On the Concept of Time

How the keeping of time was developed throughout history and how to define it


By Steve Humphrey
Illustration by Andrea Hutchinson


Last winter, I taught a course at the University of California, Santa Barbara, on the philosophy and physics of time; an extremely popular class which I have taught many times. I thought I would share some of the complexities inherent in discussions of time, one of the most perplexing issues in both philosophy and physics. There are numerous books and articles written over the last several decades that explore both our concept and experience of time, and the way time functions in contemporary physics. No clear consensus has been reached.

St. Augustine (354 AD – 430 AD) said “What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.” This sums up the position of most of us. We think we know all about time, and we use temporal language, look at clocks, worry about having too little, or too much, but we are hard-pressed to describe exactly what it is we think we know so much about. But before we get into the mystery, let us do some history.

It is not clear when people first started thinking about time, but it must have been when they noticed the cyclic nature of their experiences. The sun comes up, the sun goes down. Days pass, seasons change, the phases of the moon go through regular cycles, the patterns of the years repeat themselves. Early humans might have talked in terms of suns and moons, as in “We will meet when three suns have risen,” or “I remember what happened two moons ago.” As people became more sophisticated and began studying the heavens, they noticed the cyclical nature of the rotation of the constellations. The first calendars were developed over three thousand years ago, incorporating astronomical observations of regular phenomena. Many civilizations developed calendars, and most had to confront a particular problem, which is that a year does not contain exactly twelve full moons, nor does it contain exactly 365 days. After several centuries, the New Year would fall in the middle of summer, rather than in the winter. This was fixed by adopting the Julian calendar, which added a day every four years, and further modified by the Gregorian calendar, which adds a day every four hundred years. For example, 1700, 1800 and 1900 were not leap years on the Gregorian calendar, while 2000 was.

So, the first attempts at timekeeping involved dividing up the year into regular intervals. But what about the day? How can we know “what time it is?” The first clocks were probably sundials, where the shadow of the gnomon falls on a number on a plate. It was obvious that the sun was visible for a longer period on some days than others. The earliest clocks were water and sand clocks, which involved a container filled with water or sand emptying out a small hole. In the Roman courts, lawyers were limited in the time they could use to plead a case by the amount of water leaking out of a bucket. Records show lawyers pleading for “more water.” Notice, here, exactly what is being measured. It isn’t time, but changes in the volume of water or sand, or the movement of the sun.

In the late 16th century, Galileo Galilei noticed the swaying of a suspended lamp in the Cathedral in Pisa, and, using his pulse, determined that the period of the swinging lamp was constant, i.e., a pendulum is isochronous, meaning each period is of the same length of time. Ironically, later, doctors began using pendulum clocks to measure the pulses of their patients. He went on to invent the first pendulum clock, and soon every town in Europe had a clock tower containing a pendulum clock. But before long, civic leaders and government officials had to confront two problems involving timekeeping. The first was the problem of synchronizing different clocks to make sense of train schedules. If the clock in every town was set according to when the sun was directly overhead at noon, then since towns could be far apart, there would be disagreement about exactly when noon occurred. This creates havoc with train schedules. So, a particular clock, located in Greenwich, England, was chosen as displaying the “correct” time, and all other clocks were synchronized with it. In towns far east or west of Greenwich, the sun would not be directly overhead at noon. This is the origin of time zones.

The second puzzle had to do with navigation at sea. Finding one’s latitude in a featureless sea is fairly easy, and early charts showed safe routes across the oceans that stuck to one or another latitude. But how to determine how far east or west a ship has sailed? The British Parliament offered a cash prize of £20,000, worth about $3.17 million in today’s dollars, to anyone who could solve the so-called “problem of longitude.” Many astronomers of the day were convinced that the solution lied in the behavior of various celestial objects, including the moons of Jupiter. But one man, John Harrison, realized that if one had an accurate timepiece aboard ship, one could compare noon at sea with noon as measured in Greenwich, and calculate just how far west one had come. A huge problem arose: a pendulum clock would not work, because of the roughness of the sea. The pendulum would be bounced around to such a degree that accuracy would be lost. Harrison’s difficulty was in producing a clock that was not affected by variations in temperature, pressure or humidity, and remained accurate over long time intervals, resisted corrosion in salt air and was able to function on board a constantly-moving ship. After several prototypes, he was finally successful. His early “chronometers” are on display in the Naval Museum in Greenwich, England. I have visited this exhibit many times, and today there is still a large ball that descends a flagpole on the roof of the Naval Museum at precisely 11 a.m., which serves as a signal to ships in the Thames to synchronize their chronometers. To learn more about this fascinating period of history, one should read Dava Sobel’s “Longitude.”

In next month’s issue, I will introduce some of the puzzles and conundrums lurking in our conception of time. We will see that it is not what it appears to be.

Steve Humphrey has a Ph.D in the history and philosophy of science, with a specialty in philosophy of physics. He teaches courses in these subjects at the University of California, Santa Barbara, and has taught them at the University of Louisville.

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