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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Northing correction to Noon longitudes.
From: Henry Halboth
Date: 2005 Jun 7, 14:17 -0400
From: Henry Halboth
Date: 2005 Jun 7, 14:17 -0400
It all seem unnecessarily complicated. Within the appropriate time range, why not calculate a series of ex-meridian Latitudes for specific times and advance each to the time wanted, as the hydrographical surveyors did, and have the thing over with. I believe that Frank said at one time that he had a copy of Wharton + Fields book on hydrographical surveying, in which, if my recollection is correct, the matter is dealth with quite fully On Tue, 7 Jun 2005 15:09:29 +0100 George Huxtablewrites: > On 4 June, in "Latitude and Longitude by "Noon Sun"", Frank Reed > explained > his method for correcting "around noon" observations for longitude > for the > North-South component of vessel's speed, after explaining how to > obtain > that component, including declination change. > > "If you're moving towards the > Sun, then for every six minutes away from noon, add 0.1 minutes of > arc for > every knot of speed to the altitudes before noon and subtract 0.1 > minutes > of arc > for every knot of speed to the altitudes after noon." > > So, if there were 13 observations plotted, each of these (perhaps > only 12 > of them) must be individally adjusted, by taking the time interval > in > minutes between each point and some (arbitrary?) time-zero, dividing > by 6, > and multiplying by 0.1 x the speed in knots, adding or subtracting > the > result from the altitude, and replotting a new point. It doesn't > sound like > a trivial operation to do 12 times over, does it? > > Instead, I suggested that the original altitude data points be left > uncorrected, to provide (using Frank's folding-paper method) the > moment of > maximum altitude, on which- > > "The moment of LAN is delayed by 15.3 (tan lat - tan dec) * v where > v is the > Southerly component of the speed in knots." > > To which Frank responded, on 6 June, > > "That adds needless complication to an otherwise extremely simple > procedure." > > Well, does it indeed? It appears to be a great simplification, to my > mind. > Perhaps list members will judge for themselves. > > This correction doesn't need to be made very precisely because it's > only a > small one, but it certainly must be made. For the purpose, both Sun > dec and > ship's lat will be changing rather slowly. The simple trig > expression 15.3 > (tan lat - tan dec) can readily be precalculated (and changes only > slowly > from one day to the next). It just needs multiplying by Northing > speed to > provide a result which is the time-difference in seconds between > maximum > altitude and meridian passage. So: no fiddling with the original > graph, > just one simple multiplication, followed by one time correction. > Which is > simplest? > > By the way, I failed to mention, in previous postings, what should > be > rather obvious; that in the above expression both lat and dec should > be > taken as positive North, negative South. > > ============================ > > Then I asked- > > "The whole object of the exercise is to discover the moment of > noon. So how > does the observer know how many minutes each plotted point is away > from > noon in order to calculate that adjustment?" > > I went on to answer my own question, but that part wasn't quoted- > > "The answer is, I think, that it just doesn't matter, as far as > finding the > new centre-of-symmetry is concerned. For the purpose of making those > corrections, any point could arbitrarily be presumed to be the > moment-of-noon, and then the new centre-of-symmetry would show true > noon, > when the Sun was on the meridian." > > Frank's answer was- > > >It makes no difference. Whatever point in time is picked as the > "zero" > >point, where no adjustment for northing/southing is made, will be > the > >time of the > >fix. Being able to label the fix as "noon" is not terribly > important but it > >is nicely traditional. The real time on it, of course, is a moment > of GMT. > > It may be my fault, but I don't understand what Frank is saying > here. The > time that results from the paper-folding operation of the corrected > graph > is the moment of noon, surely, when the Sun crosses the meridian, > and what > we need to know to get the long is the chronometer reading of GMT at > that > moment (after equation of time is chucked in). I don't understand > how some > arbitrarily chosen moment, at which the corrections to altitude are > taken > to be zero, can be the "time of the fix", whatever that means. So I > suggest > that my own answer, above, to my question is the correct one. > Nevertheless, > we seem to agree that choosing a different zero-point for the > corrections > will not shift the timing of the corrected peak, which depends on > the slope > of the corrections, but not their amount. > > Then I went on to- > >"However, it looks to me as if an error in > >that initial presumption of noon would give rise to an error in > the deduced > >maximum altitude, and so in the latitude. Perhaps Frank will > comment." > > Frank did, as follows- > > "Nope. No error. See above." > > However, I urge Frank to rethink his flippant dismissal of the point > that I > have made. What's needed, to calculate latitude simply, is the Sun's > altitude AT MERIDIAN PASSAGE, and not at any other time. To obtain > that, > Frank tells us to take the altitude from the peak value of the > corrected > Sun-altitude curve, at his "folding" point, which will be at > meridian > passage. But that's not the observed altitude, it's the corrected > altitude, > at meridian passage. The correction that's been made to observed > altitude, > at that moment, depends on how far it is away in time from the > zero-point > of his corrections, and that zero-moment was chosen quite > arbitrarily. Only > if the zero-point of the corrections happened to be at the moment of > meridian passage, would the peak of the corrected-altitude curve > correspond > to the observed altitude at that moment. > > So I suggest that Frank's proposed method should be somewhat > modified. Yes, > certainly, use the corrected-altitude curve to determine, from its > symmetry, the moment of meridian passage. But then, read off, > corresponding > to that moment of meridian passage, the UNCORRECTED value of > altitude, > which will NOT in general be its peak value. > > ==================== > > Finally, there's a curious comment, as follows- > > "By the way, perhaps George could consider addressing people in the > second > person. Thanks in advance." > > Can any list member, perhaps Frank himself, kindly explain what he > is on > about here? Otherwise, that request is completely lost on me. > > George. > > ================================================================ > contact George Huxtable by email at george@huxtable.u-net.com, by > phone at > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 > Sandy > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > ================================================================ >