The second (SI unit symbol: s) is the International System of Units (SI) base unit of time[1] and also a unit of time in other systems (abbreviated s or sec[2]). Between 1000 (when al-Biruni used seconds) and 1960 the second was defined as 1/86,400 of a mean solar day (that definition still applies in some astronomical and legal contexts).[3][4]
Between 1960 and 1967, it was defined in terms of the period of the Earth's orbit around the Sun,[5] but it is now defined more precisely in atomic terms. Seconds may be measured using mechanical, electric or atomic clocks. Around 1680, London clockmaker William Clement introduced the longcase or grandfather clock that was accurate enough to measure seconds reliably as one sixtieth of a minute. This clock used an anchor escapement mechanism with a seconds pendulum to display seconds in a small subdial. This mechanism required less power, caused less friction and was more accurate than the older verge escapement. Within a few years, most British precision clockmakers were producing longcase clocks.
However, 19th- and 20th-century astronomical observations revealed that this average time is lengthening and thus the sun–earth motion is no longer considered a suitable basis for definition. With the advent of atomic clocks, it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be: 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.[1] (i.e., huh??!) Source: Wikipedia
Between 1960 and 1967, it was defined in terms of the period of the Earth's orbit around the Sun,[5] but it is now defined more precisely in atomic terms. Seconds may be measured using mechanical, electric or atomic clocks. Around 1680, London clockmaker William Clement introduced the longcase or grandfather clock that was accurate enough to measure seconds reliably as one sixtieth of a minute. This clock used an anchor escapement mechanism with a seconds pendulum to display seconds in a small subdial. This mechanism required less power, caused less friction and was more accurate than the older verge escapement. Within a few years, most British precision clockmakers were producing longcase clocks.
However, 19th- and 20th-century astronomical observations revealed that this average time is lengthening and thus the sun–earth motion is no longer considered a suitable basis for definition. With the advent of atomic clocks, it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be: 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.[1] (i.e., huh??!) Source: Wikipedia
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