Most people know of the more recent definition that the second full moon within a month is called a blue moon. I didn’t know this, but there is an older definition of “blue moon”. Most years, there are only 3 full moons per season. Every so often, because each solar calendar year contains roughly eleven days more than the lunar year, there occurs a 4th full moon in a given season. The third of those is called a blue moon, to distinguish it from the other 3 moons to fall in between the equinox and the following solstice, which in this instance are Harvest Moon, Hunter's Moon and Beaver Moon.