The present definition of the term "blue moon" is the second full moon in one calender month. Over the span of the last thousand years, a handful of observers noted that the moon appeared blue, but chances are atmospheric dust and ash thrown by massive volcanic eruptions cast the moon in the unusual shade.
Folklorist Philip Hiscock weighed in on the "blue moon" phenomenon in Sky & Telescope. He stated that long ago, the "blue moon" phase left observers mesmerized when the face of the moon appeared blue. Hiscok wrote: the moon appearing blue is a "truly modern piece of folklore, masquerading as something old" and "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."