Blue Moon

A "Blue Moon" is the name given to the second full moon in a calendar month. Because there are roughly 29.5 days between full moons, it is unusual for two full moons to "fit" into a 30 or 31 day month (and impossible to fit into a 28 or 29 day month, so February can never have a Blue Moon).

The saying "Once in a Blue Moon" means a rare occurrence, and predates the current astronomical use of the term, which is quite recent. In fact, Blue Moons are not all that rare, on average there will be one Blue Moon every 2.5 years.

The term Blue Moon is believed to have originated in 1883 after the eruption of Krakatoa. The volcano put so much dust in the atmosphere that the Moon actually looked blue in color. This was so unusual that the term "once in a Blue Moon" was coined. However, Blue Moon was also used in much the same way we use the term "Harvest Moon".

There were twelve names for full moons, one for each month, and the name Blue Moon was used in years which had 13 full moons. It referred to the third full moon of the four occuring between an equinox and solstice in that year. A misinterpretation of this led to a Sky and Telescope Magazine "Star Quiz" in July 1943 followed by an article in March 1946 which stated that the second full moon in any calendar month was called a Blue Moon (attributed to the 1937 Maine Farmers' Almanac), and this definition has now become part of the language.

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