By Dan Klepal
Enquirer staff writer

Only once in a blue moon is there controversy over exactly what constitutes a blue moon.

Tonight will be the second full moon of the month - an uncommon astrological event in its own right that occurs once every three years. Blue moons don't actually appear blue, or any other color, for that matter.

But it's not a blue moon in the strict definition of the 19th Century Farmer's Almanac.

According to the almanac, a true blue moon is the third full moon in a season that has four full moons. This also happens about every three years, but on a different cycle from the blue moon that occurs tonight.

Confused? So is the American Heritage Dictionary, which lists both definitions of the blue moon.

Richard Hamilton, a volunteer researcher of the Cincinnati Observatoryin Mount Lookout, said blue moon or not, tonight's full moon will be pretty darn cool.

"Full moons happen every 291/2 days," Hamilton said. "To have two in a single month they have to fall just right. The definition of a blue moon as the second full moon in a month is popular, but incorrect."

So how did an incorrect definition of blue moon become so popular? It's due to a mistake 58 years ago.

A March 1999 article in Sky and Telescope magazine traced the folklore and history of the expression. That article found that the definition of two full moons in a single month was created by an error published in a March 1946 article in Sky and Telescope. The author of that 1946 article had apparently misinterpreted a page from the Farmer's Almanac.

In discovering the origin of the incorrect version of blue moon, which his magazine created, author Philip Hiscock admitted that, practically speaking, there are now two definitions of blue moon.

"With... decades of popular usage behind it, the second-full-moon-in-a-month (mis) interpretation is like a genie that can't be forced back into its bottle."

Bottom line: Enjoy tonight's blue moon, weather permitting. Tonight's forecast calls for partly cloudy skies.

E-mail dklepal@enquirer.com