The ancient reason there are 60 minutes in an hour

A mysterious 5,000-year-old decision led directly to how we still count time today.

In October 1793, the newly established French Republic embarked on an ill-fated experiment. It decided to change time.

The day, the revolutionaries decided, would now be divided by 10 hours, not 24. Each hour would have 100 minutes décimales in turn made up of 100 secondes décimales.

The time system was part of a wider revolutionary calendar which aimed to rationalise (and de-Christianise) the years' structure, including a new 10-day week. Work soon began to convert existing clocks to the decimal system. Town halls mounted decimal clocks and official activities were recorded using the new calendar.

It quickly began causing no end of headaches, says Finn Burridge, a science communicator at Royal Museums Greenwich in London, UK, home of the Royal Observatory and the place where Greenwich Meantime was established.

Redesigning and converting existing clocks proved extremely tricky. The system isolated France from neighbouring countries, while the rural population hated the day of rest becoming only every 10th day. Ultimately, decimal time lasted barely more than a year in France.

To understand how we started counting, and still count today, 24 hours in a day, 60 minutes in an hour and 60 seconds in a minute, though, we need to wind the clock back to an era before the dawn of timekeeping. Because it's the story of one of the earliest number systems that started us off on this track – and explains why this awkward system has long outlived the civilisations who invented it.

A base of 60

At the origin are the Sumerians, an ancient people who lived in Mesopotamia (roughly modern-day Iraq) from around 5300-1940BC and one of the first civilisations to ever form cities. Along with many other inventions including irrigation and the plough, they are credited with creating the first known writing system. This happened to include a number system based on the concept of 60.

Hold up your hand in front of you, bend a finger, and you'll see it has three joints. Count all the joints on the fingers of one hand (not including the thumb) and you'll reach 12. Count this 12 as one using a finger on your other hand and restart the count to 12 on the first hand, until all five fingers on your second hand are used. What have you just counted to? Sixty.

This is one of the speculative theories as to why the Sumerians based their emerging mathematical system on 60, not 10 – a decision that still has implications for how we measure time today. (Find out what the way you count reveals about you in this article by Anand Jagatia.)

The Fitzwilliam Museum, University of Cambridge

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, says Martin Willis Monroe, an expert in cuneiform (the early writing systems of the ancient Middle East) cultures at the University of New Brunswick in Canada.

They began using small clay tablets, often the size of a smartphone or smaller, to keep track of numbers, impressing the details into soft clay. Other pictorial notations soon followed, developing into the Sumerians' famous cuneiform text.

It was only in the mid-19th Century that these clay tablets were uncovered and started being deciphered. They show that the Sumerians used a whole host of number systems, says Monroe, but the most prominent for mathematics, and thus ultimately astronomy and time, quickly became a so-called sexagesimal system. 

The Sumerians used 60 in a comparable way to how we now use 10. When we reach nine, we move over a space to the left, write a one and add zero to the right, says Erica Meszaros, who recently completed a doctorate in history of the exact sciences and antiquity at Brown University in the US. "[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."

Despite the tempting finger-counting theory set out above, it's not clear why the Sumerians settled on a base-60 system. "There's not a tonne of evidence where 60 itself comes from," says Monroe. Some scholars have suggested the sexagesimal system may well have predated the Sumerians.

Its ease of use, however, is clear. Sixty can be divided by one, two, three, four, five, six, 10, 12, 15, 20, 30 and 60 without the need for fractions or decimals. Contrast that with 10, which can be divided only by one, two, five and 10 and its advantages start to become clear. "If you're developing numbers for very practical purposes, like accounting, taxes or measuring fields and dividing fields for your sons' inheritance, having an easy way to do these mathematical operations can be really helpful," says Meszaros.

The origin of time

There's no clear evidence that the Sumerians used time, although timekeeping likely did exist in the region before the first documented use of sundials and water clocks by the Babylonians (an ancient Mesopotamian civilisation which came after the Sumerians) in around 1000BC, says Monroe.

The first civilisation known to divide the day into hours were the ancient Egyptians, says Rita Gautschy, an archaeoastronomer at the University of Basel in Switzerland, and this shows up in religious texts from around 2500BC. The first known objects related to hours initially referred to the 12 hours of the night: these were diagonal star clocks found on the inner lid of the coffins of noble Egyptians from between around 2100 and 1800BC, says Gautschy.

The Board of Trustees of the Science Museum, London

It's not certain why exactly the Egyptians chose a subdivision of 12 – eventually leading to 24 hours in the full day. The Egyptians had a zodiac cycle of 12 constellations, but this was likely introduced after the first references to 12 hours. Counting to 12 using the joints and fingers of one hand is another possibility. Some experts think it may have been due to how their choice of a 10-day week intersected with the visibility of certain stars. 

