The International Year of Astronomy 2009 ended with a Blue Moon and a partial lunar eclipse, as the second Full Moon of December grazed the Earth’s shadow on December 31st. The New Year’s Eve Blue Moon eclipse was visible throughout Europe, Asia, Africa and parts of Alaska, captured in this two exposure composite in cloudy skies over Saint Bonnet de Mure, France. Playing across the Moon’s southern reaches, the edge of Earth’s umbra, or dark central shadow, appears on the right side along with the prominent ray crater Tycho. At maximum eclipse, the umbra covered only about 8 percent of the diameter of the lunar disk.
A blue moon is the “extra” full moon in years that have thirteen full moons. 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 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.” – wiki