Re: [LEAPSECS] how to reset a clock

From: Rob Seaman <seaman_at_noao.edu>
Date: Thu, 4 Jan 2007 08:15:41 -0700

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
Received on Thu Jan 04 2007 - 07:21:02 PST