> > In fact, leap seconds are simply due to the earth
> > being slow. How it got to be "slow" and whether
> > it is "slowing" are another issue.
>
> Let me see if I have this right:
>
> 1) We have leap seconds because the Earth rotates more slowly
> than once every 86,400 SI seconds.
Yes. (and I know what you meant wrt solar).
Equivalently, you could say the SI second was defined
a bit too fast. The error bars on Essen's 9192631770 Hz
cesium calibration were rather large and he used an old
epoch for the ET second.
The point is leap seconds are the result of a relative
rate error. Depending on the context it isn't important
if the earth was/is slow or the SI second is fast.
> 2) Leap seconds will become more frequent in the future because
> the Earth is decelerating.
Right.
> 3) Leap seconds occur irregularly because the Earth's deceleration
> is not constant and in fact changes unpredictably.
Not quite. Leap seconds occur irregularly because the
rate of rotation is irregular over the relatively short time
span of months to years. Even if there were no secular
deceleration over the time scale of centuries -- we would
still have leap seconds; and they would continue to be
irregular.
Rob said it better in a thread a long time ago:
> 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.
> Basically we don't have leap seconds because the Earth's rotation is
> slowing down (by transfering angular momentum to the Moon). Rather,
> we have leap seconds because the Earth has *already* slowed down since
> 1900. See the rather consistent slope of about 7 seconds per decade
> on the plot of UT1-UTC (with leap seconds removed) versus date:
> ftp://gemini.tuc.noao.edu/pub/seaman/leap/noleap.pdf
>
> The current dynamical effects are the subtle wiggles imposed on this
> trend. This is actually a fairly useful bias since it guarantees (short
> of asteroid impact or armageddon) that there will be no negative leap
> seconds.
/tvb
Received on Fri Aug 04 2006 - 13:08:11 PDT