A blue moon is the second full moon to occur in a single month. Of course, whether a full moon is labelled as “blue” or not actually has nothing to do with actual astronomical phenomena. The Moon follows a regular 29.5-day path around the Earth, and is tidally locked, meaning it always presents the same face to the Earth and that its “day” lasts the same amount of time as its period of orbit around the Earth. The fact that occasionally a full moon occurs twice a month is thus an oddity produced by our calendar system, rather than by the Moon itself.
How the second full moon in a month came to be known as the “blue moon,” however, is quite peculiar. One theory claims that the medieval church, which often announced the dates of religious celebrations on the basis of positions of the Moon (since the largely illiterate population of the time made little use of printed calendars), occasionally had to dismiss an extra moon before the beginning of Lent. It did so by labelling that moon a “belewe” Moon – a word meaning blue, but also “betrayer,” or false. A medieval blue Moon was therefore a “false” Moon which occasionally disrupted the cycle of Lent. The often-quoted Middle English rhyme that “if they say the moon is belewe [blue], we must believe that it is true,” a sarcastic commentary on the authority of the Church, is therefore probably intended as a play on words (the clergy say the moon is false or “blue,” when obviously it is not blue at all).
Later, by the 19th century, farmers’ almanacs were using the phrase “blue moon” for a different purpose: to identify an extra moon in a season. Normally, a season (one-quarter of a year) would have three full moons, signifying the beginning, middle, and end of the season. The aberrant fourth moon in each season was labelled the “blue” moon. Why precisely the term “blue” was used for this is unclear, although it might relate to the same labelling of “false” moons as described above.
It was this use of the term “blue moon” by farmers’ almanacs, however, which was famously misinterpreted to create the modern definition of the “blue moon.” The idea that a blue moon is the second moon in a month is actually a relatively new idea, which is traditionally blamed on Sky and Telescope magazine writer James Hugh Pruett. In 1946, Pruett wrote an article in which he claimed that farmers’ almanacs called the second full moon to occur in a month a “blue moon.” Pruett was mistaken – the farmers’ almanacs identified extra moons by season, not by month – but his definition stuck. Today, the universally accepted definition of “blue moon” is the second full moon to occur in a single month.
To that end, Astronomy Cafe actually has a blue moon calculator available, which lists all of the eight upcoming blue moons out to 2028.