 Check out the bookstore at IRBS.com |
|
|
|
|||||
|
||||||
Subject: Re: How did Sumner navigate in 1837?
From: Herbert Prinz (hprinz@XXX.XXX)
Date: Fri May 23 2003 - 11:30:51 EDT
George Huxtable wrote:
> I don't see that [position line navigation] had to wait for the chronometer
> (which, it's true, was
> only then beginning to percolate down within the reach of ordinary
> mariners, outside the elite ranks of the Royal Navy and the East India
> company). The cleverer mariners had been finding their time using lunars
> for the previous 70 years. There's no difference between a time measured by
> a lunar and a time measured by a chronometer (except for its precision,
> perhaps). So wouldn't position-line navigation have been just as useful to
> a navigator that had obtained his time from a lunar, as to a chronometer
> user?
The one big difference between time by lunar and time by chronometer is in the
instantaneous availability of the latter. Jan Kalivoda put it succinctly: "The GMT
obtained from lunar distances was so inaccurate and so rarely obtained that the idea
of LOP hadn't soil to originate before chronometers." Let me add that once the soil
had been provided, it needed fertilizers too: The almanacs switching from apparent
to mean time must have helped. Second, the realization that celestial LOPs could be
plotted as straight lines on Mercator charts lead to graphic solutions. Thus a
position line could be visualized and was not merely an abstract mathematical
concept. This, in turn, facilitated a better integration between coastal and
celestial navigation.
Before the chronometer, a navigator who had a recent lunar observation necessarily
had gone through the whole ritual of latitude, apparent time, LD, GMT, longitude. He
would did not need a line of position. He had a position.
Sumner, on the other hand, found the position line by shooting for time with the
intention of finding longitude by chronometer. He would not have done what he did,
if he had not had a reliable specimen of such an instrument. If he could not get a
latitude during his last 600 nm, he certainly would not have had the chance for a
lunar observation for at least two or three days. When he finally got the chance to
take an altitude observation, he was faced with the choice of either trusting his
dead reckoning, or trusting his chronometer. It was an act of faith. An other
captain might have dismissed the sight as useless.
Once navigators had accepted GMT based celestial position lines as a viable
navigational concept, subsequent efforts to devise more efficient methods of
generating them made them more practical and thus even more desirable. This may have
stimulated a higher demand for good chronometers.
So, it seems we have here a classic case of what Kuhn named a paradigm shift. The
shift happened relatively fast, but by no means overnight. We remarked repeatedly on
this list how long it took from Sumner's publication to the general adoption of his
method by standard manuals of navigation. It was not so much a question of the
superiority of an isolated new reduction method as a question of confidence in new
technology and readiness to think in a totally new way.
I append here my answer to another comment by George Huxtable that is related.
> Herbert asked what a take to be a rhetorical question- "How common was it
> to do a time sight at all?". What was he implying by that? Surely any
> mariner who wanted to determine his longitude would have to take some sort
> of time-sight, involving two altitudes. It was required for any mariner who
> wished to proceed beyond latitude sailing.
So, here then I pose yet another "rhetorical" question: How many merchant mariners
wished to proceed beyond latitude sailing before the chronometer?
In fact, these two questions are not rhetorical at all, as I do not know the answer,
but they certainly express some degree of scepticism. I have studied common
textbooks on navigation of the period, but unfortunately have not had the
opportunity too read relevant ship logs of the merchant marine. I know what could be
done, or should have been done, but not what _was_ done. Thus, my honest question to
list members who have seen such logs whether the average merchant mariner took time
sights regularly in actual practice?" (I know that double or combined altitudes were
taken, but mainly for latitude. Was the time information contained in there actually
extracted?)
The value of a time sight without simultaneous knowledge of time in a reference
place (or the intention to obtain it shortly via lunars) seems questionable. But
lunars were not part of the regular routine. Time would not have been observed
merely for the purpose of telling the cook when it was proper to serve lunch? (Yes,
this time it's rhetorical!)
Best regards
Herbert Prinz
|