Many of you have probably heard, and perhaps used, the expression "Once in a Blue Moon". We use this expression to describe an activity or event that is infrequent or rare. But, how did this term originate and what does it really mean?
Well, believe it or not, there is a Blue Moon occurring tomorrow - Friday, August 31st. A Blue Moon is defined as the second full moon that occurs in a one month time period. Typically, each month only experiences one full moon. But, since the average cycle between full moons is ~29.5 days, if one occurs in the beginning of the month, there may be time for a second one to occur. That is what is happening this month. The first full moon was on August 1st, and the second one will be tomorrow (August 31st).
Now, a Blue Moon is actually not as rare as one may think. It occurs on average about every 32 months, or every 2.6 years. Also a Blue Moon is not actually blue! It will look no different than any other full moon. So, how did it get its name?
It appears that the expression "Once in a Blue Moon" actually came first, before the blue moon definition. As early as 1824 the saying was being used as it is today- to describe an unusual event. Then, sometime in the mid 1900's, the term began to be used to describe the second full moon of a month. (To be thorough, The Old Farmer's Almanac first defined a Blue Moon as the third moon that occurs in a single season in which there are four full moons. It was misinterpreted later to mean the second full moon in a month).
The moon can actually appear blue under extremely rare circumstances. There have been a few documented instances of this, one in 1950 when smoke from a large wildfire in Canada caused the moon to appear blue over parts of North America, and once in 1991 when a volcanic eruption in the Philippines caused the moon to take on a blue hue from many locations across the world. In both of these events, the moon looked blue because of the composition of smoke and ash in the atmosphere.
The moon will reach its fullest stage at 9:58 a.m. Friday.