August's full Moon has come and gone. While it wasn't the second one occurring this month, it was a "Blue Moon" according to a definition dating to the 1930s.
The Moon was full early today at 3:45 Universal Time, which was last night for North Americans. Ordinarily, its timing would have passed by largely unnoticed. But, at last check, Google has tallied more than 50,000 mentions of this celestial moment as a "Blue Moon."
Wait, what? Isn't that supposed to mean the second full Moon in a given month? That's the definition that we often hear in the news these days. For example, there were two full Moons in August 2012.
The situation is confusing, and Sky & Telescope is largely to blame — thanks to a calendrical goof that worked its way into the magazine's pages back in March 1946.
Nearly a decade earlier, "Blue Moon" had been used in the Maine Farmers' Almanac to mean the third full Moon in a season containing four of them (rather than the usual three). This occasional add-on was needed to keep the tradition sequence of names — Wolf Moon, Strawberry Moon, Harvest Moon, and so on — in sync with the calendar. In 2013 full Moons fall (reckoned by Universal Time) on June 23rd, July 22nd, August 21st, and September 19th. That's four occurrences between June's solstice and September's equinox. So, like a round of celestial musical chairs, August's is the misfit and hence a "Blue Moon."
But in a 1946 S&T article, amateur astronomer James Hugh Pruett (1886–1955) made an incorrect assumption about how these extra orbs are handled. That's where the two-full-Moons-in-a-month notion got started.
It might have remained an obscure celestial aside, except that it resurfaced during a 1980 broadcast of the "StarDate" radio program. Then it showed up in the popular board game Trivial Pursuit. The genie was out of the bottle, as celestial sleuths Don Olson, Rick Fienberg, and Roger Sinnott detail here.
In the graph at right, the occasions reckoned by the Maine Farmer's Almanac rule are actually less common than the two-per-month events (which double up in 1999 and 2018, for example). So saying "once in a blue Moon" to mean a rare occurrence will be more meaningful if we return to the MFA definition. Interestingly, the "Maine rule" dictates that Blue Moons can occur only in February, May, August, and November — whereas two full Moons can never occur in February.
The original meaning of the term is lost to antiquity, as Philip Hiscock explains, though it's been used for centuries. Conceivably the phrase "once in a blue Moon" derived from those truly rare times when the Moon takes on a pale blue tint due to atmospheric effects (such as high-altitude ash from a volcanic eruption).
Frankly, I like how the Maine Farmer's Almanac meaning ties into seasonal cycles. As pioneering aviator Antoine de Saint-Exupéry (or was it Carl Sandburg?) once said, "The time for action is now. It’s never too late to do something." So let the campaign to get those Blue Moons back where they belong start here!