Second

The **second** (symbol: **s**), is a unit of time. There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day. This tradition dates back to the Babylonians.

In science, a second is the time it takes for a caesium atom to vibrate 9,192,631,770 (around 9 billion) times. Scientists measure the second this way because the length of a day changes all the time. For example, when the dinosaurs lived, a day was about an hour shorter. Vibrations of atoms on the other hand always take the same time. This **atomic second** is also called the SI second.

Metric 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).

One heartbeat of an adult at rest, will last about one second.

## International second 
Under the International System of Units, the second is currently defined as the duration of 9,192,631,770 periods of the radiation  to the  between the two hyperfine levels of the ground state of the caesium-133 atom. This definition refers to a caesium atom at rest at a temperature of  (absolute zero). The ground state is defined at zero magnetic field.

## SI multiples 
SI prefixes are commonly used for times shorter than one second, but rarely for multiples of a second. Instead, certain non-SI units are permitted for use in SI: minutes, hours, days, and in astronomy Julian days.

Greek time periods, for example the mean synodic month, were usually specified quite precisely because they were *calculated* from carefully selected eclipses separated by hundreds of years—individual *mean* synodic months and similar time periods cannot be measured. Nevertheless, with the development of pendulum clocks keeping *mean time* (as opposed to the *apparent time* displayed by sundials), the second became measurable. The seconds pendulum was proposed as a unit of length as early as 1660 by the Royal Society of London. The duration of a beat or half period (one swing, not back and forth) of a pendulum one metre in length on the Earth's surface is approximately one second.

In 1956 the second was defined in terms of the period of revolution of the Earth around the Sun 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. they determined the orbital motion of the Moon about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as measured by an atomic clock. As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time in the International System of Units (SI) 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 ground state is defined at zero magnetic field. The second thus defined is equivalent to the ephemeris second.

The definition of the second was later refined at the 1997 meeting of the BIPM to include the statement

:*This definition refers to a caesium atom at rest at a temperature of 0 K.*

The revised definition would seem to imply that the ideal atomic clock would contain a single caesium atom at rest emitting a single frequency. In practice, however, the definition means that high-precision realizations of the second should compensate for the effects of the ambient temperature (black-body radiation) within which atomic clocks operate and extrapolate accordingly to the value of the second as defined above.

## The second in role-playing games
Sometimes in role-playing games a second is used to refer to a small period of time or a single turn of combat. It is used as a standard moment of time, and does not necessarily refer to a real second, and could be shorter or longer depending on the scenario.

## Trivia 
* Until modern times, degrees and hours were divided successively by 60 in *pars minuta prima*, *pars minuta secunda*, *pars minuta tertia* and so on. This evolved to the modern minute and second, but for smaller divisions we follow now the decimal division. In some languages, dictionaries still keep the word for *third* for 1/60 of a second, for example Polish (*tercja*) and Arabic (ثالثة).

## Related pages
* Leap second
* Orders of magnitude (time)
* UTC
* International System of Units
* Hertz
* Becquerel

## References 
## Other websites 
* Official BIPM definition of the second 
* Seconds and leap seconds by the USNO
* The leap second: its history and possible future