The Blue Moon To Shine Friday Night Won’t Be Blue

If there’s something you do “once in a blue moon,” get ready to do it tomorrow.

Yes, a blue moon will shine over our lovely Earth tomorrow – except that it won’t necessarily be blue.

The History Channel posted an explanation on why the “blue moon” is called blue while it really isn’t, and why this occurrence takes place so rarely that people came up with an expression for it.

According to the History Channel, we now consider a “blue moon” to be just a full moon shining twice during a Gregorian calendar month, or at least that’s the popular definition of the phrase now.

The second full moon in a month is known as the Blue Moon. Look for this rare phenomenon on July 31! pic.twitter.com/VIQRQKXID4

— Zion National Park (@ZionNPS) July 30, 2015

Since the Gregorian calendar is not a lunar calendar, the phases of the moon do not follow Gregorian months. The lunar cycle, which is used for lunar months of other cultures, covers an average of 29.5 days. As every month on the Gregorian calendar is longer than the lunar phase, with the exception of February, of course, some exceptional years end up having 13 full moons rather than 12.

According to the History Channel, the term “blue moon” originally referred to the third of four full moons in a season. (On a normal year, a season would only be visited by three full moons rather than four.) History, citing the Main Farmers’ Almanac, said this was the accepted definition of the phrase from 1800 until the 20th century.

The reason the third, rather than the fourth, moon was considered to be the “blue moon” is that it was the odd one out. The other full moons would land on their usual expected time periods.

One interesting fact is that our current understanding of the idea of a blue moon comes as a result of a misunderstanding. According to Sky and Telescope magazine, a contributor to the magazine by the name of James Hugh Pruett wrote an article in the mid 1940s and misunderstood the concept of the “blue moon.” Sky and Telescope says Pruett derived his idea on the fact that every Gregorian month has one full moon except for that random one that is visited by two every once in a while.

“This gives 11 months with one full moon each and one with two,” Pruett wrote. “This second in a month, so I interpret it, was called Blue Moon.”

At least he was clear that that was how he himself interpreted it.

If you’re still unsure which definition to accept, most dictionaries today, including the Merriam Webster’s Dictionary, define a “blue moon” as the second full moon in a calendar month.

As for countries and cultures that follow a lunar calendar system, tomorrow night’s “blue moon” is just going to be another normal full moon in the skies.

If you’re too tired to see the blue moon on Friday night, or miss it for any other reason, CNN says you can see it again in January, 2018.

[Photo by Bill Ingalls/NASA via Getty Images]