// the history and science of timekeeping
Where Our Idea of Time Actually Comes From
A plain-language guide to how humans went from watching the sun to splitting a second into nine billion parts.
1. Concepts of Time
Before clocks existed, time was something people experienced rather than measured. Day followed night, seasons followed seasons, and a "moment" was defined by what was happening — a meal, a prayer, a journey — not by a number on a dial. Philosophers eventually split this everyday experience into two competing ideas that still shape how we talk about time today: time as an objective, ever-flowing river that exists whether or not anyone is watching it, and time as something we construct from the order of events, with no independent existence of its own. Nearly every later debate about time, including the ones below, is a variation on this same split.
2. Ancient Greece
Greek thinkers were the first to argue about time systematically. Aristotle defined time as the measure of change or motion — without something moving or changing, he reasoned, there would be nothing to count, and therefore no time. Plato, by contrast, described time as something that came into being alongside the ordered universe itself, tied to the regular motion of the heavens. Meanwhile, the philosopher Zeno constructed his famous paradoxes — an arrow that seemingly never reaches its target, a runner who never quite catches a tortoise — specifically to expose how strange our assumptions about time and motion could be once examined closely. These weren't idle puzzles; they forced later mathematicians to think much more carefully about what it means to divide time into infinitely small pieces, an idea that becomes essential centuries later in calculus.
3. Newton & Leibniz
By the late 1600s, the debate had moved from philosophy into physics. Isaac Newton argued for "absolute time" — a single, uniform time that ticks forward the same way everywhere in the universe, independent of anything happening within it, like an invisible master clock. Gottfried Leibniz disagreed sharply. He argued time was purely relational: it was nothing more than the order in which events happen, with no independent existence apart from those events. If nothing in the universe ever changed, Leibniz argued, the question "how much time has passed?" would be meaningless. Newton's view won out for over two centuries and became the backbone of classical physics, while Leibniz's relational objections were largely set aside — until Einstein revived a version of them, as covered next.
4. Einstein
In 1905, Albert Einstein's special theory of relativity overturned Newton's single master clock. Einstein showed that time is not the same for everyone — it runs at different rates depending on how fast an observer is moving relative to something else, an effect called time dilation. Two identical clocks, one on a fast-moving spacecraft and one sitting still, will disagree with each other once reunited; this isn't a flaw in the clocks, it's how time itself behaves. A decade later, Einstein's general theory of relativity added that gravity also affects the rate of time, which is why clocks on GPS satellites (weaker gravity, high speed) have to be continuously corrected against clocks on Earth's surface just to keep GPS accurate to the meter. Time, in other words, turned out to be closer to Leibniz's relational picture than Newton's absolute one — just far stranger than either of them imagined.
5. How We Measure Time Today
Modern civilization no longer measures time by watching the sun cross the sky — it measures time by counting the vibrations of atoms. The current international standard, the second, is defined by a fixed number of oscillations of a cesium atom under precise laboratory conditions. Atomic clocks built on this principle are accurate to within a fraction of a second over millions of years, and a global network of them (coordinated through International Atomic Time and Coordinated Universal Time, or UTC) is what every phone, computer, bank transaction, and GPS satellite quietly synchronizes against. Occasional "leap seconds" are still added to keep atomic time aligned with the Earth's slightly irregular rotation — a small, ongoing patch between the ancient astronomical definition of a day and the modern atomic one.
6. Development of the Second, Minute, and the 24-Hour Day
The 24-hour day traces back to ancient Egypt, which divided daylight into 10 hours (plus twilight hours) using shadow clocks, and divided the night into 12 hours based on star observations — the two systems were later merged into a standardized 24-hour day. The division of hours into 60 minutes, and minutes into 60 seconds, comes from Babylonian mathematics, which favored a base-60 (sexagesimal) counting system because 60 divides evenly by so many numbers (2, 3, 4, 5, 6, 10, 12, 15, 20, 30). That same base-60 structure survives today not just in clocks, but in how we divide a circle into 360 degrees. For most of history, though, "minutes" and "seconds" were mostly theoretical units — mechanical clocks accurate enough to actually display them didn't become common until the 16th and 17th centuries.
7. Early Timekeeping Devices
Long before mechanical clocks, people tracked time with whatever regular process they could observe. Sundials used the sun's moving shadow and date back thousands of years across Egypt, Babylon, and China. Water clocks (clepsydras), used by both ancient Egyptians and Greeks, measured time by the steady drip or fill rate of water, which made them useful at night or indoors when sundials were useless. Other cultures used graduated candles, knotted ropes, or incense sticks that burned at a predictable rate. The real leap came in medieval Europe with the mechanical escapement — a gear mechanism that converted falling weights into a steady, countable tick — which by the 14th century made public tower clocks possible, and eventually led to the pendulum clocks, marine chronometers, and quartz movements that shaped timekeeping right up until the atomic clock took over.