How Often Is Once in a Blue Moon? More Often Than You Think, and Not Blue

The moon completes its cycle of phases in about twenty nine and a half days, while calendar months are thirty or thirty one days long, so the phases drift forward relative to the calendar. That means a full moon falling in the first day or two of a long month is followed by another before the month ends, which happens roughly every two and a half to three years. February, being shorter than the cycle, can contain no full moon at all, which happens rarely and always in a year when January and March each contain two.

The popular rule has a traceable and accidental origin:

• •An almanac published in Maine used an older seasonal rule
• •A magazine article in 1946 misread that rule
• •It described the term as a second full moon in one month
• •A radio programme and a board game repeated the definition
• •It spread through popular astronomy writing for decades
• •The error was traced and published in 1999, by which point it had won

The definition the almanac actually used counts full moons within a season rather than a month, and it exists to keep a naming system on track. Each season normally contains three full moons, each with a traditional name tied to the point in the farming year. Occasionally a season contains four, which would push every subsequent name out of step, so the third of the four was given a placeholder name and the others kept their usual ones, preserving the correspondence between names and seasons. The term therefore identified a moon that had no name of its own, which is a far more interesting thing to be than simply the second in a month.

Several other colour names attach to the moon and only some describe anything real. An ordinary full moon near the horizon appears orange or red because its light passes through a long path of atmosphere that scatters away the shorter wavelengths, which is the same reason a sunset is red and has nothing to do with the moon itself. That effect is behind the harvest moon and hunter's moon appearing coloured, though those names actually describe timing rather than colour, marking the full moons nearest the autumn equinox. A totally eclipsed moon turns red for a related reason, being lit only by sunlight bent through the edge of Earth's atmosphere.

The moon does occasionally appear genuinely blue and the cause is specific. Particles in the atmosphere of a particular size, slightly wider than the wavelength of red light, scatter red light more strongly than blue, which reverses the usual reddening and lets the moon appear blue or green. That requires an unusual particle size distribution, which is produced by large volcanic eruptions and by intense forest fires. The eruption of Krakatoa in 1883 produced blue moons reported worldwide for months, and blue moons were reported after eruptions in Mexico in 1983 and after major Canadian forest fires. Those events are genuinely rare, which is the sense the original phrase carried.

Phases drifting forward against a longer calendar month produce a second full moon roughly every two and a half to three years, which is the popular definition, and it entered circulation through a magazine misreading an almanac in 1946. The older rule counts four full moons in one season and names the third, preserving a naming sequence. Volcanic dust and smoke of the right particle size make the moon genuinely blue.