William Thompson scripsit:

> Any application which seeks to calculate the difference in time between
> two events recorded in UTC time needs to know if there are any leap
> seconds between the start and stop time.  For example, suppose you
> were studying solar flares, and analyzing some data taken in 1998,
> and you saw a burst of hard X-rays at 23:59:53 UT on Dec 31, followed
> by a rise in EUV emission at 00:00:10 UT the next day.  You'd think
> that the delay time between the two would be 17 seconds, but it's
> really 18 seconds because of the leap second introduced that day.

Thanks for the example.  Of course it is not astronomy-specific: the same
thing applies if you are calculating how long somebody spoke for in
field linguistics, or the amount of time it takes a moving part to stop
moving in engineering.  What we are dealing with here is time-zone independent
civil time.

> That's a vital difference for the scientific analysis of the data.


> And yes, part of that software package includes a list of all
> leapseconds added since 1 Jan 1972.  Currently, my software doesn't
> handle TAI/UTC conversions between 1958 and 1972, when UTC seconds
> had varying lengths.

Modern Unix time packages (both GNU and ADO) assume that TAI-UTC was 10
from the epoch until 1972-06-30T23:59:60 UTC.  Or to put it another way,
the epoch was at 1970-01-01T00:00:10 TAI.

When did the TAI timescale first come into existence?  One answer
seems to be that TAI was born on 1958-01-01T00:00:00 UT2, which was
also 1958-01-01T00:00:00 TAI.  But OTOH the definition of the SI second
changed in 1967 and again in 1997.  What did these changes do to the
uniformity of TAI?

I found the following interesting statement at
http://www.maa.mhn.de/Scholar/times.html :

#  The need for leap seconds is not caused by the secular slowdown
# of Earth's rotation (which is less than 2 milliseconds per century)
# but by irregular variations in this rotation and by the fact that the
# definition of the SI-second is fixed on the duration of the year 1900
# which was shorter than average.

Not to perambulate              || John Cowan <[EMAIL PROTECTED]>
    the corridors               || http://www.reutershealth.com
during the hours of repose      || http://www.ccil.org/~cowan
    in the boots of ascension.  \\ Sign in Austrian ski-resort hotel

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