email@example.com (joost witteveen) wrote on 22.06.97 in <m0wfr3z-000CMdC@rulcmc.leidenuniv.nl>:
> > > Posix time includes leap-year-days, but does not include the finer
> > > resolution of leap-seconds. 21 leap-seconds (number 22 is coming up)
> > > have been added since New Years Day 1970 to keep clock time in synch
> > > with astronomical time.
> > Actually, it probably was a bad idea to use "leap" for both. Leap days are
> > fixed by calendar design. Leap seconds are inserted or deleted (both are
> > possible) after comparing the atomic clocks to astronomical observations,
> > with no predictability at all. Two very different animals.
> well, depends on how you see it. The before 1752, century turns were
Make that 1582, when Gregor decided on it. Or maybe 1918, when the last
country switched over.
> still all leap years. Now, we know the length of a year/day better, and
> only 1 in for of those turn-of-century years are leap years. Maybe that
> will change again.
And before Gaius Iulius reformed the calendar, no year had leap days.
But that's the important part. All these are calendar reforms. Leap second
aren't calendar reforms.
> And about the seconds: we (currently, prossibly always)
> simply cannot calulate the length of a day accurately enough to know
> well in advance when to insert them. But I'd say the two animals are
> at least related, if not mother and daughter.
I think you got the reasoning wrong there. It's not that we can't
calculate exactly enough, it's that the rotational speed of the earth
isn't constant. It's modified by lots of influences, and these days, we
can measure the effect.
No calendar change in history was related to changes of earth's rotational
So, no, they most definitely are not closely related.
As to future changes, we know that the Gregorian calendar has an error of
about one day in 3000 years; that is, the Gregorian year averages to
365.2425 days, whereas it should be 365.2422 days. I don't know if anyone
will ever care about that.
However, note that the difference is roughly 1/3000, whereas the leap
second difference is roughly 1/90000.
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