There was a full moon on August 1, 2012, and today, August 31, marks the second full moon of the month. Using the commonly accepted calendar definition of a Blue Moon, tonight’s moon will be the last Blue Moon for nearly three years, as the next one won’t happen until 2015. But if you side with the Farmers’ Almanac definition of a Blue Moon, the next one won’t happen until 2016.

No matter which definition you ascribe to, Blue Moons happen because the lunar month, which is always about 29-1/2 days, don’t exactly match up with our Gregorian calendar months, which vary from 29 to 31 days. (I still have to count knuckles.) Confused? Here’s a brief explanation.

In the English language, the earliest recorded uses of the term “blue moon” had religious connotations, but the 19th and early 20th century history of the term is a bit closer at…