A blue moon is a very rare event. The phrase is immortalised in the 1934 lyrics by Lorenz Hart "Blue moon/you saw me standing alone/without a dream in my heart/without a love of my own". So, what is the origin of the phrase?
Sometimes the moon can appear bluefollowing a forest fire or volcanic explosion. This is because smoke passes in front of the moon and has nothing to do with the true origin of the phrase. A few famous blue moons of this type have been seen after the explosion of Krakatoa in 1883 Mount St Helens in 1980 and after Canadian prairie fires in Alberta in 1953.
The phrase "once in a blue moon" is believed to date from 1528. This was the year in which a pamphlet was produced which sharply criticised the English church. The pamphlet contained the words "if they say that the moon is belewe, we must believe it". This isambigous because the word old English word "belewe" can mean blue and can also mean betraying. The authors were using the definition of Easter as an example of how the church introduced arbitrary rules into everyday life. According to the First Council of Nicea, Easter was defined as a moveable feast which should be celebrated on the first Sunday after the first full moon following the spring equinox. This was a practical definition in an age when few could read and write. Everyday folk were expected to count the number of full moons from the start of the new year. In most years Easter was defined by thethird moon of the year, but occasionally it was defined as thefourth full moon. Sometimes the church had to tell the people in what seemed an arbitrary way to ignore the fourth new moon and define Easter by the fifth moon. The third as known as the betrayal moon. From 1528 because of the pun on the word "belewe" thethird became known as the blue moon.
The reason for the apparent misfit in which the first quarter of the year can containthree or four full moons arises from the length of the lunar month and the way in which the calendar is organised. During a lunar month, that is the time in which the moon makes a complete orbit of the Earth the Earth spins 29.5 times on its axis, and 29.5 days pass by.Furthermore there are 365.25 days in a year. Thisis the time in which the Earth completes an orbit of the sun. This means that there are twelve full moons in a year plus over half, or 17 days,of a thirteenthlunar cycle. These areastronomical facts.
The organisation of the calendar is a man made construct. The modern Gregorian calendar defines a year uses12 months of varying lengths of 30, 31 and 28 days. Seventeen days fromthethirteenth lunar cycle is completed within the calendar year the exact dates of the full moons drifts from year to year. Each calendar year has to accommodate 12.6 lunar cycles. When the drift is taken into account someyearscontain 13 new moons which means that the year must contain at least one blue moon. The drift in the lunar calendar can also align several new moons into the same calendar month and give rise to other blue moons. Depending upon the alignment of the calendar some of these blue moons could be a spurious, or betrayer, new moon, that had to be removed by church edict before Easter could be declared.
The formal definition of a blue moon very much arises from the church calendar.When the quarter contained four full moons The first moon on a season is called the first moon, the second is the mid season moon and the last is the late moon. The third of four, if it occurs, is known as the blue moon.
A more popular definition is in wide circulation. When two moons occur in the same calendar month the second full moon is commonly known as a blue moon.Thisdefinitionwaspopularised through an error in the Sky and Telescope magazine in March 1946. To be precise theseought to be called calendar blue moons.
The last blue moon occurred on 30 March 2010. The next calendar blue moons will occur on 31 August 2012 and 31 July 2015. The calendar blue moon thatoccurred on 31 December 2009 was rather special. Itcoincided with a lunar eclipse and the end of the calendar year. The next blue moon that will occur on a New Year's Ever thatcoincides with a total lunar eclipse will occur in 2028.
The next seasonal full blue moons willl occur on 21 November 2010, 21 August 2013 and 21 May 2016.
Observationally there is nothing special to see in a blue moon. They were important in the definition of Easter and are an anomaly caused by our attempt to match our calendar to astronomical observations.