[LEAPSECS] "UTC is derived from TAI"

Steve Allen sla at ucolick.org
Sun Mar 14 20:54:32 EDT 2010

"UTC is derived from TAI"

Type that phrase into google and look at the results, from
IERS, ITU, G. Petit, E.F. Arias, wikipedia
These deserve to be treated as authoritative sources.

But isn't it the case that hidden in that phrase is something that
deserves a great deal of explanation?

There is only one TAI, and it comes next month from BIPM Circular T.
There are many UTCs. Therefore the UTC that this phrase refers to can
only be the undecorated, true, just plain "UTC" which is also not
known until next month from Circular T.

As a practical matter, TAI is derived from all the various versions
of UTC(k) which are maintained by the contributing national labs.
So the only UTC which is available right now is one of the various
versions of UTC(k), not just plain UTC.

I suppose this is all just part of the explanation of why the general
notion of precision time scales is so poorly understood.

Steve Allen <sla at ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m

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