Blue moon has nothing to do with the lunar orb’s color but commonly refers to the second of two full moons that occur in a single calendar month.
According to the “Morris Dictionary of Word and Phrase Origins,” the first recorded English usage of “blue moon” dates to 1528, and saying that something happened “once in a blue moon” was equivalent to saying that it occurred on the Twelfth of Never, according to Canadian folklorist Philip Hiscock. By the 1800s, “blue moon” was being applied to a rare astronomical quirk based on the discrepancy between the lunar cycle and the calendar year. Since the lunar month averages 29.5 days—shorter than every month on the Gregorian calendar outside of February—certain years have 13 full moons instead of the typical 12. Through the 1800s and early 1900s, the Maine Farmers’ Almanac defined a “blue moon” as the third full moon in a season with four full moons instead of the typical three. The third, and not the fourth, full moon was considered to be the extra one in order to keep named moons, such as the harvest moon, linked to proper times of the year.
In a 1946 Sky & Telescope magazine article, amateur astronomer James Hugh Pruett mistakenly interpreted the Maine Farmers’ Almanac definition to be the second full moon in a calendar month. Pruett’s misinterpretation seeped into popular culture, and Hiscock traced its widespread adoption to its use in 1980 on the popular radio program StarDate. Under the currently adopted definition, a “blue moon” occurs on average every 2.7 years, and after July 2015 the next occurrence will be in January 2018.
Even rarer than a “blue moon” is a moon that actually looks blue. This unusual phenomenon can occur after volcanic eruptions, forest fires or dust storms when tiny dust particles enter the atmosphere and scatter red light while letting through the blue light. For nearly two years after the 1883 eruption of Krakatoa, the plumes of ash spewed by the Indonesian volcano made the moon take on a bluish tinge around the world, and, according to NASA the same anomaly occurred from North America to England when microscopic oil droplets entered the atmosphere after a massive forest fire in western Canada in 1950.