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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.That atomic clock employs microwave radiation, which is essentially light. You count 9,192,631,770 microwave peaks going past and call it a second.
Technically, the definition rests on the processes of the electron, not on the motion of the light. The light is how one measures the transitions, but the timing of the transitions is determined by the transition of the electron. The transition from one state to another is not a transition of motion but of energy.The reliance on light is a practical restriction. The definition of a second as the transition is a practical definition, too. It is something that we can use as a standard of accuracy.
My sundial was 20 minutes out at Xmas!
Ok, Farsight, but as I understand it light is just the tool we use to extract information about the atomic transitions, and we make all sorts of requirements to keep the light from influencing the measurement. Is there any practical difference in this case between the number of atomic transitions and the number of peaks of the light that pass by?
Farsight has already answered, but I want to remark that there is absolutely no relation at all between the two things; in theory you could get that radiation with a completely different mechanism, not involving atoms or electrons at all.
Farsight,Can you explain your last post for me please.Are you saying that atomic clocks keep time based on properties of atoms, or not?And if you are saying that they don't keep time based on properties of atoms, please explain what they are using to keep time.
But note that frequency is measured in hertz, which is defined as cycles per second, so we have to count cycles to define the second.
It jolly well is measuring a frequency; specifically, the frequency of a microwave oscillator. That oscillator is locked to the absorbtion frequency of caesium atoms in a "fountain".In principle, you could have the events that control the frequency of the oscillator happen as seldom as you like- once a day or whatever. In practice they happen very frequently, but that's just for engineering reasons. They certainly don't happen exactly 9 point something billion times a second.
so geezer to which category do you most identify with?
The only exceptions that I can think of are gravitational ones based on the rotation of planets etc that use periods of about .000078 Hz and quite accurate.
Before the development of the marine chronometer by Harrison et al the chief source of timekeeping for navigation was the passage of the Moon thru the star field.As Lee points out the eccentricity of the orbit causes a lot of problems and prediction tables had to be prepared so that adjustments could be made.When I suggested that large bodies made good time keepers I had in mind their rotational periods not of course their orbital time about other bodies.
It's the former: as I thought we'd already agreed, the second is defined by the "transition between the two hyperfine levels of the ground state of the caesium 133 atom" and most definitely not by the Earth's rotational and/or orbital periods (which are subject to stuff like earthquakes etc.).The Earth's rotational and orbital periods are defined in terms of the second.
GeezerNo the second was never defined with reference to the Earths orbital time it was defined with reference to the Earths rotational period relative to the distant stars.