Most people are familiar with the expression 'once in a blue Moon' and take it for granted that it means 'once in a great while' or 'rarely.' It is one of those reflexive, oft-spoken statements that convey meaning, though, when examined by itself, has little or no intuitive meaning.
Actually, there are two definitions to this enigmatic term, and either or both may be cited in popular astronomical literature. Professional astronomers, when they deign to mention blue Moons in texts, refer to the occasional blue color of the Moon resulting from the scattering effects of high-altitude dust lofted by forest fires or volcanic eruptions. Indeed, a blue Moon was widely observed September 26, 1950, the result of an extensive Canadian forest fire, and also August 27, 1991, from ash elevated by the volcanic eruption of Mount Pinatubo in the Philippines.
Recreational astronomers and skywatchers put a different spin on the blue Moon definition. Most of the time, each month has only one full Moon phase. Occasionally - one might even say rarely - a month will have two full Moons: one at the very beginning and one at the very end. Accordingly, a blue Moon is 'the second full Moon in a month.' Such was the case in August 1993. The first full Moon occurred on August 2; the second on August 31.
Of course, there's a slight agitation to the dual full Moon definition of blue Moons that muddies the waters a bit. Clearly, the month in which we experience a blue Moon depends on where we are located on Earth or, more specifically, where we are at the time the Moon's full phase officially occurs.
For example, in the August 1993 case, the Moon became full the first time on August 2 at 12 hours 10 minutes Universal Time, which is the mean solar time measured at the prime meridian in Greenwich, England. In the United States, however, when daylight saving time is employed, there is a four-hour time difference between the East Coast time zone and Greenwich, England (five hours when daylight saving time is not in effect). So, on the east coast, the Moon reached its full phase at 8:10 a.m., Eastern Daylight Time, August 2, 1993.
Here's the tricky part. The Moon did not reach its full phase again until 2 hours 33 minutes Universal Time, September 1. But don't forget about that 4-hour time difference between Greenwich, England, and the eastern U.S.When you factor that in, we essentially step 'backward' (westward) in time to 10:33 p.m., August 31. So, those living in North America got a blue Moon - with an hour and a half to spare - while those living in Europe did not (although they did get one in September of that year).
On average, a full Moon occurs every 29.53 days. Divide the number of days in a tropical year - 365.2422 - by 29.53 and you get the number of full Moons in a year: 12.369. The extra amount has to crop up somewhere, and it does, on average, every 2 years plus 7, 8, 9, or 10 months, when a month contains two full Moons.
The following list features upcoming years with months having two full Moon phases, thereby constituting a blue Moon. The world regions noted fall largely within the time of the full Moon phase for each date.
October 1 and October 31 - North and South America
October 2 and October 31 - Europe
November 1 and November 30 - Australia/Asia, Africa
July 1 and July 31 - North and South America July 2 and July 31 - Europe, Africa, Australia/Asia
May 1 and May 31 - Europe, North and South America
May 2 and May 31 - Africa
June 1 and June 30 - Australia/Asia
December 1 and December 31 - Europe, North and South America, Africa
December 2 and December 31 - Australia/Asia 2012
August 1 and August 31 - Europe, North and South America, Africa August 2 and August 31 - Australia/Asia
July 1 and July 30 - North and South America July 2 and July 31 - Europe, Asia/Australia
January 1 and January 31 - North and South America, Africa, Europe January 2 and January 31 - Australia/Asia March 1 and March 30 - North and South America March 1 and March 31 - Africa, Europe, Australia/Asia
October 1 and October 30 - Europe, North and South America, Africa October 1 and October 31 - Australia/Asia
August 1 and August 30 - North and South America, Europe, Africa August 1 and August 30-31 - Australia/Asia
May 1 and May 30 - Europe, North and South America, Africa May 1 and May 31 - Australia/Asia
Sometimes, the contrivance of our calendar prevents a two full-Moon-month. For example, the year 2048 sees a double full Moon in January. It would also have one in March, except that 2048 is a leap year, so the full Moon falls on February 29, not March 1, with the subsequent full Moon phase occurring March 30.
The year 2018 is unusual in having two months of dual full Moons. A similar situation last occurred in 1999, when the Moon was full twice in January (in Europe) and again in March (in Europe and western North America). After 2018, we won't see a double-double of full Moons until January and March of 2037 (for both Europe and North America). I suppose that would make these 'blue, blue' Moons the rarest of all.
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