Measurement of Time

For measurement of any time interval, we need a clock. Any phenomenon which repeats itself after a fixed interval can serve the purpose of a clock.

Atomic clock: We now use an atomic standard of time, which is based on the periodic vibrations produced in a cesium atom. This is the basis of the cesium clock, sometimes called atomic clock, used in the national standards. It is highly accurate.

• In the Cesium clock, a second is equal to 9,192,631,770 vibrations of radiation from the transition between two hyperfine levels of cesium-133 atom.

• The cesium clock works on the vibration of the cesium atom, which is similar to the vibrations of the balance wheel in a regular wristwatch and quartz crystal in a quartz wristwatch.

• Four atomic clocks maintain national standard time and frequency. Indian standard time is maintained by a Cesium clock at the National Physical Laboratory (NPL), New Delhi.

• Cesium clocks are very accurate, and the uncertainty is very low, 1 part in 1013, meaning not more than 3 μs are lost or gained in a year.
  
  

  Range and order of time intervals:

  Event	Time interval(s)
  Life-span of most unstable particle	10-24
  Time required for light to cross a nuclear distance	10-22
  Period of X-rays	10-19
  Period of atomic vibrations	10-15
  Period of light wave	10-15
  Life time of an excited state of an atom	10-8
  Period of radio wave	10-6
  Period of a sound wave	10-3
  Wink of eye	10-1
  Time between successive human heart beats	100
  Travel time for light from moon to the Earth	100
  Travel time for light from the Sun to the Earth	102
  Time period of a satellite	104
  Rotation period of the Earth	105
  Rotation and revolution periods of the moon	106
  Revolution period of the Earth	107
  Travel time for light from the nearest star	108
  Average human life-span	109
  Age of Egyptian pyramids	1011
  Time since dinosaurs became extinct	1015
  Age of the universe	1017