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

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SOURCES AND RELATED WEB SITES David Harper's Once in a Blue Moon Sky and Telescope: What's a Blue Moon? Sky and Telescope: Philip Hiscock's Once in a Blue Moon