The Second: How Humans Learned to Measure Time, From Sundials Atomic Clocks

The second is defined as 9,192,631,770 oscillations of a cesium atom. Nobody chose that number. It was measured once, under pressure, in the 1950s, and then frozen so the world's clocks would keep agreeing. Behind that number is four thousand years of people trying to make time hold still long enough to count it. Shadows on a wall. Water dripping through a hole. A monk who needed to know when to ring the bell. A falling weight caught and released, tooth by tooth. A sailor a thousand miles off course because his clock had gained four minutes. Thirty-seven chapters. No physics background required. The whole story, from the first shadow stick to clocks that would lose a second in the age of the universe. - Why noon used to stretch with the season, and why nobody minded - Where sixty came from, and why it survived everything since - Why the sun itself runs fast and slow across the year - Ten days erased by decree, and the arithmetic that made it necessary - The escapement: turning falling weight into beats you can count - How the great tower clocks were built, and what they did to the workday - A swinging lamp in a cathedral, and the hundredfold leap that followed - Heat, air and gravity defeated, one pendulum refinement at a time - Why a clock was worth a fortune to a navy, and thirty years inside one man's attempts - The watch that won at Barbados - How railways abolished local noon and drew the time zones - The hour as legislation: daylight saving and its fights - Why a cheap quartz watch beats a marine chronometer - Timing the sprint, the shutter and the experiment - The day the Earth lost its authority as a clock - Cesium, and the atom that redefined the second - The leap second wars, and the systems that broke because of one - Clocks flown around the world, and relativity you can measure - A constellation of flying clocks, and what happens when it goes dark - Optical lattice clocks, and the second's next redefinition Ending with the question the title asks. What a second actually is, why the answer is a frozen measurement rather than a fact about nature, and what changes when it is redefined again. This is the story of the most precisely known quantity in human history, and of everyone who had to get it wrong first. Scroll up and start reading.