How We Measure Time: From Shadow Clocks to Optical Atomic Clocks - and Why Satellite Navigation Needs Einstein

Twenty thousand kilometres overhead, the clock aboard a navigation satellite gains about thirty-eight microseconds a day - a number predicted by Einstein before anyone had built a clock good enough to notice. Left uncorrected, that drift would push every position fix on your phone off by roughly ten kilometres per day. The map works because someone measured time carefully enough to see gravity bending it.

How We Measure Time traces that measurement from the beginning: shadow sticks and Egyptian water clocks, the gearing of the Antikythera Mechanism, medieval tower clocks that drifted by minutes a day, and the pendulum that suddenly made seconds worth counting. It follows John Harrison's forty-year fight to win the Longitude Prize with a watch, explains why every quartz crystal in every wristwatch vibrates exactly 32,768 times a second, and shows how the second itself came to be defined by 9,192,631,770 oscillations of a caesium atom. Along the way it answers the questions most histories skip: who actually computes world time, why leap seconds are being abolished, and what an optical lattice clock accurate to one second in fifteen billion years is actually for. A history of civilisation told through the one quantity we have learned to measure better than anything else.