Blue Moon in August

August 2012 is a month with two full moons. And, by popular acclaim, that means it’s a Blue Moon month. That’s because a Blue Moon is sometimes defined as the second full moon in a calendar month. The first full moon is August 1. The second full moon is August 31, 2012.

There are two more definitions for Blue Moon. It can be the third of four full moons in a single season. Or it can be an actual blue-colored moon.

It’s very rare that you would see a blue-colored moon, although unusual sky conditions – certain-sized particles of dust or smoke – can create them. Blue-colored moons aren’t predictable. So don’t be misled by the photo above. The sorts of moons people commonly call Blue Moons aren’t usually blue. For more about truly blue-colored moons, click here.

Now on to folklore’s Blue Moons. Every month has a full moon, and, most of the time, the names for full moons coincide with particular months or seasons of the year. By either definition, the name Blue Moon accounts for times when there happen to be more full moons than is convenient.

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.

When is the next Blue Moon, according to this first definition? August 31, 2012.

The idea of a Blue Moon as the second full moon in a month stemmed from the March 1946 issue of Sky and Telescope magazine, which contained an article called “Once in a Blue Moon” by James Hugh Pruett. Pruett was using a 1937 Maine Farmer’s Almanac, but he simplified the definition. He wrote:

Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called

Blue Moon.

http://earthsky.org/astronomy-essentials/when-is-the-next-blue-moon