Photo by Kostian Iftica, courtesy of NASA, taken on July 2, 2004.
Tomorrow night, July 31, 2015, there will be a blue moon. The moon shown in the photo above is blue because the photographer shot the image through a blue filter. Nonetheless, it was a blue moon.In popular parlance, attributing something to a blue moon moon means that it seldom or rarely occurs.
What exactly is a blue moon? There are two definitions. The most recent says that a blue moon is the second full moon in a calendar month. This can occur every 2-1/2 years on average. The other, older, definition says a blue moon is the third moon in a season that has 4 full moons. This last definition is important to the ecclesiastical calendar, which needs to know when the 13th moon is going to occur in order to stay on track and determine such things as the date of Easter and Passover.
Are blue moons ever really blue? Probably not. The date of a full moon, all by itself, doesn't affect the moon's color, but volcanic activity can. Back in 1883 when the volcano Krakatoa exploded, the moon appeared to turn blue and stayed that way for years after the eruption. The particles in the ash cloud scattered particles in the visible light spectrum and allowed others to pass through, causing some objects to appear blue or green. More recently the eruptions of Mt. St. Helens and Mount Pinatubo have turned the moon blue as well. The key to having a blue moon is having lots of particles in the air that are slightly wider than the wavelength of red light. Volcanoes and forest fires can cause clouds that do that, so they can appear to be blue even if they're not. I don't want to get involved in the green cheese thing, so I'm going to move on. Thirsty? Try the Blue Martini that follows.