The Science Behind Some Popular Phrases

Once in a Blue Moon: A neat description of "not very often," it refers to the second full moon within a month - a rare thing indeed. Full moons happen about every 29.5 days, and since a typical month runs between 30 to 31 days, the likelihood of two in a month is slim. But over the course of a century there'll be 41 months with two full moons, so once in a blue moon really means - if you want to get literal - once every 2.4 years.

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