A "blue moon" is the name for the second full moon in a single month. The moon takes 29.5 days to go through a full cycle of phases, so on rare occassions (about once every 2 3/4 years), two full moons will fit in a single month. The next double blue moon will occur in 2018; the next normal blue moon will occur in November 2001.
The next blue moon is on November 30, 2001. The last blue moon was on March 31, 1999. That moon was a rare occurrance: there were two blue moons separated by only a month. The first blue moon was on January 31. Since February has only 28 days, a full lunar cycle did not fit into February, and there was no full moon that month. The next full moon was on March 1, followed by the second blue moon on March 31. Such an event happens about once every 19 years.
The term "blue moon" probably comes from the fact that, on very rare occassions, the moon actually appears to be blue. This usually happens after a volcanic eruption, such as Mount Pinatubo in 1991, when lots of dust is present high in the sky. Other times when the moon turned blue include the eruption of El Chichon in 1982, and Krakatao in 1883. These blue moons are so rare that the phrase "once in a blue moon" has come to mean a very rare event. The term "blue moon" was applied to the second full moon in a month due to an error in Sky and Telescope magazine in 1946. Read the gory details from Sky Publishing here.
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