[LEAPSECS] "UTC is derived from TAI"

Matsakis, Demetrios matsakis.demetrios at usno.navy.mil
Sun Mar 14 21:11:58 EDT 2010

The concept of whether TAI is a product in itself, or just a step to generate UTC, has led to some disagreements over time.

However, there is only one UTC, which has many realizations. The true UTC is determined not from the realizations, but from the clocks behind those realizations.


From: Steve Allen
Sent: Sun 3/14/2010 8:54 PM
To: Leap Second Discussion List
Subject: [LEAPSECS] "UTC is derived from TAI"

"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
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