Why Do We Call It a Blue Moon?

A Blue Moon is a full moon that occurs twice in a month.

We all know the saying “once in a blue moon” that is usually used to refer to things that happen relatively rarely. The origin of the phrase comes from the fact that there are usually two full moons in a month, the second being the blue moon. However, the question remains, why do we call it a blue moon when it’s actually just a simple, yellow, full moon?

This Saturday, on the 21^st of May, 2016, a full moon will fill the sky. It will be the first fool moon that occurs this month, but people are still calling it a blue moon. Even though the term is reserved for the second such occurrence, this time, it is used correctly, even though it doesn’t describe the second full moon occurring this May.

While it may sound confusing, there are a lot of facts, myths, and legends gathered around the notion of the blue moon. And as every popular habit, it can be adapted to different situations, all in the name of respecting tradition.

Usually, there are 12 full moon occurrences in a calendar’s year, each one in a different month, summing three full moons for each season. But sometimes, out planet’s orbit around the Sun and the Moon’s orbit around the Earth would make it the apparition of a 13^th moon possible.

“This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason, thirteen came to be considered an unlucky number,” reads a 1937 article from the Maine Farmer’s Almanac.

So not only is the superstition around the number thirteen centered around some very grumpy monks that had to perform extra duties, but it also explains why the expression “once in a blue moon” was originally used with a negative connotation.

The article goes on by saying that “there are seven Blue Moons in a Lunar cycle of 19 years. In older times, the almanac makers had much difficulty calculating the occurrence of the Blue Moon, and this uncertainty gave rise to the expression ‘once in a Blue Moon.”

So now you know why we call it a blue moon and what are the origins of the expression. Regarding why we are still calling today’s full moon a blue moon, the answer is quite simple. We do it in order to respect the tradition started out by the grumpy monks centuries ago.

Image source: Wikipedia