"Once in a blue moon..." means seldom or absurd. But, believe it or not, sometimes the moon really does turn blue. This happens when the air is filled with tiny particles about 1 micron (one millionth of a meter) wide. Clouds of water droplets, snowy ice crystals, fine-grained sand, dust or ash: any of these, under the right circumstances, can act like a blue filter. Seen through such a cloud, the moon looks blue. (Get the full story from Science@NASA.)
"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."
:: excerpt and picture taken from www.spaceweather.com ::
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