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I have renamed this thread, because this aspect has diverged
significantly from the original topic.

I had written, in NavList 1724, Re: lunars with and without altitudes

| "I am aware that Frank Reed has been advocating the use of
observations
| around noon to determine the moment of noon for some time, and  have
no
| wish now to rehearse once again the weaknesses in that procedure.
As
| he says, it's an inferior way to get local time."

and Frank replied, in Navlist 1728

| Well, let's not confuse matters by misquoting me, George.
Observations
| *around* noon are a very good way to determine the moment  of noon,
as I have
| described at some length in the past (and probably will again
soon!).

What I had added, but Frank didn't quote, was-

"To achieve any
precision, it needs a wide bracket of equal altitudes, before and
after noon.", and I hope that Frank will agree that is the case.

| What I
| referred to as an "inferior" way to get local time was the late
18th/early 19th
| century practice of "calling out" noon based on the *single*
observation of the
| Sun's maximum altitude.
|
| And you wrote:
| "What, then, would be the procedure for discovering the moment of
noon
| AT noon, in such a way that the moment of noon can be "called
out"?"
|
| I was referring to the "common practice" for setting local time on
pocket
| watches carried by the ship's officers.

and added

I believe you are under the impression
| that ordinary watches were rare c.1800. There's good evidence that
they were
| not. Most officers aboard ship seem to have carried them by this
date.

I don't see what the rarity or otherwise of such watches has to do
with this discussion.

| They
| set  them at noon to local apparent time as determined by "calling
out" noon
| during  the Noon sight. This was a common practice, whether we like
it from a
| modern  theoretical standpoint or not.

What evidence can Frank offer to back that statement? It seems an
absurd thing to do, if later that day they were to reset them as he
describes later.

Navigators later in the day "regulated the
| watch"  by doing a time sight. This would lead to an adjustment (or
| correction) of the  watch by a few minutes. That "regulated" local
time from the time
| sight  would then be compared with the Greenwich time from a lunar
distance
| sight or the chronometer.

That is the right time for correcting the watch from an observation;
either earlier in the day, when the Sun was rising, or later, when it
was falling. Any time, indeed, other than noon, the worst possible
moment. If the watch had been set correctly earlier, by a previous
time-sight, , what on Earth would be the point of deranging it, to an
unknown extent, by making an imprecise observation, only to reset it
again, correctly, later?

Of course, corrections to the watch reading, in order to know local
time, have to be kept in mind, and carefully noted, following the most
recent time-sight determination. These corrections are due to any
uncorrected watch error determined then, any known watch-rate error,
any change in equation of time (easily determined), and most
important, any longitude change, estimated by dead-reckoning.

I think there may be confusion between a call of "make that noon", or
some similar words, for any others, out on the bridge with their own
sextants, to freeze the measured altitude at that maximum value. That
would not imply a resetting of watches.

I ended up by asking, about the notion that watches would be reset to
local time at noon by calling out noon-

"How would that
procedure cope with any North-South component of the vessel's speed,
and the changing declination of the Sun? And what would be the
expected level of precision in the result?"

Any attempt to determine the moment of  noon from the moment of
maximum Sun altitude is, as Frank well knows, very susceptible to
large errors caused by any North-South motion of the vessel (or even
the changing Sun declination, to a lesser extent). But that question
wasn't answered.

=======================

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?

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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