A short history of timekeeping

The sun and the stars

The first clocks were the sky itself. Sunrise, noon and sunset divided the day, the phases of the moon gave the month, and the seasons and stars marked the year. Many early calendars were built around these cycles.

Ancient Egyptians used shadow clocks and sundials, and they divided daylight and night into 12 parts each, which is the origin of the 24-hour day. These early hours were not equal: a daylight hour was longer in summer than in winter. The base-60 counting of ancient Mesopotamia later gave us 60 minutes in an hour and 60 seconds in a minute.

Water, sand and fire

Sundials do not work at night or under clouds, so people built clocks that measured a steady flow:

• Water clocks (clepsydras) measured time by water dripping into or out of a vessel. They were used in Egypt, Babylon, Greece, China and elsewhere.
• Candle clocks and incense clocks burned at a known rate, with marks showing hours.
• Hourglasses filled with sand became common in Europe in the Middle Ages, notably at sea.

In China, Su Song built a large water-powered astronomical clock tower in the late 11th century, one of the most complex timekeeping machines of its era.

Mechanical clocks

Weight-driven mechanical clocks appeared in Europe in the late 13th century. Their key invention was the escapement, a device that lets a gear advance in small, regular steps. Early tower clocks often had no minute hand and could drift by many minutes a day, but they made equal hours part of town life.

In 1656, the Dutch scientist Christiaan Huygens built the first pendulum clock. A pendulum swings with a nearly constant period, and accuracy improved from minutes a day to seconds a day. For about 250 years, pendulum clocks were the most accurate timekeepers available.

Time at sea

Navigators can find their longitude by comparing local solar time with the time at a home port, but only with a clock that keeps accurate time on a moving ship. The English clockmaker John Harrison spent decades solving this problem, and his marine timekeeper H4 performed remarkably well on a voyage to Jamaica in 1761 to 1762. Marine chronometers became standard equipment on ships.

Railways and standard time

Until the 19th century, each town kept its own local solar time, so noon in one city was a few minutes off from noon in the next. Railways made this unworkable. In Britain, railway companies adopted a single time based on Greenwich in the 1840s. In North America, railroads introduced standard time zones on 18 November 1883.

In 1884 the International Meridian Conference chose the Greenwich meridian as the world’s prime meridian, which became the basis of world time zones. Read more in the history of US time zones and UTC vs GMT.

Quartz

In 1927, engineers at Bell Telephone Laboratories built the first quartz clock. A quartz crystal vibrates at a very stable frequency when an electric current passes through it. Quartz clocks were far more accurate than pendulums, and from the late 1960s small quartz watches brought that accuracy to the wrist at low cost.

Atomic clocks

Atoms absorb and emit radiation at exact frequencies, which makes them ideal timekeepers. In 1955, Louis Essen and Jack Parry built the first practical cesium atomic clock at the National Physical Laboratory in the United Kingdom. In 1967 the second itself was redefined in terms of the cesium atom.

Atomic time then exposed the small irregularities of the Earth’s rotation, which led to Coordinated Universal Time and the leap second. Today atomic clocks in satellites make GPS navigation possible, and the time on your phone is traceable to them.

Timeline

You can see where these standards end up today on the world clock.