The SI unit of time is **second** denoted by letter **s**. The fundamental unit of time was used to be defined in terms of the *mean solar day*.

A solar day is the time interval between successive arrivals of the Sun at the highest point in the sky and the mean solar day is the average time between successive arrivals of the sun at the highest point in the sky.

The second was the certain fraction of this mean solar day, that is \((\frac{1}{24})(\frac{1}{60})(\frac{1}{60})\) of the mean solar day.

This definition is not precise and not universal because it is based on the Earth's rotation only. So, this operational definition must be changed to something more precise and universal which is exactly done by the new definition based on *atomic clock*. The more precise definition of second in terms of atomic clock is

SECOND: Second is the time period equal to 9192631770 times the period of vibration of radiation from caesium-133 atom.

You may need to dive into atomic physics to understand how atomic clock works. We use this standard to measure other time intervals. For example, a particular time period is 1s, 21s, 3s, 15s and so on.