It’s not actually blue. After reading about it, it’s apparently just something that happens once every two and a half years or so. At first, I thought it was a bit silly to be going on and on about this, because obviously whatever’s going on in the sky isn’t referencing the human-imposed measurement of time. It just happens to be doing what it’s doing, and since August is a long month, it just happens to be waxing toward a full moon again while it still happens to be August.
But what about lunar calendars? In the solar calendar (the one we follow is called the Gregorian calendar, named for Pope Gregory XIII, who ordered the tweaking of the previously used solar calender named for Roman emperor Julius Caesar), our months have 31, 30, or 28 days apiece, and every four years we add one day to our year, and call it a Leap Year. Lunar calendars (such as the Hebrew calendar or the Buddhist calendar) measure their months by the moon and by the seasons. Twelve moons make one year, three moons per season.
But this particular astronomical occurrence is a thirteenth moon, which adds an extra month to a lunar year.
Since many different cultures used a lunar calendar before changing to a solar calendar, there are many different ways to deal with the extra “leap” moon in a year that had thirteen instead of twelve. Some cultures watched the seasons and others the stars to determine when this leap month should be inserted. One group of First Nations peoples living in what is now Canada simply began their year when the salmon spawned and counted the time for ten months, then left the time that followed uncounted, until the salmon spawned again.
Despite the name, a “blue moon” is not actually blue. It’s still the moon. It doesn’t change color just because our man made calendars regard it differently than other full moons. There are times when the moon looks like it is a certain color, but that’s usually because something else is in the way. The moon looks blue most often when smoke, soot, or ash is between us and the moon. The 1883 eruption of Krakatoa blasted enough debris into the sky to make the moon look blue for two years afterward.
So why do we call it blue if it’s not? We can’t say “once every two and a half years or so when there’s thirteen full moons in a year instead of twelve.” But how did it come to be called “blue” in the first place? Like many things in our culture, the reason can be traced back to the Church.
The date for Easter is traditionally the first Sunday after the Northward equinox (or March equinox). On the years when an extra moon appeared, it was up to the Church to tell the people whether the moon was indeed the Easter moon or if Lent would be extended an extra month. They called this extra moon belewe, meaning “betrayer.” I can’t seem to find whether it was called this because it betrayed the hopes of those waiting for Easter to arrive or if it was for some reason named for Judas Iscariot, the Betrayer. Either way, it’s easy to see how it became "blue;" just sound it out: be-le-we. Blue.
We say “once in a blue moon,” but we mean “once in a great while.” The time period is not defined. So maybe the phrase shouldn’t refer to the precisely calculated occurrence of a thirteenth moon in a lunar cycle, but to the less precise occasional obscuring of the moon by dust in the sky.
However we define it and whatever we call it, a full moon shining in the sky is a wonderful thing to behold. Enjoy the moon tonight, no matter what color it is.