In recent times, people have taken to calling a full moon a blue moon based on the Gregorian calendar. By this use of the term, a blue moon is the second of two full moons to occur in the same calendar month. This definition of blue moon originated from a mistake in an article in the March 1946 Sky & Telescope magazine, which failed in an attempt to infer the earlier definition used in the original Farmer's Almanac (see above). It was helped to popularity when Deborah Byrd of Earth & Sky walked into the Peridier astronomy library at the University of Texas at Austin one day, leafed through some old magazines, and found the 1948 blue moon article in Sky & Telescope. She used the definition – the second full moon in a single month – in the radio series Star Date for some years. As a result, the game Trivial Pursuit used a question and answer about blue moon. Sky & Telescope discovered the error nearly sixty years later and the magazine printed a retraction and correction.[2] By the time the correction came the calendar definition had already come into common use. As it is easier to understand, the mistaken calendar-based meaning has stuck.
Calendar blue moons occur infrequently, and the saying once in a blue moon is used to describe a rare event. However, they are inevitable because of the mis-match between the solar and lunar cycles. Each calendar year contains twelve full lunar cycles, plus about eleven days to spare. The extra days accumulate, so that while most years contain twelve full moons to match the twelve months, every two or three years there is a year with thirteen full moons. On average, this happens once every 2.72 years. Additionally, in some years there is no full moon in February at all, since February is slightly shorter than the time from one full moon to the next. This condition, known as black moon, gives additional 'blue' moons in the preceding and following months.