Ready Reference Files

Blue Moon

Blue Moon has come to mean the second full moon in a single calendar month and so by extension an infrequent event.

That definition of blue moon appears to have been generated by an article in the March 1946, Sky and Telescope, but was a misunderstanding of the original meaning. Looking at the Maine Farmers' Almanac back to 1819, the term originally referred to an extra full moon in a season. A season would normally have three months and three full moons. The label Blue Moon was applied specifically to the third full moon in a season which had four. (1) For a fuller discussion, see Hiscock, Philip. "Once in a Blue Moon", Sky and Telescope, March 1999, p52. and the follow up article in the May 1999 issue.

The following information uses the current meaning for blue moon (two full moons in one month). A month with a blue moon "happen[s] every two years or so." (2).

Some years have two months with a blue moon. "The time between full moons is 29.5 days -- so the month of February is shorter than the lunar cycle. If the first full moon of a year falls on January 1st or 2nd, there will be two full moons in January, none in February, and two in March." (3).

"The cycle of lunar phases, the Metonic cycle, is a period of 235 lunar months 236 full moons or 19 solar years. After this period, the phases of the full moon occur on the same days of the same months. In a 19 year period there are 228 months and 236 full moons. This means that each Metonic cycle there are at least 8 months that have two full moons.

About one year during the Metonic cycle there are two months that have two blue moons. The year 1999 is one such year. However, because of a slight variation in the moons orbit, this does not occur every 19 years. For example, 1980 did not have two months with blue moons but 1961 did. There were not two months with blue moons in 1942, 1923, or 1904. Starting in 1885, there were two months with blue moons. In the future, 2018 and 2037 have two months with blue moons but 2056 does not.

While having two blue moons in one year is not all that common, it does happen fairly regularly over a 76 year period." (4)

Sources:
1. Harper, David and Lynne Marie Stockman. "Blue Moon--What's the Real Definition?" Obliquity website: http://www.obliquity.com/astro/blue-st.html. Visited April 14, 1999
2. Odenwald, Sten. Ask the Scientist. NASA website: http://image.gsfc.nasa.gov/poetry/ask/a10951.html. Visited April 14, 1999
3. Transcript of Earth and Sky Radio Program for March 31, 1999. Earth and Sky website: http://www.earthsky.com/1999/es990331.html. Visited April 14, 1999
4. "More Information on "Upcoming Blue Moon". Earth and Sky website: http://www.earthsky.com/1999/esmi990301.html. Visited April 14, 1999

Verified by: RAP 4/99

Disclaimer:
While the Library has verified the information presented in these files in what it considers to be reliable and authoritative sources, it cannot take responsibility for nor guarantee the accuracy of the information presented.