Here again lies a cliche I use frequently in conversation. But how often does a “blue moon” really occur? I mean it to occur regularly, but not often. Where did this come from and what does it really mean? Thanks to the wonder of internet research, which can never be wrong, I can tell you exactly what it means.
A full moon occurs approximately once per month, but the solar calendar year contains eleven more days than the lunar calendar year. These days accumulate, and therefore about every two to three years, there is an “extra” full moon. The term “blue” moon comes from folklore–here is a couple of possibilities:
- The word “belewe” had a double meaning in Old English; either the color blue or “to betray”. In determining the dates for Lent and Easter, clergy use the lunar calendar and it is thought that when the extra moon came too early, it was the “betrayer” moon so the Lent moon could occur at its predicted time.
- The Farmer’s Almanac defined a blue moon as an extra full moon that occurred in a season. If that season had four full moons, then the third full moon was called a “blue moon”.
The most literal meaning of blue moon is when the moon appears unusually bluish to the observer. This can happen when dust particles of the correct diameter (slightly larger than the wavelength of red light, at approximately 700nm) exist in the atmosphere without many particles of differing size. Occasionally volcanic eruption can cause such a moon, as happened after the eruption of Krakatoa in 1883 which caused the moon to appear more blue for almost two years.
Most ash and dust clouds thrown into the atmosphere by fires and storms have particles of many different sizes, many smaller than one micron, which causes the moon to have a red appearance. Red appearing moons appear much more commonly than blue. This lends credence to the terminology and tradition of a blue moon being an infrequent event.