In modern times, the term "blue moon" is 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 — instead of the normal three — full moons. Either way, it's an out-of-the-ordinary phenomenon occuring only once ever 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, he says, it denoted a phenomenon even rarer than an extra full moon, one that has occurred perhaps only once or twice in recorded history: the face of moon literally appearing to turn 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 like massive amounts of volcanic dust probably account for the few times in recent millenia when the moon reportedly did present an azure face to observers on earth.
Most people don't realize that "blue moon" took on its present astronomical 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. However it is defined, the phrase will probably always retain its ancient connotations, as evinced, for example, in popular songs identifying the image with loneliness and despair.
