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Feb5-12, 06:55 AM
P: 359
Quote Quote by DaleSpam View Post
On the BIPM website I linked to under each unit definition there is a link on the evolution of the definition. It contains all of the historical information on previous standards.

The frequency detector is at rest wrt the cesium atoms. See the linked information on practical realizations. The detector and the cesium atoms may be at rest wrt the earth like the NIST, or not, like shipboard or orbital clocks. The earth centered frame is not part of the definition.
If we trace the evolution back, we can see that the earth centred frame, although not expressly part of the any definition, is, as a result of the practicalities of the measurements, a tacit assumption of the definitions.

With regard to the atomic clock, the proper second is still defined in terms of an atomic clock at rest on earth, even correcting for the velocity of the atom in the lab - as a matter of interest, the velocity relative to what? - because it isn't defined in terms of the clocks traveling relative to the earth; that is, the clocks used in the Hafele-Keating epxeriment can't be said to have counted "the proper second" when they return to rest on earth.