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.

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."

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.

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."

A timeline of accurate time

• 12th Century: The first mechanical clocks were built, accurate to an hour or so.
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• 16th Century: Even accurate pendulum clocks would drift by 10-15 minutes per day.
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• 18th Century: The H4 watch was invented which wouldn't lose minutes for weeks. "It led to the minute and the second being used in common society," says Burridge.
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• 1920s: The development of quartz clocks wildly improved accuracy, losing only a second in three years.
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• 1950s: Atomic clocks arrived, which use atoms as timekeepers and are so accurate "they won't lose a second of time in billions of years", says Burridge.

- Author: Jocelyn Timperley, BBC