Cast your mind back 40 years, if you can. You arrive in November 1978. “Rat Trap” by the Boomtown Rats is at #1 here in the UK, having displaced “Summer Nights” by Olivia Newton-John and John Travolta after its 9 weeks at the top. Jim Callaghan is still prime minister. In the US Donna Summer is #1 with her version of “Macarthur Park. Jimmy Carter is president.

I can cast my mind back to that time very easily. At home our old black and white TV had just been replaced by a colour set. We were watching “Top of the Pops” in colour for the first time. We would have snow that winter. Even at the time people called it “the Winter of Discontent” but we were happy enough. The nation did not seem to be in crisis, despite what people might have claimed in the years since then. The Revolution in Iran had begun. Within a few months the pro-Western monarchy would be abolished and a new government installed. We were advised at school to follow the news and compare what was happening there in real time with what had happened in the French Revolution nearly 200 years earlier.

If you can cast your mind back a further 40 years from 1978 you will be in 1938. The Second World War has not begun. For many people memories of the First World War, just 20 years earlier, are still fresh. I was born in the 1960s, so my knowledge of 1938 came initially from History lessons. We studied European History up to the end of the Second World War in some detail. My parents were children in Ireland throughout the war. They did not have the kind of experiences that children here in London had. They couldn’t tell us much about life in wartime.

I think often of gaps in time, comparing the distance between now and past events with similar distances further back in time. I do it especially with music and films. In 1978 people in their 50s could look back at 1938 in the same way that I can look back on those first few weeks of watching colour TV, when “Rat Trap” was at #1. I went to see live music performances more often than I went to the cinema back then, to places like The Marquee on Wardour Street and The Nashville Rooms in West Kensington. The only time I went to the cinema in late 1978 was with my dad in the Christmas holidays, to see the newly released “Superman” (Christopher Reeve in the title role). 40 years earlier two of the biggest movies in history had not yet been made. “Gone with the wind” and “The Wizard of Oz” both came out in 1939. The gap between “Superman” and “The Wizard of Oz” is the same as the gap between “La La Land” and “Grease” or “Darkest Hour” and “Apocalypse Now”.

In late 2011 I signed up for the “movies by post” service LoveFilm. It was discontinued this time last year, as noted here. Initially I filled up my “Watch List” with Oscar Winners, stretching right back to the 1920s. One of the first to arrive was “Morning Glory” (1932), for which Katherine Hepburn won the first of her four Best Actress awards. While watching it I was struck by how much time had passed since its release, nearly 80 years. If you took that gap in time and subtracted it from 1932 you would be in 1863, a time before electric lighting, moving images, the motor car, recorded sound. America was in the middle of its Civil War. Queen Victoria still had another 38 years on the throne. It is now 86 years since “Morning Glory” was made. Subtract 86 from 1932 and you arrive in 1846. The famine has begun in Ireland but is about to get a whole lot worse. At the start of 1846 Texas is still a republic.

My first opportunity to watch “Morning Glory” was in 1981. It was on TV one afternoon in the Christmas holidays. I was alone at home and settled down to watch it but was interrupted by the plumber who had been installing our central heating. (1981: the year we got central heating.) He came in to wait for my mum to get back from work. I made him a cup of tea, we had a chat, I missed the movie. At the time it was only 49 years old. It was the same age as “Midnight Cowboy”, “The Wild Bunch” and “Easy Rider” are now, though none of those films are likely to be screened in a late afternoon slot anytime soon. It took me a further 30 years after 1981 to see Katherine Hepburn’s first Oscar-winning performance.

Musically and cinematically there were huge changes in the 1960s. At the start of the decade you have “Apache” by The Shadows and “It’s Now or Never” by Elvis Presley at #1. By the end of the decade Led Zeppelin had released their first two albums. Nudity, extreme violence and foul language were all absent from mainstream cinema releases in 1960 and were commonplace by 1970. Most films released before 1960 had aged far more by the 1980s than anything released in the last 30 years. “On the waterfront” (1954) which I first saw in 1980 seemed to be from a completely different era, although only 26 years had elapsed. Pick a contemporary film from 26 years ago: “The Bodyguard”, “Scent of a Woman” or “Basic Instinct” perhaps. None of them has aged in the same way. Films set in the early 1990s don’t look too different from films set in 2018. The computer screens and mobile phones are not the same, but they’re there. They are noticeably absent from films set in 1978. Which is where we came in.