Blue Moon

Im sure you would have heard this phrase "Once in a blue moon..." which refers to a rare occasion. When there is a second full moon in the same month it is been referred has a Blue Moon and not that it is blue in color. But i got more information from Wikepedia. Have a look at the below URL.

http://en.wikipedia.org/wiki/Blue_moon

Some abstract from wikipedia ...

Calendar blue moons

In recent times, people have taken to calling a full moon a blue moon based on the Gregorian calendar. By this use of the term, a blue moon is the second of two full moons to occur in the same calendar month. This definition of blue moon originated from a mistake in an article in the March 1946 Sky & Telescope magazine, which failed in an attempt to infer the earlier definition used in the original Farmer's Almanac (see above). It was helped to popularity when Deborah Byrd of Earth & Sky walked into the Peridier astronomy library at the University of Texas at Austin one day, leafed through some old magazines, and found the 1948 blue moon article in Sky & Telescope. She used the definition – the second full moon in a single month – in the radio series Star Date for some years. As a result, the game Trivial Pursuit used a question and answer about blue moon. Sky & Telescope discovered the error nearly sixty years later and the magazine printed a retraction and correction.[2] By the time the correction came the calendar definition had already come into common use. As it is easier to understand, the mistaken calendar-based meaning has stuck.

Calendar blue moons occur infrequently, and the saying once in a blue moon is used to describe a rare event. However, they are inevitable because of the mis-match between the solar and lunar cycles. Each calendar year contains twelve full lunar cycles, plus about eleven days to spare. The extra days accumulate, so that while most years contain twelve full moons to match the twelve months, every two or three years there is a year with thirteen full moons. On average, this happens once every 2.72 years. Additionally, in some years there is no full moon in February at all, since February is slightly shorter than the time from one full moon to the next. This condition, known as black moon, gives additional 'blue' moons in the preceding and following months.

When there are thirteen moons in a year, twelve of them are given the twelve traditional names associated with that time of year (the names vary from culture to culture), and the extra one is termed a blue moon. Which of the thirteen moons is termed 'blue' depends on whether it is calculated by the old or the new method.

The months of the Gregorian calendar are all very close to the 29.5306-day period of the moon's phases: the synodic month, or lunation. Most of the months are longer than this by one or two days, except February, which is the only month which cannot contain a calendar blue moon. Since February is one or two days shorter than the moon's cycle, very occasionally it has no full moon – there is a full moon at the end of January, and the next one is at the beginning of March. What this means is that both January and March will have blue moons. This happens, on average, once every thirty-five years.

The next two calendar blue moons (based on UTC) will be on June 30, 2007 (but May 31, 2007 in the Western Hemisphere; see below); and December 31, 2009. Because February, according to UTC, will have no full moon in 2018, January and March will each have a calendar blue moon that year.

Time zone problems
Occasionally whether a moon is called blue depends on the time zone. Any full moon occurs simultaneously everywhere, but at that moment clocks and calendars are not the same.
Example, when it is early evening on August 31 in Europe, it is already early morning September 1 in New Zealand. Hence, residents of London seeing a full moon when their clocks and calendar say it is August 31 would call what they see a calendar blue moon. People seeing the same full moon from Auckland would note by their clocks and calendar that it is the early morning of September 1, and they would not term it a blue moon. But they would probably have a calendar blue moon at the end of September, or perhaps October.

Because this is confusing, astronomers worldwide and the calendar makers who rely on them typically choose the time zone of the Royal Greenwich Observatory in the United Kingdom, known as Greenwich Mean Time, or the nearly identical UTC time zone. As a practical matter, because the moon seems to the casual viewer to be full for almost three days, the use of a foreign time zone for calendar markings for full moons makes little difference.

Check this blog out... Blue moon for the Eastern countries in May...

http://spaceports.blogspot.com/2007_05_01_archive.html