03/06/1989 at 01:00 AM EST
One of the most reliable ways used to date ancient artifacts is to monitor the decay of carbon-14 isotopes. One of the most reliable ways to date more recent events is to determine what Cher was wearing, or whom she was dating, at the time. Roller skates? Probably '79. Josh Donen? Late '84, early '85. For the benefit of cultural anthropologists everywhere, here's a brief look at Cher—the woman, the actress, the chronological litmus test—as seen in 15 years of PEOPLE.