# NavList:

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

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Re: How Many Chronometers?
Date: 2009 May 6, 19:26 -0700

```Hi Jim

Chronometer rate is the gain or loss of time per day, or whatever period you
may chose to define.  The assumption is that the rate is constant is
fallicious.  The rate of a chronometer varies with many factors.  Take a
close look at the chart recently provided by George.

If the rate was constant, the line would be straight, independent of
corrections at known longitudes.  In fact, we see the converse.  The line
changes slope from correction to correction.  We even see positive going
slopes change into negative going slopes.  The rate, therefore, is changing.

The argument will be offered that modern mechanical chronometers are superior.
In fact, they are.  However, they are not perfect.  The rate still changes
with the viscosity of the lubricant with temperature. Hence the double box,
to slow the transfer of thermal energy, with a glass top, such that you can
see the chronometer without opening the box. The quartermaster is cautioned
to wind the chronometer every day, at the same time, by the same amount, such
that the chronometer utilizes the same spring region, independent of the wind
down counter that provides the hours until unwound.  All this attention to
detail, and the rate still varies.

If you remember other parts of the Royal Observatory, where H1 lives
(hooray!), then you have seen the gradual transformation to other time
pieces, away from the mechanical.  And each time piece has been superceded by
the next, each with dilemmas, problems and issues.

Back to Greg's question though, which I understand to be the determination of
when a chronometer has gone bad, we have offered the solution of solving for
time when the longitude is known. If the longitude is not known, then you
suffer from not knowing which one is wrong, the longitude or the chronometer!

Bruce Stark, when he was contributing to the list, offered a wonderful
exposition on how to recover GMT via lunars.  The details are found in a
NavList posting whose number I cannot remember.  I did keep a file of it
though, and link it herein.  All the way from not knowing any time to GMT! I
have not exercised the method and can offer no insight as to how accurately
one can recover GMT from it.  Based upon the source, however, I suspect it
works quite well!

Best Regards

File: Recovering-GMT.doc

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