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Just before midnight today, sky watchers across this country will see an event that is notable because of the quirks of the man-made calendar and happens, well, just once in a blue moon.

Few experts agree on why a blue moon - a second full moon in one month - is called a "blue moon." It has nothing to do with its color."Blue moons are the Hula Hoops of the heavens," says Jack Horkheimer, director of the Miami planetarium and host of the PBS astronomy show "Star Hustler." "They're kind of silly, but they're a lot of fun."

Blue moons have become part of our culture, largely remembered from the popular Rodgers and Hart standard: "Blue Moon, you saw me standing alone, without a dream in my heart, without a love of my own."

That song was recently encored by the singing mice in the movie "Babe."

Blue moons occur once every 2.7 years, and come about because the modern calendar's 12 months do not coincide with the moon's 29-day cycles. The last full moon was June 1.

The origin of the term itself is obscure.

"It's interesting tracking it down. We'll never know for sure," said Terence Dickinson, editor of the Canadian astronomy magazine Sky News.

Dickinson, by the way, prefers actual events like the even rarer occurrence of a moon that really is tinted blue, when something in the atmosphere blocks the red component of sunlight.

A blue-tinted moon was seen over much of the world after the 1991 eruption of Mount Pinatubo in the Philippines, Horkheimer noted.

When the last two-in-a-month, untinted blue moon was visible in this country, in September 1993, Horkheimer had a contest to find the origin of the term.

"Several thousand sent in responses," he recalled.

Horkheimer particularly liked one explanation that linked "blue moon" to the old English word "belewe" which meant "to betray." According to that theory, the second moon in one month betrays the count of one full moon in one month.

Another said it may have come from the French word for double. Others argue it came from ancient cultures in Egypt or Crete and was a symbol of good fortune.