Calendar blue moons

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 1946 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.

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 (namely January and March). The last time this occurred was in 1999. The next time this will occur will be in 2018, because February will have no full moon that year, according to UTC, which means that January and March will each have a calendar blue moon that year.

The previous calendar blue moon (based on UTC) was on June 30, 2007. The first full moon would have occurred on June 1, 2007. That was May 31, 2007 in the Western Hemisphere, making that full moon the second occurrence in May in the Western Hemisphere (see below); the next calendar blue moon will be December 31, 2009.

As stated in Wikepidia

Additional Blue Moon Information