It’s a Blue Moon

By | Nov 21, 2010

It’s commonly thought that a Blue Moon occurs when there are two full moons in the same month. This hasn’t always been the meaning of a ‘Blue Moon’. In 1946 an article by James Hugh Pruett appeared in Sky & Telescope March edition mistakenly referring to a Blue Moon as being the second full moon in a month.

Seven times in 19 years there are 13 full moons in the calendar year and with 13 full moons at least one month in that year will have two full moons.

Surprising there are times when two months, January and March, will have two full moons in a year. Of course when this happens there is not a full moon in February. The cycle of the moon is 29.5 days. This last happened in 1999 and the next time will be 2018 and only happens once every 19 years.

In old Farmer’s Almanacs the term Blue Moon referred to the third moon when four full moons occur during a season. The first full moon is called the Early (season) Moon, the 2nd the Mid (season) Moon and the last, the Late (season) Moon. This leaves the a lone seasonal moon with no name or Blue Moon.

In fact using this definition it is possible not only for the Blue Moon not to be the second moon of the month (it rarely would be), but also be in a calendar year that only has 12 full moons.

Being that now there are many more people who now consider a Blue Moon as the second full moon of the month, it is an easier to understand happenstance, it’ll be nearly impossible to not accept that a Blue Moon can be either of these. The full moon of November 21, 2010 was the third full moon in a seasonal cycle.



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