Common knowledge says that a Blue Moon refers to a month’s second full moon. But while we experienced a Blue Moon on Tuesday, Aug. 20, it was not the month’s second full moon appearance nor was it even blue. So what gives? Well, the common notion comes from a past mistake that stuck.

Three full moons typically occur each season, but every few years, a season has four full moons. The reason for this occurrence can be explained with some quick math and knowledge of the lunar cycle. The lunar cycle is 29 and a half days. Divide the 365.25 days in a year by that number, and you get about 12 lunar cycles. Each calendar year contains 11 days more than the number of days totaled from 12 lunar cycles. Those extra days add up, and just as we get an extra day in February every four years, so too do we see an extra moon in a season every couple years.

Farmers historically called the season’s fourth full moon the Blue Moon to keep track of the time throughout the year. For example, during the summer they had an “early summer,” “midsummer” and “late summer” moon. A 1937 copy of the now defunct Maine Farmers’ Almanac (not the still-published The Farmers’ Almanac) called the season’s fourth moon the Blue Moon so that the third one could keep the same name for the seasons.

The historic mistake comes from the article “Once in a Blue Moon” published in the March 1946 issue of Sky & Telescope magazine. The author of the story James Hugh Pruett referenced a previous Sky & Telescope story from the July 1943 issue by Lawrence J. Lafleur who used the Maine Farmers’ Almanac, but specified no dates to the Blue Moon. Pruett also had no dates to confirm his reference to the Blue Moon and so came up with his own interpretation.

“Seven times in 19 years there were—and still are—13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.” People accepted the interpretation, and so the new, and incorrect, definition for Blue Moon stuck.

This month’s full moon has other names including the Corn Moon, the Full Sturgeon Moon, the Full Red Moon and the Grain Moon. The next seasonal Blue Moon is expected to occur on May 21, 2016, and the next time two moons appear in one month should be July 2 and July 31, 2015.