The night sky is about to see something so rare that it’s best known as a saying: the fabled Blue Moon.
The Moon won’t look any bluer than usual, and in fact nothing will happen to the Moon itself. And it is not even a Blue Moon in the way the phrase has come to be used recently.
Instead, it is the original meaning of Blue Moon – a “true” Blue Moon – that gave rise to the saying.
The original Blue Moon occurred when a season has four full Moons in it, rather than the traditional three. When that happens, it is the third one that takes the name the Blue Moon, as will happen this August.
That was the description given in the Maine Farmers’ Almanac, which is often used as the source for the names used traditionally for the Moons in the US.
Among other things, referring to the Moon in this way helps keep the traditional way of referring to specific Moons – the last of the season can keep its name as the “late” Moon, for instance. But extra Moons can cause all sorts of problems besides, including in the calculation of the date of church festivals, which may have given rise to the popularity of the phrase.
Such an event happens roughly every 2.7 years. The last one is in February 2019, and after this one there will not be another until August 2024.
But this is not how most people use the phrase today. The more modern definition refers to a time when one particular month has two full Moons within it.
The confusion has been traced back to a mistake made in 1946, when a contributor to Sky & Telescope misread that original description and made reference to the twice-in-a-month kind of Blue Moon. The magazine recognised its mistake in 1999, in another article, but it was too late, and the newer definition has mostly stuck around.
The next chance to see a Blue Moon by the more modern definition will come in August 2023.
More often than not, the phrase Blue Moon is not used to refer to either kind of Blue Moon, or a literal Blue Moon. It is often used in songs with the aim of recalling the sense of the word used in the blues, of loneliness or sadness.
The Moon only turns the actual colour of blue very rarely indeed. Occasionally, the eruption of volcano or large fires can put smoke in the sky that changes its colour.
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