I'm glad to see such active traffic on the list - particularly
discussions such as this that are wrestling with fundamental concepts.

   On 2006-01-13, Mark Calabretta wrote:

 The point is that UTC is simply a representation of TAI.

On Jan 13, 2006, at 4:17 AM, Michael Deckers wrote:

I believe I'm now grasping what you mean:

Have spent many hours wrestling with standards documents written by
M. Calabretta.  The key to understanding what they mean is to
carefully read what is on the page.  "Simply a representation" is not
an editorial comment like, say, "just a representation" might be
taken to be.  Representations are important, too.  It is - simply -
not accurate to suggest that UTC is discontinuous at a leap second.
DST is indeed discontinuous twice a year, but the underlying standard
time never is.

Astronomers who "write UTC as a real" (eg, in JD or MJD notation)
want an approximation of UT1 to point their telescopes, they do
_not_ want TAI.

Astronomers want various flavors of time for various purposes.  It is
indeed exceptionally useful to be able to rely on simple closed form
calculations relating various quantities to "universal time", where
UTC is often a sufficiently accurate approximation.  It happens that
we also use TAI and other interval time scales for many things.  And
it turns out that our images and other data products often benefit
from the use of civil time stamps for which good old reliably
continuous UTC serves admirably.  Ultimately it is the risk to this
last category of use cases that concerns me most about the notion of
leap hours.

That said, I suspect M. Deckers and M. Calabretta could productively
collaborate on a document synthesizing the fundamental issues into a
common vision.  Or perhaps somebody is aware of such a document that
already exists?


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