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    Re: Logic of shades
    From: Paul Dolkas
    Date: 2018 Oct 26, 18:47 +0000

    On the subject of zeroing out errors before using a plastic sextant - you will probably want to re-check the errors after you take your shots, and add (or subtract) half that value to your readings. These things warp so much in the mid-day sun that I find I get significant errors in the 10 or 15 minutes it takes me to do a bunch of shots. And this is after I've let it equilibrate to outside temp for 30 or so minutes before shooting.  This is why top notch sextants are made of metal, and the ones for tropical use are painted white as well.

    At one time the upscale Davis Mk 25 was made out of a different & more stable plastic than the Mk 15, but I think all they do now is change the color.  (I could be wrong on this) The way you make a plastic less "warpy" is to mix the raw polymer with glass powder or fibers, but this wears the mold out faster.  Still, I agree that plastic sextants are a good way of getting into CN, and you can probably find decent ones on Ebay for $100 or so. (Mine cost about that)


    From: NavList@fer3.com <NavList@fer3.com> on behalf of Frank Reed <NoReply_FrankReed@fer3.com>
    Sent: Friday, October 26, 2018 10:30 AM
    To: Paul Dolkas
    Subject: [NavList] Re: Logic of shades

    Welcome aboard!

    A Davis Mk 15 is a great sextant and quite capable of getting you safely across any ocean using celestial navigation. The only serious limitation with plastic sextants is that you need to zero out (or measure) the index error before every round of sights. It's a minor nuisance, but you can expect accuracy of 2-3 miles if you're careful using it. 

    There is no real logic to the shades on sextants, and on some sextants the shades are distinctly bizarre. I owned a 70s-era MAC sextant (a Tamaya-like sextant) some years ago that had one shade so dark that the Sun could not be seen through it. The various colors are intended for convenience only. You can make your Sun green if you like... or red if you prefer. But you're right: they might as well be color-less neutral density filters of various strengths. Note that the horizon shades make even less sense.

    You're quite correct about slide rules. Common slide rules don't have quite the accuracy for full celestial navigation calculations, but you can use them for tasks like interpolation and some specialty sights. If you're a fan of slide rules, you should inquire here about the Bygrave and other cylindrical slide rules. These are designed to create very long scales by wrapping them around a cylinder. There has been great hobbyist interest in these here in the NavList community over the years.

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

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