TIME MEASUREMENT HISTORY

The International Bureau of Weights and Measures (the Bureau), near Paris, is headquarters to global scientists who calibrate the world’s units of measure that includes meter, kilogram and second. World’s timepieces including those on GPS satellites keep in step by syncing with the Bureau’s Coordinated Universal Time (UTC).

Historically, people used Earth’s rotation as their master clock, tracked with sundials and early water clocks. In 1656, Christiaan Huygens invented the gravity-driven pendulum clock. In 1670, William Clement’s clock was the first to tick-tock the seconds – like hours and minutes, a human-devised division of the day. Up to the early 20th century, Britain led the way in time standardization with “Greenwich Mean Time” set by the clock of the Royal Observatory at Greenwich. The quartz clock, measuring the second as a set number of times a quartz crystal vibrates “in an oscillating electric field” (in most applications 215 Hz which is 32,768 cycles per second), arrived in the 20th century. Using an electric field instead of gravity can make time-keeping more precise.

In the 1960s, atomic clocks redefined the second “based on the cesium atom” which undergoes 9,192,631,770 quantum vibrations each second and revealed the gradual slowing of Earth’s spin, necessitating the addition of nearly half a minute’s worth of “leap seconds” since 1972 to keep atomic time in sync with rotation-based time. America’s National Institute of Standards and Technology (NIST) provides the official time through servers synced to computers and GPS devices. Gathering readings from NIST’s atomic clock and others worldwide, the Bureau averages the differences.

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