“Once in a blue moon” is officially happening this weekend.
Earth’s satellite reaches a rare milestone on the celestial calendar this weekend with Sunday’s seasonal blue “sturgeon” moon.
What is a blue moon?
It’s 2021’s only blue moon — in this case, the third full moon to occur in a calendar season with four full moons, which is one more than the typical three. Called a “seasonal” blue moon, it appears just once every three years or so.
Today’s skywatchers often use the term “blue moon” interchangeably with a similar occurrence, in which there are two full moons in one month. Generally speaking, many years contain a floater 13th full moon, all of which are called “blue moons.”
How often do blue moons occur?
The reason for the divergence may trace back to a misinterpretation of ancient ways, when some early civilizations depended on the lunar cycle to mark the passing of time, often using the nicknames of full moons to signify their months. Due to the occasional addition of a 13th full moon in a calendar year, the term “blue” was applied to the outlier.
Now that the definition of blue moon has broadened, and the folkloric names of full moons have blended with the modern calendar, a blue moon with a bit of theatrical flair may come through — including Aug. 22’s blue sturgeon moon.
Why is this blue moon called a “sturgeon” moon, too?
Historians believe that consistent crop maintenance and rotation may have dictated the need for naming full moons, such as August’s “sturgeon moon.” The marine fish of North America was once expected to bring big catches to hunters on the Great Lakes and the Northeast’s Lake Champlain during this month, according to the Old Farmer’s Almanac.
With a lineage tracking 136 million years, sturgeon are often referred to as “living fossils.” Many serious anglers, according to RustyAngler.com, believe that bigger fish are more likely to swim inland, where small prey typically populates, around the time of a full moon, when the tide is highest. They come to get their fill, while savvy fishermen get the chance at a prize catch without heading out to deeper waters.
When is the next blue moon?
A full moon occurs about once every 29.5 days, which makes room for the occasional 13th moon in a year with 12 months. Whether it be the seasonal blue moon, in which there are four in one-quarter year, or the modern variety, meaning the second full moon in a month with two, each happens just about once every two or three years — and, even more rare, sometimes twice in one year.
The next blue moon, a modern one, occurs in about two years on Aug. 31, 2023; a seasonal blue moon will rise again on Aug. 19, 2024, according to EarthSky.org.