A Blue Moon is the term given for the rare occasions when we have two full moons within the same month -- it's the name for the second full moon.
Since full moons come about every 29 1/2 days, you can see it's hard to squeeze two in a month. But it happens about every 2 1/2 years.
And talk about 'once in a blue moon', in 1999, there were *two* Blue Moons -- one in January, and another in March. That occurs every 19 years or so.
Sometimes, different parts of the world can get their blue moons in different months. For instance, the second full moon might be 10 p.m. on Oct. 31 in Seattle, but that's 1 a.m. Nov. 1 in New York, so Seattle's blue moon would be in October while New York's would be in November.
Obliquity.com reports that if you start with the year 1600 and project over the next 10,000 years, October will have had the most blue moons with 516. As you might expect, February has never had a Blue Moon.
And no, the moon doesn't actually turn blue.
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Editor's Note: We erroneously reported in the printed P-I version on Oct. 9 that if you went back over the last 400 years, October would have 516 blue moons. It's 516 if you start in 1600, but then project out over the next 10,000 years.