The History of the Clock: How People Learned to Keep Time

Telling the time feels effortless today, but accurate clocks are a relatively recent achievement. For most of history, people read time from the sky, and the story of the clock is a story of ever-better ways to divide the day.

Key takeaways

• Sundials and water clocks were used in ancient Egypt, Mesopotamia, Greece and China.
• Mechanical clocks appeared in Europe in the late thirteenth and fourteenth centuries.
• The pendulum clock, developed by Christiaan Huygens in 1656, greatly improved accuracy.
• Marine chronometers helped sailors find longitude.
• Quartz and atomic clocks now measure time with extraordinary precision.

Sundials and water clocks

The earliest timekeepers used the Sun. A sundial tracks the movement of a shadow cast by a pointer, called a gnomon, across marked lines. Obelisks and shadow clocks were used in ancient Egypt. Because sundials do not work at night or on cloudy days, people developed water clocks, or clepsydras, which measure time by the steady flow of water out of or into a vessel. Examples are known from Egypt, Babylon, Greece and China, and in the Islamic world engineers built elaborate versions with moving figures.

Candles, incense and hourglasses

Other methods measured intervals. Marked candles and incense sticks burned at a roughly constant rate, and hourglasses, which appeared in Europe by the fourteenth century, used falling sand. They were useful for short, fixed periods, such as timing a sermon or a watch at sea.

The first mechanical clocks

Around the end of the thirteenth century, European craftsmen built clocks driven by falling weights and regulated by an escapement, a device that releases energy in small, regular steps. These early tower clocks were often unreliable and sounded bells rather than showing time on a dial. They were installed in towns and monasteries, and they gradually changed the way communities organised work and prayer.

Springs and watches

The invention of the coiled mainspring in the fifteenth century made portable timepieces possible. Early watches were luxury objects worn on chains or carried in pockets. Wristwatches became common in the early twentieth century, especially after soldiers found them practical during the First World War.

The pendulum

In 1656, Dutch scientist Christiaan Huygens built the first pendulum clock, building on Galileo's observation that a swinging pendulum keeps nearly constant time. The pendulum reduced errors from minutes per day to seconds per day, a huge leap.

Longitude and the chronometer

At sea, finding longitude requires knowing the time at a fixed reference point. In the eighteenth century, the British government offered a large prize for a solution. English clockmaker John Harrison built a series of marine chronometers that kept accurate time on long voyages, and these became vital tools for navigation.

Quartz and atomic clocks

In the twentieth century, quartz clocks used the steady vibration of a crystal when electricity is applied, making accurate timekeeping cheap. Atomic clocks, which count vibrations of atoms such as caesium, are even more precise, and since 1967 the official second has been defined by atomic properties. These clocks are used in GPS and in setting international time.

Time zones and standard time

As railways spread in the nineteenth century, local solar times caused confusion, and countries adopted standard time zones. An international meeting in 1884 established Greenwich as the prime meridian.

Water clocks and early engineering

Early clockmakers could be remarkably inventive. In 1088, the Chinese official Su Song completed a giant astronomical clock tower in Kaifeng, powered by water and including a mechanism that regulated the flow, a rotating armillary sphere and figures that struck bells and showed the time. In 1206, the engineer Al-Jazari described a series of water-powered devices in his Book of Knowledge of Ingenious Mechanical Devices, including an elephant clock that combined Greek, Egyptian, Indian and Chinese motifs with moving figures. These machines show how much sophistication was possible before springs and pendulums.

The longitude problem

Finding latitude is relatively easy, because it can be measured from the height of the Sun or Pole Star. Longitude requires knowing the time difference between your position and a reference, and that needs an accurate clock at sea. After several naval disasters, the British Parliament passed the Longitude Act in 1714, offering a large prize. Many people favoured astronomical methods, such as measuring the distance between the Moon and stars. John Harrison, a self-taught clockmaker, built a series of marine timekeepers, from the large H1 to the watch-sized H4, which performed very well on sea trials in the 1760s, and his work transformed navigation.

Daylight saving time

Many countries move their clocks forward in spring and back in autumn. The idea was proposed in the late nineteenth century and was first adopted nationally in Germany in 1916, during the First World War, to save fuel. It has been adopted, changed and abandoned in many places since. Supporters argue it provides more evening daylight, while critics point to disrupted sleep and limited energy savings, and some regions have decided to stay on one time all year.

UTC and leap seconds

Today the world's civil time is based on Coordinated Universal Time, UTC, which is kept by a network of atomic clocks. Earth's rotation is slightly irregular and slowly slowing, so a leap second has occasionally been added to UTC, a total of 27 times since 1972, to keep it close to the Sun's position. Leap seconds cause problems for computer systems, and in 2022 an international meeting agreed to stop using them by 2035 and allow the difference to grow for a time.

Clocks in daily life

Clocks are embedded in almost everything, from phones and cars to the networks that rely on precisely timed signals. Financial markets, power grids and mobile networks depend on synchronised clocks, and GPS satellites carry atomic clocks that must be corrected for the effects of relativity. Accurate timekeeping has become invisible infrastructure.

Frequently asked questions

Why is a day split into 24 hours?

The system has roots in ancient Egypt and Mesopotamia, which used twelve-part divisions of day and night.

What is an escapement?

A mechanism that lets a clock's energy escape in regular steps, producing the tick.

How accurate are atomic clocks?

The best ones lose or gain less than a second over many millions of years.

Why do we have daylight saving time?

The main idea is to shift daylight toward the evening in summer, so people can use it after work. It was introduced in many countries to save energy, though modern studies find the savings small and debated, and some places have abandoned it.

What is UTC?

UTC, Coordinated Universal Time, is the global time standard used as the basis for time zones. It is not changed for daylight saving. Time zones are expressed as offsets from UTC, for example UTC plus two hours.

Who invented the pendulum clock?

Galileo studied the regular swing of the pendulum and is often credited with the idea, but Christiaan Huygens built the first working pendulum clock in 1656, greatly improving accuracy.

Sources and further reading

• Clock (Wikipedia)
• NIST Time and Frequency Division