Re: [LEAPSECS] What problems do leap seconds *really* create?

From: John Cowan <jcowan_at_reutershealth.com>
Date: Wed, 29 Jan 2003 16:29:32 -0500 (EST)

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.

Indeed.

> 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

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.

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