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A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world , is the second , defined as about 9 billion oscillations of the caesium atom.
The second (symbol: s) is a unit of time, 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).
Second, fundamental unit of time, now defined in terms of the radiation frequency at which atoms of the element cesium change from one state to another. The second was formerly defined as 1/86,400 of the mean solar day—i.e., the average period of rotation of the Earth on its axis relative to the
- Jonathan D. Betts
The second (s or sec) is often referred to as a base time unit in the International System of Units, or the SI unit of time. The second, as per the SI, is described as a time interval equivalent to “9,192,631,770 periods of the radiation corresponding to the transition between the 02 hyperfine levels of the cesium-133 atom’s ground state”.
- Second – Unit of Time. Symbol: s. The second is defined by taking the fixed numerical value of the cesium frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s.
- Meter – Unit of Length. Symbol: m. The meter is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299,792,458 when expressed in the unit m s, where the second is defined in terms of ∆ν.
- Kilogram – Unit of Mass. Symbol: kg. The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015 ×10 when expressed in the unit J s, which is equal to kg m2 s, where the meter and the second are defined in terms of c and ∆ν.
- Ampere – Unit of Electric Current. Symbol: A. The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634 × 10 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆ν.
Oct 26, 2021 · Because cesium atoms react to microwave radiation with a frequency of 9,192,631,770 cycles per second (hertz or Hz), the international standard unit of time, the second, is defined as the duration of 9,192,631,770 cycles.
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Apr 9, 2019 · The second is currently defined using cesium atoms, which absorb and emit microwave radiation with a specific frequency. Atomic clocks count 9,192,631,770 of those microwave oscillations, and we call the elapsed time interval a second. Today, commercially available cesium clocks keep time to within 1/3,000,000 of a second per year. New ...
