(From our friends at OMSI)
A blue moon is usually explained as a full moon, which occurs twice in the same month. In August 2012, it is on the 1st (8:27 p.m. PDT) and 31st (6:58 a.m. PDT). A blue moon occurs every 3 to 4 years, when the date for one full moon falls on or near the beginning of a calendar month so that the following full moon comes before the end of the same month.
There are several different meanings for the term ‘blue moon. ‘ The phrase ‘blue moon’ has been around over 400 years, but during that time its meaning has shifted around a lot. The earliest reference was cited in The Maine Farmers’ Almanac, 1937. The almanac states that when there were two full moons in a calendar month, calendars would put the first in red, the second in blue.”
In astronomy, as stated above, a ’blue moon’ is the second full moon to appear in a single month. However, in meteorology, the correct definition of a blue moon is the physical explanation of why, on rare occasions, the moon appears blue. The scattering of moonlight causes a “blue moon” by smoke particulate. The red end of the spectrum is scattered more than the blue end of the spectrum, which causes light seen from the moon to look more blue: hence, a blue moon.
Despite the differences in meaning, in general terms, the rarity of seeing a moon that looks blue and/or the rarity of two full moons appearing in one month prompted the well-known saying “once in a blue moon,” which means something that happens very rarely.
Happy Blue Moon!