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Subject: Blue Moons
From: Daniel Lipkie

Counting Blue Moons? Think Again, Because Definition Is Wrong


It was supposed to be a banner year for blue moons, with one in January and another that rose Wednesday night, as March drew to a close.

But now the astronomy magazine Sky and Telescope has discovered that the modern definition of a blue moon -- as the second full moon in a month -- is based upon an error made more than five decades ago. And the perpetrator was none other than Sky and Telescope itself.

Rather than the twice-in-a-month definition, the magazine says in the current issue, a blue moon was originally understood to be a fourth full moon in a season, which normally has three. By that definition, neither the January nor March moons are blue; in fact, 1999 is blue-moonless.

Donald W. Olson, an astronomer at Southwest Texas State University and an author of the current article, said the error came to light as the magazine's editors were preparing an article for the March issue on blue moons, which this year had sparked much interest because of the appearance of two of them. (Most years are lucky to have one, no matter what the definition.)

In the March article, a Canadian folklorist had traced the definition to a 1946 Sky and Telescope article, which itself made reference to the use of the term in a 1937 edition of the Maine Farmers' Almanac.

Olson, who is something of an authority on old almanacs, got a copy of the 1937 edition, and immediately noticed that something was amiss. The blue moon was listed for Aug. 21, 1937. Since the lunar cycle, from full moon to full moon, is 29 1/2 days, simple arithmetic showed that this moon could not have been the second in August.

Olson fired off an urgent E-mail message to Roger W. Sinnott, one of the magazine's associate editors.

"I said, 'Roger, be very careful what you say in the article,' " Olson recalled. "The blue moon in this almanac I'm sending you is not and cannot be the second moon in a month."

The author of the 1946 article, Hugh Pruett, who lived in Oregon, did not have the 1937 almanac with him when he wrote the piece, Sinnott said. "He knew enough astronomy to realize that if there are 13 full moons in a year, one month has to have two of them," Sinnott said.

Pruett wrote in the article that "this second in a month, so I interpret it, was called Blue Moon." But he obviously did not realize the date of the blue moon in the almanac, or that 1937 in fact only had 12 moons.

Nothing came of the mistake for years, until the writer of a National Public Radio show on astronomy, having spied the blue moon tidbit while thumbing through old issues of the magazine, included it in a program in 1980. It was later picked up by the makers of the game Trivial Pursuit, who included it as a question in 1986.

Sky and Telescope published the March article acknowledging that the current definition of a blue moon appeared to be wrong. But determining what the original definition was required a bit more detective work on the part of Olson, who eventually obtained more than 40 early editions of the almanac. All told, there were more than a dozen references to blue moons. "But not a single one was the second moon in a month," he said.

Instead, he noticed that all of the blue moons in the almanacs occurred within a day or two of the 21st of a month, and that they occurred only in February, May, August and November. Olson saw a seasonal pattern.

"As soon as I saw those 21st's, I was thinking equinoxes and solstices," he said. And the four months in question, he noticed, preceded the months that contain either the equinoxes or solstices.

The editors of the Maine almanac, he realized, were going by what is known as a "tropical" year, from winter solstice to winter solstice. And some of these tropical years do have 13 moons. But more important, the additional moon was considered to come within a three-month season, not within one month. And because the almanac's editors had a convention for naming the normal 12 moons of the year, including the four that are the last in each season, the blue moon was considered the third moon of a four-moon season. By their definition, the next one is in February 2000.

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