ashley at semantic.org
Tue May 13 20:30:51 EDT 2008
On Mon, 2008-05-12 at 02:15 -0700, I wrote:
> I see the problem now. TT and TAI are the same up to isomorphism, but
> they both can be written in YMD HMS format, with a 32.184s difference.
> So how to parse?
Perhaps we shouldn't at all. This is what Steve Allen has to say:
"The forms of dynamical, atomic, and coordinate time above are not based
on earth rotation. They have no connection with days in the traditional
sense; thus they have no simple relationship with the concept of a
calendar. It is important to remember that the 24-hour cycle of tags
like 12:00:00 really only makes sense for earth rotation time."
"To their credit, the BIPM do report TAI and TT using modified Julian
date (MJD) notation. Nevertheless, it remains commonplace to use the
traditional calendrical and sexagesimal notations when counting the
seconds which are defined by these time scales. (Indeed, for the sake of
human cognition this notational convenience is very nearly a
requirement.) Unfortunately, that usage leads to even greater confusion
about the meanings of time scales."
"Denoting TAI with a tag in the form 1958-01-01T00:00:00, and denoting
TT, TCG, or TCB with a tag in the form 1977-01-01T00:00:00 should be
considered as a convenience only. They are merely counts of elapsed time
where one anonymous and indistinguishable second follows another. It is
very appropriate to count these forms of time using decimal notation
from the epochs represented by those tags. But there is no observable
event that happens cyclically at 12:00:00 for dynamical, atomic, or
coordinate time -- the entire notion of such a cyclical process is
contrary to their uniformly-incrementing conceptual definitions."
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