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Did you know ...Atomic clocks tick 9 billion times a second and the best keep time to one second in 10 million years! 13 Nobel prizewinners have contributed to the physics of atomic timekeeping. Greenwich Mean Time remains the legal time for the United Kingdom despite being abandoned for civil timekeeping in 1972. Even the venerable Greenwich time signals are locked to the atomic clocks of the United States Naval Observatory in Washington DC, Atomic clocks on navigational satellites transmit worldwide time signals accurate to 30 billionths of second. Atomic clocks of the future will be based on the vibration of a single trapped atom. Clocks on mountains tick faster than clocks at sea level. Until the 1950s, timekeeping was based on the apparent motion of the Sun which in turn reflected the rotation of the Earth on its axis. But the Earth does not turn smoothly. By the 1940s it was clear that the length of the day fluctuated unpredictably and with it the length of the second. Astronomers wanted to redefine the second in terms of the motions of the Moon and the planets. Physicists wanted to dispense with astronomical time altogether and define the second in terms of the fundamental properties of atoms.The physicists won. The revolution began in June 1955 with the operation of the first successful atomic clock and was complete by October 1967 when the atomic second ousted the astronomical second as the international unit of time. "Splitting the Second" tells the story of this revolution, explaining how atomic clocks work, how more than 200 of them are used to form the world's time, and why we need leap seconds. It tells how accurate time is distributed around the world and what it is used for. The book concludes with a look at the future of timekeeping.