Take a stroll down memory lane with a compilation of snippets from the biggest song of each year from 1940 to last year. That's 78 songs! No problem, it's easy to skip around and find the years you want to hear. But how do you define the biggest song of the year? In this case, it's the song that stayed at the top of Billboard's pop music chart the longest. In some cases, it wasn't the biggest-selling song, but if you were around, you no doubt remember it. Some of those #1 hits may surprise you (see 1974). If you want to see the UK version of the same idea, you'll find that video at Laughing Squid.
That changed in the early 1990s with SoundScan. https://en.wikipedia.org/wiki/Nielsen_SoundScan