Two full moons in a single calendar month is a relatively recent definition of a blue moon.
During the most recent blue moon by this definition — last Aug. 31 — the Washington Post even referred to it as “modern folklore.” But, in fact, it wasn’t always that way. For centuries, the term “blue moon” referred to the third full moon out of four in a given season. Typically, a season consists of three months. But because the moon works on a 27.5-day cycle, we Earthlings sometimes get a little more than our money’s worth. In 1946, however, an amateur astronomer named James Hugh Pruett wrote an article for Sky & Telescope magazine in which he mangled the definition into what we consider today to be the more familiar one. In 1980, Deborah Byrd — host of an astronomy segment on public radio — discussed the two types of blue moons on her program. The media picked up on the newer, easier-to-understand definition, which
caused it to stick. Wednesday’s blue moon is of the original type.