, although I did once observe it with an orange Sun.
I would suggest that any air traveller interested in seeing the green flash asks to visit the flight deck 鈥� the front windshield is usually clearer than the cabin windows.
The problem is that most pilots have never heard of the green flash and might question the passenger鈥檚 sanity.
A: I have taken a photograph (above) of the green flash from the Skalnat茅 Pleso Lake Observatory in the Slovak High Tatra mountains. Winter is the best time to observe the green flash 鈥� assuming the weather is good 鈥� and viewing the Sun over water helps. The picture shown above was taken over the lake at sunrise in February 1991. The green colour can be seen just above the horizon.
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