Why everyone’s talking about... A blue moon

TONIGHT it’s a blue moon, an event that happens once in… well, infrequently. But what is it?

A certain type of full moon, but not actually blue unless polluted air makes it look that way. We see roughly one full moon every month, or three a season, but they really come round slightly faster than that, with 29.5 days between them. So if we end up with four full moons in a season, the third of them is called a blue moon. Tonight’s is the third full moon this summer. Confusingly, many people think ‘blue moon’ is the second full moon in any calendar month, but the Royal Observatory says: ‘It’s a mistake, made in the 1940s and perpetuated by the Trivial Pursuit board game.’

So how long is ‘once in a blue moon’?

They occur every two to three years, with the next on August 31, 2023. The metaphorical use of the phrase to mean a long time first appeared in print in 1821, 300 years after an anti-clerical pamphlet claimed lay people had to accept whatever priests claimed, however preposterous, by saying: ‘If they say the moon is blue, we must believe that it is true.’

Why is it called a ‘blue moon’ anyway?

It may be a corruption of the Old English ‘belewe’, which means ‘to betray’, as it ‘betrays’ the usual idea of having one full moon each month.

But it’s inspired artists…

Indeed. The blue moon was a pivotal plot point in the 2011 Smurfs movie. More memorable is the 1934 Richard Rodgers and Lorenz Hart song, a doo-wop No 1 for The Marcels in 1961. It became a Manchester City anthem in the 1980s, not just because of the team colours, but because they only seemed to win away once in a blue moon. In 2015, a billion

aire paid £28.3 million for a 12carat diamond called the Blue Moon to give to his seven-yearold daughter. She’d probably have preferred a Smurf…