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The August Sturgeon Full Moon is named for the best time to catch sturgeon on the Great Lakes. It’s also a Blue Moon, but not in the way we normally think. The commonly accepted definition of a Blue Moon is the second full moon to occur in a single month. The last time this happened was on Halloween 2020.
Two full moons are possible in a month because there are 29.5 days between one full moon and the next. If the full moon date falls on the first or second day of a 31-day month, it’s time to add a second before the month ends.
The origin of a blue moon as a twice-monthly event can be traced back to an article titled “Once in a Blue Moon”, written by amateur astronomer James Hugh Pruett in Sky and Telescope magazine in March 1946. Pruett referred to the Maine Farmers Almanac, who uses the tropical year for its seasonal calculations. A tropical year is the time from one spring or fall equinox (or winter or summer solstice) to the next and lasts an average of 365.24 days. You can read more about the tropical year and why it’s relevant here.
During most tropical years, there are three full moons per season for a total of 12 per year. But sometimes a tropical year, just like a normal year, will have 13 full moons. This occurs in 2021-2022 between the two March equinoxes, with the additional full moon falling during the current summer season. According to the Almanac of Maine, the third full moon of a season with four is called a Blue Moon. Our summer full moon dates are June 24, July 23, August 22, and September 20, which makes the August moon a blue moon.
Pruett misinterpreted the Almanac’s description of a year of 13 full moons, writing: “But seven times in 19 years, there have been – and still are – 13 full moons in one year. This gives 11 months with one full moon each and one with two. This second in a month, so I’m interpreting it, was called Blue Moon. . . “
We wonder what he was thinking. I suspect he confused the traditional year, which runs from January to December, with the tropical year. No matter. In a way, it sounded true. Other writers and media picked it up and passed it on until the definition of two in a month became the norm. Granted, the other definition is an unfamiliar way of measuring time.
So now you know there are two ways to define a Blue Moon – the popular version and the seasonal version. Both are entirely human constructs, so feel free to accept or reject either. The next twice-monthly Blue Moon occurs on August 31, 2023, with the next seasonal Blue Moon on August 19, 2024.
Definitions or not, the full moon will rise in the east-southeast sky just after sunset tonight (August 21) and approximately half an hour later on August 22. To prepare for the view, check your moonrise time here. It’s just a curiosity, but the moon will cross the invisible border between Capricorn and Aquarius between midnight and 1 a.m. PT tonight. He spends the evening in Capricorn in the wee hours of Aquarius.
This weekend’s full moon also coincides with the first major hurricane expected to hit New England since 1991. Sad but true, the beautiful moon you see rise tonight will be contributing to higher tides and sea waves. larger storm. It is well known that the tides are strongest in the full and new moon, when the Earth, moon, and sun are aligned in space. The pull of the sun strengthens that of the moon and together they raise the tides. Hope everyone stays safe.
“Astro” Bob King is a freelance writer for the Duluth News Tribune. Read more of his work on duluthnewstribune.com/astrobob.
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