December 31. 2009 we had a Blue Moon. It was visible on a clear night in St. George, Utah.
A blue moon is a full moon that is not timed to the regular monthly pattern. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year. The extra days accumulate, so that every two or three years
(7 times in the 19-year metonic cycle), there is an extra full moon. The extra moon is called a "blue moon." Different definitions place the "extra" moon at different times.
The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and, notably, after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. The particles in the atmosphere have to be about one micrometer in diameter; under these circumstances, long-wavelength light, which appears red to a viewer, is scattered out of the line of sight and short-wavelength light, which appears blue to a viewer, is selectively transmitted into a viewer's eyes.[5]
