Keeping Time

Long ago, people had no clocks or watches. They told the time by looking at the sun. When the sun was high in the sky, they knew it was the middle of the day. When it was low, they knew morning or evening was near. People used shadows to help them. A tall stick pushed into the ground made a shadow that moved as the sun moved. The longer or shorter the shadow, the later or earlier in the day it was. This simple tool was called a sundial. Sundials were used in many places around the world, including Egypt, Greece and China. They were good enough for most of what people needed to do — farming, cooking and resting.

Sundials had one big problem: they only worked in sunlight. At night, or when clouds covered the sky, they were useless. So people invented other ways to track time. One of the most popular was the water clock, which the ancient Greeks called a clepsydra — a word meaning "water thief." Water dripped slowly from one container into another. Markings on the inside of the lower container showed how much water had collected and, from that, how much time had passed. Water clocks were used in courts of law to limit how long a person could speak — when the water ran out, so did your turn. This made them one of the first tools used to make sure time was shared fairly.

The mechanical clock changed everything. Invented in Europe during the thirteenth century, it used a series of interlocking gears and an escapement mechanism — a device that released the gears in controlled, regular steps — to divide time into equal, measurable units. For the first time, people could measure not just the broad movement of the sun across the sky, but the precise passing of hours, then minutes. Church towers across Europe began to display public clocks, and the rhythm of daily life began to organise itself around them. Shops opened and closed at set times. Church bells rang on the hour. The idea that a day could be divided into equal slices — and that people should arrange their lives according to those slices — was genuinely new.

Galileo is said to have discovered the principle behind the pendulum clock by watching a lamp swinging in a cathedral in Pisa, in 1582. He noticed that regardless of how wide the lamp swung, each swing seemed to take the same amount of time — a property called isochronism. It was not until 1656 that the Dutch scientist Christiaan Huygens built the first practical pendulum clock, using this principle to regulate the movement of gears with extraordinary precision. A well-made pendulum clock lost fewer than ten seconds per day — a dramatic improvement on anything that had come before. The pendulum clock became the standard of timekeeping for over two centuries, and its introduction marks the moment at which accurate time measurement became, for the first time, genuinely accessible outside of wealthy institutions.

The railway created a crisis that no clock could have anticipated. Before rail travel, each town kept its own local time — set by the position of the sun at that particular location. Bristol, for example, ran about ten minutes behind London. This was inconsequential when the fastest way to travel between them was by horse. But when trains began connecting cities at speeds of forty or fifty miles per hour, railway companies realised that operating a national timetable using dozens of different local times was both logistically impossible and potentially catastrophic. A train arriving in Bristol "on time" by London's clock might be ten minutes late — or ten minutes early — by Bristol's. The solution, adopted progressively across Britain from 1847, was to standardise all railway clocks to Greenwich Mean Time. By 1880, Greenwich Time was the legal time across Great Britain. The imposition of a single, uniform time across an entire nation was, in historical terms, a radical act.

The atomic clock, developed in the 1950s, did not merely improve on previous timekeeping — it redefined what measurement itself could mean. Where mechanical and pendulum clocks relied on the regularity of physical motion, atomic clocks exploit a property of certain atoms — most commonly caesium-133 — that transition between energy states at a frequency so consistent that it varies by less than one second in three hundred million years. The International System of Units defines the second not by the movement of celestial bodies, but by exactly 9,192,631,770 oscillations of the caesium-133 atom. This is not simply a more accurate clock. It is a fundamentally different kind of measurement — one grounded not in the observable behaviour of the cosmos, but in the intrinsic properties of matter itself. The practical implications are profound: GPS satellites, internet infrastructure and global financial systems all depend on atomic synchronisation to function.

The Global Positioning System — GPS — is, among other things, a lesson in the strangeness of time. Each of the thirty-one satellites in the GPS constellation carries an atomic clock, and the system determines a receiver's position on Earth by calculating the difference in the time it takes signals to arrive from multiple satellites simultaneously. The precision required is extraordinary: a timing error of one microsecond — one millionth of a second — translates to a positional error of approximately three hundred metres. But there is a complication that Einstein predicted and engineers are obliged to correct for: time passes at different rates depending on gravitational field strength and velocity. Satellites in orbit experience weaker gravity and greater speed than receivers on Earth; as a consequence, their clocks run faster — by approximately thirty-eight microseconds per day. Without correction, GPS would accumulate an error of roughly eleven kilometres every twenty-four hours. The mundane act of navigating to an address is, invisibly, an exercise in relativistic physics.

Beneath the history of timekeeping lies a question that measurement has never fully resolved: what is time itself? Newton conceived of it as absolute — a container within which events occur, flowing uniformly and independently of any observer or any matter. Einstein dismantled this conception, demonstrating that time is neither absolute nor uniform, but is relative to the observer's velocity and gravitational context — a dimension of spacetime, not a backdrop to it. Contemporary physics complicates the picture further. Quantum mechanics, which governs the behaviour of matter at the subatomic scale, has no directionality of time built into its fundamental equations; the laws apply equally in both temporal directions, raising the uncomfortable question of why time appears — to all macroscopic observers, without exception — to move in only one direction. The arrow of time, as physicists call it, remains one of the deepest unsolved problems in science. What our instruments measure with increasing precision is the interval between events. What that interval actually represents — what time is, in itself — remains, after millennia of inquiry, genuinely and profoundly open.