A brief, 20,000-year history of timekeeping

As clocks got more accurate, we had to redefine the second.

Photo by Fabrizio Verrecchia.

By Kelsey Atherton

Over millennia, humankind’s time-tracking has grown increasingly precise. Sundials divided days into hours. Clocks broke hours into quarters and ­minutes, and finally minutes into seconds. As timepieces evolved, so did scientists’ need for…

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