This entry is largely taken from information at WIKIPEDIA.org

The second (SI symbol: s), sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time

SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g., the millisecond (one thousandth of a second) and nanosecond (one billionth of a second). Though SI prefixes may also be used to form multiples of the second (such as “kilosecond,” or one thousand seconds), such units are rarely used in practice. More commonly encountered, non-SI units of time such as the minute, hour, and day increase by multiples of 60 and 24 (rather than by powers of ten as in the SI system).

The second is currently 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.

Originally the second was defined as 1/86420 of a solar day (60x60X24), but of course the time between two noons varies enormously in the course of a year (Equation of time), so it was specifed as 1/86420 of a mean solar day, but eventually it became clear that the mean solar day has variations as well.

In 1956 the second was defined in terms of the period of revolution of the Earth around the Sun (i.e. as a fraction of the solar year) for a particular epoch, because by then it had become recognized that the Earth's rotation on its own axis was not sufficiently uniform as a standard of time. The Earth's motion was described in Newcomb's Tables of the Sun, which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made between 1750 and 1892.[9] The second thus defined is

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

Language wise 'second' is derived from the fact that the second is the 2nd sexagesimal division (i.e. divided by 60) of the hour (the minute being the 1st).

The second first became accurately measurable with the development of pendulum clocks keeping mean time (as opposed to the apparent time displayed by sundials), specifically in 1670 when William Clement added a seconds pendulum to the original pendulum clock of Christian Huygens.[8] The seconds pendulum has a period of two seconds, one second for a swing forward and one second for a swing back, enabling the longcase clock incorporating it to tick seconds. From this time, a second hand that rotated once per minute in a small subdial began to be added to the clock faces of precision clocks.

SI multiples for second (s)

Value Symbol Name

Submultiples

1/10th s ds decisecond

1/100th s cs centisecond

1/1000th s ms millisecond

1/1 000 000th s µs microsecond

1/1000 000000 s ns nanosecond

10^–12 s ps picosecond

10^–15 s fs femtosecond

10^–18 s as attosecond

10^–21 s zs zeptosecond

10^–24 s ys yoctosecond

Multiples

10 s das decasecond

100 s hs hectosecond

1000 s ks kilosecond

1000 000 s Ms megasecond

1 000 000 000 s Gs gigasecond

1 000000 000000 s Ts terasecond

10^15 s Ps petasecond

10^18 s Es exasecond

10^21 s Zs zettasecond

10^24 s Ys yottasecond

Common prefixes are in bold

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