Heralding the change in season, a full Moon will rise in the eastern twilight sky on August 31st for the second time this month (the first time came on August 1st). Many people use the expression "once in a blue Moon" to mean something that occurs rarely, and you might be tempted to call August 31st's big, bright orb a "Blue Moon" too.

While the former meaning can be traced back centuries, the latter definition is much newer - and it's wrong! At least if you're a stickler about these things.

"In modern usage, the second full Moon in a month has come to be called a 'Blue Moon.' But it's not!" says Kelly Beatty, Senior Contributing Editor for Sky and Telescope magazine.

"This colorful term is actually a calendrical goof that worked its way into the pages of Sky and Telescope back in March 1946, and it spread to the world from there."

Sky and Telescope admitted to its "Blue Moon blooper" in its May 1999 issue. Canadian folklorist Philip Hiscock and Texas astronomer Donald W. Olson had helped the magazine's editors figure out how the mistake was made, and how the two-full-Moons-in-a-month meaning spread into the English language.

Before 1946, a Blue Moon always meant something else. For example, says Hiscock, sometimes it referred to an obvious absurdity. Quite a few old songs use it as a symbol of sadness and loneliness.

There's even a cocktail called a Blue Moon; it's a mix of curacao, gin, and perhaps a twist of lemon. And, exceedingly rarely, the Moon actually does turn blue in our sky - when a volcanic eruption, forest fires or dust storms send lots of fine dust into the atmosphere.

Our 1946 writer, amateur astronomer James Hugh Pruett (1886-1955), made an incorrect assumption about how the term had been used in the Maine Farmers' Almanac - which consistently used "Blue Moon" to mean the third full Moon in a season containing four of them (rather than the usual three).

By this definition, there is no Blue Moon in August 2012; instead, the last one was in November 2010, and the next happens in August 2013.

But there's no turning back now. The concept of a Blue Moon as the second full Moon in a month with two, as well as the third full Moon in a season with four, are now both listed as official definitions in the 4th edition of the American Heritage Dictionary (Houghton Mifflin, 2000).

By either definition, Blue Moons happen about once every 2.7 years on average. The last occurrence of two full Moons in a calendar month was on New Years' Eve in 2009. After August 31st, it won't happen again until July 2015.

If you want to tell your readers, listeners, or viewers that this Thursday's full Moon is a Blue Moon, go right ahead. Pretty much everyone else will too. The newer, "wrong" definition is simpler and handier for most people to grasp and use.

"That's how the English language shifts. You can't beat back the tide," quips Sky and Telescope Senior Editor Alan MacRobert. "Not when the Moon is pulling the tide."