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"Once in a blue moon..." means seldom or absurd.
But, believe it or not, sometimes the moon really does turn blue, usually after volcanic eruptions or major forest fires. Volcanoes and wildfires fill the air with ash and dust. If the airborne particles are just the right size--about 1 micron (one millionth of a meter) wide--they act like a color-filter, tinging the moon blue. Clouds of water droplets, ice crystals or fine-grained sand can do the same thing.
There are other reasons for odd-looking moons, notes atmospheric optics expert Les Cowley. "Our eyes have automatic 'white balances' just like digital cameras. Go outdoors from a cosy cabin lit by an oil lamp (yellow light) and the moon will appear blue until your eyes adjust."
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