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Thread: How much sextant ($$$) is needed to learn the basics?

  1. #36
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    Polishing should be a crime.

  2. #37
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    Why is polishing the arc so harmful? I don't have a sextant to polish. I'm simply curious.

    Jeff

  3. #38
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    Quote Originally Posted by jpatrick View Post
    Why is polishing the arc so harmful? I don't have a sextant to polish. I'm simply curious.

    Jeff
    Ah… There are in fact two related crimes that can be carried out against a sextant with polish and a rag. One relates to micrometer sextants and is polishing the thread on the arc that the worm gear moves on, because the accuracy of the instrument depends on this and if the thread is worn down with polish the accuracy will deteriorate. The other crime relates more particularly to older vernier sextants. These usually have the graduations engraved on a strip of silver that is let into the bronze arc. People who want to get it nice and shiny will do so and in doing so will make the arc difficult or even impossible to read, rendering the instrument useless.
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  4. #39
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    DSCF6863.jpg


    DSCF6867.jpg

    I bought a Davis MK 15 from Toplicht in Hamburg for 200,- in 2005 but much later picked up a Russian SNO-T from ebay for about 260,- , not much more than a Davis would have cost by then. The Davis is functional but the SNO-T is a much better instrument in direct comparison. Every now and then the Russian sextants come up at affordable prices and it may be worth to hunt for one of them.
    By the way, I also wanted them to learn the basics and am still not fully competent in using them. Being landlocked does not help too much either, although I also bought an artificial horizon from Davis.
    Attached Images Attached Images
    Last edited by SOF64; 09-30-2022 at 03:15 PM. Reason: swapping pictures

  5. #40
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    Just returned this afternoon from a road trip to Eau Clare, WI and Minneapolis, MN. Davis Mark 15 had arrived Sat (neighbor took it off the porch) and it looks just like the one SOF64 shows above. Class #2 is this evening. From what has been posted it would seem risky to buy a used sextant, originally od high quality, on the internet as it would be impossible to identify mistreatment by former users. Is that a correct assumption?

  6. #41
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    The old books are full of warnings about buying second hand sextants. When I bought my first one I had it re-certified by the venerable firm of nautical instrument suppliers from whom I bought it. That is practically impossible now. The makers are out of business and only a firm like Potters (who had an upstairs room devoted only to sextants
    as recently as 1982), could ever have afforded a collimating instrument and the skilled staff to operate it.

    My other one hasn’t been checked but since it came unused from German Navy stores I reckoned it was OK.

    I think that if a sextant depends essentially on:

    Four things that can’t be fixed:

    1. The accuracy of the centering of the arm pivot.
    2. The accuracy with which the worm and thread have been cut (for a micrometer sextant)
    3. The accuracy of the gradations and of the clamping screw (for a vernier sextant)
    4. No distortion in the frame

    Three of these are manufacturing errors and the last one is what happens if it’s drop kicked across a steel deck in which case there will be other visible damage.

    Four things that might be fixed.
    5. The absence of collimation error in the telescope(s).
    6. The condition and quality of the index and horizon mirrors
    7. The condition and quality of the telescope lenses and prisms
    8. The state of the box and accessories
    5 and 7 depend on seeing the instrument, 6 and 8 can be judged from a picture on line.

    And finally the three errors which the user is expected to check and correct himself anyway…

    Then if you keep these things in mind you can probably take an educated guess about the condition of a sextant that you see for sale on line.
    Last edited by Andrew Craig-Bennett; 10-03-2022 at 05:05 PM.
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  7. #42
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    I may just have been lucky with the SNO-T sextant, but with the information provided by W. J. Morris' books I could verify that it works precisely, and I could even tweak it a bit to further increase precision. It looked fine in the ebay pictures, and I took a deliberate gamble at the price it was advertised for.
    I have done angular measurements of stars and of the diameter of the moon and such, but I haven't done more than a bit of reading on celestial navigation so far.
    (By the way, the sextant books by W. J. Morris are so good that they put the idea in my head I could try and build one for myself, perhaps with a precision equivalent to the Davis sextants.)

  8. #43
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    I agree. W.J.Morris’s books are a “must have”.
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  9. #44
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    Just saw this. Frank Worsley’s Hezzanith vernier sextant, in the museum of the Scott Polar Research Institute in Cambridge.

    https://www.spri.cam.ac.uk/museum/sh...les/y2010.75a/
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  10. #45
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    Hi, Andrew

    This is a beautiful sextant with a fascinating story attached to it.

  11. #46
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    Hi Horst,

    I think that sextant is in the same category as Bligh’s chronometer:



    Larcum Kendall’s “K2” his cheaper (relatively!) copy of John Harrison’s “H4”. Both are now in the British National Maritime Museum in Greenwich.
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  12. #47
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    When I last saw them, Harrison's H3 and H4 were displayed next door at the Royal Observatory. I stood there in awe and admiration for quite some time.

  13. #48
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    So did I.

    Kendal’s “cheap” copy omitted the seven and a half second remontoire and was correspondingly less accurate so Bligh had to go ashore and recover GMT by observation (which he was capable of doing). K2 was taken by the mutineers and was eventually swapped for other stuff with a Yankee whaler whose Master sold it in Chile and after sixty years the Captain of a visiting RN ship bought it and presented it to the British Museum, who have enough clocks,and gave it to Greenwich.
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  14. #49
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    Default Re: How much sextant ($$$) is needed to learn the basics?

    Bligh may not have been the most competent leader, but he must have been a remarkably capable navigator. Fletcher Christian's leadership also ended in a catastrophe on Pitcairn Island.
    The story of the Harrison chronometers has fascinated me for a long time. First I found a picture of one of his wooden clocks in a book on the history of woodworking. Later I read extensively about the subsequent development of his marine chronometers H1 to H4 and about his fight for recognition and the promised reward. More recently I read about the further fate of the chronometers and the efforts of Rupert T. Gould to restore them and bring them back to life.

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