The song was released a month before the legendary artist was shot dead in December 1980, and it was known to have been influenced by a film about baseball that Lennon saw during a visit to Bermuda earlier that year.
Kenneth Womack told The Observer he had watched dozens of films before realising that the key inspiration was A Love Affair: The Eleanor And Lou Gehrig Story, a 1978 movie about a baseball player who died after battling a rare nervous system disorder.
“I wanted to know what film had inspired him to compose such a beautiful song,” Womack explained. “For John, the use of such ‘found objects’ in life and art was essential to his composition practices.”
He added: “For decades, American filmmakers had put out one baseball film after another, and there are hundreds of them.
“After watching dozens in search of the mysterious film in question, I began to study TV guides from that period. John was a regular subscriber.”
Upon realising A Love Affair had been screened at the time Lennon was staying in Bermuda, Womack made the connection, concluding: “The mystery, quite suddenly, was solved.”
Womack’s new book, John Lennon 1980: The Last Days in the Life, is out now.
Speaking to Lennon’s son Sean for a new BBC Radio 2 documentary to mark what would have been the late singer’s 80th birthday, McCartney recalled how the pair fell out when the Beatles split in 1970.
McCartney told Sean: “It really, really would have been a heartache to me if we hadn’t have reunited. It was so lovely too that we did, and it really gives me sort of strength to know that.”