[The Moon]

The Blue Moon

A "Blue Moon" is the name given to the second full moon in a calendar month. Because there are roughly 29.5 days between full moons, it is unusual for two full moons to "fit" into a 30 or 31 day month (and impossible to fit into a 28 or 29 day month, so February can never have a Blue Moon). The saying "Once in a Blue Moon" means a rare occurrence, and predates the current astronomical use of the term, which is quite recent. In fact, Blue Moons are not all that rare, on average there will be one Blue Moon every 2.5 years. After 1999, the next Blue Moons will be in November 2001; July 2004; and June 2007. The last one before 1999 was in July 1996.

The term Blue Moon is believed to have originated in 1883 after the eruption of Krakatoa. The volcano put so much dust in the atmosphere that the Moon actually looked blue in color. This was so unusual that the term "once in a Blue Moon" was coined. However, Blue Moon was also used in much the same way we use the term "Harvest Moon". There were twelve names for full moons, one for each month, and the name Blue Moon was used in years which had 13 full moons. It referred to the third full moon of the four occuring between an equinox and solstice in that year. A misinterpretation of this led to a Sky and Telescope Magazine "Star Quiz" in July 1943 followed by an article in March 1946 which stated that the second full moon in any calendar month was called a Blue Moon (attributed to the 1937 Maine Farmers' Almanac), and this definition has now become part of the language.

The Double Blue Moon of 1999

In January and March of 1999, we had a situation which only takes place about four times a century: two Blue Moons occurring in the same year. The last time this happened was in January and April of 1961 and it will not happen again until January and March of 2018. A double Blue Moon most commonly occurs in January/March but is also possible in January/April or January/May and only when there is no full moon at all in February. It is also possible to have a Blue Moon in December of one year and March of the next year, again, there is no full moon in the intervening February. See the links below for more details.

Full Moons January-March 1999
Eastern Standard Time Universal Time (GMT)
January 1, 9:51 p.m. January 2, 02:51
January 31, 11:08 a.m. January 31, 16:08
March 2, 1:59 a.m. March 2, 06:59
March 31, 5:49 p.m. March 31, 22:49

[Blue bullet] The Blue Moon Page - including a Blue Moon Calculator
[Blue bullet] The origin of the term "Blue Moon" - Sky and Telescope magazine
[Blue bullet] Phases of the Moon - U.S. Naval Observatory

[Red bullet] NSSDCA Moon home page
[Red bullet] NSSDCA Moon fact sheet
[Red bullet] Planetary home page

Dr. David R. Williams, dave.williams@nasa.gov
NSSDCA, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771

NASA Official: Dave Williams, david.r.williams@nasa.gov
Last Updated: 08 November 2016, DRW