If you're getting ready to croon about the blue moon Wednesday night, well, hold that tune.
According to Sky & Telescope magazine, Wednesday's full moon will indeed be the second this month, but it won't be a real blue moon. And the magazine claims that it's to blame. Apparently it published an erroneous definition in 1946 and nobody noticed until now.Blue moons are a hot topic this year because both January and March have two full moons, an unusual event. Editors discovered the magazine's blunder while preparing to highlight the phenomenon for the magazine's March issue.
The editors found a 1946 Sky & Telescope article by amateur astronomer James Hugh Pruett that defined a blue moon. But when they went back to the article's source, the Maine Farmer's Almanac from 1937, they saw Mr. Pruett had misinterpreted the almanac's definition.
"We knew we had a problem," says Roger Sinnott, a co-author of an article retracting the 1946 definition for the magazine's May issue. The retraction's other authors are Richard Fienberg, the publisher, and Don Olson, a professor of physics at Southwest Texas State University.
The team discovered that the almanac had originally defined a blue moon as the third full moon of a season in which four appear if a total of 13 full moons appear in a 12-month period. Mr. Pruett, who died in 1955, incorrectly concluded that if there were 13 full moons in one year, then one month must have had two of them. His conclusion even found its way into a 1986 edition of the Trivial Pursuit board game.
Sky & Telescope mentioned the error in its March issue, but says it decided to search for more details for its May issue. The researchers have since located 42 copies of Maine almanacs dating back to the 1800s, but the 1937 edition holds the earliest mention of a blue moon. None of the later editions changed the definition.
The team is also researching the origin of the almanac's findings. Its conclusion thus far: The almanac was probably trying to account for the phrase "once in a blue moon," which dates back centuries.
Under the almanac's definition, 1999 has no blue moons and the next one isn't until Feb.19, 2000. But that may not matter. "I suspect that people will continue to think of blue moons as the second full moon in a month," says Dale Smith, president of the International Planetarium Society. "It'll be the typical case of terms being defined by how they're most commonly used."