Abstract
A brief green or blue flash can often be seen at sunset or sunrise, lasting usually between 0.5 and 2.5 seconds and visible at perhaps one-half the clear sunsets or sunrises. The ancient Egyptians were probably aware of it and drew some of their notions of the underworld from it. D. Winstanley showed in 1873 that it was due to refraction of the sun's light by the earth's atmosphere. Other theories suggesting that the green flash is caused by light shining through water or by retinal fatigue are untenable. The reported durations of the flash are widely variable due to subjective factors and variations in the index of refraction in the air which also accounts for the capriciousness of appearance. The lack of transition in the spectrum is largely due to Brewster's lines. The brighter planets sometimes have a green flash visible to both naked eye and telescopic observers. Careful attention and the objective factors of a clear atmosphere, a free water line, a temperature inversion and a telescope all increase the chance of seeing the green flash.
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