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----- Original Message -----

From: Hanno Ix

To: NavList@fer3.com

Sent: Thursday, August 06, 2009 7:34 PM

Subject: [NavList 9388] Re: Working a lunar

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George: Thank you for taking the time to think through this concept of mine - as of yet still half baked. Let me get to the core of the concept in form of a couple of questions: 1. Is it true that: on land, the culmination of a heavenly body, including the moon, can be established rather accurately in reference to altitude and in reference to time as indicated by a stable local clock? Neglect for now culminations near the horizon. Typcally, on land a transit is being used for this measurement. How well the observed culmination refers to the real local Noon or GMT is not at issue in this question. The "stable" clock need only proceed accurately from second to second, but may not indicate GMT or even the local time. 2. Is it true that: for any given time, there exists on the surface of the Earth a Geometric Locus of all those positions that have the same time difference DT between the culminations of two heavenly bodies? (The Geometric Locus need not be contiguous over the surface of the Earth and will change with GMT.  I assume it to be close to a straight line locally.) Affirmative ansers to these two questions form my basic assumptions. If they are not valid I will hold my peace. Best regards. H PS: Found your prior article and enjoyed it. --- On Thu, 8/6/09, George Huxtable <george@hux.me.uk> wrote: From: George Huxtable <george@hux.me.uk> Subject: [NavList 9385] Re: Working a lunar To: NavList@fer3.com Date: Thursday, August 6, 2009, 1:29 AM Hanno Ix suggested a different approach to lunars. I don't follow some details of that proposal. He wrote- "A ship reaches land of unknown coordinates. Land makes it practical for the navigator to measure the meridian passages of Heavenly Bodies rather reliably. Given GMT, he can calculate LAT and LONG. (One shot method.)" What is this "one shot method", which allows both lat and long to be obtained? Does it exist? However, leaving that aside, Hanno seems to be reinventing the wheel, as he suspected, when writing "I doubt if it is new". Obtaining time, and thus longitude can, in theory at least, be done by measuring simultaneous (or nearly so) altitudes of the Moon and another-body, rather than the lunar distance, across the sky, between them. Effectively, two-time-sights are being taken, which, because of the motion of the Moon with respect to any other-body, will only correspond to each other if the correct GMT has been assumed. Just as with any other time-sight, it's most accurate when the altitude changes rapidly with time, so is worst anywhere near meridian passage. Such a method has been proposed several times over the years, notably by Francis Chichester, in an article in "Journal of Navigation", misleading named "longitude without time", and followed up in other forums. It's described best, I think, in chapter 17, "Time by lunar lines of position", of John Letcher's book "Self-contained celestial navigation with H.O. 208" (1977). The method has been discussed on this list a few times, under various threadnames which I can't now recall, but that's no reason why Hanno shouldn't raise it again. The proposed method has the advantage of using the sextant, and making the corrections, in a familiar way, unlike lunar distances; hence its appeal. But it has serious snags. The lunar-distance method itself has the great drawback of lack-of-precision. A lunar distance, measured with an accuracy of 1 arc-minute, can establish longitude only within an error 30x greater, or 30' of longitude. So it has been useful in marginal circumstances when any rough notion of longitude is better than no notion at all: but not much better than that. The lunar-by-altitudes method is, most of the time, significantly less accurate, even, than lunar distances, for several good reasons, and that's enough to rule it out of court, or it least to outweigh any advantage it might have.  With a lunar, the thing hinges on a measurement of a single quantity, the angle in the sky itself. Provided the observer possesses enough skill, and the ship's motion is kind, this can be measured with some accuracy, because the horizon isn't involved (except for use with auxiliary corrections, not needed to high precision). In contrast, in any altitude observation, uncertainties in the horizon itself provide most of the inherent errors, and there have to be two such altitude measurements, not one, so increasing the scatter. With a lunar distance, angles between Moon and other-body are changing, increasing or decreasing, at a rate that's always in the region of 30' per hour, provided certain simple rules are followed. That can also apply to altitude measurements made in the tropics, when the Moon and other-body will pass nearly overhead, but not from higher latitudes, when the rate-of-change will always be less, and indeed much less when anywhere near meridian passage. These are practical problems that conspire against the altitude method; sufficient to explain why it has never been adopted in practice. If I've misunderstood what Hanno was proposing, perhaps he will explain further. George. contact George Huxtable, at  george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Hanno Ix" <hannoix@sbcglobal.net> To: <NavList@fer3.com> Sent: Thursday, August 06, 2009 6:57 AM Subject: [NavList 9382] Re: Working a lunar Gentlemen: I am a novice to CelNav, and I certainly have no experience in lunars. Some algorithm occurred to me, though, that I would like to share and discuss. However, given the age of this business, if it is a valid one I doubt it is new. If anyone has seen it before, please let me know, so I could read up on it. The objective is to find GMT and location. Let's make a Gedanken experiment: A ship reaches land of unknown coordinates. Land makes it practical for the navigator to measure the meridian passages of Heavenly Bodies rather reliably. Given GMT, he can calculate LAT and LONG. (One shot method.) But now we pose GMT as unknown. Sitting on land, measure the meridian passages of, say, sun and moon which moves. Can I find GMT, too, using the now available data not using the classical moon distance methods? If I see things right, there must be a LOP which connects all locations on Earth with a given, fixed difference DT between the meridian passages of sun and moon. However, along this LOP, the same DT occurs at a different GMT. In this scenario, the LOP referring to a given DT is pre - calculated, listed in an almanac and annotated with GMT at each LAT. So, by having found the LAT before we just read the GMT of the DT-specific LOP. There is another opportunity: By accepting preliminarily this GMT, we can calculate LAT again, namely from the meridian passage of the MOON, and compare both values found. If there is a gross difference we must have made an error. This, by itself, would be of value. Otherwise, though, we have good reasons to accept the GMT we found. I appologize if I am talking about a method I have not gone through myself yet! I fear there is a hick-up in this somewhere. But I would like to hear the critique of you specialist navigators before I spend alot of time trying to do something long known as wrong. If, however, you find it sound, and has not done before I will pusue it. Best regards

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