BluemoonphotoOn July 31st, there will be a “blue moon,” visible from anywhere on Earth where the sun goes down. You’ve probably heard the saying “once in a blue moon” more times than you can count and it might bring to mind images of a night sky with a shiny blue moon, or the enticing flavor of Blue Moon beer. What is less commonly known is where this saying comes from and what a blue moon actually is.

What is a Blue Moon?

Contrary to its name, a blue moon is not actually blue. Occasionally the moon appears to have a blue hint to it, which only occurs when there are certain types of dust particles scattered throughout the atmosphere that create blue light. Generally, volcanic activity causes this, which explains why people reported seeing a blue colored moon after El Chichón’s eruption in Mexico in 1983.

Blue moons are actually defined in the Gregorian calendar as being the second full moon in a month. Normally there is one full moon a month because a lunation (time it takes for one lunar cycle to be complete) is 29.53 days. However, with 365.24 days in a year, the number of lunations per year averages to 12.37, which means that every two or three years there is a month with a second moon, called a blue moon.

Because the moon was also full on July 1, July 31’s full moon will be a blue moon.

Origin of the Expression

The saying “once in a blue moon” originated in Britain around the 16th century to describe something absurd that would most likely never happen, since the thought of the moon being the color blue seemed ridiculous to most people. Slowly the expression evolved into meaning a rare event.

Evolution of a Blue Moon’s Meaning

When the Farmer’s Almanac was being published in Maine, the writers used “blue moon” to be the name of the third moon of a season containing four moons. Since a season normally just has three full moons, a “blue moon” was a good way to describe the rare event of there being a fourth.

Additionally, the Farmer’s Almanac gave names to each full moon of the year as a way to keep track of the seasons. For example, September’s full moon is called the “harvest moon” to remind farmers that early autumn is harvest time. Naming the occasional extra moon was an important way to ensure that it wouldn’t throw off farmers’ lunar calendar.

Eventually, as the number of farmers in the U.S. declined, the term “blue moon” shifted from being defined within the context of the seasonal calendar to being defined within the context of the Gregorian calendar. This explains its current definition as being the second full moon in a month.

Just as solar calendars have had trouble accounting for the extra quarter day of the year and have had to create leap years, lunar calendars have sometimes encountered difficulties accounting for blue moons. This explains why the Islamic calendar slips back through the seasons and why today’s Jewish calendar adds an extra “leap month” from time to time.

The Incan Calendars

The Incas had a complex time keeping system that had multiple calendars for both solar and lunar calendars to keep track of celestial movements. Their lunar year was made up of 30-day “moons.” Every third year would be 13 moons long and the two others would be 12 moons long to account for blue moons.

Because the sun and moon have different cycles, humans have tried for centuries to reconcile their patterns into one, comprehensive calendar without much luck.

Take Time to Connect with the Cosmos

Taking time to look at the blue moon this month is a good way to remind yourself of the complexity of celestial bodies, how little we know about what lies beyond, and to reflect upon how tiny us humans are in the great web of the universe.

To deepen your connection with the cosmos and spend time with people who have always viewed themselves as part of an interconnected web, sign up for one of our journeys to the Amazon. This is an opportunity to learn from our indigenous partners who have invited us stay with them and bring home the lesson of living in harmony with the universe that we are all part of.

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Photo Credit: Flickr user Randen Pederson