I'm curious after reading this Feb 2008 article...
How does the NIST determine a certain apparatus is accurate to within a certain tolerance? Do they have a telescope aimed at a particular star so that one rotation of the earth can provide a precise standard period? Is there some other precise period that can be measured? The rotation of the earth is said to be slowing so I don't even know how well that idea would work. Thanks.
Relative to modern atomic clocks, the earty is pretty poor at timekeeping.
I think the trick to measuring how good your clock is uses 3 clocks that are roughly all the same quality. If you compare all pairs, the statistics will tell you how much noise comes from each one. (or something like that)
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You can now get atomic clocks of various types in fairly high quantities. The more you average them together the better the results at a rate of about sqrt(N).
They compare it with several other atomic clocks some of which are more accurate over short periods but subject to the occasional glitch. there is a bit more on the strontium clock online at:
The IERS has a nice webpage on the science of how the Earth's rotation is measured and related back to atomic clock time. The atomic clocks are now way better than the Earth's rotation, seasonal variations are easily visible. The new kit can detect duirnal changes caused by oceanic tides. The main measurements now come from radio astronomy VLBI - radio astronomers using VLBI have H-maser clocks at every station and the solution for exact timekeeping comes out in the wash. Provided that they are observing a suitable unresolved distant point source.
In the old days optical transit instruments like the one at Greenwich were used to define the length of a sidereal day. Some of them had a pool of mercury in front to determine the local horizontal plane.
Lots of them. Pulsars rotating are extremely good mechanical clocks if you have a big enough aerial to detect the signal. Unfortunately, star quakes and drag cause them to vary a bit.
It wouldn't. The Earth is slowing, precessing, nutating and wobbling about from lunar and planetary perturbations. Even the early atomic clocks were way more stable than the Earth's rotation.
The only standard good enough for atomic clocks is an ensemble average of the best atomic clocks on Earth.
Regards, Martin Brown
Tim
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So it basically comes down to an exercise in counting.
Sylvia.
Kinda like the time Superman flew counter-clockwise around the Earth faster than the speed of light and went back in time to save Lois? ;-)
Cheers! Rich
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