Although I know the expression “once in a blue moon”, up to yesterday I didn’t know -nor wonder- what a blue moon actually is. But yesterday, all of a sudden, both Belgians and Argentines mentioned it to me, on the same blue moon night. August 31st.

It is supposed to be rare. It is a second full moon in one month. Isn’t it obvious that once in a while there are 2 full moons in one month? Every 29 days there is a full moon, a month lasts 30 or 31 days, so there is a ‘blue moon’ every 2 and a half years, which is not so exceptional. And as for the color blue, -that really is a big dissapointment- it looks exactly the same as any other full moon. Nothing blue-ish about it.

So where does the expression “once in a blue moon” come from?

I did some research and it seems like its only the last 25 years or so, that the blue moon is identified with this 2nd full moon in a year. But long before that, lets say about 400 years ago, the expression was already in use.

In the 16th century, saying that the moon was blue, was arguing the impossible. It was like saying that black was actually white. In the 18th century ‘a blue moon’ was used in a similar way : it was saying that something was impossible. For example “I will stop drinking when the moon is blue” which actually meant he would never stop drinking.

But it seemed that its only in the 19th century that people realized that it actually possible that the moon turns blue, and that this is something very rare. When the volcano Krakatoa in Indonesia exploded in 1883, its dust turned sunsets green and the Moon was blue all around the world for about two years. In 1927, the Indian monsoons arrived late and the extra-long dry season blew up enough dust for a blue Moon. And the moon also turned blue in 1951 in northeastern North America, when huge forest fires in western Canada threw smoke up into the sky.

So yes, sometimes the moon turns blue, and yes, it is extremely rare.

At the end of the 19th and in the beginning of the 20th century, the Maine Farmers’ Almanac listed blue moon dates. These were the third full moon in a quarter of the year when there were four full moons (normally a quarter year has three full moons). In each season, the full moon had a name, for example, the first moon of summer is called the early summer moon, the second is called the midsummer moon, and the last is called the late summer moon. When a season has four full moons the third is called the blue moon so that the last can continue to be called the late moon.

This ‘blue moon’ was misread by Pruatt in the ‘sky and telescope’ in 1946, and repeated in 1950 in the same magazine, as the 2nd blue moon in a month.

It is, according to the folklorist Philip Hiscock, only in the ’80 that the story of the blue moon -as in 2nd full moon in 1 month- became widespread after a broadcast on the radio as a piece of old folklore, and this actually struck peoples imagination. It looked like a new legend was born.

So “once in a blue moon”  still refers to something rare,  although the “blue moon” of today really isn’t. And the meaning of today finds its origin in …a mistake.

But when it comes to special moons, we should look out for the blue moon on December 31, 2028. It will definitely be worth staying up for, as it will not only be blue, it will also disappear in a total eclipse. Just put it in your agenda and hope for clear skies!