Astronomical Blue Moon
Technically speaking, there are twelve names given to the full moon - one for each month of the year. Since a lunar month is shorter than a solar month, there are sometimes 13 full moons in one year. The name "Blue Moon" was used in years which had this extra full moon. It specifically refers to the third full moon of four that occurs between an equinox and solstice of that year. In 1943, this technical definition was misinterpreted by Sky and Telescope Magazine, where they defined a Blue Moon as the name given to the second full moon in a calendar month. Since then, this mistaken definition has become the more commonly known definition for the term.
Because a lunar month is approximately 29.5 days, it is somewhat uncommon for a blue moon to occur, and in fact they occur with almost the same frequency under each definition. While the term "once in a blue moon" has come to signify a very rare occurence, on average, they actually occur about every 2.5 to 2.75 years.
Using the more modern definition, it is possible for two blue moons to occur in a single calendar year. This event is much more rare, occurring only about four times per century in a year where February has no full moon. This double blue moon can occur in January/March, January/April, or January/May. It is possible for a blue moon to occur in December of one year and one to occur in March of the next year as well. This December/March combination also requires that February has no full moon.
Blue Moon Calendar
1950-1999 | 2000-2050
--------------------+--------------------
May 31, 1950 | November 30, 2001
December 31, 1952 | July 31, 2004
October 31, 1955 | June 30, 2007
July 30, 1958 | December 31, 2009
January 31, 1961* | August 31, 2012
April 30, 1961* | July 31, 2015
November 30, 1963 | January 31, 2018*
August 31, 1966 | March 31, 2018*
May 31, 1969 | October 31, 2020
December 31, 1971 | August 31, 2023
October 31, 1974 | May 31, 2026
July 30, 1977 | December 31, 2028
March 31, 1980 | September 30, 2031
December 30, 1982 | July 31, 2034
July 31, 1985 | January 31, 2037*
May 31, 1988 | March 31, 2037*
December 31, 1990 | October 31, 2039
September 30, 1993 | August 31, 2042
July 30, 1996 | May 30, 2045
January 31, 1999* | January 31, 2048
March 31, 1999* | September 30, 2050
*A double Blue Moon (i.e., two Blue Moons in one calendar year)
See also: Black Moon
Literal Blue Moon
While the definition of a blue moon above has nothing to do with the actual appearance of the moon, there are phenomena that can cause the moon to actually appear bluish in color. High concentrations of dust particles in the atmosphere can cause the light of the moon to appear blue. It is said that following the eruption of Krakatoa in 1883, such a blue moon (sometimes green moon) could be seen around the world for nearly two years. In some cases during this time period, even the sun itself appeared blue.
References
- http://www.infoplease.com/spot/bluemoon1.html
- http://www.griffithobs.org/IPSBlueMoon.html
- http://nssdc.gsfc.nasa.gov/planetary/lunar/blue_moon.html
- http://www.farmersalmanac.com/astronomy/moonnames.html
- http://www.opticsforkids.org/resources/Scattering_2.pdf