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, in Sumer. The Ancient 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 the Ancient Greeks, who called them clepsydrae. The Shang Dynasty is believed to have used the outflow water clock around the same time; the clocks were introduced from Mesopotamia, possibly as early as 2000 BC. Other ancient timekeeping devices include the candle clock, used in China, Japan, England and Iraq; the timestick, widely used in In...more >>dia and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock. The earliest clocks relied on shadows cast by the sun, so they were not useful in cloudy weather or at night, and required recalibration as the seasons changed if the gnomon was not aligned with the earth's axis. The first clock with an escapement, which transferred rotational energy into intermittent motions, appeared in China in the 8th century, and Arabic engineers invented water clocks driven by gears and weights in the 11th century.Mechanical clocks employing the verge escapement mechanism were invented in Europe at the turn of the 14th century, and became the standard timekeeping device until the spring-powered clock and pocket watch in the 16th century, followed by the pendulum clock in the 18th century. During the 20th century, quartz oscillators were invented, followed by atomic clocks. Although first used in laboratories, quartz oscillators were both easy to produce and accurate, leading to their use in wristwatches. Atomic clocks are far more accurate than any previous timekeeping device, and are used to calibrate other clocks and to calculate the proper t