Once in a blue moon
The notion of a blue moon has been around for a very long time. During the 18th century, if someone said, “I’ll marry you when the moon is blue” it meant never because there was no such thing as a blue moon. During the 19th century, however, that thinking was revised because, although extremely rare, there were times when the moon did in fact appear to be blue. This was sometimes due to the after-effects of huge forest fires and abnormal dust clouds after storms and monsoons. Thus, the expression ‘once in a blue moon’ meaning a rare occurrence made its way into the language. The most notable blue moon of all was after the Krakatoa eruption in 1883, when for a period of two years, sunsets were green and full moons were indeed blue. The definition ‘blue moon’ has also been given to the appearance of the second full moon in one month, a phenomenon that happens once every three years because of the difference between lunar and calendar months. This definition, however, dates only from 1946 and therefore cannot be the origin of the expression, ‘once in a blue moon’ which dates from the mid-19th century.