About two centuries ago, a traveler leaving London for another British city might have to adjust his watch upon arrival, not because of a time zone difference as happens today between countries, but because each city set its time according to the position of the sun in its sky.
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When trains appeared, this difference became a real problem. A train that leaves a city at a certain time may arrive in another city whose local time is several minutes different, which confuses travel schedules and makes it difficult to organize train traffic.
No one could have imagined back then that the world would reach a point where the one-second adjustment, which humans used to add to Coordinated Universal Time to correct for the Earth's rotation, would become a source of concern for communication networks, satellites, and digital systems. But that is exactly what happened.
On October 15, 2026, representatives of member states at the General Conference on Weights and Measures, held in Versailles, France, made a decision to make Coordinated Universal Time (UTC) continuous from May 20, 2027, without the need to add or remove leap seconds.
Thus begins a new phase in the history of time measurement, which may last for centuries before it calls for a re-examination of the relationship between the time measured by atoms and the time determined by the movement of the Earth.
When the sun was the clock of every city
Before the invention of modern clocks, the sun was the natural reference for time. Humans knew the succession of night and day, and then learned to measure the passage of hours from the change in the length and direction of shadows.
The ancient Egyptians used sundials, and later water clocks emerged, measuring time by the flow of water at a relatively regular rate. These methods had clear limitations; sundials did not work at night or when clouds obscured the sun, while water clocks were affected by temperature and the regularity of the flow.
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