From AMS Glossary
- Those rare occasions when the moon takes on a markedly blue cast.
Usually, when the moon (or sun) is seen low in the sky or even high in a polluted one, it is yellowish or reddish. This is a consequence of the greater extinction of short wavelength radiation by small particles and molecules. Yet, there are particle sizes for which extinction is greater for long wavelengths, and the moon seen through a sky populated by such particles is bluish. It is rare that a large population of the particles of the appropriate size (and only that size) are produced, so the blue moon is very rare: 1883 (Krakatoa), 1927 (a late monsoon in India), 1951 (forest fires in Alberta, Canada). Because of the rarity of blue moons, the phrase, "once in a blue moon," has been used metaphorically since the midnineteenth century to denote a real, but rare event.
- In recent times, often used incorrectly to mean the occurrence of a second full moon within a calendar month.
This strange new use of the term, popularized by a board game in the mid-1980s, arose from a mistake made by the author of a magazine article written in the 1940s. Since two full moons in one month occur fairly regularly, it should not be considered a rare event.