But, as it happens, it is not just the public who are in error, the news media have got it wrong too. They are using a popular, but totally incorrect definition of Blue Moon; that is “any month where there are two full moons in the same month”. By this reckoning, a blue moon will occur any time the full moon coincides with the beginning/end of a month with 30 or 31 days. This will occur reasonably often, and not as rarely as you would think when considering the phrase “once in a blue moon”.
The correct definition of a Blue Moon is when a season contains 4 full moons, instead of the normal 3. When this occurs, the third full moon of the season is the blue moon. So although we have had 2 full moons in December, neither is the Blue Moon. This happens every 2.7 years, the last Blue Moon was in May 2008, and the next Blue Moon will not be until November 2010.
It now seems that the false definition is now more popular than the correct one. But it doesn’t make it right.