Re: [ISO8601] Digest Number 227
- Fred Bone wrote:
>Adjusting the definition of the second would:
> Tex Texin said:
> > The problem for software is the unpredictability of when leap seconds
> > will be added.
> > If they were added at predictable times, then software could easily
> > accomodate them.
> > We need to add one about every year and a half. I wonder if it wouldn't
> > be better to schedule them out a hundred years so most software could
> > take them into account and let the astronomical software that needs
> > accuracy greater than 1 second deal with the special cases of where
> > astronomical time is different from utc.
> If it were possible to schedule them that far in advance, doubtless
> either it would have been done or the definition of the second would
> have been adjusted to eliminate the need.
a) be a really big deal because of the knock-on effect on all the
other SI units and derived standards and
b) not solve the problem in the long term anyway because the
variation in day length contains a trend towards longer days (as
well as short term unpredicatable fluctuations).
The reason we have had positive leap seconds but no negative leap
seconds is that, on average, days are now a few milliseconds longer
than they were when the measurements were made on which the original
definition of the second was made (I think I've seen both 1870 and
1820 quoted as the date on which the Earth's rotation period was
actually 86400.000 seconds). This slowing down is caused primarily
by the dissipation of energy caused by tides.
> The fact is that years are not all the same length, and the variationLeap seconds are more to do with variation in day length than year
> is inherently unpredictable.
length, i.e., they relate to variation in the rotation of the Earth
around its own axis, not round the Sun.
> Consequently the decision about leap
> seconds is made some six months ahead. The last one was at the end of
> December 1998, and we are currently experiencing the longest period
> with no leap second since the concept was introduced.
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/