Through the ages, man has used many methods of time measurement such as rotation of the earth, rising and setting of the sun, movement of the moon and stars and the change of seasons. Perhaps the earliest measurement of time was based on the regular cycle of night and day. The 24 hour period between one mid night 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 hours, 9 minutes and 9.54 seconds. Sidereal time is more accurate than solar time.
The oldest methods for measuring time include sundial, candle clocks, water clocks and hour glasses. In a sun dial, time was measured by the length of the shadow of a stick casted by the sun. With candle clocks, time was measured by the rate of a burning candle. The water clock was a leaking bowl. In the hour glass, sand flowed 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 passed.
The development of clocks that worked by springs started in the late 1400. The two main types of modern clocks are – mechanical clocks and electronic clocks. Mechanical clocks are powered by various devices that must be wound while the electronic clocks are battery powered. Quartz based clocks are also battery driven. Most of the quartz clocks which are based on quartz crystal vibrations are accurate upto 60 seconds in a year.
Digital clocks and watches which became popular in the 1970s are also very accurate time measuring devices. Most of them have liquid crystal display system or light-emitting diode display system. These are also quartz based.
The most accurate means of measuring time is an atomic clock. An atomic clock measures the vibrations of certain atoms of cesium or ammonia gas which keep extremely accurate time. In 1000,000 years an atomic clock may loose or gain only a few seconds. The world time changed to atomic time standard in 1972.