"Once in a blue moon." It's an event rare enough that it has become a common phrase invoked to imply the rarity of other situations. And this Friday is the last time you'll be able to catch the cosmological event until 2015.
As you might already know, a blue moon is not one in which the moon takes on a melancholy hue, but when a full moon occurs more than usual during a certain timeframe. There are two definitions, according to the Farmers' Almanac, as to when a full moon is considered a blue moon.
Here's one way to look at it:
One explanation connects it with the word "belewe" from the Old English, meaning, "to betray." Perhaps, then, the Moon was "belewe" because it betrayed the usual perception of one full Moon per month. However, in the March 1999 issue of Sky & Telescope magazine, author Phillip Hiscock revealed one somewhat confusing origin of this term. It seems that the modern custom of naming the second full Moon of a month "blue," came from an article published in the March 1946 Sky & Telescope magazine. The article was "Once in a Blue Moon," written by James Hugh Pruett.
On the other hand though, the Farmers' Almanac points out this article was based on information in the Maine Farmers' Almanac (no relation to the Farmers' Almanac in general). It states Pruett misinterpreted what the Maine Farmers' Almanac meant by blue moon, which is really considered when four full moons happened during one seasonal period. Based on this concept, a blue moon is the third full moon of four set to occur during one season of the year -- generally only three occur per season.
"So, basically the current use of 'Blue Moon' to mean the second full Moon in a month can be traced to a 55-year-old mistake in Sky & Telescope magazine," the Farmers' Almanac concludes.
It is this "mistaken" definition though that is widely held today. Based on this definition, the next blue moon to occur after the August 31 event, will be in July 2015.
Blue moons might not even be considered that rare given they occur on average every two and a half to three years. Live Science points out what is actually rare is when there are two blue moons in one calendar year. Also based on the "new" definition of blue moon, the last time two months in the same year each had two full moons was 1999. The next time it will occur is 2018.
Watch this video about blue moons from Live Science:
Overall, the blue moon has little scientific value. Harvard University astronomer Avi Loeb said the moon is far more important to lovers, literature and folklore than to science.
Still, perhaps a fitting nod to Neil Armstrong, the famed astronaut's private funeral service will be held Friday. Armstrong, the first man to walk on the moon, died last Saturday in Ohio at age 82.
Armstrong's family has suggested paying tribute to him by looking at the moon and giving the astronaut a wink.
The Associated Press contributed to this report.