The Rarest Full Moon of the Past 19 Years

As if a pandemic, white supremacy, and an upcoming election weren’t spooky enough, 2020 also features a full “blue” moon on Halloween at 7:49 a.m. PT/10:49 a.m. ET. Though what is more unusual than a Blue Moon is having a full moon on Halloween — something to see only every 18–19 years. In all of the 21st century, we will only have six Halloween full moons: 2001, 2020, 2039, 2058, 2077, and 2096. If you combine a blue moon and a full moon on Halloween, it certainly adds extra astrological and witchy intrigue.

While several news outlets reported it has been 76 years since the last full moon on Halloween, this inaccuracy can confusion as with my calculations the last one was on October 31, 2001, at 9:43 p.m. PT. So those in later times zones missed it, though Pacific, Central, and Mountain Times have only waited 19 years.

The modern, more common meaning of blue moon — two full moons in the same calendar month occurs around every 2.7 years. In fact, the current usage of the term “Blue Moon” is relatively new in the past, when a journalist — obviously a man who wasn’t listening — misunderstood the phrase. However, what many don’t know is that there are several different types of blue moons, listed below. So while the phrase “once in a blue moon” means a rare occurrence, depending on the definition it can happen more or less frequently.

Here are the four types of Blue Moons (to my knowledge) with meanings that have evolved over the past century:

1) A seasonal blue moon or four full moons in a season instead of three. Between 1600 and mid-1960s, this was the prevailing definition. That last full moon was considered an added bonus and was determined by the Farmer’s Almanac decided by growing seasons. Farming cycles often use meteorological seasons based on the annual temperature cycle and the 12-month calendar rather than astronomical seasons that focus on the position of the Earth’s tilt in relation to the Sun. A true, Blue Moon much rarer, around every three years, and apparently was misunderstood by a journalist in 1946 and then printed in a scientific magazine. I’m annoyed by this journalism, so I can’t imagine how the interviewee must feel. According to the traditional meaning of Blue Moon, the most recent one occurred May 19, 2019, and then again next year on August 22, 2021.