Hey readers, Keshav here.

Welcome to **part 3** of the series **Measuring time : What, Why and How** where
we deal with these baby units of time : **Minutes and Seconds**. So as
we all know a minute is one sixtieth of an hour and a second is one sixtieth of
a minute. There are 2 factors in the development of these divisions. First is
the inclusion of an uncommon number 60 and second is the words minute and
second. **The number 60 comes from Babylonians** who in turn have
got it from the sumerians. As we use 10 in our number system today they
used to have 60. Similar to 12, sixty is a number countable on hands, L.C.M. of
1^{st} 5 natural numbers and angle of an equilateral triangle.
More specifically, number sixty gave them the ability to use fractions easily.
But they did not divide the hour in 60 parts; instead they divided a day in 60
parts as well in 360 parts. In the system of 60 divisions, each division was
again divided into 60 parts, then again in 60 parts and so on. So their 1^{st} division
was equal to 24 modern day minutes, 2^{nd} was 24 modern day
seconds and the 3^{rd} one was 0.4 seconds. The adjective **“minutum”** in
Greek and Latin represented a tiny part of time which was not exactly fixed as
well as not equal to 1/60^{th} of an hour. In a later sexagesimal
system hour was divided as primary **minute**, **second**ary
minute and tertiary minute. So from here we got the word minute for the first
division of hour and the word second for second division of hour. The system of
360 divisions did not survive in timekeeping but did survive in mathematics as
a circle has 360 degrees. If you want to know more about this and the relation
of time and circles do let me know in the comments. But as we saw in the first
video, the solar day is not a constant period of time, these minute and second
were defined using mean solar day and two systems of mean solar day as well as
apparent solar day were used side by side for a long period of time. But even
the mean solar day is not constant. So instead of dividing the day in hours,
minutes and second, we define a second today without using sun or any other
terrestrial body so that it is always same and then reverse manufacture the
minutes, hours and day. So a second as defined by IBPM in 1967 is duration
of **9,192,631,770** periods of the radiation corresponding to the
transition between two hyperfine levels of the ground state of cesium-133 atom
at 0K. For common people, a second is now defined using internal properties of
a metal named caesium. So a minute is 60 seconds, an hour is sixty minutes and
a day is 24 hours. And the selection is done to match the mean solar day as
much as possible and as I told you in the 1^{st} video a mean
solar day is only 0.002s longer than a civil day. Done with the small things we
will be moving on to big periods of time i.e. months and year which will have
enough information to skip some days off the calendar.

Sources :

Wikipedia : Time , Second

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