Blue moon day 2009
A rare "blue moon" is pictured in Buenos Aires, capital of Argentina, on Dec. 31, 2009. A "blue moon" is the second full moon in a month and has nothing to do with moon's color
A blue moon is a Full Moon that is not timed to the regular monthly pattern. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year. The extra days accumulate, so that every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. The extra moon is called a "blue moon." Different definitions place the "extra" moon at different times.
* In calculating the dates for Lent and Easter, the Clergy identify the Lent Moon. It is thought that historically when the moon's timing was too early, they named an earlier moon as a "betrayer moon" (belewe moon), thus the Lent moon came at its expected time.
* 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 – bringing the correct seasonal timings for future moons.
* The Farmers' Almanac defined blue moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon.
* Recent popular usage defined a blue moon as the second full moon in a calendar month, stemming from an interpretation error made in 1946 that was discovered in 1999.For example, December 31, 2009 was a blue moon according to this usage.
Stargazers seeing out 2009 were treated to a spectacular 'blue moon' last night.
Blue moons occur about once every two and a half years, which is the origin of the saying 'once in a blue moon'. Furthermore a blue moon falling precisely on December 31st is extremely unusual.
The last time it happened was in 1990, and the next time won't be until 2028.
What space enthusiasts saw was the 13th moon of the year. Most years have 12 full moons, with one around every four weeks.
However, each solar calendar year, which is our normal calendar, consists of 365 days - the time it takes for Earth to go around the Sun. This is around 11 days longer than a lunar year which gradually shifts when a full moon falls.
It is because of this continual shift that it is so rare for the 'blue moon' to occur on New Year's Eve.
For astrologers looking for portents for their 2010 horoscopes there was also a partial lunar eclipse to consider.
Party goers in Europe and Asia were able to spot the eclipse on the same evening. Those in Wales and western England had the clearest skies in Britain but breaks in the cloud meant it was visible in London as well.
The Moon passed partly into the shadow cast by our planet, which is known as the umbra. The peak of the eclipse was at 7.23pm (GMT), when the Moon darkened and a portion from the bottom disappeared completely. Such an event can only occur when the Sun, Earth and Moon are closely aligned.
The term 'blue moon' is steeped in British folklore. In Shakespeare's day, blue moon simply meant rare or absurd.
Folklore expert Philip Hiscock said: 'The phrase 'blue moon' has been around for more than 400 years, and during that time its meaning has shifted.'
A partial lunar eclipse above Dublin around 7pm last night. About eight per cent of the Moon was covered by Earth's shadow
A couple looks at the moon during a partial lunar eclipse under the Roman pillars of the Temple of Hercules in Amman, Jordan
The modern definition sprang up in the 1940s. In those days, the Farmer's Almanac of Maine offered a very convoluted definition of a blue moon that included factors such as ecclesiastical dates.
The Sky & Telescope published an article in 1946 to try and clear up the meaning. Author James Hugh Pruett interpreted the Almanac as a blue moon being the 'second (full moon) in a month.'
That was not correct but could be understood and the modern blue moon was born.
Those living in the late 19th century literally saw a blue moon in the years following the eruption of the Indonesian volcano Krakatoa in 1883.
Plumes of ash had risen to the top of Earth's atmosphere which strongly scattered red light, while allowing other colour wavelengths to pass through.
The view of the Moon in China (pictured) and India, showed the bottom left corner blacked out - the reverse side to Europe