It's fitting 2012 includes Leap Day, the Summer Olympics, and a presidential election.

After all, they only happen about once in a blue moon.

On Friday night, a blue moon will happen, too.

York County residents are encouraged to look skyward tomorrow night for the rare - only once every 3.5 to 4 years or so - event of a second full moon in one month.

The last full moon was Aug. 2.

Bill Kreiger, professor of Earth Sciences and Science Education at York College, said the term blue moon comes from a long-ago saying, as people wanted a name for the extra full moon.

The event happens because calendars and the moon orbital cycle aren't exactly in sync.

Every month already has a name for its full moon, Kreiger said; August is the Green Corn Moon, while September is the Harvest Moon.

That's where the expression "once in a blue moon" comes from, he said, since it only happens every few years. The next one comes in 2015.

Yorkers might get an extra treat, too.

Extra particles in the air from hurricanes and forest fires might be enough to tint the atmosphere so that the blue moon will appear a little blue, black or red.

- Reach Andrew Shaw at ashaw@yorkdispatch.com