It is easy to take for granted the familiar tick of a clock or the digital sweep of a second hand.

Yet the structure of our time-24 hours in a day, 60 minutes in an hour, 60 seconds in a minute-is a product of decisions made thousands of years ago. Surprisingly, these ancient choices have proven more resilient than many modern attempts to rationalise time.

The failed decimal experiment

In October 1793, the French Republic embarked on a bold experiment. As part of a wider revolutionary effort to rationalise the calendar and remove religious influence, France introduced “decimal time,” dividing the day into ten hours, each consisting of 100 minutes and 100 seconds, according to the BBC.

“It quickly began causing no end of headaches,” recalls Finn Burridge, a science communicator at Royal Museums Greenwich. The system lasted barely more than a year. Existing clocks could not be easily converted, France became temporarily isolated from other nations, and rural communities resented a ten-day work week.

Burridge adds: “It was tried, but it was unsuccessful, it didn’t take off.” The decimal system would fade into history, leaving the ancient 24-hour, 60-minute, 60-second structure intact.

Sumerian roots of 60

The origin of our 60-unit divisions can be traced back to the Sumerians of Mesopotamia, who flourished between c. 5300 and 1940 BC. They developed a sexagesimal, or base-60, number system. The exact reason for choosing 60 remains uncertain.

“There’s not a tonne of evidence where 60 itself comes from,” notes Martin Willis Monroe, an expert in cuneiform cultures. One theory suggests it derives from finger-counting: the twelve joints on one hand counted by the thumb, multiplied by the five fingers on the other hand, gives 60.

The system was far from arbitrary. Erica Meszaros, historian of the exact sciences, explains: “[It’s the] same thing with sexagesimal: they get up to 59 and instead of having a number higher than 59 they just use a one, but one place over.” Its practical benefits were clear. “If you’re developing numbers for very practical purposes, like accounting, taxes or measuring fields… having an easy way to do these mathematical operations can be really helpful,” Meszaros adds.

Monroe highlights the link to Sumerian agriculture: “Their development of written numbers was driven by a need to keep records for the increasingly large and complex farming system supporting their growing cities.”

The Egyptian division of day and night

The Egyptians, around 2500 BC, were the first to divide the day into hours. They initially focused on the 12 hours of the night, based on star clocks.

“We broke down the day into 12 because we break down the night sky into 12 months and 12 zodiacal signs,” says Meszaros. Early “seasonal hours” varied in length depending on the season, but by the Roman period, the hour had become a standardised unit.

Rita Gautschy, archaeoastronomer at the University of Basel, notes that many Egyptian timekeeping instruments may have had a ritual purpose: “Personally, I think that a lot of them were gifts to the gods, votive gifts.”

Babylonian refinement and astronomy

The Babylonians (c. 2000–540 BC) inherited the Sumerian sexagesimal system and applied it to astronomy. They subdivided time into units such as the ush (roughly four modern minutes) and ninda (roughly four modern seconds), though originally these measured distances in the sky or planetary velocities rather than daily life.

“They’re thinking about it as subdividing numbers that measure distance in the sky or velocity of planets,” Monroe explains.

Greek adoption and Hellenistic exchange

The ancient Greeks adopted the Babylonian system, recognising the value of continuity. “They kept the same division because this allowed them to just add new observations to existing ones,” Meszaros says. From around 330 BC, Egypt became a Hellenistic melting pot, where people and ideas from across the region merged, further consolidating these astronomical methods.

“It’s a system that worked well enough for the Babylonians that the people who came after them took it wholesale in order to take the astronomical data and traditions as well,” Meszaros adds.

Precision in the modern era

Timekeeping as a practical necessity advanced significantly with mechanical clocks in the 17th and 18th centuries. The H4 watch, a marvel of precision, finally made minutes and seconds relevant in daily life. “It led to the minute and the second being used in common society,” Burridge notes.

Today, atomic clocks measure the second with unparalleled precision, based on the radiation of caesium-133 atoms. Burridge marvels at their accuracy: “They won’t lose a second of time in billions of years.” Despite this, the fundamental structure of our hours, minutes, and seconds-a human construct dating back over 5,000 years-remains embedded in society.

From ancient Mesopotamia to modern laboratories, our clocks are more than machines: they are a testament to the enduring power of human ingenuity, and the strange, resilient logic of numbers chosen long ago.