Although the phrase “once in a blue moon” is often used to describe an extremely rare event the origin of the phrase is less widely known. In fact few people know what is meant by a blue moon. The blue moon in this phrase has nothing whatsoever to do with the colour of the moon.
The phrase is believed to date from 1528. In that year a pamphlet was produced which criticised the English clergy. It said that “if they the say that the moon is belewe, we must believe it”. Although the word belewe can mean blue the dissenters probably used the word to mean betraying. They were referring to the definition of Easter. At the First Council of Nicea the date of Easter defined as a movable feast. It should be celebrated on the first Sunday after the first full moon following the vernal equinox. At a time when most people counted the seasons by the passage of the moon this was difficult to calculate. In most years the reference moon was the fourth of the year. Very occasionally it referred to the fifth moon. The church has to tell people whether the fourth moon was appropriate or was a betrayal moon. For everyday folk the rules seemed arbitrary.
The problems encountered in the calendar came from a mismatch in the length of a calendar and a lunar month. The calendar month is variously 30,31 or 28 days long. The lunar month is 29.5 days long. This means that it is occasionally possible to have two full moons on the same calendar month. When two moons occur in the same calendar month the second full moon is commonly known as a blue moon. These are calendar blue moons. Although popular this is an incorrect definition which was first popularised in the Sky and Telescope magazine in March 1946.
When we talk of a calendar year believe that there are 365 days in a year. In fact the Earth spins 365 and a quarter times in the time it takes to complete one orbit of the sun. In every fourth year we add a day to a leap year to keep the calendar in alignment. In a similar way there are 12 lunar cycles in a year with eleven days left over. In every second or third year there is an extra full moon in the year. To ensure that there the correct number of lunar cycles in the religion calendar the blue moon was often treated as a leap moon which could be ignored.
Historically, the church and farmers planned their calendars on the assumption that each quarter of the year contained three full moons. When the quarter contained four full moons The first moon on a season is 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. These are seasonal blue moons.
The last calendar blue moon occurred on 31 December 2009. Tnext calendar blue moons can be seen in March 2010, August 2012 and Jult 2015. The next blue moon that will occur on a New Year’s Ever will coincide with a total lunar eclipse in 2028.
The next seasonal full blue moons can be seen on 21 November 2010, 21 August 2013 and 21 May 2016.
Sometimes smoke in the atmosphere, arising from forest fires and volcanic events can give the appearance of a blue moon. These are caused by totally different circumstances to those causing the astronomical blue moons.