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    Re: Longitude by Time Sight...good enough at sea?
    From: Doug MacPherson
    Date: 2012 Nov 21, 16:06 -0800
    Thank-you gentleman for your in depth responses.  Frank, I do actually get very good accuracy on land with my Astro III sextant. Typically within 1 to 2 nautical miles. Glad to hear that the time sight is still appreciated.

    Cheers,

    Doug

    On Wed, Nov 21, 2012 at 2:39 PM, Frank Reed <FrankReed@historicalatlas.com> wrote:

    Oh, yes, there's nothing wrong with the time sight! Well, ok, there are a few things wrong with it, but nothing that we can't deal with. :)

    You wrote:
    "This seemed to be the normal procedure prior to the distribution of Ho 229 and others."

    It was in gradual decline from the late 19th century forward and the rate at which it was replaced by the intercept method varied from one navigational culture to another. Note that there were intercept methods with plotted LOPs long before the short tables like HO 229. But you're absolutely right that navigators were doing latitude by Noon Sun and longitude by Time Sights right through the Second World War, especially in the US merchant marine. By contrast, a significant fraction of British merchant vessels were already plotting celestial LOPs in the late 19th century. But this does again highlight one of the great myths of celestial navigation, namely, that Sumner's lines were in wide use (at least on American vessels) from the middle of the 19th century forward. That just wasn't the case.

    I have taught a class at Mystic Seaport in Connecticut for several years now called "Celestial Navigation: Nineteenth Century Methods". I teach these very methods, and though the class is focused on the 19th century, and we look at historical logbooks, I try to emphasize that "Latitude by Noon Sun" and "Longitude by Time Sight" never became obsolete and a modern navigator can certainly use them today as long as we know a few limitations of these methods. They were no less inaccurate than other methods of celestial navigation when used correctly.

    You added:


    "Not a lot of computation involved and you can plot your fix directly onto the chart."

    Exactly. And part of the historical attraction was the fact that plotting was not required. In fact, some late 19th century navigators considered any method that involved plotting as "fancy navigation". And of course plotting burned through paper which was not necessarily widely available.

    You wrote:
    "I consistently get within 5 NM of my actual position when shooting from land..all be it, with the correct Latitude, but varying the Latitude by a few nm keeps me within the 10 nm."

    As long as the azimuths of the Sun are roughly East/West, the variation of the latitude should not introduce much error. In any case, you should perhaps expect better accuracy than this. The math is every bit as accurate as any other method (e.g. HO 229). With a metal sextant, you should expect positions within 1 nautical mile. With a plastic sextant, within about 3 nautical miles. The idea that you wouldn't worry about accuracy better than ten nautical miles is a rather recent notion for NavList posts, and it's really an exaggeration. It's true that you could "get by" with that accuracy (it's a big, empty global ocean and all that), but that doesn't mean that you can't do better consistently, especially if you're practicing from land, and it doesn't mean that you shouldn't try to do better.

    You asked:


    "Any reason why traditional sailors travelling the seas would not use this method when far from land today when they are inclined to turn off the GPS?"

    It's mostly a question of navigational culture. Students of celestial navigation today tend to learn late 20th century methods. There is one big non-cultural advantage of any of the intercept methods. You can get an LOP for any celestial object that you can see: the stars in twilight, the Moon in daylight, the Sun at azimuths closer to the meridian, etc. And they ALL are processed in the same way and yield immediately transparent navigational information --a celestial line of position for each. While it's true that you can extend the methodology of Lat by Noon Sun to other bodies on the meridian and while you can do time sights with stars in twilight, this was relatively rare in practice and it's much more work.

    You concluded:


    "Would seem to save a lot of plotting."

    Yes. Absolutely. One small scale (large area) chart for an ocean crossing, and a few large scale charts for your destination and alternates, and you're all set. No fussing with little plotting sheets.

    -FER

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