Rare 'True' Blue Moon 2021: What Makes This Year's Full Moon Different From Others?

This weekend is your last chance to see the uncommon full blue moon until 2024, when it will become "only a memory."

Sky & Telescope magazine says this upcoming "true" blue moon is the third full moon in a season with four full moons rather than the usual three.

The sky phenomenon has inspired music, art, and language, including Elvis Presley's "When My Blue Moon Turns to Gold Again" and "Blue Moon," and the phrase "once in a blue moon," describing a rare occurrence.

NASA said the true blue moon would reach its highest point in the sky early Sunday morning at 1:04 a.m. Eastern time. People in the Americas will be able to see a nearly full moon Saturday night before the true blue moon reaches its highest point in the sky early Sunday morning at 1:04 a.m. Eastern time. But Almanac said the moon would reach its brightest point at 8:02 a.m. Eastern time on Sunday and be close to full after sunset.

Why The "True" Blue Moon

According to NASA, the phrase "blue moon" was first used in 1528. But Sky & Telescope traces its origins to the Maine Farmers' Almanac published in the 1930s.

"Introducing the 'Blue' Moon meant that the traditional full Moon names, such as the Wolf Moon and Harvest Moon, stayed in (sync) with their season," said Diana Hannikainen, Sky & Telescope's observing editor, in a news release per ABC News.

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This was before late amateur astronomer, and Sky & Telescope writer Hugh Pruett misinterpreted the term in 1946. According to Sky & Telescope, it resulted in the popular meaning of blue moon: the second full moon in a month, the last of which will come on Halloween 2020, according to Sky & Telescope.

Space.com, however, said it is "somehow" a Blue Moon, but unless we follow what has become a fairly esoteric rule.

Is it a Blue Moon if the moon appears to be blue? Not really, Digital Trends said. Particles in the atmosphere, generally from dust or smoke, cause the moon to seem blue at times. This is particularly common in the area of a volcano or wildfire, which spews a lot of ash into the air. This can, but does not always, coincide with a seasonal Blue Moon.

Other Names For Blue Moon

This moon has been referred to by a variety of names. According to NASA, the Maine Farmers' Almanac was the first to publish Native American terms for full moons in the 1930s, including the Algonquin tribes' "Sturgeon Moon," named after large fish that were easier to catch in the Great Lakes and other bodies of water at the time, and the "Green Corn Moon."

However, the name of the August full moon varies by culture. The Canadian Encyclopedia said Anishnaabe called the blue moon the "berries moon" and Cherokee the "drying up moon" among the Cherokee. August's full moon is known as the "summer moon" by the Comanche people. The "great harvest" moon is how the Creek people refer to it. The Hopi people refer to it as the "joyous moon."

The Western Washington University Planetarium has a list of all the Native American names for this full moon.