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- The SI unit of time is the second (s).
- The SI unit of time is the second (s).
- For many years, duration of one mean Solar day was considered as reference.
- 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.
- 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.
- Average duration of a day is taken as 24 hours.
- One hour is of 60 minutes and each minute is of 60 seconds.
- One hour is of 60 minutes and each minute is of 60 seconds.
- Thus a mean Solar day = 24 hours = 24Í60Í60 = 86400 s.
- Thus a mean Solar day = 24 hours = 24Í60Í60 = 86400 s.
- Accordingly a second was defined as 1/86400 of a mean Solar day.
- 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.
- 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 non-varying (constant) unit for measurement of time, a cesium atomic clock is used.
- Hence, to get more standard and non-varying (constant) unit for measurement of time, a cesium atomic clock is used.
- It is based on periodic vibrations produced in cesium atom.
- 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.
- 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.
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