An everyday experience with small fractions of a second is a 1-gigahertz microprocessor which has a cycle time of 1 nanosecond. In scientific work, small fractions of a second are counted in milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second. Fractions of a second are usually counted in tenths or hundredths. Multiples of seconds are usually counted in hours and minutes. īecause the Earth's rotation varies and is also slowing very slightly, a leap second is added at irregular intervals to clock time to keep clocks in sync with Earth's rotation. The second is equal to the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the hyperfine levels of the unperturbed ground state of the 133Cs atom. The second is also part of several other units of measurement like meters per second for speed, meters per second per second for acceleration, and cycles per second for frequency.Īlthough the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units ( SI) is a much steadier timekeeper: Digital clocks and watches often have a two-digit seconds counter. Analog clocks and watches often have sixty tick marks on their faces, representing seconds (and minutes), and a "second hand" to mark the passage of time in seconds. The second (symbol: s, also abbreviated: sec ) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄ 86400 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 (24 × 60 × 60 = 86400). A pendulum-governed escapement of a clock, ticking every second
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