When’s the next blue Moon?

When's the next blue Moon?

To read the full story with a schedule that shows all blue Moons through 2020, see our article "What is a Blue Moon?"

Here's the abbreviated version: In 1999 Sky & Telescope admitted to its “blue Moon blooper,” an error that had crept onto the magazine’s pages 53 years earlier (see the March 1999 issue, page 52, and May 1999, page 36). Canadian folklorist Philip Hiscock and Texas astronomer Donald W. Olson helped us figure out how the 1946 mistake was made, and how the erroneous meaning of blue Moon (as the second full Moon in a month) eventually spread around the world. Before 1946, a blue Moon always meant something else.

Our 1946 writer made an incorrect assumption about how the term had been used in the Maine Farmers’ Almanac, where it referred to the third full Moon in a three-month season containing four. That happened in August 2005.

There’s no turning back now. The above two concepts of blue Moon are listed as definitions 1a and 1b, respectively, in the American Heritage Dictionary (Houghton Mifflin Co., 4th edition,2000).

— Roger W. Sinnott

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