Wednesday, August 29, 2012

Look in the Sky on August 31st to see the Blue Moon

August 2012 is a month with two full moons. Which means, 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 on August 1st. 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, someday, you might actually see a blue-colored moon.

It is very rare that you would actually see a blue-colored moon, although with unusual sky conditions – certain-sized particles of dust or smoke can actually create them. 

Every month typically has a full moon (although sometimes February doesn’t). In fact, the word for “month” even comes from the word “moon.” Most of the time, the names for full moons coincide with particular months or even seasons of the year. So whether you define a Blue Moon as the second full moon in a month or the third full moon of four in a season the name Blue Moon accounts for times when there are more full moons than is ordinary.

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.

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.

The next year of double blue moons is coming up in 2018.

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