Enjoy July's blue moon early Friday

A common term for a rare event is “once in a blue moon.”

There are several astronomical definitions for a “blue moon.” The most common is a second full moon occurring in a calendar month.

Along similar lines, most years have 12 full moons. About every three years there is a 13th full moon. The extra full moon is called a blue moon — but is not necessarily the last full moon in the year.

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Each full moon has a folklore name according to its time of the year, such as the full (Buck) moon, which occurred July 1. An extra full moon that does not fit into the scheme for one of these folklore names is a blue moon like the one that will be officially full at 3:44 a.m. Friday.

Most seasons (spring, summer, fall, and winter) have three full moons. Occasionally, there will be four full moons in a season. The third full moon in this case is sometimes called a blue moon.

A looser definition for a blue moon is a moon that appears blue due to atmospheric conditions.