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Blue Moon: Is There Really Such a Thing?

The saying "once in a blue moon" has nothing to do with color. Instead it refers to the relatively rare occurrence of what is called a "blue moon". (However, an actual blue-colored moon could occur from atmospheric effects such as a big forest fire on Earth causing a lot of haze. This has happened in the past!)

The definition of a Blue Moon has changed over the years (see below). However, the commonly accepted definition of a Blue Moon is simple: it is a full moon that rises twice in one month. It happens because sometimes the number of days in a calendar month are greater than the cycle of the moon, which is 29.5 days. This can cause the moon to rise twice in one month, near the first and the last days of the same month. It is said that blue moons happen every 33 months or about every 3 years. This relatively rare occurrence has spawned the saying "once in a blue moon".

A Blue Moon cannot happen in February because the calendar month never has enough days. Furthermore, sometimes the occurrence of a blue moon depends on your time zone. During 1993 a blue moon occurred in either in August or September depending on where you lived. If you lived east of the line that runs through the Atlantic Ocean, the blue moon occurred in September, but west of that line it occurred in August.

Origins of The Definition of a Blue Moon

A year can be divided into quarters, or seasons. In most years, each season contains 3 full moons. Just as the lunar cycle causes some months to have 2 full moons, some seasons will have an extra full moon. The Farmer's Almanac calls the third full moon of any season containing four in total a "Blue Moon".

However, a series of cascading misinterpretations and unfounded assumptions led to a change in definition. A 1943 article followed by a 1946 article in the magazine Sky and Telescope essentially declared the second full moon in the same month to be a "Blue Moon" in reference to data found in an edition of the 1937 Maine Farmerís Almanac. But the interpretation was incorrect. Over the course of decades, this interpretation was repeated until it became "fact". You can read the entire story in detail here on the Sky and Telescope website.

Note that a season isn't exactly a quarter of a year. It is slightly altered because of the archaic Christian ecclesiastical calendar. This was the calendar used by the Christian church to determine the exact date for certain holidays such as Easter, and it did not use the same lunar cycle that modern science recognizes.

Upcoming Blue Moon Dates
These dates are based on the more well-known definition of a Blue Moon (the second full moon in a calendar month).

1999 January & March
2001 October & November
2004 July
2007 May
2009 December
2012 August
2015 July
2018 January & March
2020 October