[ntpwg] Timing Definitions
seaman at noao.edu
Tue Feb 20 17:01:52 PST 2007
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
>> coinage on the pendulum. Presumably, the "accuracy" of Big Ben
>> on the residual frequency error and the interval between coinage
>> updates. You set your watch to Big Ben and inherit its
>> 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
> 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
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
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...
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
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
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
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