Don't ask why, but every once in a great while we feel compelled to revisit the question of what constitutes a blue moon. Language lovers know blue moon as a metaphor for a very long period of time. Skywatchers know blue moon as a reference to the bluish appearance of the moon that is attributable to dust particles in the high atmosphere. And folklorists know blue moon as a modern nickname for the not-so-rare occurrence of a second full moon in a calendar month. "Not-so-rare"? That's right. Astronomers calculate two such moons come around seven times every 19 years.

But readers of the astronomy magazine Sky & Telescope know this modern usage was born in error. It seems that in March of 1946, a writer for that magazine misinterpreted some data from an earlier article based upon the Maine Farmers' Almanac of 1937. His concept that blue moon named the two-full-moons-in-a-month event soon caught on.

But in fact, researchers now believe blue moon originally referred to a seasonal event: the very occasional third of four full moons that could occur in a season when measured according to a tropical year calendar. Why give that moon a name of its own? To ensure that the final moon of a season would retain such traditional nicknames as Harvest Moon and Hunter's Moon.