Skip to Content

Astronomers shed light on Blue Moon

Date of release: 03/26/99

SAN MARCOS, TEXAS — How often does a respected national scientific magazine retract information it first published 53 years ago?

Probably only once in a Blue Moon.

But that’s what Sky & Telescope magazine will do in its May 1999 issue which will redefine popular folklore surrounding the definition of a Blue Moon.

For more than half a century, public opinion has held that a “Blue Moon” is a second full moon in any given month, like the one that will occur March 31 this year. In fact, that particular definition was born in the pages of Sky & Telescope in 1946, and from there its acceptance seemed to grow, well, astronomically.

Unfortunately, that definition is based on a misunderstanding, according to an article in the May 1999 edition of the magazine.

The article is authored by Don Olson, professor of physics at Southwest Texas State University, Rick Fienberg, publisher of Sky & Telescope, and Roger Sinnott, associate editor of Sky & Telescope.

The authors say a Blue Moon occurs when a particular season contains four full moons. In that event, the third full moon is the Blue Moon. All other full moons of the year are named after seasonal activities, such as the Harvest Moon, the Lenten Moon or the Hunter’s Moon.

The two-full-moons-in-a-month definition was first set forth in a March 1946 Sky & Telescope article on the origin and evolution of the term “once in a blue moon,” written by amateur astronomer James Hugh Pruett.

In that article, Pruett mentions an earlier Sky & Telescope piece based on a 1937 Maine almanac, which called the 13th full moon of a year a Blue Moon. Pruett speculated that if there were 13 full moons in a year, then one month must have contained two full moons. He then incorrectly asserted, “This second in a month, so I interpret it, was called Blue Moon.”

That interpretation was wrong, but from it the folklore grew. A headline over a later Sky & Telescope story on second-in-a-month full moons carried the term “Blue Moon.” Based on the magazine’s stories, the popular radio program StarDate featured Pruett’s definition. It then found its way into a children’s almanac, and even into an edition of the board game Trivial Pursuit as it gradually became cemented in popular culture.

However, Olson, Fienberg and Sinnott say Pruett erred in his article, and they contend that had Pruett had the almanac handy while he was writing, the misinterpretation never would have happened.

The authors located a copy of the 1937 Maine Farmers’ Almanac and found that it called a full moon of Aug. 21 that year a Blue Moon. But since a lunar cycle is 29 1/2 days, a second full moon could not have happened on the 21st day of the month, a fact Pruett would certainly have recognized had he seen the almanac. Clearly, Pruett and the almanac’s author were working from different definitions of a “Blue Moon.”

In fact, Olson, Fienberg and Sinnott found a dozen references to Blue Moons in various editions of the Maine almanac, and not a single one was a second full moon in a month.

The almanac based its calculation on a seasonal year -- winter solstice to winter solstice -- rather than a calendar year. Therefore, the Aug. 21 moon, while not the second full moon of the month, was the third of a summer season that would contain four full moons. The authors say that makes it fit the original, but seemingly forgotten, definition of a Blue Moon.

Forgotten, at least, until now.

Considerable media attention has been devoted to the Blue Moon phenomenon in 1999 because both January and March exhibit two full moons. Under the popular definition, both are Blue Moons. But using the older, now rediscovered definition using a seasonal year, neither qualifies as “blue” because neither was the third of four full moons in a single season.

Using the seasonal year calculation, the next Blue Moon will take place on Feb. 19, 2000. A Blue Moon occurs about seven times every 19 years under either definition, but not on the same dates.

The authors say that the two-full-moons-in-a-month definition that now has such widespread acceptance “is like a genie that can’t be forced back into its bottle.”

But Olson adds, “That’s not necessarily a bad thing. Rather than argue over whether to celebrate the dawn of the new millennium on January first in 2000 or 2001, those with the sunniest outlooks will celebrate twice. Why not treat Blue Moons the same way? Both occurrences are interesting. I think we can appreciate both.”

The Sky & Telescope article provides additional insight that defines Blue Moons, but the origin of the term “blue” itself is still uncertain.