[LEAPSECS] Nit-pick: SI second

Tom Van Baak tvb at LeapSecond.com
Mon Feb 7 12:58:46 EST 2011

> Doesn't it depend upon gravity (aka sea level)? Is that standardized?


Yes and no.

The SI second is defined in the local reference frame. So your
cesium clock at sea level ticks SI seconds for you there. Your
cesium clock on a mountain ticks SI seconds for you there. The
fact that the two clocks will disagree when compared with each
other is true; but both clocks are still ticking SI seconds in their
own frames.

Now to make a time-scale out of these identical but differing SI
seconds does require some statement of common elevation. So
this is why reported clock data is all adjusted to sea level in the
computation of TAI (and hence UTC).


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