How the measurement of time has shaped history


Humans have tracked time in one way or another in every civilization we have records of, writes physicist Chad Orzel. In his new book A Brief History of Timing (BenBella Books, 2022), Orzel chronicles Neolithic efforts to predict solstices and other astronomical events, the last atomic clocks that keep time to ever more precise decimal places, and everything in between. It describes the evolution of clocks, from water clocks that timed intervals based on how long it took for water to flow from a container to hourglasses filled with sand to the first mechanical and pendulum clocks of our modern era. Each episode is filled with interesting physics and engineering, as well as insight into how different ways of keeping time have affected how people have lived their lives at various times and places in history.

American Scientist talked to Orzel about the coolest clocks in history, the most complicated calendar systems, and why we still need to improve today’s best clocks.

[An edited transcript of the interview follows.]

How did the advent of clocks change history?

There is an interesting democratization of time as it goes. The oldest monuments are things like Newgrange in Ireland. It’s this massive man-made hill with a passage in the middle. Once a year, sunlight reaches this central chamber, and that tells you it’s the winter solstice. I’ve been there, and you can fit 10-12 people in there, maybe. It’s an elite thing where only a few people have access to this information. As you start getting things like water clocks, it’s something people can use to time things. They’re not super accurate, but that makes it more accessible. Mechanical clocks make it even better, and then you get public clocks – clocks on church steeples with bells ringing the hours. Everyone is starting to have access to time. Mechanical watches begin to become reasonably accurate and reasonably inexpensive in the 1890s. They cost around a day’s wages. Suddenly everyone has access to accurate timing at all times, and that’s a really interesting change.

Is it difficult to follow the advent of different types of clocks?

When people write about clocks in history, they use the same word for a bunch of different things. There is a famous example: there was a fire in a particular monastery, and the story says that some of the brothers ran to the well, and others ran to the clock. It tells you it was a water clock because they go there to fill buckets to put out the fire. There is another reference which says that a clock was installed above the rood screen of a church. If it’s 50 feet in the air, it probably wasn’t a water clock but a mechanical clock because no one would make a device where you have to get up there and fill it with water.

What is your favorite clock in history?

I really like this Chinese tower clock. It was built around the year 1100 by a court official, Su Song. It’s a water clock, based on a constant flow of water, but it’s a mechanical device. It’s this giant wheel that has buckets at the end of its arms, and the bucket is positioned under this constant-flow water source. When it fills beyond a certain point, the bucket tilts, which releases a mechanism that allows the wheel to turn. The wheel spins and brings a new bucket that begins to fill. The regulation of the timing is really done by the water, which also provides the driving force. It is the weight of the water that turns the wheel. It’s that strange hybrid between an old school water clock and the mechanical clocks that would be developed in Europe a century or two later. It’s an incredibly complex system, a monumental thing that has worked incredibly well. But it didn’t last long – it lasted about 20 years. It was located in a capital of a particular dynasty, which fell, and the successors could not operate it.

It’s a great episode in history. The origin is that Su Song was sent to offer his winter solstice greetings to a neighboring kingdom. But the schedule was off by one day. He arrived and greeted the wrong day, which would have been very embarrassing. When he came back, the calendar makers were punished and he said, “I’m fixing this.”

How does a society’s way of keeping time reveal what it values?

Every civilization we have decent records of has its own way of keeping time. It’s very interesting because there are all these different approaches. None of the natural cycles you see are proportionate to each other. A year is not a whole number of days, and it is not a whole number of moon cycles. So you have to decide what you prioritize over what else. You have systems like the Islamic calendar, which is strictly lunar. They end with a calendar which is 12 lunar months, which is short [compared with about 365 days in a solar year]so the dates of the holidays change according to the seasons.

The Jewish calendar makes things complicated because they want to keep both: they want the holidays to be associated with the seasons, so they have to fall in the right part of the year, but they also want them to be in the right phase of the moon.

The Gregorian calendar kind of splits the difference: we have months whose length is kind of based on the moon, but we fix the months, so the solstice will always be June 20, 21 or 22. We prioritize the position of the year versus the seasons above everything else.

Then you have the Mayans doing something completely different. Their calendar implies this interval of 260 days, and no one really knows why 260 days was so important.

How accurate are clocks today?

The official time is now based on cesium: one second corresponds to 9,192,631,770 oscillations of the light emitted when cesium moves between two particular states.

I can’t believe you know it by heart!

I’ve taught this a bunch of times [laughs].

Time is defined in terms of cesium atoms, so the best clocks in the world are cesium clocks. Cesium clocks are good at one part in 1016. If it indicates a second, there are 15 zeros after the decimal point before arriving at the first uncertain digit. There are experimental clocks that are two, or even three, orders of magnitude better than this. They are not officially clocks. They measure a frequency and measure it with better accuracy than the best cesium clocks.

These are good enough for an aluminum ion clock to do a relativity test. [Researchers] held the ion fixed in each other, they were shaking back and forth, and they could see that the one moving was ticking a little slower. Then they held one in position and moved one about a foot higher, and they could see that the one at a higher altitude ticked faster. They fit perfectly with relativity.

Is there a limit to the accuracy of clocks?

There is a limit in the sense that there are a lot of things that affect accuracy. Einstein’s general theory of relative tells you that the closer you are to a large mass, the slower your clock will slow down. At some point, you are sensitive to the gravitational pull of graduate students walking in and out of the lab. At that point, it becomes impractical.

This is already a problem because atomic time for the world is a consensus of atomic clocks around the world. Here in the United States there are two major standards laboratories: one is the Naval Observatory in Washington DC, around sea level, and the other is in Boulder, Colorado, about a mile away. Their cesium clocks run at different rates because they are at different distances from the center of the Earth. They have to take that into account. So we kind of made a reckless choice locating it in Boulder.

After all you know about clocks, I have to ask you what kind of watch you wear.

I have two: one is a quartz watch, nothing special. I probably spent more on replacing the band several times than I spent on the watch. The other one I have is a mechanical watch from the 1960s. It’s an Omega watch, purely mechanical. It’s an incredibly complex watch, an engineering marvel, but I can go to a dollar store and buy a watch that can keep time just as well because quartz is so precise.


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