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A Brief History of Timekeeping

A Brief History of Timekeeping PDF

Author: Chad Orzel

Publisher: BenBella Books


Publish Date: January 25, 2022

ISBN-10: 1953295606

Pages: 336

File Type: PDF

Language: English

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Book Preface

The picturesque campus of Union College in Schenectady, New York,where I teach, features lush, green quads surrounded by columned buildings full of classrooms, labs, and offices. Like most college campuses, it’s also full of clocks. There’s the tower clock in Memorial Chapel, whose chimes regularly ring out to mark the hours (and showcase the talents of the student musicians who occasionally take over to play tunes on the chimes). A large decorative clock, a gift of the class of 1997, stands in front of the Reamer Campus Center, where it serves as a landmark for meeting people. Almost every classroom sports an analog clock on the wall, for students and faculty to track the progress of class hours (too fast for the faculty, too slow for the students). And, of course, there are innumerable unseen clocks: embedded in every computer, worn as wristwatches, carried in smartphones.

Even beyond the physical presence of clocks, the tracking of time is ubiquitous on campus. Classes are scheduled down to the minute—start at 9:15 AM, end at 10:20 AM, start again at 10:30 AM in a different room—and meetings and appointments fill the day. Student work is often timed—10 minutes allotted for an in-class presentation, an hour to complete an exam—and athletic feats are recorded down to fractions of a second. On a slower scale, the academic year progresses through a stately cycle of marked dates: the opening convocation, the start and end of the academic term, the annual Steinmetz Symposium celebrating student research. Every day brings students and faculty closer to the commencement ceremony that ends the year and sends a new class of graduates off into the world.

All of this tracking and measuring of time is generally regarded as too routine to be commented upon. What’s too often overlooked in the process of keeping on schedule (and fretting about being overscheduled) is the depth of history and rich variety of science underlying all this activity. We tend to think of our preoccupation with time as a modern phenomenon, but in fact the tracking of time has been a major concern in essentially every era and location that we find evidence of human activity. Every historical society we know of had its own ways of tracking the passage of time, some of them dizzyingly complicated. Some of the most ancient architectural monuments we know of are calendar markers.

A Brief History of Timekeeping is a virtual journey through the science that grew up alongside centuries of human efforts to measure the passage of time. Starting with the oldest known solstice-marking monuments, we will explore the astronomy of the solar system and the features that determine the sun’s path across the sky. Efforts to better understand the motion of the sun and planets led to the development of Newtonian physics, and we’ll see how this enables the technology to build mechanical clocks. And we’ll discuss revolutionary developments in the physics of electromagnetism and quantum mechanics that eventually led to the replacement of standards of timekeeping based on the motion of physical objects (like the pendulum of a mechanical clock) by modern atomic clocks that count the oscillations of light.

The history of timekeeping is not only a story of abstract science and technology: it also includes intriguing elements of politics and philosophy. We’ll look at the social aspects of calendar systems like the intricate mix of cycles in the Mayan calendar at their peak around 500 CE, the theological considerations that led Europe to adopt the Gregorian calendar nearly a thousand years later, and the political negotiations that produced our modern system of time zones. And we’ll see how practical issues involved in these processes raised philosophical questions that in turn reshaped our understanding of our place in the universe as well as the nature of space and time.

The tracking of time is, in a deep way, a signature preoccupation not just of modern society, but of human civilization in general. The process of building and refining timekeeping devices has been one of the great drivers of progress in science and technology for millennia: from Neolithic solstice markers through mechanical watches to ultra-precise laser frequency standards, we are and always have been a species that builds clocks.


As you read through this book, you will encounter occasional sections highlighted, as this one is, by shaded bars along the sides of the page. These sections go a little deeper into the scientific principles underlying particular methods of timekeeping and have less historical content.

I’ve made every effort to make all the sections of the book approachable and engaging for as broad an audience as possible—you won’t find extensive math or undefined jargon—but recognize that individual readers may find particular topics more or less congenial. While I hope you’ll read every word on every page, the shading is a convenient way to mark sections that are slightly more technical. Those who are primarily interested in the scientific content may want to seek out the highlighted sections; those who are primarily interested in the historical aspects may want to pass over them a bit more lightly. (There are also some shorter boxes, which house topics or anecdotes that fall to the side of the book’s primary thrust but were too good not to share.) Of course, a statement that sweeping depends on a particular definition of terms. In this case, we need a definition of “clock” that encompasses all these devices over a span of several thousand years. So, what is the common feature shared by solstice markers, mechanical watches, and laser spectroscopy? At the most basic level, a clock is a thing that ticks.

The “tick” here can be the audible physical tick we associate with a mechanical clock like the one in Union’s Memorial Chapel, caused by collisions between gear teeth as a heavy pendulum swings back and forth. It can also be a more subtle physical effect, like the alternating voltage that provides the time signal for the electronic wall clocks in our classrooms. It can be exceedingly fast, like the nine-billion-times-a-second oscillations of the microwaves used in the atomic clocks that provide the time signals transmitted to smartphones via the internet, or ponderously slow like the changing position of the rising sun on the horizon.

