At the thirteenth General Conference on Weights and Measures in 1967, one second was redefined as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.” In April of this year, the National Institute of Standards and Technology in Boulder, Colorado, unveiled a new atomic clock to act as the United States’ primary time standard; it will not gain or lose a second in 300 million years.

Along with an elephant whose name in Arabic meant “the father of intelligence,” Abbasid caliph Harun al-Rashid presented Holy Roman emperor Charlemagne with a mechanical clock powered by water in 807. At noon a weight dropped, bells sounded, and twelve brass horsemen emerged from twelve windows.

In 2013 a picture taken by the Planck telescope of fluctuations in the cosmic microwave background—radiation generated by the Big Bang—resulted in the estimation that the universe is 13.82 billion years old.

In 1863, four years before publishing the first volume of Das Kapital, Karl Marx wrote to Friedrich Engels that apart from “the discoveries of gunpowder, the compass, and the printing press—these necessary preconditions of bourgeois development—the two material bases on which the preparations for machine industry were organized within manufacture...were the clock and the mill.” He elaborated: “The clock is the first automatic machine applied to practical purposes, and the whole theory of production of regular motion was developed on it.”

The duke of Milan, Azzo Visconti, commissioned a clock to be built in the campanile of San Gottardo; upon its completion in 1336 his secretary, Galvano Fiamma, wrote that the “admirable” timepiece had bells that struck “twenty-four times according to the number of the twenty-four hours of the day and night.” He concluded, “This is exceedingly necessary for people of all estates.” It is the first documented hour-striking clock in a public setting. A Milanese chronicle later reported Visconti’s time of death as August 14, 1339, in the twentieth hour—the first modern reference to an hour indicator in such a context.