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

(N/A) In ancient times,time was estimated based on the length of the shadow cast by an object in sunlight. The Jantar Mantar in Jaipur is a historical example of such a device.
Time was also measured using pendulum clocks.
Currently,we use atomic clocks for standard time measurement.
These clocks are based on the periodic vibrations produced in a cesium atom.
In a cesium atomic clock,one second is defined as the time taken for $9,192,631,770$ vibrations of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-$133$ atom.
The vibrations of the cesium atom regulate the rate of the clock,just as the vibrations of a balance wheel regulate an ordinary wristwatch.
Cesium atomic clocks are extremely accurate. In principle,they provide a portable standard. By using $4$ cesium clocks,national standards of time and frequency are maintained.
For Indian Standard Time $(IST)$,the cesium atomic clock at the $NPL$ (National Physical Laboratory) in New Delhi is used.
The uncertainty in time resolution is $\pm 1 \times 10^{-13} \text{ s}$.
These clocks lose or gain no more than $3 \mu\text{s}$ in one year.
Due to this tremendous accuracy in time measurement,the $SI$ unit of length has been defined in terms of the path length light travels in a specific time interval.
The distance traveled by light in $\frac{1}{299,792,458}$ seconds is defined as $1 \text{ meter}$.