The earliest known instruments to measure time, sundials and water clocks, appeared in Egypt around 1500BC. Some were used during daily work, but most were "likely more related to the religious sphere and rituals" than timekeeping, says Gautschy. "Personally, I think that a lot of them were gifts to the gods, votive gifts," she says. "We don't have much information about scientific time keeping [from the era]."

Initially, in texts about the business of daily life, the smallest time unit was generally the work shift, says Gautschy – usually imagined as being either morning or afternoon. But by the Roman period of ancient Egypt (from 30BC), hours became the standard, with half hours also starting to appear, she says.

The arrival of minutes

Meanwhile, the Babylonians had also been developing their use of hours. They would ultimately be the first to break the hour down into much smaller units – albeit not for timekeeping purposes. 

The Babylonians, who thrived from 2000BC to 540 BC, adopted both the cuneiform script and sexagesimal number system from the Sumerians. By around 1000BC, says Meszaros, they had developed a calendar based on how long it took for the sun to return to the same position in the sky – a little more than 360 days.

This was a handy number for a civilisation already using a counting system based on 60. "Wow, isn't that nice in a sexagesimal system?" says Meszaros. "In fact, it led really nicely to 12 months of 30 days each", which also fit in with the moon cycle, she says.

The Babylonians developed a practical time system for day-to-day use which divided both day and night each into 12, as the Egyptians did. The lengths of these "seasonal hours" would vary with the length of day and night. "We broke down the day into 12 because we break down the night sky into 12 months and 12 zodiacal signs," says Meszaros.

Many other ancient civilisations used seasonal hours and they were still in use in 15th Century Europe and 19th Century Japan. This seasonal time was never divided into smaller units for practical use, though, notes Monroe. "[That] isn't really a thing until the early modern period… It doesn't exist in Mesopotamia and other ancient cultures, because there's really no need for it."

Cuneiform Digital Library Initiative

The Babylonians also developed another time system for calculating and measuring astronomical events, which wasn't for daily use. This divided the day into 12 "beru", which we can think of as the same as two modern hours. Babylonia wasn't the only ancient culture to use them: they also appeared in ancient China and Japan, for example.

Driven by the need to measure more granularity in their calculations, the Babylonians started breaking down these beru double-hours into 30 ancient minutes known as ush, each equal to four of our present-day minutes. These were further divided by 60 into smaller units called ninda, each worth about four modern seconds. These subdivisions were likely used "because we break things into groups of 60 in the sexagesimal system", says Meszaros.

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However, the Babylonians were "not thinking about it as like subdividing time", notes Monroe. "They're thinking about it as subdividing numbers that measure distance in the sky or velocity of planets."

It can be hard to say exactly who built upon whom amongst all these ancient developments of time, says Gautschy. "From around 330BC onward, Egypt, with the new centre of science in Alexandria, became a melting pot where people, and with them their ideas from all regions, amalgamated," she says. "That's what we call the Hellenistic world."

Still, it's clear the ancient Greeks adopted the Babylonian astronomical time system, says Meszaros. "They kept the same division because this allowed them to just add new observations to existing ones… 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."

Counting seconds

While the Greeks had sand clocks at court "to make sure that people had the same amount of time to speak", the Babylonian time system they adopted was only used conceptually by astrologers and largely "not really relevant for daily life", says Gautschy.

But the concepts of hours, minutes and seconds that emerged from the Hellenistic melting pot were passed through the centuries to the present day. It was only a few hundred years ago, however, that timekeeping devices became accurate enough for minutes and the seconds to start being used day to day.

The second is now used in countless scientific definitions, and once we started counting time units smaller than a second, scientists did move to a metric system, breaking it down to milli and microseconds (a thousandth and a millionth of a second, respectively).

In the 20th Century, atomic clocks allowed scientists to redefine the second more precisely, moving from defining it based on rotations of the Sun to a precise value based on the absorption and emission of microwave radiation by caesium-133 atoms. Today, our global network of atomic clocks keep the time of pretty much every modern clock, and is behind everything from the internet to GPS to super accurate MRI imaging.

Tracing the history of timekeeping, though, reveals that it is actually a human construct, determined by human decisions. Hours, minutes and seconds arrived with us through a series of choices, coincidences and happenstance. But they stayed with us as useful heritage through the centuries, a hangover from ancient times so deeply ingrained that changing the system now would probably just be too much to handle. 

Even during France's 18th Century attempt to decimalise time, in practice the new system was barely used, even while the Republic's similar efforts to decimalise distance measurements and currency were adopted and are used to this day. Decimal time lasted just 17 months, although the calendar stayed in some use for about a decade. "It was tried, but it was unsuccessful, it didn't take off," says Burridge.

A 1795 speech by Claude-Antoine Prieur, a member of the French National convention, may have been what put the final nail in the coffin of decimal time. As well as offering hardly anyone a marked advantage, he argued, it cast a bad light on the other new metric measurement systems – which were, in contrast, he said, useful.

* Jocelyn Timperley is a senior journalist for the BBC's science features team. Find her on X @jloistf.

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