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On 24 Nov 2006 at 11:40, George Huxtable wrote:

> Of course, it never matters exactly what a watch-dial actually reads, as
> long as any error on that reading is known.
>
> I think that's what Ken Muldrew was getting at, when he wrote, in
> [NavList 1729]-
>
> "When keeping apparent time on a watch, it seems to me that using the
> approximate time of noon through a sextant altitude observation is
> perfectly satisfactory. In my own attempts to keep apparent time on a
> mechanical pocket watch, I found it impossible to set accurately from a
> time sight. It was much easier to simply set the approximate time (taking
> care to match the position of the minute hand between tick marks to the
> position of the second hand) and then use a time sight to get the error.
> Between the equation of time and any East/West motion, the watch will be
> off so quickly that there is really no point in taking pains to set it
> accurately."
>
> He seems to accept, then, that the setting at noon was to no more than an
> approximate time, to be put right at a later time sight. After having set
> it approximately, around noon, would he then be prepared to use that
> approximate time for navigational purposes, prior to determining those
> errors with a proper time-sight?

Let's agree that if you're using a mechanical pocket watch to navigate,
then you're also stuck somewhere about 200 years ago. This means that you
are not doing position line navigation but independent latitude and
longitude measurements. Since the noon sight has given you a good
latitude, and since the best you can hope for with longitude will come
from a lunar distance (at least as far as celestial observations go),
which is pretty innaccurate even with good sights, then I would say that I
would be perfectly happy to navigate using the watch that shows only
approximate time.

My watch is showing local apparent time, so it is being used to measure
intervals for updating the dead reckoning; I wouldn't be trying to infer
longitude from the time. If land is nearby, then surely I would be
navigating by latitude + observation + soundings and not by anything that
requires knowing the precise local apparent time. I'm not sure what I
might miss by having a watch that is, say, 10 minutes off.

Ken Muldrew

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