Before you go out and try to photograph this blue hue, and then wonder why it doesn’t look blue to you, you might want to read a little further.
A “Blue Moon” refers to the 4th full moon in a season, OR the 2nd full moon in a month. It is completely unrelated to the color blue. Since there is a full moon every 29 days, there are 13 in every 365 day cycle. About once every 2-3 years, that results in a second full moon in one month.
Not so dramatic, is it?
The reason it’s called a “Blue Moon” is because all of the full moons are named. You’ve probably heard of the Wolf Moon – that’s February’s full moon – or the Harvest Moon – that’s October’s. Likewise, the August full moon is called the Grain Moon because that’s the time of year when Indian corn is edible. The first moon this month, on August 2nd, was the Grain Moon, leaving tonight’s full moon without a name. Thus, this full moon is called the Blue Moon.
So what does it mean when someone says “Once in a blue moon” to indicate something rare? While Blue Moons are rare, occurring about every 33 months on average, a moon that is actually colored with a blue tint is even more rare. When the moon appears blue, it is because of high altitude dust particles. This happened from 1950 to 1951 because of some large forest fires in Europe, and also happened from 1883 to 1884 because of particles from the eruption of Krakatoa. There were some reports of a blue moon was reported in 1983 because of the eruption of El Chichon, in 1980 from Mount St. Helens, and 1981 from Mount Pinatubo.