Once in a blue moon

The English language contains many expressions relating to astronomical events, which is not all that strange, when you realise that before humans had clocks, radar, satellites, and so on, people were completely reliant on their own observations in order to keep time and predict seasonal changes. 

The term ‘blue moon’ is one of many ‘full moon’-expressions. You may have also heard of ‘wolf moon’ or ‘harvest moon’, for example. Many of these terms come from ancient cultures where celestial observations guided local timekeeping.

A blue moon does not actually tell you anything about the colour of the Moon. The expression is commonly used when two full moons happen in one calendar month.  

Our calendar months are based on the lunar cycle. The Moon takes 29.5 days to go through a full cycle. Our calendar months range from 28 to 31 days. As you can tell, the two cycles don’t match up completely. As a consequence, some months will have an extra full moon. The second moon in the same calendar month is often referred to as a ‘blue moon’. 

This is a relatively rare event, only happening about once every two or three years. Hence the connection to the expression: something that seldomly happens. 

So, if the Moon doesn’t actually change colour, where did the word ‘blue’ come from? It’s believed this word may have come from the Old English word ‘belewe’, which means ‘betray’. The moon ‘tricked’ people to believe a new calendar month had started, when in fact, it was still the same month. 

Incidentally, there have been moments when the moon has taken on a slightly more blueish hue than what we’re used to. This sometimes happens as a result of increased ash or smoke particles in the atmosphere after a volcanic eruption, like when Krakatoa erupted in 1883. But such events only occur once in a … blue moon.