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Rare ‘seasonal blue moon’ rising this weekend

Sunday’s full moon is a blue moon according to the original definition of the phrase.

NEW ORLEANS (CN) — The full moon that will chart across the sky Sunday is a blue moon by the original definition, rather than by what you might think you know blue moon to mean.

This month’s full blue moon, known by Native American tribes as the Sturgeon Moon because August is a month when sturgeon were readily available to Algonquin tribe members who fished them from Lake Superior and other large bodies of water, is also the third full moon in a season when there will be four full moons, rather than the typical three.

The modern definition of the blue moon — as a second full moon in a single month — originated from a widely circulated misinterpretation of the term that was printed in Sky & Telescope in 1946 and spread from there throughout the English language.

The original phrase “Blue Moon” appeared regularly in the Maine Farmers’ Almanac in the late 1930s and referred to the third full moon in a season with an unusual number of four full moons, rather than three.

“Introducing the ‘Blue Moon’ meant that the traditional full moon names, such as the Wolf Moon and Harvest Moon, stayed in synch with their season,” Diana Hannikainen, Sky & Telescope’s Observing Editor, said in a statement.

For centuries, people across Europe as well as Native American tribes, tracked the changing seasons by following the lunar month rather than the solar year on which our modern calendar is based. The months were named after things associated with particular seasons in the Northern Hemisphere, and that’s where some of the most popular names for full moons also originated.  

Some years have 13 full moons, which means one of them doesn’t fit within the traditional full moon naming system, and that’s where the original definition of a blue moon began.

There are still just 12 moons in 2021, but four of them — rather than the typical three — occur during the summer season, on June 24, July 23, Aug. 22 and Sept. 20. Thus, according to the rule used by the almanac, August’s full moon, which is the third of four this season, is a blue moon.

In 1946, amateur astronomer and frequent Sky & Telescope contributor James Hugh Pruett incorrectly interpreted the Almanac’s description as being the second full moon in a month.

Sky & Telescope acknowledged the error in March 1999 in an article in which Texas astronomer-historian Donald Olson teamed up with research librarian Margaret Vaverek at Texas State University, along with the magazine’s editors at the time to trace back to the origin of the mistake as well as how the modern meaning of two full moons in a single month spread so widely in the English language.

According to either definition, blue moons are still pretty rare, happening only about once every 2.7 years or so.

A true blue moon occurs when the cycle of lunar phases causes the full moon to happen within a few days after an equinox or solstice. The last time this happened was in February 2019, and the next after this month’s will be in August 2024.

This blue moon will be exactly full — or directly opposite the sun — Aug. 22 at 8:01 a.m. Eastern Time.  

Because of this moon’s placement in the sky, to sky watchers in the Americas, the moon will appear full from Aug. 21 through 23, with the moon appearing the fullest before dawn and after dusk on the 22.

Prior to the 1930’s interpretation, a blue moon was typically not an astrological term at all. More often, it applied to loneliness, while “once in a blue moon” meant something that rarely happened.

An actual blue moon — in which the moon appears blue in the sky — is extremely rare and typically only occurs during instances when a volcanic eruption or forest fire sends smoke and fine dust particles into the Earth’s atmosphere.

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