What Happens During a Blue Moon?

Terminology

• People originally used the term "blue moon" to describe the third of four full moons during a calendar season -- spring, summer, winter or fall. In the 19th and 20th centuries, the "Maine Farmers' Almanac" referred to blue moons in this way. In 1946, an article in "Sky and Telescope" magazine incorrectly reported a "blue moon" as the second full moon in a calendar month. Over time, the 1946 definition of the term gained widespread use among the general public. Today, people still use the term "blue moon" in reference to a month in which two full moons occur.

Moon Facts

• In a single month, the moon travels through its various phases from new to full to new again. There are approximately 29.5 days between full moons. Because almost all the months have 30 or 31 days, two full moons are bound to occur in the same month. When this happens, one full moon is at the beginning of the month and the second is at the end. According to "From Blue Moons to Black Holes: A Basic Guide to Astronomy, Outer Space, and Space Exploration" by Melanie Melton Knocke, blue moons typically occur once every two or three years.

Turning Blue

• Although the term "blue moon" may suggest the moon appears blue in color, this does not happen. A second full moon in a calendar month looks like any other full moon: a round, white orb in the sky. However, dust and smoke from volcanic eruptions and atmospheric pollution can sometimes create rare occurrences where the moon appears to be a shade of blue. For centuries, people have used the term "once in a blue moon" to describe rare events or occurrences.

Folklore and Magic

• People associate blue moons with different folk beliefs. Some consider the blue moon to be a predictor of future events. In "The Moon: Myth and Image," author Jules Cashford says a common folk belief is that "two full moons in a calendar month bring on a flood." Cashford also notes that in England, a familiar rhyme warns, "Two moons in May, no corn, no hay." A blue moon event also supposedly plays a role in magic. In modern witchcraft, blue moons bring added spiritual energy and luck. "A Witch's Book of Answers" by Eileen Holland and Cerelia states the blue moon is a time to perform wish spells.