By Julie Sevrens Lyons
San Jose Mercury News
May 31, 2007, 1B

First thing's first: It isn't blue.
And really, it isn't all that rare.
    But the astronomical event that was the inspiration for the Marcels' timeless doo-wop hit - and is the namesake of a beer, a bluegrass band and even an alpaca breeding ranch - will appear in North America at 6:04 PDT tonight, after an absence of several years.
    It's a blue moon, the second full moon in a calendar month.
    What makes tonight's lunar occurrence particularly cool is that only the Americas will have one this week: Europe, Africa, Asia and Australia won't get a blue moon until late June. And residents of New Zealand will have to wait until the end of July to see one.
    "It's just an accident of the calendar," said Marni Berendsen, education project coordinator for the Astronomical Society of the Pacific in San Francisco.
    While there have been many meanings to the term "blue moon" over the ages, it has taken on its most current one only since the end of World War II.
    In 1946, an article in Sky & Telescope magazine inadvertently deviated from the widely understood definition of the time and called it the second full moon in a calendar month. Previously, blue moons had been known in farmers' almanacs as the third full moon in any season with four full moons.
    The current definition, even though it was a mistake, was accidentally repeated in a National Public Radio program on astronomy in 1980, and then appeared as the answer to a question in the board game Trivial Pursuit. Since then, the definition has been an accepted part of the lexicon.
    Explains a sheepish Roger Sinnott, senior editor of Sky & Telescope, "It wasn't until some 56 years later that we found out that we'd made a mistake."
    Blue moons are infrequent, occurring about once every 2.7 years. But because they are no bigger, fuller or blue-er than your standard full moons, they tend to be a bit of a snoozer, star gazers say.
    "It's really not of astronomical interest," said Charles Meegan, an astrophysicist with NASA's National Space Science and Technology Center in Huntsville, Ala. Further professing his lack of interest in the occasion, he added: "The best time to look at a moon with a telescope is not when it's full because there aren't any shadows. Wait until it's halfway full and it's much more interesting to look at."
    OK, so maybe a blue moon isn't up there with Halley's comet, but it's still the source of so much fascination in our modern-day society.
    It will be celebrated in many pockets of the country tonight. Goldendale, Wash., is having its Blue Moon Music Festival. The Blue Moon Community Farm in Dunn, Wisc., is having a harvest dinner. And there's the Ann Arbor Witches Meetup Group, a Michigan coven of witches, wiccans, pagans and sorcerers. But they actually held their event on Monday, the Memorial Day holiday - because, after all, the Blue Moon is taking place in the middle of the work week.
    The fact the event falls on a Thursday, however, wasn't enough to deter the owner of the Sacred Grove, a metaphysical bookstore in Santa Cruz, from planning his wedding on the special day.
    "A lot of people in the community get excited with the blue moon," said his friend, Debbie Silvia, who has been tending the bookstore in his absence. "Some believe that they are very magical. Others look at them just as another full moon to celebrate under."

A full moon is visible in this view above Earth's horizon and airglow, photographed by an Expedition 10 crewmember on the International Space Station (ISS). Photo courtesy of NASA (NASA)


"Blue moon" has been part of the English language for more than 400 years, originally referring to something absurd or impossible. Now it's used to mean something occurs rarely. In astronomy, it refers to the second full moon in one month.

Blue moons aren't blue. But moons have appeared that way at least a couple of times in history. After the eruption of Krakatau in Indonesia in 1883, vast amounts of volcanic dust not only made the moon seem blue but also turned sunsets that color for nearly two years. The moon also looked blue in September 1951, when forest fires swept through Canada.

Blue moons occur, on average, about once every 2.7 years.

It's impossible for February to have one - even in a leap year - because the shortest interval between one full moon and the next is 29 days, six hours and 35 minutes.

The last blue moon in California was on July 31, 2004. After tonight's, the next one will be on Dec. 31, 2009.

Contact Julie Sevrens Lyons at or (408) 920-5989.