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Ken Muldrew wrote:

> The way I read the passage from Bowditch that Bruce cites is that the
> navigator of old regulates the chronometer by bringing it into unison with
> local apparent time. The error in the mechanism is immaterial since LAT is
> constantly changing due to the movement of the navigator. Since the local
> time is only known approximately anyway, there is no need to worry over
> the running of the mechanism itself (within limits). Presumably this is a
> different chronometer than the one being used to track Greenwich time,
> being used just as a common watch was before chronometers were available.


But a chronometer is a chronometer, not a watch to be re-set daily.

What I understand Bowditch to have meant is that Time (unless otherwise
qualified) means local time -- LMT, not LAT, as is clear from his words.
The chronometer (after any corrections for its rate of going) tracks GMT
but that is not Time. The difference between the chronometer's corrected
indication and LMT is then an error in its representation of Time, from
which error one can calculate longitude at 15 degrees to the hour.

To a modern navigator, of course, Time (the true, fundamental time) is
GMT, as shown by the chronometer (after whatever correction for its rate
of going). Zone time is just a convenient way of reconciling GMT to
local daylight hours, while local time is something involved in figuring
out the GMT of meridian passage and the like. We wouldn't think of, for
example, keeping a logbook in local time as Bowditch's contemporaries
did. But if we know GMT and longitude, figuring out LMT is trivial.

And that, I think, is Bruce's point: Where we use Time=GMT plus
longitude to find LMT when we need it, an earlier generation of
navigators used Time=LMT (or even LAT) plus the difference between LMT
and GMT to find longitude. Which serves to explain why a time sight is
called a time sight not a longitude sight: It was used to find Time when
Time was considered to mean local time.


Trevor Kenchington


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Trevor J. Kenchington PhD                         Gadus@iStar.ca
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