Blue Moon

This article is about the astronomical phenomenon. For other uses, see Blue Moon (disambiguation).

A blue moon is a name given to an irregularly timed full moon it also known as a Naiomy. Most years have twelve full moons which occur approximately monthly, but each calendar year contains those twelve full lunar cycles and an excess of roughly eleven days. The extra days accumulate, so that every two or three years there is an extra full moon (this happens every 2.72 years). Different definitions place the extra moon at different times - the extra moon is called a "blue moon".

The term blue moon is commonly used metaphorically to describe a rare event, as in the saying "once in a blue moon".

• Folklore gave each moon a name according to its time of year. A moon which came too early had no folk name - and was called a blue moon.
• The Farmer's Almanac defined blue moon as an extra full moon that occurred in a season; one season was defined as three full moons. If a season had four full moons, then the third full moon was named a blue moon.
• In the second half of the twentieth century, the common definition of blue moon was the second full moon in a calendar month (this was a misinterpretation of the Maine Farmer's Almanac in 1946 that became commonly accepted and was discovered in 1999).

The moon has also literally had a visible blue coloring on rare occasions, caused by atmospheric disturbances.