Looking to recharge? How about watching a full moon rise? This month you'll have two nights to do so because full moon phase falls between tonight and tomorrow night (August 21-22) at 7:01 a.m. Central Time Sunday morning, August 22. Tonight, the moon will be about 99.6 percent illuminated and tomorrow night, 99.2 percent — full enough to appear round with the unaided eye.

August's Full Sturgeon 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 of one. 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.

Every 29.5 days the moon repeats its phase. That's the time it takes for the moon to go around the Earth once and line up at the same angle to the sun. Contributed / CMG Lee — CC SA 4.0 International
Every 29.5 days the moon repeats its phase. That's the time it takes for the moon to go around the Earth once and line up at the same angle to the sun. Contributed / CMG Lee — CC SA 4.0 International

Two full moons are possible in a month because there are 29.5 days separating one full moon from the next. If the full moon date happens to land on the first or second day of a 31-day month, there's time to squeeze in a second one before month's end.

The origin of a Blue Moon as a two-in-a-month event dates back to an article titled "Once in a Blue Moon," written by amateur astronomer James Hugh Pruett in Sky & Telescope magazine in March 1946. Pruett referenced the Maine Farmers' Almanac, which 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 averages 365.24 days. You can read more about the tropical year and why it's relevant here.

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During most tropical years, there are three full moons per season for a total of 12 a year. But occasionally, a tropical year, just like a normal year, will have 13 full moons. This happens in 2021-2022 between the two March equinoxes, with the extra full moon falling in the current summer season. According to the Maine Almanac, the third full moon in a season with four is called a Blue Moon. Our summer full moon dates are June 24, July 23, August 22 and September 20, so that makes the August moon a Blue Moon.

Pruett misinterpreted the Almanac's description of a 13-full moon-year, writing: “But 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 . . . “

One wonders what he was thinking. I suspect he confused the traditional year, which runs from January through December, with the tropical year. No matter. Somehow it rang true. Other writers and media picked it up and passed it on until the two-in-a-month definition became the standard. Admittedly, the other definition is an unfamiliar way to measure time.

So now you know there are two ways of defining a Blue Moon — the flubbed, popular version and the seasonal one. Both are entirely human constructs so feel free to accept or reject either. The next two-in-a-month Blue Moon occurs on August 31, 2023, with the next seasonal Blue Moon on August 19, 2024.

The nearly full moon will be in conjunction with Jupiter Saturday night, August 21, 2021. Wait till around 10 or later when they're well-placed in the southeastern sky for the best view. Contributed / Stellarium
The nearly full moon will be in conjunction with Jupiter Saturday night, August 21, 2021. Wait till around 10 or later when they're well-placed in the southeastern sky for the best view. Contributed / Stellarium

Definitions or not, the full moon will rise in east-southeast sky just after sunset tonight (August 21) and about a half-hour later on August 22. To prepare for the sight, check your moonrise time here. It's just a curiosity, but the moon will cross the invisible border separating Capricornus and Aquarius between midnight and 1 a.m. Central Time tonight. It spends the evening in Capricornus the wee hours in Aquarius.

Both the sun and moon work together to boost tides when all three bodies are lined up in space as they are now. Tides are extra strong when the full moon happens to be closest to the Earth, which thankfully won't be happening this weekend. Contributed / oceanservice.noaa.gov
Both the sun and moon work together to boost tides when all three bodies are lined up in space as they are now. Tides are extra strong when the full moon happens to be closest to the Earth, which thankfully won't be happening this weekend. Contributed / oceanservice.noaa.gov

This weekend's full moon also coincides with the first large hurricane expected to lash New England since 1991. Sad but true, the beautiful moon you see rise tonight will contribute to higher tides and larger storm surges. It's well known that tides are strongest at full and new moon, when the Earth, moon and sun are lined up in space. The sun's pull reinforces the moon's and together they drive up the tides. Let's hope everyone stays safe.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.