In the early 19th century, as the sun moved across Britain from east to west, people set their clocks to local mean time, so that noon in Greenwich would occur about 16½ minutes before noon in Plymouth. Back then, travel on foot, by horse, or by coach was slow and inconvenient, so having to adjust their pocket watch, for the few who even had one, was the least of travelers’ concerns.

However, with the advent of railway travel, keeping track of time differences became confusing and impractical. In 1845, Henry Booth, a railway businessman involved with the Liverpool and Manchester Railway, petitioned parliament for a “Uniformity of Time,” arguing that when “the great bell of St. Paul’s strikes ONE, simultaneously, every City clock and Village chime, from John of Groat’s to the Land’s End, strikes ONE, also.”

In addition to rail travel, advances in industrialization and automation also increasingly required time standardization, synchronization, and optimization. With the advent of satellite navigation, the requirement for accurate time reached the order of nanoseconds, because a signal delay of one nanosecond corresponds to roughly one foot of distance on the ground. This is why atomic clocks were one of the enabling technologies for GPS.

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