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Once in a Blue Moon

What exactly is this? According to star watchers, if there are two full moons in a month, which is a rare phenomenon, the second is called a "blue moon". It may appear blue, depending on how unclear or foggy the city skyline is but the name is given not so much due to the colour but because of the rarity of the phenomenon.

The moon goes through one complete cycle every 29.5 days but the solar calendar months are longer, usually 30 or 31 days. This makes it very unlikely that any given month will have two full moons, though it does sometimes happen. If there is a full moon on the first or second day of the month, a second full moon will occur at the end of the month.

The well-known Metonic Cycle of lunar phases is 19 years long. During this time, there are 235 lunar months, and hence 236 Full Moons. There are also 228 calendar months, so at least 8 of those months must have seen two Full Moons. So, Once in a Blue Moon as a mathematical probability is 8 chances in 228. Therefore, every 19 years or so a single year will offer two Blue Moons. In 1999, February will have no full moon, but January and March will both boast a full moon and a Blue Moon.

Some of the recent / upcoming blue moons: January 31, March 31, 1999: December 30, 2001: July 31, 2004: June 30, 2007 and December 31, 2009.