Okay, the "blue moon" tonight was not really blue - that was just me playing with the colors. However, tonight's blue moon is the only full moon this month. I always thought that a blue moon is the second full moon in a month. It seems that in reality, the blue moon is the third full moon in a season (i.e. spring, summer, etc.) that has four full moons. Allegedly, a misinterpretation in 1943 of an old farmers' almanac gave us the "two-full-moons-in-a-month" definition of blue moon that has stuck around in popular culture. The Christian Science Monitor has an interesting story about the blue moon: http://www.csmonitor.com/Science/2013/0820/Blue-moon-Five-amazing-facts.