Blue Moon

A Blue Moon, in the sense this site (and most people) mean it today, is the second full moon to occur within a single calendar month — since a full moon cycle (synodic month) averages about 29.5 days, slightly shorter than most calendar months, most months only fit one full moon, but occasionally one lands early enough in the month that a second one squeezes in before the month ends. That's the exact, computable rule powering the isBlueMoon flag on this site's full-moon pages.

This "second full moon in a month" definition is, honestly, a relatively modern misunderstanding of an older tradition, and it's worth explaining the actual history rather than just stating the popular rule as if it were ancient. The older definition, documented back to the Maine Farmers' Almanac, defined a Blue Moon as the third full moon in an astronomical season (the roughly three-month stretch between an equinox and a solstice, or vice versa) that happened to contain four full moons instead of the usual three — a subtly different, seasonal rule rather than a calendar-month rule.

The now-dominant calendar-month definition traces to a specific, documented error: in a 1946 Sky & Telescope magazine article, amateur astronomer James Hugh Pruett misread the older seasonal definition from a 1937 Maine Farmers' Almanac and described a Blue Moon simply as the second full moon in a calendar month. That misreading went largely unnoticed for decades until a 1980s trivia game and, later, popular almanacs and media picked it up and spread it widely enough that it became the meaning nearly everyone uses and expects today — a genuine, documented case of a mistake becoming the accepted popular definition, which this site states plainly rather than picking a "correct" one silently.

The Moon itself doesn't actually turn blue during a Blue Moon under either definition — the name has nothing to do with color. The idiom "once in a blue moon," meaning something rare, actually predates both formal definitions: 16th-century English used "the Moon is blue" as a stock phrase for an obviously absurd, impossible claim, and the phrase's modern "rare event" meaning developed later from that older, sarcastic usage, only getting attached to actual second-full-moon events well after the idiom itself already existed.

A genuinely blue-tinted Moon is a separate, real, and much rarer phenomenon than either Blue Moon definition — caused by fine airborne particles of a very specific size (around one micron across) scattering red light more than blue, the opposite of the usual atmospheric scattering that reddens sunsets. This has been documented after major volcanic eruptions (Krakatoa in 1883 produced blue moons visible worldwide for nearly two years) and large wildfires, and is a completely different mechanism from either the calendar-month or seasonal Blue Moon rule.

The astronomical mechanism that makes a calendar-month Blue Moon possible at all is straightforward once stated plainly: a synodic month averages about 29.53 days, while most calendar months run 30 or 31 days (only February, at 28 or 29, is shorter than a synodic month), so a full moon landing on or before roughly the 1st or 2nd of a 31-day month leaves just enough room for a second one before the month closes.

James Hugh Pruett, the astronomer behind the 1946 Sky & Telescope misreading that produced today's dominant definition, wasn't attempting to invent a new rule — by all accounts he was simply summarizing an older source inaccurately, an honest mistake that happened to spread widely enough through later popular use to effectively become the new standard, a genuinely unusual case of an error outcompeting the original correct definition in public understanding.