The August 31, 2012 full moon is a Blue Moon
What is a Blue Moon?
There are in fact two definitions for a blue moon. According to the more recent definition, a blue moon is the second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month (the average span between two moons is 29.5 days). The full Moon on Aug. 31, 2012, will be this type of blue moon; it will be the second full moon in one month.
Analysis: There will in fact be two full moons during the month of August 2012, the second of which, per the currently most popular definition of the phrase, will be a blue moon. It will occur on August 31, 2012.
Blue moon as second full moon in a month. In recent decades, many people have begun using the name Blue Moon to describe the second full moon of a calendar month.
The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every 2-3 years, so these sorts of Blue Moons come about that often.
How Often Does a Blue Moon Occur?
Over the next 20 years there will be about 15 blue moons, with an almost equal number of both types of blue moons occurring. No blue moon of any kind will occur in the years 2011, 2014, and 2017.
The more recent phenomenon, where the blue moon is considered to be the second full moon in a calendar month, last occurred on December 31, 2009. Two full moons in one month may occur in any month out of the year except for February, which is shorter than the lunar cycle.
The other, older blue moon event, which happens when there are four full moons in a season, last occurred in May 2008 and will again on Nov. 21, 2010. Since this type of blue moon is reckoned according to the seasons, it can only occur in February, May, August, or November, about a month before the equinox or the solstice.
