<Tr> <Th> Symbol </Th> <Td> s </Td> </Tr> <P> The second is the SI base unit of time, commonly understood and historically defined as 1 / 86,400 of a day--this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each . Another intuitive understanding is that it is about the time between beats of a human heart . Mechanical and electric clocks and watches usually have a face with 60 tickmarks representing seconds and minutes, traversed by a second hand and minute hand . Digital clocks and watches often have a two - digit counter that cycles through seconds . In common parlance, a "clock tick" is a second, though most modern clocks are digital electronic, and do not actually tick . The second is also part of several other units of measurement like velocity, acceleration, and frequency . </P> <P> Though the historical definition of the unit was based upon this division of the Earth's rotation cycle, the formal definition in the International System of Units SI is a much steadier timekeeper: 1 second is defined to be exactly 9 192 631 770 cycles of a Caesium atomic clock . Because the Earth's rotation varies and is also slowing ever so slightly, a leap second is added to clock time to keep clocks in sync with Earth's rotation . </P> <P> Multiples of seconds are usually counted in hours and minutes . Fractions of a second are usually counted in tenths or hundredths . In scientific work, small fractions of a second are counted in milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second . An everyday experience with small fractions of a second is a 1 - gigahertz microprocessor which has a cycle time of 1 nanosecond . Camera shutter speeds usually range from 1 / 60 second to 1 / 250 second . </P>

Where does the length of a second come from
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