A Blue Moon is an astronomical phenomenon that refers to various types of unusual moon events. The term “Blue Moon” was traditionally applied to an “extra” full moon that occurred in a season; usually a season has three full moons, but in some years there are four. The third of the four full moons was called a Blue Moon so that the last full moon can still fall near the start of the following season and be called the “late moon.”
However, the modern interpretation, which is most commonly used today, comes from a mistake printed in a 1946 article in “Sky and Telescope” magazine. The author of the article, James Hugh Pruett, misunderstood the basis for calculating the seasonal Blue Moon and stated that a Blue Moon was the second full moon in a calendar month that has two full moons. This ‘misinterpreted’ definition proliferated after it was used on the popular radio program StarDate on January 31, 1980, and then again in a Trivial Pursuit game in 1986.
Despite the origin, the definition of a Blue Moon as the second full moon in a calendar month with two full moons has been widely accepted and used. This scenario happens approximately once every 2.7 years, though on rare occasions a single calendar month can actually contain two Blue Moons; this requires February to be missing a Full Moon and both January and March to have two. The most recent occurrence was in 2018.
It’s important to clarify the “Blue Moon” doesn’t mean the moon appears blue. The term simply refers to the “extra” full moon. The moon can appear bluish in color, but this is an infrequent occurrence and has to do with the composition of dust and smoke particles in the atmosphere, rather than the phase of the moon itself.
At the end of the day, whether you stick with the older, traditional definition of a Blue Moon or go with the more recent and commonly understood definition, these events do remind us of the fascinating rhythms and patterns of our celestial companion.
