BLUE MOONS AND GREEN LIGHT
Sin,—Mr. A. R. Clough expresses a wish to understand the cause of the " green flash " seen at times as the sun sets into a clear sea, for example in the Mediterranean and Indian Ocean. One not infrequently hears of this phenomenon, and of requests for its explanation, but surely Sir William Bragg in his book The Universe of Light beautifully and simply explains its cause :
" The refraction of the sun's rays by the atmosphere has an effect on the apparent time of sunset. Since light travels more slowly in the air than in space empty of material substances, the sun's rays are bent out of their original course before they reach the earth. The change in direction is at the maximum when the rays strike the surface of the atmosphere at their greatest obliquity, and this happens when the sun is on the horizon. The sun sets geometrically before it does so visually ; the amount of the change of direction is such that the sun has set geometrically at about the time that the lower edge just touches the horizon."
But white light may be separated into its constituent colours and, during its refraction, " blues and greens are more refracted than yellows and reds. Consequently the rays which reach us from the setting sun are of the former kind unless, which is nearly always the case, other conditions interfere. Hence arises the sq-called ' green ray " which sometimes flashes out for a few seconds just before sunrise or just after sunset, and is looked for when air is very still and uniform."
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