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Concepts of Time- What is the definition of One Second ? - ItsMyAcademy.comItsMyAcademy.com

Concepts of Time- What is the definition of One Second ?

Concepts of Time- What is the definition of One Second ?





The concept of time is very old. The idea of passage of time occurred first from the motion of moon across the sky, then from the formation of day and night as a result of rotation of earth around its axis and so on. Today, even a child is familiar with what time is. But it is very difficult to define time precisely.


According to man of the 20th century, a great mathematician and physicist Albert Einstein, “time is what a clock reads”.


In fact any phenomenon that repeats itself regularly can serve as a measure of time.

Human heart which beats regularly is an inbuilt clock in every human being. Rotation of earth around its axis, revolution of earth around the sun are some other examples of repetitive phenomena, which serve as measures of time.


The most common unit of time is Second.


Originally, one second was defined as the time taken by simple pendulum of length of one meter in going from one extreme position to the other extreme position.


Later on, this definition was modified and one second represented one mean solar second.


One solar day is the time interval between two successive noons  i.e  the time interval between two successive passages of the sun across the meridian. The length of a solar day averaged over an year is called mean solar day.

One mean solar day is made  up of 24 hours or 24 * 60 minutes or 24*60*60 + 86400 seconds.

Hence we define one mean solar second as – 1/ 86400 part of a mean solar day.



Careful observations have revealed that astronomical standards of time have limited accuracy due to irregularities in the motion of earth.

Further, tidal friction is slowing down the rotational motion of earth about its axis.


Therefore definition of second is modified again as – the duration of 9,192,631,770 periods of radiation corresponding to unperturbed transition between the two hyper fine levels (F = 4, M = 0 and F = 3 , M = 0 ) of the ground state of Cesium s -133 atom


time - defination of time

a continuous cold caesium fountain atomic clock in Switzerland, started operating in 2004 at an uncertainty of one second in 30 million years.