If you want to measure time more specifically than just “daytime” and “morning” and “around noon,” you need something to measure it with.
This is easier to do at night. From prehistoric times, humans have paid attention to the stars, and their rising and setting gives you a way to mark the passage of time. Evidence in Egyptian tombs suggests that the twelve divisions of night were based on certain stars or constellations. But of course this comes with the complication of seasonal changes, and also you can’t export your system to a different latitude, because they’ll be seeing a different slice of the universe.
A similar difficulty applies to sundials, our next-oldest method of timekeeping. In the daytime you don’t have as visible a set of markers — just the one big, incredibly bright one — but once you have the concept of dividing time into units, you can build a sundial, and use the apparent motion of the sun and the shadow cast by the gnomon across the demarcated lines to tell you what time it is.
But designing a good sundial isn’t a simple matter of drawing some lines on a disc and sticking a wedge in it to cast a shadow. I’m not going to attempt to describe the various factors you have to take into account (like orientation relative to the celestial poles) and all the different designs you can build, because quite frankly I don’t know my astronomy or mathematics well enough to really parse everything on this page. Suffice it to say, if you want your sundial to not only be accurate but remain so throughout the year, you need to do more than just put it on a plinth in your backyard.
There’s a reason timekeeping was often considered sacred in antiquity. Doing it right requires quite a lot of education, and such education was often more available to the priesthood than anyone else. You have to be a close observer of celestial phenomena — to which nearly every human culture has ascribed spiritual significance — and to the layperson, your pronouncements are every bit as mysterious as divination or other sacred rites.
After you come up with your units of measurement based on the stars or the sun, you can start measuring elapsed time with other devices. Water clocks are extremely ancient; they mark the passage of time via the flow of water. In the most basic design, the water is flowing either into or out of a container with lines that measure whatever unit is desired. They don’t have to be basic, though; more advanced designs include gearing and automata, essentially using the water to drive a whole mechanism rather than simply rise and fall to the lines.
Hourglasses come a great deal later. They require good enough glassmaking to create the bulbs and the narrow neck between them, but unlike water clocks, they don’t suffer inaccuracy from things like evaporation or changes in temperature (which alter the speed at which water flows). In fact, you can take them on board ships — which made them the timekeeping device par excellence for seagoing vessels for many centuries. Other options include candles with marked lines, oil-lamp clocks, and incense clocks, some of which dropped weights as they burned to make noise and signal the intervals.
Clocks as we think of the term now have been around for a long time, if you count things like water-powered devices. But the invention of the verge escapement in thirteenth-century Europe allowed for a transition to wholly mechanical clocks, using oscillating elements like balance wheels or pendulums to drive the process.
Building something like that takes an even higher degree of education and skill, not to mention the technological sophistication necessary to craft precise and durable gearing. And once people had designed mechanical clocks, they began working on two improvements: first, making them more precise, and second, making them smaller — all the way from tower clocks down to pocket watches.
This gets especially interesting when you look at the role of timekeeping in navigation. Calculating one’s latitude is easy if you understand astronomy, but longitude is a whole other kettle of fish . . . so much so that governments offered monetary rewards to anyone who could improve the technology. Pendulum clocks were much more accurate than the mechanisms of pocket watches, but they’re miserably bad at sea; the motion of the ship disrupts the swing of the weight. Credit for solving the issue usually goes to John Harrison — who essentially brought what people thought was a knife to a gun fight, designing a pocket watch that was accurate enough for navigation, and far more stable than a pendulum at sea.
Those older technologies are still around today, all the way from pendulums to sundials, but we haven’t stopped inventing new ones. Quartz clocks use the oscillation of a crystal to achieve much more precise results than a simple mechanical design, such that “quartz” became an advertising point in the selling of wristwatches; then you got digital watches, which are being superseded by more powerful fitness trackers and smart watches these days (or just people’s cell phones).
At this point our most accurate form of timekeeping is the atomic clock, which is accurate to a few seconds over the course of millennia. These aren’t commercially available, and probably never will be, but they don’t have to be: we can transmit information from them to other devices, setting our own clocks and watches and phones against atomic time in the same way people used to check sundials to calibrate their water clocks. There’s some slippage, but for normal purposes that more than suffices.
So far I’ve been talking about concerns of technology and accuracy, but I would be remiss if I didn’t also mention aesthetics. Going all the way back to sundials and water clocks, people have worked to make them beautiful as well as functional. Given the expense of a good mechanical clock, it isn’t surprising that individuals who could afford one in the first place often went the extra mile to decorate it with precious metals and jewels, or put it in a case of exotic wood, or cover it in carving and filigree. Whether it was a businessman’s gold Rolex, a Victorian lady’s pocket watch, the pendulum clock in a Renaissance family’s home, or the enormous public clock in the town square, such things have been status symbols and aesthetic objects since the very beginning.
The advent of cell phones has put a big dent in that, as many people give up wristwatches entirely. But it’s making a small comeback as the makers of fitness trackers and smart watches start offering bands and cases that weren’t designed with the gym in mind; we may see more of that going forward.