July 31, 2015 will be the occasion of a blue moon — the last blue moon anyone will see, in fact, until 2018. But what exactly is a blue moon, and why does anyone care?

These days, the term “blue moon” is most often defined as the second of two full moons that occur within the same 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, a blue moon is an out-of-the-ordinary phenomenon that occurs only once every few years, hence the expression “Once in a blue moon.”

Historically, the phrase was understood in a much more literal way, writes folklorist Philip Hiscock in the pages of Sky & Telescope. Once upon a time, he says, it denoted an even rarer phenomenon than an extra full moon in a given month or season, a phenomenon that has occurred perhaps only once or twice in recorded history — the face of moon actually 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 volcanic eruptions probably account for the few times in recent millennia when the moon presented an azure face to observers on earth.

One such incidence occurred over a period of several days in December 1883, when geologist W. Jerome Harrison, among others, reported viewing a striking “electric-blue” crescent moon against a copper-colored sky from his home in Birmingham, England. He and other scientists attributed it to lingering debris from the explosion of the massive volcano Krakatoa, which had occurred a little more than three months earlier. In fact, oddly-hued skies, sunsets and moonrises were observed for up to two years after the eruption.

Most people don’t realize that “blue moon” took on its present meaning only recently. It’s a “truly modern piece of folklore, masquerading as something old,” says Hiscock. The second-full-moon-in-a-month definition is only slight more than 50 years old.

No matter how it’s defined, “blue moon” will probably always retain its ancient connotations, as evinced in popular songs identifying the image with loneliness and despair.