‘True’ Blue Moon to appear on August 22

A blue moon is an additional full moon that appears in a subdivision of a year: the third of four full moons in a season.

Washington: If you are a selenophile, then this news might interest you! The American Astronomical Society has just indicated that if the skies would be clear this weekend, then the full moon of Sunday, August 22, will be a ‘Blue Moon’.

A blue moon is an additional full moon that appears in a subdivision of a year: the third of four full moons in a season.

In modern usage, ‘Blue Moon’ has come to refer to the second full moon in a month (the last of these occurred on October 31, 2020) – but that has not always been the case. This colourful term is actually a calendar goof that worked its way into the pages of Sky & Telescope in March 1946 and spread around the world from there.

Editors and contributors to Sky & Telescope have traced the traditional astronomical definition to the Maine Farmers’ Almanac in the late 1930s. The Almanac consistently used the term to refer to the third full Moon in a season containing four (rather than the usual 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,” said Diana Hannikainen (pronounced HUHN-ih-KY-nen), Sky & Telescope’s Observing Editor. But in 1946, amateur astronomer and frequent contributor to Sky & Telescope James Hugh Pruett (1886-1955) incorrectly interpreted the Almanac’s description, and the second-full-Moon-in-a-month usage was born.

Sky & Telescope admitted to its ‘Blue Moon blooper’ in the March 1999 issue (“What Is a Blue Moon in Astronomy?”). Canadian folklorist Philip Hiscock and Texas astronomer-historian Donald W. Olson worked with the magazine’s editors at the time to figure out the origin of the mistake, and how the two-full-Moons-in-a-month meaning spread into the English language. By either definition, Blue Moons are still relatively rare. They happen about once every 2.7 years on average. We get a ‘true’ Blue Moon when the cycle of lunar phases causes the full Moon to occur within a few days after an equinox or solstice. The last such occurrence was in February 2019, and the next after this month’s will be in August 2024.