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Last updated on April 19, 2014 at 18:42 EDT

Once in a Blue Moon

May 31, 2007

By Clara Moskowitz

This moon isn’t any bluer than usual, or any more impressive than your average full moon.

But whenever there is a second full moon in a month, as there will be today at 6:04 p.m., we call it a blue moon.

“There really is no scientific importance ascribed to a blue moon,” said Greg Laughlin, an astronomer at the University of California, Santa Cruz.

Most astronomers, he said, would probably be aware of a blue moon only if they read about it in a newspaper. “It’s really an artifact of our own calendar, and the fact that our months are fairly close to a lunar cycle.”

Because a full moon comes about once every 29.5 days, sometimes one month will squeeze in two full moons. This happens almost every three years, which is probably less rare than most people mean when they say “once in a blue moon.”

Although it has no astronomical significance, the blue moon has a fascinating cultural history.

Our modern conception of a blue moon is largely due to a magazine’s mistake — and a board game.

In 1946, Sky and Telescope published a story that described a blue moon as the second of two full moons in one month. The author based this definition on the 1937 Maine Farmer’s Almanac, but he misinterpreted the almanac’s meaning.

Instead of being an extra full moon in a month, a blue moon was counted as an extra full moon in a season. The almanac divided each year into four quarters, based on where the equinoxes and solstices fell. Each quarter normally contained three full moons. But when there were four, the third was designated the blue moon. Thus the full moons that qualified as “blue” under the almanac definition were not always the same as the second full moon of a month.

The error was perpetuated when a popular radio program, StarDate, included the new definition on one of its shows in the 1980s. Public knowledge of this interpretation got a boost when Trivial Pursuit produced its Genus II edition in 1986, which included a question about blue moons.

Nearly 60 years after the original article, Sky and Telescope noticed its mistake and printed a correction. By then, the cat was out of the bag and the new meaning of “blue moon” was here to stay.

The phrase “blue moon,” however, is much older than this astronomical definition, and originally had nothing to do with the rarity of two full moons in a month.

Four hundred years ago, a blue moon meant something absurd and clearly untrue, according to folklorist Philip Hiscock of Memorial University of Newfoundland, who wrote an article tracing the lineage of the blue moon.

“If someone said ‘He would argue (that) the moon was blue,’ the average 16th-century man would take it the way we would understand, ‘He would argue that black is white,’” writes Hiscock.

This meaning eventually morphed into something that would never happen, like pigs flying, or hell freezing over, and the phrase “once in a blue moon” emerged.

If you ask decades of lonely singers, a blue moon represents lost and longed-for love.

“Blue Moon of Kentucky keep on Shining / Shine on the one that’s gone and left me blue,” Elvis sang in Bill Monroe’s famous bluegrass waltz.

In the song made famous by Frank Sinatra, the blue moon turned to gold when the singer finally found love.

There even seems to be a superstition that under a blue moon, women should propose to men instead of letting them pop the question.

For astrologers, a blue moon doesn’t have any extra significance beyond any other full moon, although it does mean twice as much craziness in one month.

“A full moon is just a more intense time,” said Capitola astrologer Mary-Patrick Lioux. “If people are inclined to be angry, they’re extra angry. If people are loving, they’re extra loving.”

Though blue moons aren’t really blue, the moon can occasionally appear that color. Smoke and dust in the atmosphere can tint objects in the sky. After the eruption of the volcano Krakatoa in 1883, the moon looked blue for months.

Because of the difference across time zones, only the Western Hemisphere will see a blue moon tonight. When the full moon appears in Europe, Africa and Asia, it will already be June 1, and hence not a second full moon in a month.