What Exactly is a “Blue Moon”?

Image

The phrase “blue moon” is used metaphorically to refer to a rare event, as in the expression “once in a blue moon.”  It has nothing to do with the actual color of the moon, although any moon can appear blue if the air is full of particles from volcanic eruptions or forest fires.

A “blue moon” is usually defined as the second full moon occurring during a calendar month. But until 1946, a “blue moon” was the third full moon in any season in which there were four full moons.  It was a sort of Leap Day for moons, necessitated by the fact that the average lunar cycle is 29.53 days, slightly shorter than an average month.

Continue reading