Blue moon is a folklore term that has many definitions. As early as 1528, this term was used to represent an absurd belief. Later, people described uncommon events as occurring “once in a blue moon.” Additional definitions of blue moon refer to rare types of full moons.
Two types of full moons qualify as blue moons. According to one definition, a blue moon is the second full moon in a month that has two full moons. According to an older definition, a blue moon is the third full moon in a season that has four full moons. The older definition was developed using a calendar in which spring always begins on March 21. In this calendar, the beginnings of summer, autumn, and winter are based on the position of an imaginary sun called the dynamical mean sun. Today, astronomers use a different rule for determining the beginnings of the seasons, as explained in the World Book article Season .
In the early 1900’s, the Maine Farmers’ Almanac published dates on which blue moons would appear, using the older definition. The newer definition of the second full moon in a month originated in Sky & Telescope magazine in 1946. Other publications adopted the definition, and many people came to accept it. In 1999, however, Sky & Telescope published an article explaining that the 1946 definition actually originated from a misreading of the Maine Farmers’ Almanac.
