# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Chronometer Suggestions
From: Alexandre Eremenko
Date: 2009 Jan 5, 15:39 -0500

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Dear Irv,

I am not a real expert on chronometers, but let me explain
what I do know. A chronometer is not expected "to show time"
in the same sense your wristwatch is expected. They always
applied corrections to chronometer time, and did not attempt
to adjust its "going" to zero.

The main difference between a chronometer
and an ordinary watch is that a chronometer is expected to
have a very constant RATE.
For example, it can be slow 1.5 seconds per day, and
this is fine as long as this 1.5 remains constant.

When chronometers were used for navigation, they applied
daily correction to them. Once you check it in a harbor,
and obtain an error, say T(0), the correction on the n-th
day is
determined by the formula T(n)=T(0)+Rn,
where R is the daily rate. They did not try to adjust it
to make R=0.

Earlier (in 19-th and early 20-s century) they used more
complicated formulas, taking into account the temperature
of the chronometer which was measured by a termometer in
the chronometer box.
This was not done with the later chronometers because
they are very well temperature compensated. This should apply
to Hamiltons and Mersers as well.

I never tried to adjust my chronometer,
and it has daily rate of -1.88 sec per day
(determined from
two runs about 2 months each). When I use the above
correction formula,
it gives me the true time within a second on most of
the days, say only 1-2 days in a month I obtain an error of
more than 1 second, and never obtain more than 2.

All this conforms to the factory specification.
I did not test it at
various controlled temperatures yet,
as they do in the maitenance
shops.

I compare it with a good modern mechanical wristwatch
(Swiss mvmnt ETA 2824-2). This watch looks BETTER
for daily use:-)
It deviates by less than 20 seconds in a month run. But it
is not usable for navigation, without daily corrections.
Because the rate is not constant: it goes one-two
on one day and then one second behind on the next day, and
this is UNPREDICTABLE. On one 15-day run, the average error
was zero, but the maximal error
on some days was up to 8 sec.

And one more remark on your message:

On Mon, 5 Jan 2009, Irv Haworth wrote:

> I have two chronometers (both Thomas Mercer)
> (Hamilton is more or
> less the USA version)

I doubt that Hamilton is a US version of Mercer.
I read that Hamilton company delepoped their chronometer
independently, starting from their railroad watch,
and when I look at the movements
they look completely different. I saw many claims that
Hamilton 21 was the most accurate mechanical chronometer
in the world in its time.

Best regards,
Alex.

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