What's a blue moon, anyway? Once in a blue moon...
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 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 60 years later and the magazine printed a retraction and correction. But by the time the correction came, the calendar definition had already come into common use. Because it is so much 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 12 full lunar cycles, plus about 11 days to spare. The extra days accumulate, so that while most years contain 12 full moons to match the 12 months, every 2 or 3 years there is a year with 13 full moons. On average, this happens once every 2.72 years.
When there are 13 moons in a year, 12 of them are given the 12 traditional names associated with that time of year (the names vary from culture to culture), and the extra one is termed a blue moon. Which of the 13 moons is termed 'blue' depends on whether it is calculated by the old or the new method.
The next two calendar blue moons will be on May 31, 2007 in the Western Hemisphere and December 31, 2009. Because February will have no full moon in 2018, January and March will each have a calendar blue moon that year.
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 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 60 years later and the magazine printed a retraction and correction. But by the time the correction came, the calendar definition had already come into common use. Because it is so much 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 12 full lunar cycles, plus about 11 days to spare. The extra days accumulate, so that while most years contain 12 full moons to match the 12 months, every 2 or 3 years there is a year with 13 full moons. On average, this happens once every 2.72 years.
When there are 13 moons in a year, 12 of them are given the 12 traditional names associated with that time of year (the names vary from culture to culture), and the extra one is termed a blue moon. Which of the 13 moons is termed 'blue' depends on whether it is calculated by the old or the new method.
The next two calendar blue moons will be on May 31, 2007 in the Western Hemisphere and December 31, 2009. Because February will have no full moon in 2018, January and March will each have a calendar blue moon that year.