11 Pivotal Moments In The History Of Timekeeping

The US release of my new book, A Brief History of Timekeeping, is tomorrow (Tuesday, Jan 25), and in honor of that, I thought it would be worth a post giving quick summaries of some of the most important developments in the history of the science and technology of measuring time. As discussed in previous posts here, this is a long and deep history, stretching back thousands of years, so let’s start at the beginning:

1) The Invention of Solstice Markers, more than 5,200 years ago: While we often talk about consciousness of time as a modern problem, the actual history of timekeeping is remarkably deep, stretching well back before the invention of language. The oldest functioning clock we have rely on the slow motion of the rising and setting sun along the horizon, tracked by monumental structures aligned to highlight the position corresponding to a particular date, usually one of the solstices. The most famous of these is probably Stonehenge in the UK, but I’m more fond of the passage tomb at Newgrange (near Dublin, Ireland), which consists of an artificial hill with a vaulted central chamber reached by a narrow 20-meter passage aligned so that the rising sun on the December solstice (the shortest day of the Northern Hemisphere year) casts a narrow ray all the way to the center. This was constructed around 3200 BCE, and still functions today— you can enter a lottery to win one of a handful of spots in the central chamber to see the sunrise (no refunds in the event of cloudy weather, alas).

2) The Invention of Constant-Flow Water Clocks, more than 3,500 years ago: Incredibly, we can actually put a name to a person who claims credit for this: an inscription in the tomb of an Egyptian court official named Amenemhet from around 1500 BCE brags that he had presented the pharaoh Amenhotep I with a water clock that could track the hours of the night accurately through all the seasons of the year. We don’t have this specific clock, but we have a good idea what it was, thanks to the “Karnak Clepsydra” from a couple centuries later: a tapered vessel shaped something like a modern flowerpot, with a hole at the bottom and twelve columns of lines on the interior to mark the water level corresponding to different hours for each of the months of the Egyptian civil calendar. Modern studies suggest that the taper corrects for the tendency of the outflow to slow as the water level drops, and this combined with the position of the lines suggests that it could be accurate to within around 15 minutes, which is remarkably good for such a simple device, well worth a mention in your epitaph.

3) The Introduction of “Leap Years,” 8 BCE: We don’t necessarily think of calendars and clocks as being the same thing, but they are: they’re both tools for tracking the passage of time by counting ticks, and predicting the repetition of certain natural cycles. In the case of a clock, the “tick” is the motion of a pendulum or a quartz crystal, and the repeating cycle is the rising and setting of the sun for each new day; in the case of a calendar, the “tick” is a day, and the repeating cycle is the change of seasons for each new year. A tropical year isn’t an integer number of days, though, so we need to do something clever to keep track, and the introduction of the Julian calendar (completed in 8BCE by the Emperor Augustus) provides an elegantly simple method of aligning fixed-length months with the seasons, adding one day to February every four years. Averaged over a full cycle, this comes within 11 minutes of the true length of a tropical year; this did need to be corrected by the Gregorian reform in 1582 (as previously discussed here and here), but is remarkably good for its time.

4) The Tower Clock of Su Song, 1094 CE: This is a bit of a cheat, since it’s not actually a step in a continuing process (as nothing that follows was directly built on its principles), but it was a sufficiently remarkable achievement that I have to give it a mention. Su Song was an official in the court of the Northern Song dynasty in Kaifeng who, after narrowly avoiding a diplomatic incident caused by a calendrical error, built a monumental public clock powered by a constant-flow water source turning a massive wheel that turned a giant celestial globe and an armillary sphere in time with the stars. Su Song’s clock also had elaborate public-facing displays to show the time to the citizens of Kaifeng, and ring bells and beat drums to announce the hours. It was only in operation for around 30 years before the Northern Song collapsed, but as we can see from modern reconstructions, it was an extremely impressive device in its day.

5) The Invention of Mechanical Clocks, 1200 CE (give or take): The exact origin of mechanical clocks is a little obscure, but clocks based on a “verge and foliot” system, marking time by counting the swings of a weight twisting back and forth, powered by falling weights, spread across Europe during the 1500s. The early examples weren’t terribly accurate— they didn’t have minute hands for the first few centuries— but offered significant advantages over both sundials (which don’t work when it’s cloudy) and water clocks (which can freeze up in the winter). This is the point where the “tick-tock” sound of gear teeth colliding with each other enters the world of timekeeping.

