Blue Moon

Blue Moon
Name	Blue Moon
Type	lunar phenomenon and idiom
First attested	19th century (idiomatic), 20th century (astronomical sense)

Blue Moon is a term applied both to a rare lunar appearance and to a colloquial idiom denoting rarity. The expression has been documented in print and periodicals and has been formalized in astronomical usage, appearing in calendrical guides and popular astronomy outreach. It intersects with topics in observational astronomy, chronology, meteorology, folklore, and media such as The Atlantic and Sky & Telescope.

Definition and Types

The term denotes two principal senses: the traditional calendrical sense where a month contains an extra full moon and the observational sense describing an unusual bluish hue. In calendrical usage one finds the "seasonal" and the "calendar month" types: the seasonal type arises when a meteorological season contains four full moons and the calendar month type occurs when a single civil month contains two full moons. Both senses are referenced by publications like National Weather Service advisories, features in Smithsonian Magazine, and columns in The Guardian; astronomers and chronologers in organizations such as the Royal Astronomical Society and the International Astronomical Union discuss calculation conventions.

The color-related type refers to a moon that appears blue due to scattering by particulate matter in the atmosphere after major volcanic eruptions such as Krakatoa (1883) or large wildfires like those near California in various years. Reports of blue-hued moons have been cataloged by observatories including Mount Wilson Observatory and in bulletins of the U.S. Naval Observatory.

Etymology and Usage History

Earliest uses of the phrase in English-language print appear in 19th-century periodicals and plays; writers and lexicographers in institutions such as the Oxford English Dictionary trace idiomatic meaning to expressions like "once in a blue moon" emerging in American and British newspapers. Literary uses by authors discussed in bibliographies from Cambridge University Press and entries in the Online Etymology Dictionary show the phrase adopted into idioms for improbability and rarity, later amplified by columns in Harper's Magazine and The New York Times.

The calendrical definition gained force following a 20th-century misinterpretation printed in the magazine Sky & Telescope, which cited a misreading of a 1946 article in The Maine Farmer's Almanac and spread through broadcasters at organizations like BBC and National Public Radio. Lexicographers at Merriam-Webster and editors at Encyclopædia Britannica document the conflation of seasonal and monthly definitions and the subsequent popularization.

Astronomical Causes and Calculation

Astronomically, full moons recur roughly every 29.53 days according to the lunar synodic period measured by observatories including Greenwich Observatory and calculations published by the Jet Propulsion Laboratory. Because twelve synodic months (about 354 days) are shorter than a solar year (about 365 days), a thirteenth full moon may fall within a single calendar year or a given season; this underlies the calendrical "extra" full moon. Calendrical computation uses ephemerides prepared by agencies such as NASA and the International Earth Rotation and Reference Systems Service.

Coloration arises from Mie scattering and Rayleigh scattering phenomena first formalized in studies by physicists at Cambridge University and Max Planck Institute; large aerosol particles from eruptions like Mount St. Helens (1980) or volcanic events such as Mount Pinatubo (1991) preferentially scatter red wavelengths, shifting apparent lunar color toward blue. Atmospheric optics research published in journals from American Geophysical Union and data from satellite missions like MODIS (Moderate Resolution Imaging Spectroradiometer) quantify particulate optical depth correlated with sightings.

Cultural Significance and Idioms

The idiom "once in a blue moon" permeates popular culture, song titles in discographies of artists covered by Billboard, television episodes across networks such as BBC and NBC, and marketing campaigns by corporations including breweries and publishing houses. Folkloric traditions in communities documented by ethnographers at Smithsonian Institution sometimes attach omens or rites to rare lunar events, while poets in anthologies from Faber & Faber and collectors at institutions like Library of Congress use the phrase metaphorically.

Journalistic treatment in outlets such as Time (magazine) and The Washington Post often pairs the idiom with reports on calendrical curiosities; legal and civic calendars compiled by municipal governments and registries reference scheduling effects when two full moons fall within a month. The expression also appears in translations and cross-cultural studies in departments at Harvard University and University of Oxford.

Occurrences and Notable Examples

Notable calendar occurrences recorded by almanacs include years when a calendar month contained two full moons—events tracked by The Old Farmer's Almanac and observational logs at Palomar Observatory. Historical blue-hued lunar sightings were reported after the 1883 eruption of Krakatoa and the 1983 eruption of El Chichón, with photographs archived in collections at Smithsonian Institution and case studies in bulletins from the American Meteorological Society.

Contemporary examples cited in press releases by NASA and public outreach from European Space Agency note specific dates when the calendrical blue moon fell in modern calendars, often prompting media coverage from outlets like CNN and BBC News.

Misconceptions and Clarifications

A persistent misconception conflates the idiom with the strict astronomical condition; style guides at Associated Press and lexicons at Merriam-Webster clarify that the phrase predates the formal calendrical meaning. Another common error is believing the moon turns blue frequently; atmospheric science from NOAA and volcanology reports from United States Geological Survey show that true bluish coloration requires substantial aerosol loading, typically following major eruptions or intense fires. Astronomical societies such as Astronomical League provide educational material to distinguish photographic color balance effects and atmospheric optics from genuine spectral shifts.

Category:Lunar phenomena