These days the term “blue moon” is most often defined as the second full moon occurring within a single month. By a somewhat older definition, it’s the third full moon in a season that has four full moons instead of the usual three. Either way, it’s an out-of-the-ordinary phenomenon that occurs only once every few years.
“Blue moon” was understood in a much more literal way historically, writes folklorist Philip Hiscock in the pages of Sky & Telescope. Once upon a time, it denoted an even rarer phenomenon than an extra full moon in a given month or season, he says, one that has occurred perhaps only once or twice in recorded history: the face of moon appearing to turn bright blue in color.
“In fact,” notes Hiscock, “the very earliest uses of the term were remarkably like saying the Moon is made of green cheese. Both were obvious absurdities, about which there could be no doubt. ‘He would argue the Moon was blue’ was taken by the average person of the 16th century as we take ‘He’d argue that black is white.'”
Unusual atmospheric phenomena such as dust and ash thrown up by massive volcanic eruptions probably account for the few times in recent millennia when the moon presented an azure face to observers on earth. In December 1883, geologist W. Jerome Harrison reported viewing a striking “electric-blue” crescent moon against a copper-colored sky from his home in Birmingham, England. He attributed it to lingering debris from the explosion of Krakatoa.
Most people don’t realize that “blue moon” took on its present meaning fairly recently. It’s a “truly modern piece of folklore, masquerading as something old,” says Hiscock. The second-full-moon-in-a-month definition only dates back about 50 years. No matter how it’s defined, the phrase will probably always retain its ancient connotations, as evinced in popular songs identifying the image with loneliness and despair. Via What Is a Blue Moon?.