It’s like the volcanic version of using solar eclipses to date historical records. Solar eclipses are a stunning experience even today, so it’s no surprise that ancient humans often wrote about them as mysterious cosmic or religious events. And because we know they’re visible in specific spots at predictable intervals, we can work backwards to figure out exactly which day every eclipse that’s ever happened occurred. If Plato wrote about an eclipse in his diary, we’d just need to know when an eclipse happened in Athens around the time Plato lived, and then we’d know exactly what day he wrote that diary entry. We could then use that reference point to date the surrounding pages, which might help us understand when historically significant cultural events occurred.