happen as it takes some very large and catastrophic event to cause it. Strangely, this is not where the saying came from, however. A real blue moon was thought to be impossible, centuries ago when this phrase came into popular use, and that is what they meant when they said it. Impossible. “Once in a blue moon,” “the moon is made of green cheese,” and “when pigs fly” all were synonyms for something so absurd it couldn’t possibly be real.