Can you imagine what would have happened if we could not measure time? Time would pass without us knowing what time of the day it was. But from the very beginning, man has known the value of time and has found various methods of measuring it. In ancient times, man used many methods of time measurement such as the rotation of the Earth, rising and setting of the Sun, movement of the moon, stars and the change of seasons. Perhaps the earliest method of time measurement was based on the regular cycle of night and day (called the ‘circadian cycle’), that influences life forms on the Earth. The 24-hour period between one midnight and the next was called the mean solar day. Time measurement by the Earth’s rotation with respect to the Sun is called sidereal time. A sidereal day lasts 23 hours, 56 minutes and 4 seconds. The time the Earth takes to complete one revolution around the Sun is called the sidereal year. The sidereal year lasts 365 days, 6 hour, 9 minutes and 9.54 seconds. Sidereal time is more accurate than solar time. The oldest means of measuring time included sundials, candle clocks, water clocks and hour glasses. In a sundial, the length of the shadow of a stick (gnomon) is used to measure time by the Sun, while in candle clocks, time was measured by the rate of a burning candle. The water clock was a leaking bowl. In an hour glass, sand flows down from one container into another at a steady rate. By measuring the amount of sand in either container, a person could tell how much time had elapsed. The development of clocks that worked using springs started in the late 1400s. The two main types of modern clocks are mechanical clocks and electronic clocks. In mechanical clocks, various types of springs must be wound, while the electronic clocks are battery operated. Quartz based clocks are also battery-driven. Most of the quartz clocks, which are based on quartz crystal vibrations, are accurate up to 60 seconds in a year. Digital clocks and watches, which became popular in the 1970s, are very accurate time-measuring devices. Most of them have liquid crystal display devices or light-emitting diode display system. These are also quartz crystal based. The most accurate means recently invented for measuring time is an atomic clock. An atomic clock measures the vibrations of certain atoms such as cesium, ammonia gas or mercury ions, which are extremely accurate. In 1,000,000 years, an atomic clock may lose or gain only a few seconds. The world time was changed to atomic time standard in 1972.