It may not happen often but will happen this week. A blue moon will light up the sky on Friday, August 31st, for the last time until 2015.
And no, the moon will not be blue.
The term blue moon describes the fourth full moon of a season. It can also refer to the second full moon in a single month. August had its first full moon on August 1st and this Friday it will have its second.
EarthSky.org reports that the modern definition of blue moon stems from a 1946 issue of Sky and Telescope Magazine when author James Hugh Pruett simplified a 1937 Maine Farmer’s Almanac definition. Pruett wrote:
“Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.”
According to Live Science blue moons occur because it takes about 29.5 days for the moon to go from new to full. Since most months are longer than 29 days some months will occasionally get two full moons.
Blue moon can also refer to a rare occurrence, as in: “It only happens once in a blue moon.” If the phrase was interpreted literally, it would be: “It only happens once every 2.7 years.”
MSNBC reports that a blue moon occurs about once every 2.7 years. So they aren’t exactly rare.
The last time a blue moon occurred was on December 31st, 2009. The next will appear on July 31st, 2015.