Most people are familiar with the term "Once in a blue moon", meaning "very infrequently". Variations on this phrase have been in use for some 400 years, and certainly a visibly blue moon is a rare event. They can be caused by smoke or dust particles in the atmosphere, such as happened after forest fires in Sweden in 1950 and Canada in 1951 and, notably, after the eruption of Krakatoa in 1883 which caused blue moons for nearly two years.

However, the best known physical definition is the one which states that the second full moon in a month is called a blue moon. This happens about seven times every 19 years, though the dates vary around the world because, even though a full moon occurs at the same moment for the whole planet, there are many variations on local time. In 1999, for instance, there were no fewer than four different scenarios depending on your location and variations in daylight saving time.

Blue moons, by this formula, happen roughly once every three years. February, being the only month shorter than a lunation, is the only month in which it is possible for there to be no full moon, a phenomenon which goes hand-in-hand with the "double blue moon" (usually one in January and one in March). Based on Universal Time calculations, these are the months in which blue moons will occur over a full 19 year cycle -

blue moons 1999 - 2020: "second in a month" rule

month

J

F

M

A

M

J

J

A

S

O

N

D

1999

2001

2004

2007

2009

2012

2015

2018

folklore?

This formula is usually quoted as a piece of ancient folklore, and it does appear to tie in with genuinely old naming conventions for full moons. However, when Philip Hiscock of the Memorial University of Newfoundland Folklore and Language Archive explored the history of this usage of the term he found no reference older than a few decades!

In a remarkable series of investigations, he and Sky and Telescope writers Donald Olson, Roger Sinnott and Richard Tresch Feinberg traced the origins of the phrase and discovered (no doubt with a mixture of amusement and embarrassment) that it stemmed from an error in their own magazine some 50 years before.

In 1943, S&T writer Laurence J Lafleur answered a reader's question by quoting from the 1937 Maine Farmers' Almanac that "occasionally the Moon comes full thirteen times in a year." In 1946 James Hugh Pruett, in an article in the same magazine, referred to Lafleur's earlier item. However, he added that the thirteenth full moon in a year "gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon."

If Pruett had seen the full text of the Maine Farmers' Almanac, or if Lafleur had included dates in his article, it would have been obvious that this interpretation could not have been correct. But the genie was now out of the bottle. In 1980 Deborah Byrd quoted Pruett's assertion in a National Public Radio programme Star Date. This in turn was picked up by the international news wire, and now has been thoroughly spread via the Internet.

formula

Olson, Sinnott and Feinberg managed to track down some 40 editions of the Maine Farmers' Almanac, ranging from 1819 to 1962. Of the dozen Blue Moons mentioned in them not one was the second in a calendar month. But there was a pattern: they all fell almost exactly a month before the equonoxes and solstices. So what formula was the Almanac using?

A great deal of mathematical modeling has revealed that the calculation is essentially seasonal but with some unusual refinements. It uses the tropical year, which begins and ends with the winter equinox around the 21^st of December, and instead of following the eliptical path of the Sun it employs a fictitious dynamical Sun, an imaginary body which travels at a uniform rate and creates seasons of precisely ¼ year.

In the dark ages which followed the fall of the Roman empire, one of the few lifelines for science and mathematics in Europe was the desire to calculate the correct date of Easter, following clues in the bible. This must fall on the Sunday immediately following the 14^th day of the Paschal Moon, which in turn falls on or after the day of the ecclesiastical vernal equinox which is fixed as 21^st March. It can never occur before 22^nd March or after 25^th April. Lent, which begins 46 days before Easter, contains the Lenten Moon.

By tradition all twelve full moons of a year have names, some religious like those above and the Moon Before Yule and the Moon After Yule, and some relating to seasonal activities and events such as Flower Moon, Hay Moon, Harvest Moon and Hunter's Moon. The main function of a Blue Moon is to preserve the relationships between these religious and lunar events in those years when there is a thirteenth full moon.

By this rule, a Blue Moon is the third full moon of four in a fixed season usually containing three, and will always fall on the 20^th - 23^rd of May, February, August or November. Here, again using Universal Time, are the months in which blue moons will occur over a full 19 year cycle using this more traditional rule -

blue moons 1999 - 2020: farmers' almanac rule

month

J

F

M

A

M

J

J

A

S

O

N

D

2000

2002

2005

2008

2010

2013

2016

2019