It is difficult to pinpoint the exact occurrence rate of a blue moon because the concept has a checkered past. According to The Maine Farmers’ Almanac, which began publication in 1819, a blue moon was considered the third full moon in a season of four. This concept stood for 118 years, until a misinterpretation of the 1937 edition of the Almanac was written up by James Hugh Pruett. Through some unfortunate miscalculation, Pruett took the Almanac’s view to mean that a blue moon was the second full moon in a month. How he arrived at this conclusion is anyone’s guess, but it oddly became the go-to blue moon definition for many years. It was further popularized in an influential volume of Sky & Telescope magazine in 1946, which was cited throughout the decades, most notably on an episode of the radio show StarDate in 1980. These inconsistencies have persisted, alongside the idiom of improbability, ‘once in a blue moon.’ And then, of course, there is the observed atmospheric phenomenon wherein smoke and dust particles of a very particular size rise up from the Earth and cast the moon in blue overtones. All this being said, determining the probability of ‘a blue moon, in a blue moon, in a blue moon’ defies all calculation.