[LEAPSECS] Nit-pick: SI second

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Feb 7 05:40:54 EST 2011

In message <20110207101736.BA8A780003B at ip-64-139-1-69.sjc.megapath.net>, Hal Mu
rray writes:

>>> That may all be true. But is the SI second a unit of measurement?


>> yes, one that can be reproduced a priori anywhere in the universe that

>> someone has a cesium atom.


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

The in-force definition of the SI second does not state it explicitly.

The TAI timescale was redefined to be at mean sea level (aka: "MSL"
aka "on the geoid") sometime in the middle of the 1970-ies.

Since TAI is a count of SI seconds, presumably that (de facto) adds
a "at MSL" footnote to the SI definitions.

I don't know the exact wording of the MSL adjustment text, and therefore
it is not clear if the MSL increase from polar icecap
melting will have a measurable impact on TAI. Probably not...


Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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