What is a blue Moon?







            Once in a Blue Moon… is a common way of saying not very often, but what exactly is a Blue Moon? It is the second Full Moon to occur in a single calendar month. 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 contain two Full Moons, though it does sometimes happen: If you have 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, we can define Once in a Blue Moon as a probability: 8 chances in 228, or about 3.5 per cent!

Thus every 19 years or so a single year will offer two Blue Moons. In 1999, when the next double-Blue-Moon bonanza rolls around, February will have no full moon, but January and March will both boast a full moon and a bona fide Blue Moon. There are several other meanings ascribed to the term “blue moon” (the most common being “an uncommon event”) According to  reports, when the Indonesian volcano Krakatoa exploded in 1883, its dust turned sunsets green and the moon blue all around the world for the best part of two years. In 1927, a late monsoon in our country set up conditions for a blue moon.

Even by the 19th century, it was clear that although visually blue moons were rare, they did happen from time to time. So the phrase “once in a blue moon” came about. It meant then exactly what it means today: that an event was fairly infrequent, but not quite regular enough to pinpoint.” Some of the recent/upcoming blue moons: January 31, 1999; March 31, 1999; December 30, 2001; July 31, 2004; June 30, 2007 and December 31, 2009.