In every one of these clocks, though, there is a tick: a regular, repeated action that can be counted to mark the passage of time. When the microwave field in an atomic clock completes an oscillation from up to down and back, we know that 1/9,192,631,770th of a second has passed. When the shadow cast by a building on campus sweeps across the quad and then returns to pointing due north, we know that one full day has passed. When the rising sun completes its seasonal cycle and returns to its northernmost position on the horizon, we know that one full year has passed. Each of these events is a metaphorical tick, and we quantify the passage of time by counting the number of these ticks that happen between two events of interest.

It follows that one of the core themes in the history of timekeeping is the process of identifying and understanding these ticks. To make a high-quality clock, we must first identify a process that might serve as a tick, and then understand all the factors that change its rate and how those factors affect the ultimate accuracy of that clock. The drive to understand the processes by which various kinds of clocks tick has led to revolutionary discoveries in fundamental science, particularly in physics and astrophysics (as we’ll see in Chapters 6 and 8).

In order to time events by counting ticks, a practical clock must also have some sort of readout, a means by which humans can keep track of the count. While the audible ticks of a mechanical clock can, in principle, be counted directly, many of the physical systems used as clocks work at speeds that are impractical for human observers; the quartz crystal in my wristwatch oscillates much too rapidly to be counted directly, so its motion must be recorded electronically and slowed down to move the second hand one mark for every 32,768 vibrations.

Many of the signature features we associate with practical clocks are these readout devices: the hands and numbers on a watch, or the gnomon and markings of a sundial tracking the slow motion of a shadow through the day. The history of timekeeping is thus also a history of innovation in readout technologies, finding clever ways to display the passage of time in human-readable ways that minimize sources of error. One of the most dramatic examples is the competition between methods for finding longitude at sea (described in Chapters 9 and 10), a contest decided in part on the ability of ordinary sailors to use them.

Another problem plaguing the makers of clocks through the centuries is that many excellent standards are impractical to transport. A solstice marker is fundamentally bound to a particular point in space, recording the position of the sun as seen from a particular location; if you shift the marker to another position, its references no longer apply. At the opposite technological extreme, a precision atomic clock requires a significant infrastructure for the preparation and interrogation of atoms, keeping the very best atomic clocks confined to laboratories.

Many of the practical clocks we surround ourselves with are thus actually models of clocks, a way of keeping track of time that approximates the time recorded by a better standard tick that is inconveniently immobile. The clock on the wall of my classroom displays only an approximate time based on the oscillation of the electric current supplied from the power grid; the official time is determined by an assortment of atomic clocks in national standards laboratories scattered around the globe and overseen by an international governing body (we’ll discuss the origin of global time zones and the determination of the official time in Chapter 11).

Because most practical clocks are models of time, a third core theme in the history of timekeeping is the idea of comparing clocks to one another, and using that comparison to refine our models of time. The fundamental process is universal, employed by every civilization that has modeled time: you synchronize your model clock with some standard, allow it to run freely for many ticks, then check it against the standard again and see if they’re still in synch. It’s the same process you might use to check the performance of your watch: you set it to the correct official time (say, using the US time web page provided by the National Institute of Standards and Technology [NIST]), wear it around for a while, and then some days or weeks later check it against the official time again, and adjust as needed. Even though the quartz crystal in your watch is not connected in any direct way to the cesium atoms in atomic clocks at NIST, this process ensures that you carry with you a very good model of the actual official time.
Through the long history of timekeeping, this process has been used to produce dramatic results on a variety of timescales. As we’ll see in Chapter 12, the process of synchronizing two clocks at different locations through the exchange of telegraphic signals was central to the development of the theory of relativity, with momentous consequences for our understanding of time and space. At the other extreme of duration (as we’ll see in Chapter 3), the Gregorian calendar reform grew out of this synchronize–free-run–check process applied over the course of centuries.

The Gregorian calendar is also an important illustration of a fourth theme: the way the sheer depth of the history of timekeeping enables incredible precision, even with crude tools. Thanks to calendar records spanning thousands of years, from Roman times through the medieval period and into the Renaissance, astronomers could detect a difference between their model of time and the cycle of the seasons that amounts to just 11 minutes per year. Following the “synchronize” phase of calendar reform in 8 CE, the Julian calendar ran freely for more than 1,500 years before its final “check,” which led to the introduction of our modern calendar by Pope Gregory XIII in 1582. The Gregorian calendar will stay in synch with the seasons for another 3,000 years, despite being implemented two decades before the invention of the telescope. You don’t need atomic clocks with nanosecond precision to make highly accurate measurements of time, provided you can wait long enough.

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