What Is A Blue Moon?
August 1, 2012

Once In A Blue Moon: August Is A Blue Moon Month

April Flowers for redOrbit.com - Your Universe Online

People say things will happen "once in a blue moon" when they mean it's unlikely to happen or something very rare. In fact, we've been using a "blue moon" as synonymous with "never" for about 400 years. But what is a blue moon, really?

A full moon that actually appears blue is very rare. It happens because of ash or dust in the air, sometimes from volcanic eruptions or major forest fires, which act like a color filter for your eyes making the moon appear blue. Because these types of blue moons only happen after a major event, they aren't predictable.

There are a few other reasons the moon could appear blue, according to Les Cowley, an atmospheric optics expert. "Our eyes have automatic 'white balances' just like digital cameras. Go outdoors from a cozy cabin lit by an oil lamp (yellow light) and the moon will appear blue until your eyes adjust."

In 1883, though, people saw blue moons, green moons, lavender suns and even noctilucent clouds that glowed at night. Sounds impossible, right? The reason was Krakatoa, an Indonesian volcano that spewed ash to the top of Earth's atmosphere. The explosion was equivalent to a 100 megaton nuclear bombs and the atmospheric affects lasted for years.

Modern folklore gives us two other possible meanings for a blue moon: the third of four full moons in a single season or the second full moon in a calendar month.

The second type of blue moon happens when there are four full moons in a season. A season generally only has three full moons. The third moon out of four is the blue moon, because it comes at an unusual time and doesn't fit the pattern of naming moons according to where they fall in relation to the solstices and equinoxes. From 1932 to 1957, the Maine Farmer's Almanac listed the blue moons that this convoluted seasonal rule created.

The last definition for a blue moon occurs when there is a second full moon in one calendar month. The time between one full moon and the next is nearly the length of a full calendar month, approximately 29.5 days. So, sometimes, if a full moon happens in the first few days of a month, then a second full moon can happen at the end of the month. This type of blue moon comes around every two to three years.

Can there be more than one of this type of blue moon in a year? Yes, there can. In 1999, there were TWO blue moons and it will happen again in 2018.

Confusing, isn't it?  Where did this idea come from?

In 1943, and again in 1946, Sky and Telescope magazine ran articles on blue moons. The 1946 article by James Pruett, a Eugene, OR amateur astronomer, simplified the Maine Farmer's Almanac definition to "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." Although this definition was not quite correct, Sky and Telescope adopted it and it entered into the popular consciousness.

August 2012 just so happens to be a blue moon month. There will be a full moon on August 2nd, and then again on August 31st. So it seems that a really rare once in a blue moon event is about to happen.