Every day, billions of people glance at a clock, check a phone, or ask someone the time, and every single one of them is using a system rooted in the arithmetic of a civilization that vanished more than four thousand years ago. Sixty minutes in an hour. Sixty seconds in a minute. It seems arbitrary, even a little odd in a world that long since standardized on tens. Why not 100 minutes in an hour? Why not 10 seconds in a minute? The answer is a story that spans millennia and crosses continents, a story of farmers, astronomers, priests, and mathematicians, with each generation inheriting and refining the work of the last. To understand why time is divided the way it is, you have to go back to the very beginning of civilization itself.
The Sumerians: The People Who Invented Almost Everything
Around 5,000 years ago, in the fertile river valleys between the Tigris and Euphrates, the land of modern-day Iraq, a people called the Sumerians built something the world had never seen before: cities. Not just settlements, but organized urban centers with specialized workers, laws, and administration. Sumer, as this region was known, is often called the Cradle of Civilization, and with good reason. The Sumerians invented irrigation systems that could channel floodwater across dry fields, transforming marginal land into some of the most productive farmland in the ancient world. They invented the plough, dramatically multiplying the amount of land a single farmer could work. They built the wheel. They developed the arch. And perhaps most significantly, they created the first known writing system in the world.
That writing system was called cuneiform, a name derived from the Latin cuneus, meaning "wedge." Sumerian scribes would press a reed stylus into soft clay tablets at different angles, leaving wedge-shaped marks that represented sounds, objects, and quantities. It was a practical invention born out of necessity. When you're administering a city with thousands of residents, tracking grain stores, livestock counts, temple offerings, and tax records, you need a reliable way to write things down. Cuneiform was that way. And because the Sumerians needed to write down numbers constantly, they also needed a robust numerical system.
The system they chose was extraordinary. Rather than counting in tens, as most modern cultures do, almost certainly because humans have ten fingers, the Sumerians built their mathematics around the number 60. This is called the sexagesimal system, from the Latin sexagesimus, meaning "sixtieth." Evidence of this system appears on clay tablets from at least 3000 BC, and some scholars place its origins even earlier.
Why 60? Historians and mathematicians have debated this for centuries, and no single explanation has won universal agreement. One compelling theory involves counting on your hands. If you use the thumb of one hand as a pointer, the four remaining fingers each have three joints, giving you 12 countable segments. Use the five fingers of the other hand to track how many times you reach 12, and you can count up to 60 on your two hands alone. This could explain why 12 and 60 both appear so frequently in ancient measurement systems.
Another theory is purely mathematical. The Greek astronomer and scholar Theon of Alexandria, writing in the fourth century AD, argued that 60 was chosen because of how extraordinarily divisible it is. Sixty can be divided evenly by 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30, twelve divisors in total. Compare that to 10, which can only be divided evenly by 1, 2, 5, and 10. For a society doing a lot of practical mathematics, splitting rations among workers, dividing land, and calculating trade, a number that can be halved, thirded, quartered, and split five or six ways without leaving awkward remainders is enormously useful. In fact, 60 is the smallest number divisible by every whole number from 1 to 6. For a pre-calculator civilization, this was a gift.
There is also a theory proposed by the German mathematician Moritz Cantor in the 19th century: the Babylonians (who inherited the system from the Sumerians) observed approximately 360 days in a year and divided the sky's circle into 360 corresponding degrees, making 60 a natural sixth of that total. Whatever the true origin, the Sumerians clearly found the number highly practical. Their weights and measures were organized around it. Their merchants traded in units of 60. Their scribes taught multiplication tables in base 60. By the time their empire began to fade, the sexagesimal system was so deeply embedded in the culture of Mesopotamia that no one could simply abandon it.
The Babylonians: Timekeepers of the Ancient World
The Sumerian empire did not last forever. Around 2400 BC, Sumer was conquered by the Akkadians, and later, around 1800 BC, the region fell under the control of the Amorites, better known as the Babylonians. Like most conquering civilizations, the Babylonians absorbed and adapted the best of what they found rather than destroying it. They inherited the Sumerian sexagesimal system, and, critically, they put it to work in ways their predecessors never had.
The Babylonians were extraordinary astronomers. They observed the night sky with meticulous discipline, tracking the positions of planets, the phases of the moon, and the movements of the stars over years and decades. Their records were detailed enough that modern astronomers can still use them. And it was in service of this astronomical ambition that they made their most important contribution to our modern experience of time.
For daily and practical timekeeping, the Babylonians had real tools. Water clocks, called clepsydrae in Greek, meaning "water thieves", were among the earliest timekeeping devices that did not depend entirely on the sun. The oldest written descriptions of clepsydrae from Mesopotamia survive in ancient texts, and the first water clocks date back more than 3,500 years to ancient Babylon and Egypt. These devices worked on a simple principle: a vessel with a small hole through which water dripped at a steady rate. As the water level fell, markings on the inside of the vessel indicated the passage of time. This was enormously useful for measuring intervals at night, during religious ceremonies, or in any situation where a sundial was useless.
