By Danny Keating, Director of the Louisiana/Mississippi HVAC Insider

Editor’s note: I originally wrote this for the September 2013 editorial. Our Blue Moon on October 31st is both the 2nd full moon in October and the 3rd full moon in the autumn season. A true Blue Moon.

I awoke Tuesday, August 20th in the middle of the night–actually it was early Wednesday morning–and I thought I had left on an outside light in the rear of my house. I climbed out of bed and to my surprise instead of a light there was a big bright full moon illuminating my back yard, my boat dock and reflecting off Lake Moonraker which lies behind my house. It turns out this wasn’t just a full moon, but was in fact a Blue Moon.

Blue Moons don’t happen too often, which is why we say “once in a Blue Moon,” to indicate when we mean “very rarely.” The next Blue Moon isn’t set to occur until July, 2015. The last was in November, 2010.

Many of us mistakenly define a Blue Moon as the second full moon in a given month. This is because in 1946 an article in “Sky & Telescope” magazine mistakenly defined it as the second full moon in a single month and the definition stuck. Because August will only have one full moon it won’t meet the mistaken but commonly used definition of a Blue Moon. This August’s full moon actually does qualify as a Blue Moon because the actual definition of a Blue Moon is that it is the “the third full moon in a season (i.e. summer) that has a total of four full moons (most seasons have only three full moons, one per month). For what it’s worth Thursday, September 19th will be the fourth full moon of this summer season occurring just before the first day of autumn, September 22nd.

Since full moons occur every 29.5 days and there are 365.25 days in a year, there is a need to do some intercalating to make everything round out evenly, like we do every four years with a leap year. In the case of the leap year, this is necessary to ensure that the months will continue to occur within the correct season.

In the case of the Blue Moon, the problem was that the extra moons would throw off the count when the seasons (spring, summer, autumn & winter) were assigned 3 months each and every couple of years an extra moon would appear, thereby confusing the people as to the proper date to celebrate, for example, the beginning of spring. The suggestion has been made that the term “blue moon” came into existence to replace the outdated term belewe, ‘to betray’. The original meaning would then have been “betrayer moon,” which would indicate that the people could not trust the moon in counting the months for that season in which the Blue Moon occurred.

As I said full moons occur every 29.5 days on average, when the moon is directly opposite the sun from the perspective of us Earthlings. This causes the moon to be fully illuminated as a large, bright circle. Usually, when the moon is full, it passes either above or below Earth’s shadow, but sometimes, when it is perfectly aligned, it travels right through the shadow, causing a lunar eclipse which tends to get us Earthlings really excited and always has throughout recorded history.

By the way, August’s Blue Moon was not actually blue, but there have been blue moons in history. According to the National Oceanic and Atmospheric Administration, the last one appeared over Edinburgh, Scotland in 1950 and was due to moon light traveling through a cloud of particles coming from forest fires a long distance away in Canada.