Measurement of Time

•  The SI unit of time is the second (s).

• For many years, duration of one mean Solar day was considered as reference.

• A mean Solar day is the average time interval from one noon to the next noon.

• Average duration of a day is taken as 24 hours.

• One hour is of 60 minutes and each minute is of 60 seconds.

• Thus a mean Solar day = 24 hours = 24 * 60 * 60 = 86400 s. Accordingly a second was defined as 1/86400 of a mean Solar day.

• It was later observed that the length of a Solar day varies gradually due to the gradual slowing down of the Earth's rotation.

• Hence, to get more standard and nonvarying (constant) unit for measurement of time, a cesium atomic clock is used.

• It is based on periodic vibrations produced in cesium atom.

• In cesium atomic clock, a second is taken as the time needed for 9,192,631,770 vibrations of the radiation (wave) emitted during a transition between two hyperfine states of Cs¹³³ atom.