What is a Blue Moon?

A lot of myths surround the blue moon. But in general, it’s agreed that the blue moon is a very rare thing, hence the phrase, “once in a blue moon.” However, the blue moon is not remarkable for its color, but rather, in the way that it turns up in the year.

The official definition of a blue moon is that it is a full moon that is not timed to the regular monthly pattern of one full moon per month. This situation arises because while there are twelve full lunar months in a year, there is also a spillover of about ten to eleven days beyond that. Since these days accumulate, there will eventually come a time about once every two to three years (once every 2.7154 years, to be precise) where a year has thirteen full moons. The occurrence of this extra moon often played havoc with the calendars of the time, which were drawn up by the assumption that there were only twelve full moons in a year (complete with names for them) and so this extra moon gained the title of “a blue moon”.

Depending on when exactly in the year a blue moon turned up, and in what context it appeared, it would have different names. The English church used the cycle of the moons to determine when Lent would come, and the moon heralding Lent was called, no surprisingly, the Lent Moon. However, when the extra moon appeared before the Lent Moon, the church would be forced to tell the people that it was not a Lent Moon, or rather a “betrayer moon.” In old English, the word “belewe” meant “betrayer.” It does not take a great leap of logic to see how it could be corrupted to “blue” over the years.

However, sometimes the moon can literally have a blue color due to the presence of dust and smoke particles in the air. These particles diffract the light from the moon to produce a bluish color, much in the way the atmosphere diffracts the light from the Sun to make the sky appear blue (a phenomenon known as Rayleigh’s Scattering). These particles can be generated during forest fires, as was observed in Sweden and Canada in the 1950s, and by volcanic eruptions, such as when the volcano Krakatoa erupted in 1883. That last event caused the moon to appear blue for over two years.

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