Today’s Full Moon is the second this month – the Moon was also full on Thursday August 2nd. In recent years, the second Full Moon in a single calendar month has become known as a ‘Blue Moon’, as in something that happens only rarely. However, this occurrence is not especially rare – we get two Full Moons in a calendar month every two or three years – and also the definition of an astronomical ‘Blue Moon’ is slightly more complex than this straightforward definition.
It’s possible to get two Full Moons (or two of any other particular phase) in a month because the period between two similar phases – New Moon to New Moon, say, or Last Quarter to Last Quarter – is just a little over 29½ days. This is known as the Moon’s synodic period, and as it is slightly shorter than most calendar months, a phase that occurs at the beginning of the month can be repeated again at the end. February is the only month that is too short for this effect – in fact, since February’s 28-day length is less than the Moon’s synodic period, it’s possible for February to miss out altogether on a particular phase. This was notable in 1999, when both January and March had two Full Moons and February had none (if you missed that interesting event, it will happen again in 2018). Even this year, February lost out on a First Quarter, with one falling on January 31st and the next on March 1st.
So where did the ‘Blue Moon’ description come from? Well, it all started with Sky and Telescope magazine, which ran an article in its March 1946 issue in which the author referred to a ‘Blue Moon’ as the second Full Moon in a month. This then became accepted lore in future issues of the magazine. However, the author of the 1946 article had misinterpreted a definition given in the American Farmer’s Almanac, which appears to be the original source of the expression. It’s perhaps not surprising that this happened – the definition given by the Farmer’s Almanac is rather convoluted.
Basically, the original definition says that in years when there are 13 Full Moons rather than the usual 12, and one of the seasons has four Full Moons while the others have just three, the ‘Blue Moon’ is the third one in the season with the ‘extra’ Full Moon. The full story can be found in this Sky and Telescope article, which explains it all in more detail than I can here.
Nonetheless, even though this may be the ‘correct’ definition of a ‘Blue Moon’, I think it’s too complicated to catch on and the ‘second Full Moon in a calendar month’ version will be the one that’s most often used. So watch out for August’s ‘Blue Moon’ – the next will be in July 2015.