Origins: Inventing Time

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This section, Origins, is devoted to the archaeology of daily life, those small, unassuming vestiges of the remote past that have quietly insinuated themselves into our everyday lives.

If someone tells you, for example, that the Galápagos Islands, in the Pacific Ocean far off the coast of Ecuador, lie directly south of Chicago, you would probably consult one of the commonest objects of daily life, a map. The science of mapmaking, indeed, has its origins in the ancient world; the Spring issue of Archaeology Odyssey tells how the second-century A.D. geographer Ptolemy of Alexandria plotted sites on a map by using the coordinates longitude and latitude.

Now if you learned that Rio de Janeiro, Brazil, unexpectedly, is a couple of time zones ahead of New York, you might start thinking about time—and you might wonder about one of its basic units, the hour. Why are there 24 hours in a day, rather than 10—given that we use a decimal system? Stranger yet, why 60 minutes, and why 60 seconds?

It is perfectly possible to divide the day into 10 hours (or 100, or any number that is convenient) and the hour into 10 minutes (or 100). Over the millennia and around the globe, time has indeed been parceled out in different ways. As late as the fourth century A.D., the Romans divided daylight hours into only two parts—before midday (ante meridiem, or a.m.) and after midday (post meridiem, or p.m.)—though they later added designations for early morning, forenoon, afternoon and evening. Until the 19th century, the Japanese counted only six daylight “hours.”

Our stern taskmistress Nature has determined some of the ways in which we reckon time: The year is governed by the earth’s revolution around the sun (some calendars, like the Jewish calendar, rely on lunar cycles, but these calendars generally have mechanisms that reorient them to the solar year) and the day by the earth’s rotation. But Nature is silent concerning the hour. The 24-hour day and 60-minute hour are mere conventions, human inventions, quirks of history. This messy system for counting time, ultimately, should be blamed on the ancient Babylonians.

As early as 1300 B.C., the Egyptians were dividing the day into two parts: 12 equal daylight hours (sunrise to sunset) and 12 equal nighttime hours (sunset to sunrise). The Egyptians had apparently borrowed this system, along with much of their mathematics, from the Mesopotamians—pushing the origins of the hour even further back in time.

Why the number 12? The ancient Babylonians counted in units of 60 (whereas we count by tens); this is called a sexagesimal number system. In The Discoverers (Random House, 1983), Daniel Boorstin suggests that the Egyptians (or the Babylonians) may have divided the perfect number 60 by the number of 007then-known planets—Mercury, Venus, Mars, Jupiter and Saturn—producing the number 12. In any event, 12 was convenient as a factor of 60.

But this is not the hour we know, with its 24 regular 60-minute intervals pulsing around the clock. Rather, it is a variable, or seasonal, hour: Each of the 12 daylight hours is longer in the summer (when days are long) than in the winter; the duration of the seasonal hour also changes as one moves north or south. Daylight and nighttime hours are equal in length only twice a year, on the vernal and autumnal equinoxes. This variable division of the day suited the timekeeping tools available to the ancients: the sundial and gnomon (the Greek name for the vertical pole that casts a shadow on a dial), which pointed out hours not in regular intervals but according to the time of year and geographical location. Sundials are attested in late-second-millennium B.C. Egypt; literary evidence suggests that they were used during the early second millennium B.C. in Mesopotamia.

The creation of the standardized hour, what scholars call the equinoctial hour, came later—perhaps as a result of a new kind of technology, the water clock (called by the Greeks a clepsydra, literally meaning “water thief”). Water clocks measure time in absolute, rather than variable, intervals; they were used, for instance, to keep orators from speaking too long in Athenian law courts.

Exactly when the day was divided into 24 equal hours remains under debate: It may have coincided with the creation of Hellenistic astronomical schools, or it may have come much later, around the 13th century A.D. Whether a Greek or medieval invention, it was based on the 24-hour day used by ancient Egyptians and Babylonians.

Perhaps in the 13th century, with the invention of accurate mechanical clocks, the hour was subdivided into minutes (from the Latin pars minuta prima, or “first small part”) and seconds (partes minutae secundae).

Although this development moves us out of the ancient world, we should note that it, too, was a tribute to the old Babylonian sexagesimal system. Medieval chronologists divided the hour into 60 minutes and 60 seconds, following Ptolemy’s division of each degree of a circle into 60 minutes, with each minute containing 60 seconds. (The convention of dividing the circle into 360 degrees, still in use today, was probably invented by the ancient Egyptians, who again made use of the Babylonian sexagesimal system. The small circle we use to designate degrees, as in the 360° of a circle, is probably an Egyptian hieroglyphic, perhaps representing the sun, meaning “day.”)

Whether you knew it or not, as the second hand of your watch moves steadily around the minute, and as the minute hand moves around the hour, you are counting as the Babylonians did, by 60s.

This section, Origins, is devoted to the archaeology of daily life, those small, unassuming vestiges of the remote past that have quietly insinuated themselves into our everyday lives.