The International System of Units (SI) is founded on seven units, from which all other units are derived. They are the meter, kilogram, second, ampere, kelvin, mole and candela (units of length; mass; time; electric current; temperature; amount of substance; and luminous intensity).
The second is perhaps the most important unit, since so many other units depend on it. It’s also the unit we can measure the most accurately, using atomic clocks. The second is defined as “the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.” The frequency that is measured and forms the basis of our unit of time is the oscillation between two energy states of an electron in Cs-133. The exact number of periods is derived from the earlier unit Ephemeris time, which tries to define the second so that it corresponds to the movement of the Earth around the Sun.
Cesium clocks are so accurate they’re only off by one second every thirty million years, or roughly two seconds since the dinosaurs went extinct.
dailymeh posted this on August 28, 2011
