Blue Moon Science: Friday's Full Moon Explained

Night sky watcher Kalani Pokipala sent this image of the full moon from Honolulu, HI, taken August 2, 2012. Pokipala writes: "All is well and still in Hawaii. Aloha!" CREDIT: Kalani Pokipala View full size image

With a so-called "blue moon" set to rise in the night sky this Friday (Aug. 31), you may find yourself wondering: Just what is a blue moon, exactly? And where does the term come from?

Somewhat confusingly, the answers have nothing to do with the moon's color. The "blue moon" tale is a convoluted one, with much of the action taking place in the relatively recent past.

In 1946, "Sky and Telescope" magazine traced the term to the "Maine Farmer's Almanac," where it apparently referred to the third full moon in a season that contains four full moons instead of the usual three.

The year is divided into four equal seasons, each 91 or 92 days long. Because there are 29.5 days between full moons, four full moons occasionally get squeezed into a single season. [Once-in-a-Blue-Moon Not Really Blue (Infographic)]

But the author of the "Sky and Telescope" article misintrepeted this complicated definition, declaring that a "blue moon" is actually the second full moon in a month with two full moons.

The new (and wrong) meaning didn't attain widespread usage until decades later. It was used during a broadcast of the radio show "Stardate" in January 1980, according to a much more recent article in "Sky and Telescope."

Then, in 1981, the designers of the board game Trivial Pursuit came across the 1946 magazine article. They put the definition into the game, and suddenly this incorrect explanation of an unscientific term became “general knowledge.”

So that is why Friday's full moon is known as a "blue moon." It really is no different than any other full moon, except that an accident of the calendar causes it to be the second full moon in the month of August, the first having been on Aug. 1.

While the phrase "once in a blue moon" implies that the skywatching phenomenon should be exceedingly rare, that's not the case. On average, blue moons come along once every 2.7 years.

Moon Master: An Easy Quiz for Lunatics

For most of human history, the moon was largely a mystery. It spawned awe and fear and to this day is the source of myth and legend. But today we know a lot about our favorite natural satellite. Do you?

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Moon Master: An Easy Quiz for Lunatics

For most of human history, the moon was largely a mystery. It spawned awe and fear and to this day is the source of myth and legend. But today we know a lot about our favorite natural satellite. Do you?

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While we're on the subject of weird moon myths, here's another: the fable that the moon is made of green cheese.

This really did start out as a fable in the 16th century. In the original story, a simple-minded person sees the reflection of the full moon in a pond, and is made to believe that this is a wheel of new-made cheese. This children's story has been repeated so many times through history that it has become something of a cultural touchstone.

But the moon, like the Earth, is made of ordinary rock. In fact, most researchers believe that the moon coalesced from pieces of the ancient Earth that were blasted into space after a Mars-size body struck our planet billions of years ago.

Please don't tell Wallace and Gromit. We wouldn't want to disillusion them.

If you snap any good photos of Friday's full moon and would like them to be considered for a future story or gallery, please send them to SPACE.com assistant managing editor Clara Moskowitz at cmoskowitz@space.com.

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions. Follow Starry Night on Twitter @StarryNightEdu.