It was probably the Babylonians who first divided the day into 24 hours, with 60 minutes in each hour. These are numbers that can easily be divided by 2, 3 and 4. The very first clocks, like the first calendars, were based on the Sun, using the movements of its shadow to read the time from a marked area of Earth or stone. Later methods of time measurement were based on actions that happened at a fixed rate, such as the pull of gravity on grains of sand.
For thousands of years, devices have been used to measure and keep track of time. The current sexagesimal system of time measurement dates to approximately 2000 bc from the Sumerians. The Egyptians divided the day into two 12-hour periods, and used large obelisks to track the movement of the sun. They also developed water clocks, which were probably first used in the Precinct of Amun-Re, and later outside Egypt as well; they were employed frequently by Persians and the Ancient Greeks, who called them clepsydrae. The Zhou dynasty is believed to have used the outflow water clock around the same of the time, devices which were introduced from Mesopotamia as early as 2000 bc.
Other ancient timekeeping devices include the candle clock, used in ancient China, ancient Japan, England and Mesopotamia; the time-stick, widely used in Persia, India and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock. The sundial, another early clock, relies on shadows to provide a good estimate of the hour on a sunny day. It is not so useful in cloudy weather or at night and requires recalibration as the seasons change (if the gnomon was not aligned with the Earth’s axis).
The earliest known clock with a water-powered escapement mechanism, which transferred rotational energy into intermittent motions, dates back to 3rd century bc in ancient Greece; Chinese engineers later invented clocks incorporating mercury-powered escapement mechanisms in the 10th century, followed by Arabic engineers inventing water clocks driven by gears and weights in the 11th century.
The first mechanical clocks, employing the verge escapement mechanism with a foliot or balance wheel timekeeper, were invented in Europe at around the start of the 14th century, and became the standard timekeeping device until the pendulum clock was invented in 1656. The invention of the mainspring in the early 15th century allowed portable clocks to be built, evolving into the first pocket watches by the 17th century, but these were not very accurate until the balance spring was added to the balance wheel in the mid-17th century.
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