The opening lines of the song ("I was 21 years when I wrote this song/I'm 22 now, but I won't be for long") are identical to the opening lines of Paul Simon's song "Leaves that are Green," which appears on Simon and Garfunkel's 1966 album Sounds of Silence. In concert, in Winnipeg, Canada, on 27 September 2006, Bragg stated that Simon and Garfunkel had a strong influence on him, and that he took the line from their song intentionally.
Bragg says that the song had its origins in seeing two satellites flying alongside each other. Searching for romantic inspiration, he had to make do with "space hardware". He told a BBC interviewer that he "stole" the melody from Thin Lizzy's Cowboy Song.
Bragg's original version of the song had only two verses. MacColl thought the song was too short, and so Bragg wrote a further two verses for her, which she consolidated into one. Since MacColl's death, Bragg has included the additional verse in performances of the song as a tribute.