Sky Watch: "Blue Moon" Due Early Saturday

John Roach
for National Geographic News

July 30, 2004

Get ready. If you live anywhere in Europe or the Americas, the "blue moon" is coming to a sky near you Saturday. The phenomenon is mainly due to astronomical arithmetic (and a few mix-ups, but we'll get to those later).

"It's how the math works out," said Philip Hiscock, a folklorist at Memorial University of Newfoundland in Canada. Hiscock is recognized as an authority on the history and folklore surrounding the phrase "blue moon." The phrase has come to refer to those rare occasions when a second full moon appears within a single calendar month.

Why two full moons in a single month? It's because the natural cycle of moon phases doesn't perfectly match up with our human-made calendar.

"The lengths of the months in our calendar were set arbitrarily by Julius Caesar and Augustus Caesar 2,000 years ago," said English astronomer David Harper, who is a fellow of the Royal Astronomical Society and author of the Once in a Blue Moon Web site.

For example, 19 years equals 228 calendar months, which is almost exactly equal to 235 lunar months. A lunar month is the amount of time from one full moon to the next.

"As you can see, there are 7 more lunar months—and hence full moons—than there are calendar months. So in a 19-year period, there are bound to be 7 months which have 2 full moons," Harper said.

A month with two full moons occurs about once every two and a half years. Some years, though, have two months with two full moons. In 1999, for instance, blue moons occurred in both January and March.

This July people living in Europe, Africa, and North and South America will experience a blue moon on Saturday, July 31. People in New Zealand, Australia, and the East Asia will see the blue moon on August 30.

Why do Australia, New Zealand, and East Asia get their blue moon in August? It's a matter of time zones.

A full moon will occur in the relatively early evening of July 31—the last day of July—in the Americas and Europe. But in Australia, for example, the moon will not be full until after midnight. So technically, this full moon will fall on August 1 in Australia, making it the first full moon of August, rather than the second full moon of July.

Mistaken Identity

Today people define a blue moon as the second full moon of the month, a definition inadvertently introduced by Sky and Telescope magazine in 1946 and subsequently popularized in the 1980s by a Trivial Pursuit board game question.

But the saying "once in a blue moon"—meaning "not very often"—has been around for at least 150 years. And the phrase "blue moon" has been used for at least four centuries, according to Hiscock's research, which he reported in the March 1999 issue of Sky and Telescope.

The earliest known meaning of the phrase was to indicate the absurd, according to Hiscock. This eventually led to equating the blue moon—like flying pigs—with something that would never happen.

Then again, there are instances when the moon does appear blue, as it did after the Indonesian volcano Krakatau (Krakatoa) erupted in 1883. Forest fires and monsoons have also caused the moon to appear blue. Such occurrences likely helped change the meaning of "once in a blue moon" from "never" to "not very often."

The "not very often" meaning of " blue moon" is still the most common. But Hiscock also cites associations of the phrase with sadness and loneliness, often in song lyrics.

Sky and Telescope's 1946 mislabeling of the blue moon as the second full moon of the month came from a misinterpretation of a page from the 1937 Maine Farmers' Almanac. The almanac used the term to refer to the third full moon of a season that has four full moons, which has to do with the Christian ecclesiastical calendar.

The magazine acknowledged and explained their mistake in the May 1999 issue.

The late Charles Federer, founder of Sky and Telescope said of the misinterpretation: "Even if the calendrical meaning is new, I don't see any harm in it. It's something fun to talk about, and it helps attract people to astronomy."

Cause for Celebration?

This year's blue moon will look the same as any other full moon. And despite superstition and rumor, rates of crime, poor driving, dog bites, and childbirths will be about the same as any other night of the year. But some will see the phenomenon as cause for celebration.

Colorado-based Coors Brewing Company, for example, plans to launch a promotion this July around their Blue Moon brand of Belgian-style wheat beer.

The beer was named at a 1995 branding meeting, when a marketer said, "Flavors like these are ones that come around once in a blue moon," according to Amy Valdez, a spokesperson for the company.

The promotion includes table displays and posters adorned with the Blue Moon logo and the phrase "7.31.04. Discover it."

How did the moon form? Why do we always see only one side of it? For these answers and more, see our moon facts >>

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