6) Kepler’s Laws of Planetary Motion, 1609: This might not seem like it fits with the others, but Johannes Kepler’s introduction of his Laws of Planetary Motion, based on the data collected by Tycho Brahe over the preceding decades, allowed the rigorous prediction of the motion of the planets, the moons of Jupiter, and eventually the Moon, all of which were to prove incredibly useful for the determination and tracking of time in years to come. His work also helped inspire the work of Newton and other natural philosophers that set physics on the path to the modern ultra-precise science that it has become.

7) The Invention of Pendulum Clocks, 1657: Using a pendulum to make a more reliable version of a mechanical clock was first proposed by Galileo Galilei in the 1630s, but he was more or less completely blind by then, so never built a working model. The first working pendulum clock was made by Christiaan Huygens and Salomon Coster in the Netherlands in 1657 (Robert Hooke in England had some priority disputes with Huygens over early clocks, because that’s what Robert Hooke did, but historians agree Huygens and Coster were first). Within a decade, pendulum-based clocks accurate to seconds per day, and became an indispensable tool for astronomy.

8) The Longitude Problem, 1700s: As European empires expanded across the globe, navigation became a critically important problem, one with a close connection to timekeeping. Knowing your longitude requires knowing the difference between the time (as measured by the position of the sun) at two different places on the rotating Earth, but keeping accurate time on board a ship in the age of sail was a formidable challenge. This eventually prompted a number of offers of royal prizes, most famously the UK’s Longitude Prize in 1714, which in turn inspired scientists and engineers to work toward two solutions: the more celebrated in modern times is the marine chronometer invented by John Harrison, but the Method of Lunar Distances made possible by the measurements of Tobias Mayer and turned into the Nautical Almanac by the astronomer Nevil Maskelyne was just as important at the time.

9) The Invention of Quartz Clocks, 1927: These days, we take high-quality timekeeping for granted: you can walk into a supermarket in the US and buy a clock for a few dollars that will keep time better than the best mechanical watch produced by John Harrison and his contemporaries. These are powered by the exceptionally regular motion of vibrating crystals of quartz, which was first used to power a clock in 1927 by Warren Marrison and Joseph Horton. These became commercial products over the next few decades, with the first quartz watches arriving in the late 1960s, and are now essentially ubiquitous. Almost any electrically powered clock you can find nowadays has a quartz oscillator inside, shaking back and forth at exactly 32,768 oscillations per second.

10) The Invention of Cesium Clocks, 1955: The modern definition of the second, central to the SI system of units, is 9,192,631,770 oscillations of the light produced when cesium-133 atoms move between two particular energy states. This frequency is determined by the laws of quantum mechanics, discovered in the 1920s, and makes every cesium atom in the universe a potential reference for a clock based on light, with no physical moving parts. The first cesium atomic clock was made by Louis Essen and Jack Parry in the UK’s National Physical Laboratory in 1955, and there are now hundreds of cesium clocks in operation in standard labs all around the world, and orbiting the Earth as part of the Global Positioning System. The best of these are so accurate they would need to be run for something like a billion years before they’d drift off by one full second.

11) The Optical Frequency Comb, 1990s: Incredibly, those one-second-in-a-billion-years cesium clocks aren’t the best clocks in the world today— not even close. There are experimental clocks in development that are a hundred times more accurate than the best cesium clock, based on the oscillation frequency of light in the visible or even ultraviolet range. The key enabling technology for this is the optical frequency comb, which was recognized with half of the 2005 Nobel Prize in Physics for John Hall and Theodor W. Hänsch. These combs use ultrafast lasers to make a system allowing the determination and comparison of frequencies at the 18-decimal-place kind of level. They’re not used for time standards yet, but if cesium is ever supplanted by another element as the definition of the second, you can bet that a frequency comb system will be at the heart of the clock.

And there you have it, a timeline of timekeeping. All of these topics, plus more are discussed in much more detail in A Brief History of Timekeeping, available tomorrow wherever books are sold.