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    Re: Longitude by star clock
    From: Frank Reed
    Date: 2021 Aug 19, 10:14 -0700

    Duane Smith, you wrote:
    "Greetings all, first time here, just wanted to share something that might be interesting."

    In fact, you posted about this topic almost exactly a year ago. Here are the messages from that discussion last year: Finding longitude from LST.

    You also wrote:
    "It's doubtful anyone out there has used the star clock to get longitude. It seems God and I are the only ones who know about and approve this method."

    No, it's not new, but it doesn't work the way you think it does. First of all, visual observation of Local Sidereal Time (by the way, this is exactly the same as LHA Aries which is more familiar to navigators) has been available in the navigators' toolbox for centuries. It has some limited use. There used to a tool for this called a nocturnal, which was understood, rather literally, as a night-time or "nocturnal" version of a sundial. Four centuries ago, navigators at sea used nocturnals as simple local-time clocks after dark. Hobbyists sometimes still experiment with nocturnals. In the modern world, one can use a makeshift nocturnal or equivalent to get a visual estimate of LHA Aries to adjust the altitude of Polaris (for latitude). But no, you can't get longitude this way beyond a grossly crude estimate that would have been counted as useless even 250 years ago.

    You continued:
    "It is quite simple. Measure the LHA of a star directly on the sky. Combine the LHA with the stars' Right Ascension to get LST. Combine LST with GST to get longitude. This can be done All Night. Polaris is the pivot point to measure by, the center of the clock."

    This is a good example of armchair theorizing. A portion of what you have written here is founded on a valid concept, but you have glossed over the difficult part with a single word: measure. This can't be done to better than roughly 3-5°. Therefore a longitude derived from the process will also have a typical error of 3-5° or more. And that's useless.

    You wrote:
    "that star's RA is your Local Sidereal Time."

    Absolutely correct. And this principle is "baked in" in standard celestial navigation. It's nothing new. The principle is correct, but your proposed application of it is, unfortunately, misguided. Sextants exist because the observation you propose can't be done in any practical way.

    You added:
    "What may scare many readers is the list of things not needed for this method. You will NOT need; sextant, sight reduction tables or forms, plotting sheets, DR, AP, EP positions, atmospheric refraction, altitude above sea level. It is amazing nobody seems to have figured this out before."

    Well.... no..... People have figured this out before. The reason you don't need corrections and other details is because this method is hopelessly poor as far as direct position-finding (longitude determination) is concerned. Estimating LHA Aries (again, identical to LST) visually to the nearest handful of degrees does have some value when correctng the altitude of Polaris or when setting up a traditional star finder. It's also a handy local-time clock accurate to +/- 15 minutes. But you can't get longitude this way. Not in any useful sense.

    So why not? What's the fundamental problem? It's really much the same as the sundial problem. Why can't we get longitude by comparing a clock set to Greenwich Time with the local time determined by a sundial? In fact you can do this on land to low accuracy. At sea? It's hopeless. Sundials don't work on rolling decks. Similarly, holding a nocturnal properly vertical is quite difficult at sea. Instead we use a sextant to replace a sundial and nocturnal. With a little math, the process of observing the Sun's altitude effectively converts a sextant into a fancy, accurate sundial. That's exactly how the "Old Navigation" worked.

    Frank Reed

       
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