Sundials, too, were part of the Babylonian toolkit. The historian Herodotus, writing in the fifth century BC, credited the Babylonians with the invention of the hemispherical sundial, a bowl-shaped instrument carved from stone, with a pointer in the center whose shadow tracked an arc across the interior surface as the sun moved. The Babylonian astronomer Berosus (who flourished around 290 BC) is specifically credited with this design. The path of the shadow was divided into 12 parts, marking the time of day. This was a significant refinement of earlier, cruder instruments, and it became widely used throughout the ancient world.
But perhaps the Babylonians' most lasting contribution to timekeeping was their division of the sky itself. They divided the day and night each into twelve double hours called kaspu or beru and then began breaking these larger units into smaller subdivisions using, naturally, their beloved base-60 system. The Babylonians divided each beru into 30 ush, each of which was equal to approximately four modern minutes. These were further divided by 60 into even smaller units called ninda, each worth roughly four modern seconds. Crucially, these subdivisions were used not because the Babylonians were thinking about practical timekeeping for farmers and merchants, but because they were thinking about measuring the sky. They were tracking the angular velocity of the moon, the arc of a planet across the heavens, and the position of stars relative to the horizon. For that kind of precision, you need fine-grained numbers — and in a base-60 system, dividing by 60 is the most natural thing in the world.
The Babylonians also gave us the 360-degree circle, dividing the full rotation of the sky into 360 parts that matched their approximate year length. This choice reinforced the sexagesimal system at every level: 6 × 60 = 360. The circle, the year, the sky, the count, all of it interlocked around the number 60.
The Egyptians: The First People to Divide the Day Into Hours
While the Babylonians were measuring the arc of the heavens, the ancient Egyptians were doing something more immediately practical: they were dividing the day into hours. The Egyptians are recognized by scholars as the first civilization to systematically slice the daily cycle into smaller named parts. The earliest evidence of this comes from religious texts dating to around 2500 BC, but the practice became well-documented during the period archaeologists call the New Kingdom, roughly 1550 to 1069 BC.
The Egyptian approach to dividing the day was rooted in observation of the night sky. Each night, Egyptian astronomers, who were also priests, watched the rising of particular stars called decans. There were 36 of these stars in total, selected because each one disappeared below the horizon for approximately 70 days each year, reappearing at sunrise after that absence (the same behavior as the star Sirius, which was sacred to the Egyptians). Over the course of a year, one new decan star would rise every ten days, matching the Egyptian civil calendar, which was divided into 36 ten-day weeks of three per month. By 2150 BC, the Egyptians were charting 12 of these decan stars per night on the lids of coffins, the so-called diagonal star clocks or coffin lid star charts found in tombs from around 2100 to 1800 BC. Each star governed one portion of the night, and there were exactly 12 of them. The night had 12 parts.
This choice of 12 was not obviously tied to Babylonian sexagesimal thinking. Instead, it seems to have arisen from Egypt's own astronomical patterns, the 12 constellations of their zodiac, the 12 months of the year, and the 12 decan stars visible in a given night. The Egyptians then divided the daytime into a matching 12 parts, giving a total of 24 divisions of the day. These were not equal hours as we know them today. They were temporal hours or seasonal hours, longer in summer when daylight lasted longer and shorter in winter. A summer daytime hour in ancient Egypt could last nearly 80 of our minutes; a winter one might barely stretch to 45. What mattered was the structure, not the fixed length.
To measure these daytime hours, the Egyptians used a range of ingenious instruments. Shadow clocks were among the simplest: a horizontal bar raised slightly at one end so it cast a shadow along a marked scale. In the morning the raised end faced east; in the afternoon it was turned to face west. The earliest surviving example of a sundial proper, a limestone tablet found in 2013 in the Valley of the Kings, in the area where workmen building royal tombs once lived, dates to around 1500 BC and is divided into twelve sections to mark the working hours of the day. Water clocks (called clepsydrae) were also used, especially at night; the oldest surviving example was found in the tomb of the pharaoh Amenhotep III, dating to around 1350 BC. The Egyptians also used an instrument called a merkhet, a sighting tool made from a palm rib that, when used in pairs, could align with stars to determine the time at night.
There is no evidence that the Egyptians used minutes or seconds. For them, the hour was the smallest named unit of time in common use. But by establishing the 24-hour framework, they made the most important structural decision in the history of timekeeping, the one that every civilization after them would inherit.
The Greeks: Giving Precision to Time
The Egyptians built the frame. The Babylonians filled it with numbers. But it was the ancient Greeks who synthesized everything into the system we actually use today, complete with hours, minutes, and seconds of fixed, equal length.
