There’s a blue moon this weekend, and not just over Kentucky. The next full moon is on Sunday, Aug. 22. There are two definitions for “blue moon,” and the one this weekend is the rarer of the two. You should be able to see it Sunday night unless you have a cloudy or stormy night that could block your view of the moon. The moon will not really look blue this weekend though.
The more common blue moon happens when there is a full moon during a single calendar month. However, August 2021 does not include a second full moon, so the blue moon we will see this weekend comes under the second definition. According to Space.com, this second definition comes from a July 1943 issue of Sky & Telescope magazine, which itself cited the 1937 issue of Maine Farmers’ Almanac.
The almanac’s page for August 1937 explained that the full moon usually only happens 12 times a year, and three times each season. However, there are some years with 13 full moons, which would also mean that one season would have four full moons instead of just three. A third of the four would be called a Blue Moon, with the fourth one called the “final moon.” According to the almanac, “This was considered a very unfortunate circumstance … and it upset the regular arrangement of church festivals. For this reason, 13 came to be considered an unlucky number.”
In 2021, summer does not start until Wednesday, Sept. 22. That is two days after September’s full moon on Monday, Sept. 20. Therefore, summer 2021 will have four full moons! The first was on June 24 and the second on July 23.
“Blue Moon” came to have a second, more common meaning thanks to the March 1946 issue of Sky & Telescope. In the article “Once in a Blue Moon,” author James Hugh Pruett misinterpreted the July 1943 article. “Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon,” Pruett wrote. The misinterpretation might have been forgotten had it not been referenced in a 1980 episode of Deborah Byrd’s NPR show StarDate. This led to the original 1937 definition of “Blue Moon” being forgotten.
The next time we have a month with two full moons is August 2023, when that month’s second full moon comes on Aug. 30, 2023. Coincidentally, the next time we have a Blue Moon by the 1937 almanac definition is also in August! It will happen on Aug. 19, 2024.