The old saying "once in a blue Moon" means not very often. But how often do we really see a blue Moon? According to astronomical definitions, it happens on the second full Moon in the month.

The blue Moon photographed above was taken a few years ago by Tom King of Watauga, Texas. The blue hue was probably due to tiny water droplets in the air. "The air was damp and heavy with moisture," King said. 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.

There are other reasons for blue Moons, King 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."

:: NASA