The first step was taken by the astronomer Hipparchus of Nicaea (c. 190–120 BC), widely regarded as the greatest astronomer of antiquity. Hipparchus, born in Bithynia (now northern Turkey), spent much of his career on Rhodes, where his observations were unmatched for over a thousand years. He is credited with discovering the precession of the equinoxes, the slow wobble of Earth's axis that causes the position of stars to shift over thousands of years, and he calculated the length of the tropical year to within six and a half minutes, a remarkable achievement using nothing but careful observation and geometry.
Hipparchus was deeply familiar with Babylonian astronomical methods, and he systematically incorporated them into Greek science. He adopted the Babylonian practice of dividing a circle into 360 degrees, then dividing each degree into 60 arc minutes, and he was the first Greek astronomer known to use this full system consistently. More significantly for our purposes, Hipparchus proposed that the hours of the day should be of equal, fixed length, what he called equinoctial hours, based on the length of day and night at the equinoxes, when they are equal. Up to this point, seasonal hours that varied with the sun had been the norm across the ancient world. Equal hours were a radical idea. They required you to treat time as an abstract mathematical quantity rather than a reflection of the natural world. Most people ignored the concept for centuries, preferring the intuitive seasonal hours they'd always used. But for astronomers doing precise calculations, equal hours were essential.
The next crucial step came from the astronomer and geographer Claudius Ptolemy (c. 100–170 AD), who synthesised virtually all Greek astronomical knowledge into a monumental work known as the Almagest. Ptolemy took Hipparchus's framework and extended it. He divided each of the 360 degrees of the celestial sphere into 60 equal parts. He called these partes minutae primae, "first small parts", which is where we get the word minute. He then divided each of those into 60 still smaller parts, calling them partes minutae secundae , "second small parts", which gives us the word second. The terminology and the structure were directly inherited from the Babylonian sexagesimal system, which had by then passed through centuries of Greek astronomical use and been thoroughly naturalized into the fabric of scientific thinking.
The Greeks also made important practical contributions to timekeeping as an observable, public activity. The Tower of the Winds in Athens, built around 150 BC and still standing today, was one of the ancient world's most sophisticated timekeeping monuments. This octagonal marble tower featured nine sundials on its exterior walls, facing different compass points, so the time could be read from any direction. Inside it housed a water clock fed by a spring on the Acropolis. The tower was built in the Agora, the marketplace at the heart of Athenian public life, meaning that ordinary citizens could know the time simply by looking up.
Another remarkable Greek contribution was the Antikythera Mechanism, a bronze device recovered from a first-century BC shipwreck near the Greek island of Antikythera and now housed in the National Archaeological Museum in Athens. This device used a sophisticated system of interlocking gears to track the positions of the sun, moon, and planets, predict eclipses, and calculate astronomical cycles. It is the oldest known analog computer, and it drew directly on both Babylonian astronomical data and Greek mathematical models, a perfect symbol of the synthesis that the Greeks achieved.
It is worth pausing to note that Hipparchus himself calculated the mean lunar month at 29 days, 12 hours, 44 minutes, and 2.5 seconds, a figure that differs from the modern accepted value by less than one second. Think about that. Using naked-eye observations, geometry, and a number system borrowed from ancient Mesopotamia, a Greek astronomer working more than 2,000 years ago measured a month to within a heartbeat of what atomic clocks confirm today.
The Legacy: Five Thousand Years in Every Clock
By the time the Roman Empire adopted Greek astronomical standards, and by the time those standards were preserved and extended by Islamic scholars through the medieval period, the structure of time was essentially fixed. Hours are divided into 60 minutes, and minutes are divided into 60 seconds. The invention of mechanical clocks in 14th-century Europe made equal hours the practical standard for everyone, not just astronomers, and the industrial revolution made precision timekeeping a matter of economic necessity.
Today, when a phone tells you it is 3:27:14 PM, you are reading a number in the Sumerian sexagesimal system. The 27 minutes are the direct descendant of the Babylonian ush and the Greek partes minutae primae. The 14 seconds echo Ptolemy's partes minutae secundae. The 3 hours come from the Egyptian priests who watched decan stars rise over the Nile. The entire structure is 5,000 years old.
The reason 60 survived while most other ancient mathematical conventions vanished is ultimately the same reason the Sumerians chose it in the first place: it works extraordinarily well. A number with 12 divisors is simply more useful than any alternative for a world of fractions, shares, and subdivisions. One hour divides cleanly into halves (30 minutes), thirds (20 minutes), quarters (15 minutes), fifths (12 minutes), sixths (10 minutes), and many more. Try the same exercise with 100 minutes in an hour, and a third becomes 33.333..., an infinitely repeating decimal that would have made ancient scribes furious and would make your daily schedule a mathematical nightmare.
So the next time you set an alarm, calculate a journey, or ask a colleague how long something will take, remember that you are thinking in the arithmetic of Sumerian farmers, Babylonian stargazers, Egyptian priests, and Greek mathematicians. The humble hour, with its 60 minutes, is one of humanity's most durable and elegantly practical inventions, and it was never really about minutes at all. It was about a number so perfectly divisible that every civilization that encountered it decided, independently, to keep it.