History of Blue Moon


“Do you see that full moon in the sky tonight? That happens once in a blue moon.”

Scientifically, this happens 7 times in 19 years or about once in 3 years.


I first wrote about this on July 31, 2015. We had blue moons in January and March 2018 and October and November 2020. Blue moons never occur in February, which has only 28 days. The time between two successive full moons is approximately 29.5 days.

We had a full moon earlier this month, on August 1. Like the prior full moon, this is also considered a supermoon. This occurs when a full moon’s orbit is closest to Earth, as in August, in its elliptical orbit around the Earth.

You won’t see a blue moon again until August 2024; the last time was October 31, 2020. Technically, the full moon occurred at 8:43 AM ET today, about 20 minutes after it set for the U.S. East Coast, but it can still be again when the moon rises at 8:23 PM ET.


What is a Blue Moon?

The primary definition of a Blue Moon, according to the now-defunct Maine Farmers’ Almanac, is the 4th occurrence of a moon in a 3-month season rather than the more regularly occurring 3.

An alternative and more recent definition appeared in a 1943 Sky and Telescope Magazine article referring to a blue moon as a second full moon in a month. This second definition has stuck due to a StarDate radio show reinforcing it in 1980 and the popular board game Trivial Pursuit.


How does a Blue Moon fit into the Calendar?

That’s the problem; there are usually 12 full moons in a year. The aforementioned Maine Farmers’ Almanac posited that a 13th full moon “upset the arrangement of Church festivals.” Some church holidays in the ecclesiastical calendar are tied to the phases of the moon.

Historically, Easter was set – at the church Council of Nicaea in 325 A.D. – to be observed on

the first Sunday after the full moon following the vernal equinox.

Hence, Lent, which is set 40 days before Easter, is also determined by the phase of the moon.


What are other names?

It is also called a Full Sturgeon Moon (if both full moons occur in August), Green Corn Moon, Grain Moon, or Full Red Moon.

And, of course, it is also the name of a 1961 #1 hit song by the doo-wop group The Marcels, originally written in 1934 by Rodgers & Hart.


Bill Petro, your friendly neighborhood historian

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About billpetro

Bill Petro writes articles on history, technology, pop culture, and travel. He has been a technology sales enablement executive with extensive experience in Cloud Computing, Automation, Data Center, Information Storage, Big Data/Analytics, Mobile, and Social technologies.

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