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
The Blue Moon Page
- including a Blue Moon Calculator
The origin of the term "Blue Moon"
- Sky and Telescope magazine
Folklore of the Blue Moon
Moon illumination for a given year
- U.S. Naval Observatory
Moon Phase Calculator - NASA Marshall Space Flight Center
NSSDC Moon home page
NSSDC Moon fact sheet
Planetary home page
Dr. David R. Williams, firstname.lastname@example.org
NSSDC, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771
NASA Official: Ed Grayzeck, email@example.com
Last Updated: 11 December 2003, DRW