Posted by Brian Ventrudo
“Once in a Blue Moon”. A common saying about an uncommon event. But is a Blue Moon really blue? Actually, no. A Blue Moon has nothing to do with the Moon’s colour, but with how calendar months stack up against the slightly shorter lunar cycles. And to make things even more confusing, there are two definitions of a Blue Moon. Here’s how it works…
A Blue Moon simply results from the build-up of extra days caused by the lunar month and the calendar month. Except for February, calendar months have 30 or 31 days. But the Moon reaches its full phase every 29 days. So eventually full Moons pile up and one of two things happens, both of which are now called Blue Moons.
First, there’s the Maine Farmer’s Almanac definition of a Blue Moon. That’s when, during a single three-month season, there are four full Moons. The third is the Blue Moon. By this definition, the next Blue Moon occurs on August 21, 2013.
The second definition is more recent, and says that a Blue Moon is simply the second full Moon of a calendar month. That’s what happens on August 31, 2012 and won’t happen again until July 2015.
Both types occur, on average, every 2.7 years. So Blue Moons are uncommon, but not rare.
One more point… sometimes the Moon really does appear blue. This happens when the atmosphere contains high concentrations of fine dust, usually from a volcanic eruption. Blue Moons were observed in the weeks after the eruptions of Mt. Pinatubo and Mt. St. Helens. And the Moon appeared blue for nearly two years after the enormous eruption of Krakatoa in 1883. Let’s hope we don’t see that type of blue Moon for a long while…