Watch Out! "BLUE MOON"s rarity

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Coming December 31, 2009 - 11:59pm

Rare New Year's Eve 'blue moon' to ring in 2010
By ALICIA CHANG, AP Science Writer Alicia Chang, Ap Science Writer – Tue Dec 29, 7:03 pm ET

Once in a blue moon there is one on New Year's Eve. Revelers ringing in 2010 will be treated to a so-called blue moon. According to popular definition, a blue moon is the second full moon in a month. But don't expect it to be blue — the name has nothing to do with the color of our closest celestial neighbor.

A full moon occurred on Dec. 2. It will appear again on Thursday in time for the New Year's countdown.
"If you're in Times Square, you'll see the full moon right above you. It's going to be that brilliant," said Jack Horkheimer, director emeritus of the Miami Space Transit Planetarium and host of a weekly astronomy TV show.

The New Year's Eve blue moon will be visible in the United States, Canada, Europe, South America and Africa. For partygoers in Australia and Asia, the full moon does not show up until New Year's Day, making January a blue moon month for them.

However, the Eastern Hemisphere can celebrate with a partial lunar eclipse on New Year's Eve when part of the moon enters the Earth's shadow. The eclipse will not be visible in the Americas.
A full moon occurs every 29.5 days, and most years have 12. On average, an extra full moon in a month — a blue moon — occurs every 2.5 years. The last time there was a lunar double take was in May 2007. New Year's Eve blue moons are rarer, occurring every 19 years. The last time was in 1990; the next one won't come again until 2028.

Blue moons have no astronomical significance, said Greg Laughlin, an astronomer at the University of California, Santa Cruz.

"`Blue moon' is just a name in the same sense as a `hunter's moon' or a `harvest moon,'" Laughlin said in an e-mail.

The popular definition of blue moon came about after a writer for Sky & Telescope magazine in 1946 misinterpreted the Maine Farmer's Almanac and labeled a blue moon as the second full moon in a month. In fact, the almanac defined a blue moon as the third full moon in a season with four full moons, not the usual three.

Though Sky & Telescope corrected the error decades later, the definition caught on. For purists, however, this New Year's Eve full moon doesn't even qualify as a blue moon. It's just the first full moon of the winter season.

In a tongue-in-cheek essay posted on the magazine's Web site this week, senior contributing editor Kelly Beatty wrote: "If skies are clear when I'm out celebrating, I'll take a peek at that brilliant orb as it rises over the Boston skyline to see if it's an icy shade of blue. Or maybe I'll just howl."

July 7, 2004: When you hear someone say "Once in a Blue Moon…" you know what they mean: Rare. Seldom. Maybe even absurd. After all, when was the last time you saw the moon turn blue?

[On July 31st, you should look, because there's going to be a Blue Moon.
According to modern folklore, a Blue Moon is the second full moon in a calendar month. Usually months have only one full moon, but occasionally a second one sneaks in. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month. This happens every two and a half years, on average.
Right: One way to make a blue moon: use a blue filter. That's what Kostian Iftica did on July 2nd when he photographed this full moon rising over Brighton, Mass.

July has already had one full moon on July 2nd. The next, on July 31st, is by definition a Blue Moon.

But will it really be blue? Probably not. The date of a full moon, all by itself, doesn't affect the moon's color. The moon on July 31st will be pearly-gray, as usual. Unless....

There was a time, not long ago, when people saw blue moons almost every night. Full moons, half moons, crescent moons--they were all blue, except some nights when they were green.
The time was 1883, the year an Indonesian volcano named Krakatoa exploded. Scientists liken the blast to a 100-megaton nuclear bomb. Fully 600 km away, people heard the noise as loud as a cannon shot. Plumes of ash rose to the very top of Earth's atmosphere. And the moon turned blue.
Krakatoa's ash is the reason. Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide--the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green.

Blue moons persisted for years after the eruption. People also saw lavender suns and, for the first time, noctilucent clouds. The ash caused "such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration," according to volcanologist Scott Rowland at the University of Hawaii.

Above: Still smoldering after all these years: a recent picture of Krakatoa. Credit: Robert W. Decker of Volcano World. [More]

Other less potent volcanos have turned the moon blue, too. People saw blue moons in 1983, for instance, after the eruption of the El Chichon volcano in Mexico. And there are reports of blue moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.
The key to a blue moon is having in the air lots of particles slightly wider than the wavelength of red light (0.7 micron)--and no other sizes present. This is rare, but volcanoes sometimes spit out such clouds, as do forest fires:

"On September 23, 1950, several muskeg fires that had been quietly smoldering for several years in Alberta suddenly blew up into major--and very smoky--fires," writes physics professor Sue Ann Bowling of the University of Alaska. "Winds carried the smoke eastward and southward with unusual speed, and the conditions of the fire produced large quantities of oily droplets of just the right size (about 1 micron in diameter) to scatter red and yellow light. Wherever the smoke cleared enough so that the sun was visible, it was lavender or blue. Ontario and much of the east coast of the U.S. were affected by the following day, but the smoke kept going. Two days later, observers in England reported an indigo sun in smoke-dimmed skies, followed by an equally blue moon that evening."

Right: Smoke from forest fires can cause blue moons, too. Photo credit: John McColgan of the Bureau of Land Management, Alaska Fire Service.
In the western U.S., there will be wildfires burning on July 31st. If any of those fires produce ash or oily-smoke containing lots of 1-micron particles, the Blue Moon there could be blue.
More likely, it'll be red. Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with a wide range of sizes. Most are smaller than 1 micron, and they tend to scatter blue light. This kind of cloud makes the Moon turn red; indeed, red Blue Moons are far more common than blue Blue Moons.

Absurd? Yes, but that's what a Blue Moon is all about. Step outside at sunset on July 31st, look east, and see for yourself.

more info:

Blue Moons and Lavender Suns (Alaska Science Forum)

The eruption of Krakatoa in 1883 threw enormous plumes of ash as much as 80 km high into Earth's atmosphere and caused spectacular sunsets for years. It was not long after the eruption that noctilucent clouds were first observed, as well as many blue-colored moons.