NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant Accuracy and anomalous dip
From: Trevor Kenchington
Date: 2003 Mar 23, 15:56 -0400
From: Trevor Kenchington
Date: 2003 Mar 23, 15:56 -0400
Fred Hebard wrote: > This leads us back to Jared's question, where they did seem to be > establishing their observatory in a random spot. However, perhaps it > wasn't too difficult to measure the distance to a known spot in the > vicinity, or perhaps the spots were known and had been located by > observing the moons of Jupiter or some such. Is it possible that, contrary to the written account, they were making their observations not at some convenient random point but at either established survey triangulation stations that had already been tied in by geodetic surveyors or benchmarks surveyed relative to such stations? That might explain why they ended up at forts: Trig stations need to be on hilltops with good visibility all round, which tend to also be favoured spots for forts, while it helps to have the trig station on government property. Streets in the middle of towns, which the cable-ship navigators also found themselves in, are likely places for benchmarks. The best place to check the going of your chronometers would obviously be a proper time observatory. If that isn't possible (and it would rarely be -- time observatories being few and far between), your own observations for time from a point of known location would be the next best thing, with stations established by geodetic survey being the only ones known with adequate precision for cable-laying purposes. But to try checking the going by observation without knowing where you are, and without setting up an observatory to determine the time (which the cable ship's personnel do not seem to have done), appears hopeless. I note George's suggestion that they found time from cable signals and hence the precise location of the cable hut. But if, as George suggests, the written account relates to checking the chronometers following a cable break, using the known location of the cable hut and observations for time, why were they taking the observations at forts and on town streets, rather than alongside cable huts? Unless all they were checking was the rate of going of the chronometers, not the accumulated error, the written version seems to be deficient in at least one detail: It seems that they must either have known the time (from cable signals) or else their position (from prior geodetic survey or from observations using time from cable signals). And if they knew the time precisely, they would not have been taking observations to check it. Hence, I suggest that (by one means or another) they knew exactly where they were when taking their observations. Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus