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 Mean Time 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.

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.

- Author: Jocelyn Timperley, BBC