You may have heard (or used) the popular saying “Once in a blue moon”. It’s a catchy phrase but what really is the “blue moon”.
Contrary to what that name suggests, the name “blue moon” has nothing to do with the actual color of the moon. Full moons can be seen in the usual white or yellow shades or even more rare reds, oranges, and even blues depending on atmospheric conditions. To get the name “blue moon” it’s all about the numbers.
Normally there are 12 full moons per year. Therefore each of the four seasons should contain three full moons. Every once in a while there are four full moons in a season. The third of the full moons is traditionally called a Blue Moon.That was the case last night. (August 21st, 2013)
There is however, a secondary “blue moon” definition… Traditionally, the 12 full moons per year fall in place with one per month. Occasionally two full moons will fall within the same month. The second full moon is also often called a Blue Moon, but this is not the original meaning.
So is “Once in a blue moon” really that rare? Well… it depends on which definition you use. Using the traditional (seasonal) definition, yes it is fairly rare. A blue moon happens on average about once every 2.7 years. Using the more modern (monthly) definition, it’s not as rare.
-Meteorologist Myles Henderson