Explain how time was measured in ancient times. Also write note on cesium clock (atomic clock).

Explanation

In ancient times from length of shadow of an object casted by sunlight time was estimated.
Jantarmantar is located in Jaipur present also.
Time is measured by pendulum clock also.
Now we use atomic clock for standard time measurement.
It is based on the periodic vibration produced in cesium atom.
In the cesium atomic clock the second is taken as time needed for 9,192,631,770 vibration of the radiation corresponding to transition between the two hyperfine levels of the ground state of cesium-133 atom.
The vibration of cesium atom regulate the rate of this cesium clock just as the vibration of a balance wheel regulate an ordinary wrist watch.
The cesium atomic clock are very accurate. In principle they provide portable standard.
By using 4 cesium clock national standard of time and frequency are maintained.
For Indian Standard Time (IST) cesium atomic clock at NPL (National Physical Laboratory) at New Delhi is used.
Uncertainty in time resolution is ` +-1 xx 10^(-13)` sec obtained.
They lose or gain no more than `3mus` in one year.
In view of tremendous accuracy in time measurement SI unit of length has been expressed in terms of path length light travels in certain interval of time.
Distance travelled by light in `(1)/(2999792458)` second is called I meter.
The time interval of events that we come across in the universe vary over a very wide range. Table 2.5 gives the range and order of some typical time intervals.