"I had never paid any real attention to the term 'Blue Moon' until one October evening in 2003," he recalls. "I had my telescope set up in the backyard and the moon began rising in the east with a strange blue tint I had not seen before."

The cause of the blue was probably tiny droplets of water in the air. "The air was damp and heavy with moisture," notes King. When water droplets are about 1 micron (one millionth of a meter) in diameter, they strongly scatter red and green light while allowing other colors to pass. A white moonbeam passing through such a misty cloud turns blue.

Clouds of ice crystals, fine-grained sand, volcanic ash or smoke from forest fires can have the same effect. "The key," notes atmospheric optics expert Les Cowley, "is that the airborne particles should all be of very similar size, a micron or so in diameter." Only then do they scatter the correct wavelengths of moonlight and act as a blue filter.

There are other reasons for blue Moons, he notes. "Our eyes have automatic 'white balances' just like digital cameras. Go outdoors from a cozy cabin lit by an oil lamp (yellow light) and the Moon will appear blue until your eyes adjust."

What kind of Blue Moon will you see this week? There's only one way to find out!

 Author: Dr. Tony Phillips | Production Editor: Dr. Tony Phillips | Credit: Science@NASA

SOURCE: NASA Science May 30, 2007

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