Measuring Time - Sundials, Clocks, Timekeeping

Sundial measuring time — A sundial tells the time of day by the position of the sun. As the sun moves across the sky, the shadow-edge aligns with different hour lines.

Modern life depends on accurate time. People can now count on GPS satellites to know the precise time, but until about 150 years ago, the exact time varied from town to town, and until about fifty years ago, people did not carry highly accurate timepieces.

It is now possible to buy a watch or clock that synchronizes itself with the United States Naval Observatory. More than fifty clocks at the Naval Observatory measure the frequency emitted by atoms of the element cesium. These atomic clocks are accurate to within one second every 100,000,000 years. In fact, our ability to measure time is more accurate than the stability of the Earth.

Diagram showing how vibrations of cesium atoms are used to keep time in an atomic clock — Atomic clocks keep extremely accurate time by counting the steady vibrations of cesium atoms.

The Origins of Timekeeping

Humankind first measured the time of the day with sundials. In ancient Rome, lawyers would be scheduled to speak in the Forum ante medium—before the sun reached its highest point, or post medium—after the sun had reached its highest point. The abbreviations a.m. and p.m. reflect this practice. But sundials were usable only on sunny days, so people developed other ways to measure time.

Some ancient civilizations used surprisingly accurate water clocks, which measured time by allowing water to drip slowly from one container into another. Hourglasses later became common aboard ships because they could measure time even when the sky was cloudy or the sea was rough.

The First Mechanical Clocks

The first mechanical clocks in Europe had no numbers. Most people could not read, and few people needed to know the exact minute of the day. By about 1300 CE, most European towns had tall clock towers. A bell-ringer would await the first view of the sun and announce the beginning of the day by ringing a large bell.

The white marble Leaning Tower of Pisa tilting next to the multi-tiered Romanesque Cathedral of Santa Maria Assunta under a clear dawn sky. — Built beginning in 1173, the Leaning Tower of Pisa served as a cathedral’s bell tower. Its bells once announced religious services, warned that the city gates were about to close, marked important holidays, and helped organize daily life in a world where few people owned clocks or watches. Photograph by the author.

Bell ringers would later return to the tower to ring out the later hours. Early timekeeping was not precise and varied from town to town. Bells might announce the opening of a market, the beginning of work, a religious service, or a warning that the town gates were about to close for the night.

A pendulum is a weight that swings back and forth at a steady rate. In 1656, the Dutch scientist Christiaan Huygens built the first practical pendulum clock, inspired by the work of Galileo. Earlier clocks could lose many minutes each day, but pendulum clocks were often accurate to within a minute a day. The day could now be measured with precision.

Illustration of a pendulum clock showing the pendulum swinging back and forth beneath the clock face. — A pendulum clock works because the pendulum swings back and forth at essentially the same speed every time. Each swing allows the clock’s gears to move forward a tiny amount. Because the swings stay steady, the gears move steadily and the clock keeps more accurate time.

Hours and Minutes

Dividing the day into parts was imprecise before the invention of the pendulum clock. People used hourglasses and water clocks to measure shorter units of time. The Greeks and later the Romans borrowed the ancient Egyptian idea of a twenty-four-hour day. The number 24 was useful because it could be evenly divided by 2, 3, 4, 6, and 8.

Minute and second are terms mathematicians used to divide circles long before clocks used these terms to measure time. Medieval scholars divided circles into “pars minuta prima,” a Latin term which means “the first small part” and explains why minute, when pronounced my-NOOT, refers to something tiny. They divided minutes into “pars minuta secunda,” which means “the second small part.”

The Week and the Month

People also divided time into larger units. The seven-day week comes to us from the Hebrews, who believed in a Sabbath, or day of rest, one day out of seven. The idea of a weekly day of rest spread through the Middle East and was later adopted by the Romans after Christianity became popular in the Empire. Rome once controlled much of Europe, but the Romans also traded with cultures far beyond Europe. In time, the seven-day week became a worldwide standard.

Our months roughly correspond with the phases of the moon. A full moon occurs about every 29½ days, but twelve lunar months add up to only about 354 days. Calendars were adjusted so that the months stayed aligned with the solar year and the seasons. In 1582, Pope Gregory XIII introduced a revised calendar that corrected errors in earlier calendars. The Gregorian calendar eventually became the calendar used throughout most of the world.

Full moon rising above silhouetted trees at dusk — The ancients were fascinated by the regular pattern of the waxing (growing) and waning (shrinking) of the moon. Cultures from the Babylonians to the Mayans built their religious beliefs around the moon’s cycles. In fact, the Roman goddess of the moon was Luna, from which we get terms like lunar. The word lunatic comes from the old belief that the full moon could affect human behavior.

Winding Your Watch

Today our watches and phones are synchronized and it is easy to know the exact time, but older people remember when timekeeping for everyday people was not so precise. Highly accurate watches did not become common until the 1970s. Before that, people wound their watches by turning a small knob at least once a day to keep the mainspring tight. Many radio and television stations broadcast a tone at the beginning of the hour so that people who needed the exact time could set their watches. Just before the top of the hour an announcer might say, “At the tone, the time will be two o’clock.”

3…2…1…WAIT!

In 1972, a group of scientists recognized that clock time could lose a second every few years due to earthquakes, tides, movement inside Earth’s core, and the moon’s gravity. They proposed adding leap seconds to official timekeeping when necessary. The last leap second was added at 11:59:60 p.m. UTC on December 31, 2016, so people celebrating around the world were actually a second early.

Businesses and governments did not like having leap seconds because adding a second to the clock created problems for computer systems that depend on exact timing. In 2022, the International Telecommunication Union recommended that we no longer have leap seconds. This means that clock time will soon no longer sync with solar time. The two could be out of alignment by about a minute over several centuries. But unless the human lifespan suddenly extends, it will not be the concern of anybody living today.

Resources

Download this lesson as a worksheet with activities

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Mr. Donn has an excellent website that includes a section on Geography.

Cite this page:
Dowling, Mike, “Measuring Time: A History of Time,” mrdowling.com, September 17, 2026. Web. [Date of access].

Sources:
Christiaan Huygens, Horologium (1658)
David S. Landes, Revolution in Time: Clocks and the Making of the Modern World (Harvard University Press, 1983)
International Telecommunication Union, Resolution 655 (WRC-23), on the discontinuation of leap seconds (2022)

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Calendars and Years