It’s 40 years this week since Pink Floyd released ‘The Dark Side of the Moon’. In that time, it has sold over 50 million copies and famously entered the Billboard US album charts on 31 March 1973 only to slip out 741 weeks later, sometime in 1987. As it happens, it slipped back in only a couple of months later when it was re-released on CD and everyone and anyone with a decent hi-fi system who had already worn out their vinyl version, went out and bought a new copy in this exciting new format.

The 40th anniversary seemed as good a reason as any to feature the story of making the album in my regular Rock’n’Roll Routemaster slot of the Robert Elms show on BBC London yesterday. I know he quite likes the early whimsical, psychedelic, poppy Floyd, the Floyd of ‘See Emily Play’ and ‘Arnold Layne’, probably the best song about a nocturnal, clothesline knicker-nicking pervert ever written. But he rather stunned me when he revealed that he was actually a huge fan of ‘Dark Side…’ and that despite pressure from his Soul Boy older brothers Barry and Reggie, he’d actually bought himself a copy back in 1973. Regular listeners would also have been stunned and would be forgiven for checking their radio tunings when they heard him playing ‘Us & Them’ just before the one o’clock news. I had asked him to play ‘The Great Gig In The Sky’ but apparently when he played it during a Floyd Fourfer last year, senior BBC London management advised him in no uncertain terms never to play that damn caterwauling song ever again. Harsh but fair, it is a bit scary out of context I suppose.

It also surprised me that he’d never heard of ‘The Dark Side of the Rainbow’, a hilariously fascinating but totally bogus internet meme, which proposes that if you press ‘play’ on your ‘Dark Side of the Moon’ CD at the precise moment that the MGM lion roars for the third time at the start of your ‘The Wizard of Oz’ DVD, there are all sorts of weird and spooky coincidences between audio and video. You know, Dorothy starts running away during ‘On The Run’, Dorothy screams during wordless shouty piece ‘The Great Gig In The Sky’,  the Wicked Witch of the West appears at the precise moment David Gilmour sings “which is which” in ‘Us & Them’, that kind of thing. One theory is that it’s as a result of apophenia, which is the mind’s tendency to focus on the eight spooky matching moments and completely disregard the 992 moments which don’t correspond in the slightest. Another theory – not necessarily mine, but I’m happy to claim it – suggests that it’s more likely as a result of the effects of the tons of recreational drugs the viewer/listener may have taken in order to enjoy the album and the film together. And the amount of spare time they have. And the amount of access to the internet they have.

Apparently some people take it quite seriously. Cable channel Turner Classic Movies (TCM) aired ‘The Wizard Oz’ in 2001 with the Floyd’s magnum opus as the soundtrack rather than the real one. I’ve no idea what they did after 42 minutes when the album ended – the film’s over an hour and a half long. Lit up another one and pressed play again, I imagine.