# Second (physics)

*This article is about*

**Second (physics)**. For other uses of the term**second**, please see second (disambiguation).The **second** is a unit of time, currently defined in the 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 cesium 133 atom.*^{[1]} Previously, the second had been defined as 1/86 400 of a day, with 60 seconds making one minute, 60 minutes making one hour, and 24 hours making one day, and the day being defined as one mean solar day using astronomical observations. The SI second is 1/86 400 of a mean solar day as measured in 1820; since then, the rotation of the earth has slowed, and the mean solar day is approximately 86 400.002 seconds long.

Practical details in achieving a realization of the second are described by the BIPM.^{[2]}

The word "second" is often used colloquially to mean any very short amount of time.

## References

- ↑ International Bureau of Weights and Measures
- ↑
Practical realization of the definition of the unit of time.
*SI Brochure Appendix 2*. Bureau International des Poids et Mesures. Retrieved on 2011-03-08.