[LEAPSECS] Looking-glass, through

[Warner Losh]

On 01/12/2011 10:30, Steve Allen wrote:

> On Wed 2011-01-12T16:36:35 +0000, Tony Finch hath writ:

>> Yes, but how accurately do you need clocks to track it? How frequently do

>> you need to make adjustments to correct for the atomic/angular rate error,

>> and what size of adjustment is acceptable?

> It would appear that making adjustments every 10 days is not

> often enough, at least in the US, viz:

> http://www.nist.gov/pml/div688/grp50/NISTUTC.cfm

> http://www.nist.gov/pml/div688/grp50/nistusno.cfm

>

> Even if we abandon the leap second, we have issues at the nanosecond level.


These are the computed errors between the 'paper clock' that is UTC and 
the various realizations of UTC.  This data shows an error of about 
.2ns/day (give or take) over the periods listed.  This is an error of 
about 2.5e-15.  This likely corresponds to the accuracy of the cesium 
standards used by NIST to realize time, since high precision 5071A's are 
good to something less than a nanosecond per day.

This is several orders of magnitude smaller than the UT deviation from 
86400 SI seconds, which is on the order of 1ms per day with variations 
on the order of 1ms over time (The 10 day estimates are in the 10's of 
microsecond range as far as accuracy).


>>> For instance, what authority will historians or lawyers consult to learn

>>> the applicable timezone offsets that were in force in some location(s)

>>> during some epoch(s) in question?

>> That problem exists whether universal time is atomic or angular so it

>> makes no difference to the proposal.

> When the leap second was invented there were countably few systems which

> could count every second, so a second was not a problem.  Now it is.

>

> Right now there are countably few systems which can count every nanosecond.

> Unless there is some sort of conceptual barrier which prevents a need

> for nanoseconds, when such systems do become common the problem of

> historic time zone offset reconciliation will be trivial by comparison

> to the issues of systems which believe that nanosecond (picosecond)

> synchronization is possible without table lookups and continuous

> effort to track the table values.


Most phones need to be synchronized to the microsecondish level, so the 
day is coming when that will be more of an issue.

But keep in mind that when you get down to the nano-second level, all 
those crazy things you learned about in physics start to matter.  You 
have relativistic effects between different frames of reference.  You 
have gravitational effects if you change altitude from sea level, you 
even have Sagnac effects due to the rotation of the earth, etc.  All of 
these effects can cause changes in time elapsing at the nanosecond 
level.  I think puts a practical limit on how closely clocks can and 
will be synchronized and syntonized.

Of course, reading this now, I may be suffering from the 'Nobody will 
ever need more than 640k of RAM in their personal computer' argument.

Warner


> Abandoning leap seconds simply sweeps the need for good timekeeping

> practices under a rug rather than giving ongoing incentive to design

> systems which match the way chronometers actually work.

>

> --

> Steve Allen<sla at ucolick.org>                 WGS-84 (GPS)

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