# [ntp:questions] Leap second functional question

Darren Dunham ddunham at taos.com
Mon Feb 25 17:53:28 UTC 2008

Unruh <unruh-spam at physics.ubc.ca> wrote:
>>> While it may be slowing on average, it is noise and it could well speed up
>>> as well.
>
>>Noise implies that the changes are both positive and negative, in
> They are
>>which case a leap second won't be needed. If it is systematic,
> They would still be needed. Just because your computer's drift rate is both
> positive and negative does not mean that compensation is not needed.
> They do not necessarily average out on the time scale of years.
>
> There is a net drift to longer days. but superposed on that is a noise
> which even over the time scale ofyears makes a difference. An earthquake in
> Java rearranges the moment of inertia of the earth and changes the rotation
> rate of the earth, and it can be positive or negative.
>
>>i.e. the changes are more in one direction than another, a leap second
>>will be needed.
>
> It is needed even if it equal in both directions over a long time.

It is needed if the noise is equal in both directions *and* if the
average is somewhere near zero.

Besides the noise, there is a strong forward bias as well.  The second
is based on earth rotation in 1900.  The past century has allowed for
the average rate to slow significantly since then.

So a negative leap will only be needed if the noise is strong enough to
wipe out the entire bias.  The recent speedup has only been strong
enough to delay the positive second additions, not reverse them.

<http://en.wikipedia.org/wiki/Image:Leapsecond.ut1-utc.svg>
<http://maia.usno.navy.mil/lplot1.gif> (Excess Length of Day plot).

--
Darren Dunham                                           ddunham at taos.com
Senior Technical Consultant         TAOS            http://www.taos.com/
Got some Dr Pepper?                           San Francisco, CA bay area
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