[ntpwg] Timing Definitions

[Rob Seaman]

On Feb 20, 2007, at 4:14 PM, Kurt Roeckx wrote:

> On Tue, Feb 20, 2007 at 12:56:05AM +0000, David L. Mills wrote:
>
>> Stand next to Westminister Abbey and set your watch to Big Ben.  
>> Big Ben
>> clockkeepers keep the clock in tune by listening to the BBC and  
>> putting
>> coinage on the pendulum. Presumably, the "accuracy" of Big Ben  
>> depends
>> on the residual frequency error and the interval between coinage
>> updates. You set your watch to Big Ben and inherit its  
>> characteristics
>> and some of your own. How "accurate" is your watch?

This example is also given in Dr. Mills' excellent book.

> Since I have no idea about what the accuracy or precision is of Big
> Ben, it's going to be rather hard to say anything about it.  And I
> think that's the point we're both trying to make.

I don't think so.  Some protocol could be implemented to convey  
information regarding Ben's current load of coinage and about how far  
it leads or lags the BBC.  It isn't ignorance of these metadata that  
causes trouble in estimating accuracy, it is the lack of a  
fundamental standard.

> Anyway, there are 2 ways to synchronize your clock to Big Ben.

I'm sure there are many more than two.  This reminds me of the  
barometer:

	http://www.snopes.com/college/exam/barometer.asp

I can even add one more.  I've got a nifty new MacBook with the  
accelerometer chip.  The SeisMac app comes with a library that can  
poll three axis values at a couple hundred Hz.  Placing a MacBook in  
close enough proximity to the tower's clock would allow ticks to be  
detected.  Ben is actually the name of the bell, and one might well  
be able to count the striking of the hours from outside the  
building.  Anybody in London want to try?

> If I'm going to set my watch by any of those 2 methods, what  
> matters is
> which way has the smallest total error.

What matters is whether you have a handy fundamental reference to  
consult - see the appended message from the leapsecs list, for  
example.  (Any followups about leap seconds should be directed that  
way - not for the faint of heart.)  The utility of NTP is precisely  
in conveying an estimate of a fundamental standard to a remote  
locale.  At NOAO we rely on NTP on Kitt Peak in Arizona and Cerro  
Tololo in Chile.  Imagine if the weights and measures folks had to  
recalibrate a meter, a liter or a kilo from 5000 miles away -  
continually, 24/7.  The nice thing about UTC (or other flavor of  
Universal Time) is that we're all sitting on top of the fundamental  
reference.

To return to the point in question.  The current draft looks good.   
Wrap it up with a pretty bow.  It's about time you guys started  
working on ntpv5...

Rob Seaman
National Optical Astronomy Observatory
------

  	From: 	  seaman at noao.edu
	Subject: 	Re: [LEAPSECS] how to reset a clock
	Date: 	January 4, 2007 8:15:41 AM MST
	To: 	  LEAPSECS at ROM.USNO.NAVY.MIL

Peter Bunclark wrote:

> Indeed isn't this Rob's ship's chronometer?

Actually, I think it was Mr. Harrison's.  (And Steve Allen has been  
basing his arguments more recently on this distinction.)  This  
healthy debate between astronomical time and clock time has happened  
before.  The answer is the same as before - both types of time are  
needed.  (Some things never change.)  I'm sure Pete is more familiar  
with this story than I am, but others may not be.

Harrison attempted to build a perfect clock to win the Longitude  
Prize.  Folks who haven't read Sobel's book should do so - my  
classmates at Villanova and I learned the story from an Augustinian  
priest who appeared old enough to have known Harrison personnally.   
Harrison's first glorious shipboard clock failed to take the prize  
due to a lack of compensation for centrifugal effects on a sailing  
vessel that must tack when sailing against the wind (or must wear  
through an even larger angle, bringing the wind across its stern).   
Compensation was needed for relativistic effects, if Newtonian rather  
than Einstein.  (Some things never change.)

Harrison invented or improved a variety of familiar mechanical  
doodads like the roller bearing and bimetallic temperature  
compensation.  He likely could have succeeded in solving this  
particular problem, but there would always have been another physical  
improvement needed.  (Some things never change.)  Each improvement  
would have made the clock more complicated and eventually too fragile  
to possibly work on a constantly moving platform buffeted and often  
bathed by the salty sea.

He created a second clock and was working on a third round of  
improvements when the idea we're discussing first occurred to him.   
He had been using a pocket watch as a mechanism to transfer time from  
stationary standard clocks (many built by himself) to his portable  
prototypes.  He would reset the clock in one place and physically  
carry it to where the time was needed.  If a roundtrip correction  
were needed, presumably he would note the time on either end and  
halve the difference.  This is the "standard synchrony" or  
"conventionality of simultaneity" of special relativity - familiar to  
anyone who has looked under the hood of NTP.  (Some things never  
change.)

What Harrison recognized was that he didn't need to build a perfect  
clock - he merely needed to quantify and log the error inherent in  
the clock.  By replacing a large and finicky "better" clock, with a  
small and robust, but more even-tempered, one, the rate of the clock  
could be regularized and its random and systematic errors could be  
minimized.  That the rate of the clock was now guaranteed not to  
match the rate of the spinning Earth was no longer a bug, but a  
feature.  By carefully calibrating the clock rate before leaving on a  
voyage, and checking it against astronomical observations throughout  
the voyage, it was possible to compute the mean solar time at the  
home port.  (Some things never change.)  Comparison with the local  
time, measured by sextant, then recovered the longitude directly.

And, of course, a ship would not carry a single clock, but two or  
more.  Friendly ships meeting at sea would also exchange clock  
readings - creating the first ensemble time scale.  (Some things  
never change.)

Thus was the chronometer born - and thus did Britannia rule the waves.

The point is that time isn't just an unending count of seconds - it  
is the epoch of when the count was zero.  That epoch often has  
significance in some periodic natural phenomena, usually related to  
Earth orientation.

Rob

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