ver 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 ever-more-exact tickers. They developed devices that relied not on Earth’s wobbly rotation, but on microscopic atomic movements. At the heart of it all is an ever-advancing appreciation for our smallest temporal unit, the second. Modern systems like GPS and cellphones rely on keeping this interval consistent, which makes defining and refining it, well, of the essence.