Blue Moon

Lore & Background

The phrase 'blue moon' in the metaphorical sense of a long time dates from the 1820s. The specific calendrical term originated with the 1937 Maine Farmers' Almanac, possibly invented by editor H. P. Trefethen. The almanac's explanation—that an extra moon was called 'blue' and considered unlucky—is unsupported and likely a hoax. James Hugh Pruett misinterpreted the almanac in 1946, leading to the modern definition of the second full moon in a month. The term 'blue moon' for a calendrical event is not from Native American tradition.

Reader's Guide

A calendrical blue moon (by Pruett's definition) occurs 7 times in every 19 years, or about once every 2 or 3 years. It can happen in any month except February. A visually blue moon is rare and unpredictable, caused by particles of about 1 micrometre in diameter scattering red light. Such events have been recorded after volcanic eruptions (e.g., Krakatoa in 1883, El Chichón in 1983, Mount St. Helens in 1980, Mount Pinatubo in 1991) and forest fires (e.g., Sweden and Canada in 1950–1951).

Did You Know?

• The earliest known use of 'blue moon' in the metaphorical sense of a long time is from 1821, in Pierce Egan's Real Life in London.
• A blue moon can appear green; after the 1883 Krakatoa eruption, observers saw a green crescent moon.
• The phrase 'once in a blue moon' is recorded from 1833.