# What is time measured against? [duplicate]

Today I was observing a clock and its movement, every second is an exact second on every clock.

I was making a comparison between a second and a meter. I know in France there is a metal stick one meter long and is a basic measure valid for every place on earth.

So I was wondering what is time measured against?

I don't know if my question is clear enough, but how do you measure one second?

## marked as duplicate by John Rennie, Kyle Kanos, ACuriousMind♦, Qmechanic♦Oct 18 '15 at 12:31

The "standard meter" in Paris you refer to is obsolete and an interesting historical relic only. It was the definition of the meter from 1870 until 1960, when the meter was redefined as a number of wavelengths of a certain emission line from krypton 86. Since 1983, the meter has been wholly defined in terms of the second: it is the length such that the universal Lorentz covariant speed $c$ (experimentally found to be the speed of light) is precisely 299792458 meters per second. Otherwise put, it is the distance travelled by light in 1/299792458 second.

This leaves the second to be defined, as in your question's title. Currently the second is defined as (quoting from Wikipedia)

"... the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom ... at rest at a temperature of 0 K"

So it is defined in terms of a vibration frequency that can be measured relatively simply in any well kitted laboratory. Notice how the definition altogether gets rid of the need to keep a "standard length" or "standard clock" anywhere: the description suffices for anyone to reproduce the standard.

The second is defined as

The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

The number of periods seems a bit arbitrary, but yo may understand that is has an historical background.