Everyone has heard the old term “once in a blue Moon”, which is used to reference a rare event. The origin of the term comes from the fact that when a month has two full moons in it, the second one is called blue. This will occur this month appropriately on October 31, Halloween, so be sure and watch for it. This second full moon blue thing is not old folklore. A March 1999 issue of Sky and Telescope describes the term as recent occurrence.

For the record, while blue moons are uncommon, they are not that rare, occurring seven times every 19 years. The earliest documented use of a second monthly moon being referred to as blue is a Maine Farmer’s Almanac in 1937. It didn’t gain public interest until the 1980’s, when the phenomena was described in some high profile publications, including The Kid’s World Almanac, and Star Date, a popular radio program. In 1988 the media in general picked up on the term and it’s almost always mentioned by TV meteorologists when it occurs.

The kicker of all this is that “once in a blue Moon” was used prior to 1937 and originated for other reasons than two full moons in a month. One possibility is the term “He’d argue the Moon was blue”, meaning the individual would argue the absurd. Another use was something like: “he’ll change his mind when the Moon is blue”, meaning almost never. Still another possible origination of the term referring to rare events is the fact that the moon does occasionally turn a blue color. This results from a lot of solid particles being blown into the atmosphere by volcanic eruption (1882 in Indonesia), extended drought (1927 in India), and forest fires (1951 in Canada).

No matter the origin, a blue Moon is a good excuse to step outside on Halloween and take a look at Earth’s night-light. This particular full moon is commonly called the Hunter’s Moon, which follows the Harvest Moon that occurred on October 1.

By: Steve Roark
Volunteer Interpreter, Cumberland Gap National Park