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The glittery ball in New York's Times Square drops every New Year's Eve. But this year it will be joined a second glowing orb in the sky: The last night of 2009 will boast December's second full moon, otherwise known as a blue moon.A blue moon, according to folklore, is the appearance of a second full moon in a given month, said Bob Hayward, an astronomer and educator at Pisgah Astronomical Research Institute, the observatory in Rosman.
It's more a trick of the human calendar than any mischief on the part of the moon, which waxes and wanes on a steady schedule in its 29 1/2-day orbit around the Earth.
Given the months with an uneven number of days in the Gregorian calendar, every couple of years will see an extra full moon in the calendar year, Hayward said. “We can safely say there are about eight blue moons in any 18-year period,” he said.
That extra moon gave rise to the folklore saying “once in a blue moon,” meaning a rare occurrence.
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