Once in a Blue Moon

Why does the phrase “once in a blue moon” mean “rarely”? Is it because, on unusual occasions, atmospheric conditions conspire to create a moon that appears blue? Or does a “blue moon” occur when there are two full moons in a single month—another uncommon occurrence?

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Once in a Blue Moon

Why does the phrase “once in a blue moon” mean “rarely”? Is it because, on unusual occasions, atmospheric conditions conspire to create a moon that appears blue? Or does a “blue moon” occur when there are two full moons in a single month—another uncommon occurrence?

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